contamination and cleanliness imaps 2010 06
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Craig Hillman
Contamination and CleanlinessDeveloping Practical Responses to aChallenging Problem
IMAPS-Nordic Gotenborg, Sweden June 8, 2010
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Outline
Overview of Contamination
and Cleanliness
Motivation
Trends
Drivers
Temperature
Humidity / Moisture
Voltage and Electric Field
Contamination
Sources of Contamination
Printed Board Fabrication
Fluxes / Assembly
Handling
Use Environment
Detection and Measurement
Types of Equipment and
Processes
Industry Best Practices
Contamination Limits
Mitigations
Cleaning
Conformal Coating
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Motivation / Trends
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Why Contamination and Cleanliness?
Believed to be one of theprimary drivers of fieldissues in electronics today
Induces corrosion and metal migration(electrochemical migration ECM)
Intermittent behavior lends itself tono-fault-found (NFF) returns
Driven by self-healing behavior Difficult to diagnosis
Pervasive Failure modes observed on batteries, LCDs, PCBAs, wiring,
switches, etc.
Will continue to get worse
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Future of Contamination / Cleanliness
Continued reductions in pitch between conductors will makefuture packaging more susceptible
Increased use of leadless packages (QFN, land grid array, etc.)results in reduction in standoff
Will reduce efficiency of cleaning, which may lead to increasedconcentration of contaminants
Increased product sales into countries with polluted and tropicalenvironments (East Asia, South Asia, etc.)
ECM occurrence very sensitive to ambient humidity conditions
Pb-Free and smaller bond pads Require more aggressive flux formulations
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The Future: Reduced Spacings
Critical distance effect? Two studies by DfR staff
NaCl seeding
Conformal coating over no-clean flux residues Over 300 coupons tested (IPC-B-25)
6.25, 12.5, and 25 mil spacings; 40C/93%RH, 65C/88%RH,85C/85%RH
No ECM at 25 mil spacings
Experience based onolder designs may notbe valid
Test coupons must
have the same spacing andsame electric field as actualproduct 2 mil (48 m)BGA Substrate Traces
4 mil (100 m)PCB External Traces (low
voltage line)
7 mil (170 m)Thin Shrink Small Outline
Package (TSSOP) Leads
5 mil (120 m)Peripheral Flip Chip Solder
Bumps
SpacingParameter
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QFN and Dendritic Growth
Large area, multi-I/O and low standoff can trap fluxunder the QFN
Processes using no-clean flux should be requalified Particular configuration could result in elevated weak
organic acid concentrations
Those processes not using no-clean flux will likelyexperience dendritic growth without modification ofcleaning process
Changes in water temperature Changes in saponifier
Changes to impingement jets
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QFN and Dendritic Growth (cont.)
The electric field strength between adjacent conductors is astrong driver for dendritic growth
Voltage / distance
Digital technology typically has a maximum field strength of0.5 V/mil
TSSOP80 with 3.3VDC power and 16 mil pitch
Previous generation analog / power technology had a maximumfield strength of 1.6 V/mil
SOT23 with 50VDC power and 50 mil pitch
Introduction of QFN has resulted in electric fields as high as
3.5 V/mil 24VDC and 16 mil pitch
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QFN and Dendritic Growth (cont.)
Some component manufacturers are aware of thisissue and separate power and ground Linear Technologies (left) has strong separation power and
ground Intersil (right) has power and ground on adjacent pins
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Higher Activity Flux
Reduced bond pads = smaller volumes of solder paste (less flux) Oxide thickness remains the same
Requires higher activity fluxes
Copper
Copper
Oxide
Solder
Paste
6 mil stencils
4 mil stencils
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Drivers
Background on ECM
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Failure Mode
Why do you care about excessive
contamination or insufficient cleanliness lead
to?
Electrochemical Migration
(note: not Electromigration; completely different mechanism)
Understanding the mechanism provides
insight into the drivers and appropriatemitigations
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Electrochemical Migration (ECM)
Defined, per IPC-TR-476A, Electrochemical
Migration: Electrically Induced Failures in
Printed Wiring Assemblies (1997), as The growth of conductive metal filaments or
dendrites on or through a printed board under the
influence of a DC voltage bias
Too narrow
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What is ECM?
Alternative definition
Movement of metal through an electrolytic solution under
an applied electric field between insulated conductors
Electrochemical migration can occur on or in almost
all electronic packaging
Die surface
Epoxy encapsulant
Printed board
Passive components Etc.
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ECM Mechanisms
Some ECM Mechanisms have more definitivedescriptions
Dendritic growth Descriptor for ECM along a
surface that produces adendrite morphology
Tree-like, Feather-like
Conductive anodic filaments(CAF) Descriptor for migration within
a printed circuit board (PCB)
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ECM Steps
Traditional electrochemical migration involvesfour steps
Path formation Electrodissolution
Ion migration
Electrodeposition
In ECM along internal surfaces (e.g., CAF),ion migration / electrodeposition co-exist
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Path Formation
Physio-chemical changes necessary to initiate ECM
Different meanings for different mechanisms
Believed to be the rate-limiting step
Dendritic growth
The creation of an electrolytic solution sufficiently
conductive Driven by relative humidity, contaminants,
delamination
Conductive anodic filaments (CAF) Degradation of the epoxy/glass interface
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Electrodissolution
Minimum requirements Electric field and a polar liquid
Usually water
Voltage > Anodicelectromagnetic force (EMF) 0.34 volts for Cu
Electrochemical reactionusually occurs adjacent toanode 2H
20 O
2+ 4H+ + 4e-
Increased concentration of H+
reduces pH to below 7.0
Dissolution of anode material (Cu) Cu Cu+ + e- or Cu Cu2+ + 2e
Halides are not required to induce dissolution Can increases susceptibility through changes in ion formation
Reduction in pH; increases solubility of metal ions (i.e, copper)
Cu+2
H+ H2
H+
H+
Copper
e e
H+
H+
Cu2
O
CuO Cu2O
NaClNaCl
Na+ Cl-
Cl-
Na+
CuCl H2O+
Cu2O
Cl-
CuCl++ H+
H2
H+
Cl-
Cu+2Pure Water --
Cu+1Chloride --
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Ionic Migration
DecreasingpH
v = E = q/(6r)
v is velocity, is mobility, E is field strength, q is charge, r is ionic radius, is viscosity
Also known as electrophoresis
Migration of charged
particles through a solution
under the influence of anelectric field
Positive ions travel along the
field lines From anode to cathode
Electrons travel in reverse
direction
Velocity can be computed
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Electrodeposition
Once ions reach the cathode, electrochemical reactions occur:
O2 + 2H20 + 4e- 4OH -
2H20+ 2e- 2OH- + H2
Cu
+
+ e
-
Cu or Cu2+
+ 2e
-
Cu Because cathodic electrodeposition is largely diffusion controlled, rate
depends on the metal ion concentration in the aqueous medium
Production of hydroxyl ions (OH-) at cathode can reduce the rate of
electrodeposition by combining metal ions with hydroxyl ions to formhydroxides Hydroxides remove metal ions from aqueous medium through partial
precipitation
Thus, ECM rate is correlated with the solubility product of the metal ionhydroxide
Ionic contaminates, such as Cl -, can further change the ECM rate byforming alternate reaction paths and by forming additional ionic species, Example: CuCl in the case of copper and Cl-.
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Dendrite Propagation
Growth along preferentialcrystallographic directions
Creates tree-likeappearance
Primarily along (111) plane
Branches in the [211] and
[110] directions
(111) is a close-packed (CP)plane in FCC structures.
Most metals are FCC CP planes are slip planes
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Drivers
Temperature / Moisture
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ECM Drivers: Electrolytic Solution
Comprised of water and ions (thin film of moisture)
Where does the water come from? Ambient moisture in the air
Evaporation of absorbed moisture (surface ECM)
Measurement (techniques) Adsorption -- Quartz crystal, Ellipsometry
Absorption -- Weight gain
Measurement (units) Adsorption -- Monolayers of moisture, or areal mass density (ng/cm2) 1 monolayer = 31 ng/cm2
Absorption -- Percent change in weight
How much water?
Very dependent upon relative humidity Relatively insensitive to temperature
Even for moisture absorption (numerous internal interfaces)(faster diffusion)
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Water Adsorption (Metals)
Leygraf and Graedel, 2000Lee and Staehle, 1997
Metals
2
4
6
8
10
Monolayerso
fMoisture
0 Copper
Marcus, 2002
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Water Absorption
0.00
0.05
0.10
0.15
0.20
0.25
0 5 10 15 20 25 30 35
Time1/2
(hour1/2
)
Moist
ureContent(%)
85oC/85%RH
40oC/93%RH
Epoxy encapsulant
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Monolayers of Moisture
Insulator Material
_
AnodeCathode
Most measurements performed onnon-porous solids (metals, ceramics)
Surface of interest for ECM tends
to be polymeric (solder mask, FR4,
encapsulant)
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Moisture Adsorption (Polymer)
Monolayers over polymerdifficult to measure
Dueling processes ofadsorption and absorption
Influenced by surfaceproperties (hydrophilic,hydrophobic), bulk properties,and porosity
Parameters rarely measuredby materials supplier
Actual number of monolayersover polymeric surface as afunction of relative humidity is
debatable Estimate of few to several
hundred
ECM possible> 80%RH
ECM likelyCondensation
ECM unlikely60 80%RH
ECM rare< 60%RH
General guidelines
Partially dependent upon ionic solubility
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Deliquescence
Dissolution of ionisableresidues Resistance change can be
several orders of magnitude(der Marderosian, 1977)
Each inorganic compoundhas a different critical RH
HCl contaminatedsubstrates Dilution of contaminants at
70%RH (Zamanzadeh,1989)
%RH
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Condensation
What is condensation?
When surface moisture becomes visible?
The amount of adsorbed moisture at 100%RH?
Definition somewhat unclear Film thickness can range (metals)
Minimum (10 monolayers of moisture)
Dew (~1,000 monolayers of moisture)
Raindrops (~10,000 monolayers of moisture) When does condensation occur?
At 100%RH
When the surface temperature is below the dew point
temperature Presence of cracks and delaminations
Hygroscopic
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Problems with Condensation
Mimics behavior of bulk water Large decrease in resistance
5 monolayers has conductivity two orders of magnitude lowerthan bulk water
20 monolayers has a conductivity one order of magnitudelower
Dissolution of additional surface contaminants
Rapid decrease in time to failure
Days to seconds
Changes in migration behavior
Larger distances Over alternative surfaces (e.g., conformal coating)
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*From Contamination Studies Laboratory, Inc., http://www.residues.com
Elapsed time 12 sec.
Dendritic Growth during Water Drop Test
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Pin to pin migration over conformal coating
DendriticGrowth
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Cracks and Condensation
Vapor pressure of water in a crack is reduced
Condensation of liquid occurs when partial
pressure matches reduced vapor pressure
Water molecules adsorb to surface; first
layers bonding energy is comparable to
chemical bond
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Capillary Condensation
ln RH = -2v/RTd, where is surface tension of water (~80 dyne/cm)
v is molar volume (18 cm3/mole)
R is gas constant (107 dynecmmole-1K-1)
T is absolute temperature
d is crack opening
d
k d
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Cracks and ECM
Greatly reduces humidity
necessary to induce
condensation Why ECM in cracked chip
capacitors happens
relatively rapidly
Why conformal coatingmust adhere to the board
surface
Why popcorning results in
elevated leakage currents in
plastic encapsulated
microcircuits (PEMs)
H i R id
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Hygroscopic Residues
Certain contaminants create conditions thatincrease moisture film thickness
Increase risk of condensation Ionic and non-ionic contaminants
Examples: Polyglycols
When present, turns surface from hydrophobic(water repelling) to hydrophilic (water attracting)
Non-ionic: Not detectable using ion
chromatography or Omegameter
L L
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Lessons to Learn
Do not underestimate relative humidity in controlledenvironments (telecom, server, storage)
Macro environment controlled to 40-60%RH
Micro environments can easily exceed 80%RH or greater forlong periods of time
Case study
Failure of hard drive controller due to ECM internal to the epoxyencapsulant
Environmental tests by DfR Solutions staff indicated ECM wouldnot initiate below a certain percentage of moisture absorption
Equivalent to 82%RH
Drivers
Minimal power dissipation during standby Use of metal enclosure (minimal air flow)
Possible outgassing of absorbed moisture from polymeric materials
L L ( )
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Lessons to Learn (cont.)
Do not overestimate potential for condensation inuncontrolled environments
Temperature rise of 5C often sufficient to preventcondensation Constant power on
Excessive condensation can initiate corrosion over
conformal coating Critical focus
Cleanliness
Housing design (prevent physical water from coming into
contact with board surface)
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Drivers
Voltage / Electric Field
El t i Fi ld (V lt /Di t )
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Electric Field (Voltage/Distance)
Voltage is a primary driver in two processes
Electrodissolution (oxidation reaction)
Ion Migration Electrodissolution
Applied voltage must exceed EMF
0.13 V for Sn/Pb, 0.25 V for Ni, 0.34 V for Cu,
0.8 V for Ag, and 1.5 V for Au
Ion migration
Force on the ions, and therefore velocity, is a function of
electric field strength
El t i Fi ld St th (N i l)
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Electric Field Strengths (Nominal)
Logic devices
Previous generation 6.4 V/mm (SO32 --
1.27 mm pitch, 5 VDC)
Current generation 20 V/mm (TSSOP80 --
0.4 mm pitch, 3.3 VDC)
Power devices
Previous generation 64 V/mm (SOT23 --
1.27 mm pitch, 50 VDC)
Current generation 140 V/mm (QFN
0.4 mm pitch, 24 VDC)
Copper traces: 240 V/mm (0.5 mm spacing, 120 VDC)
IPC-2221A allows 300 V/mm (0.1 mm spacing, 30 VDC)
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Copper traces
120 VDC with 20 to 30 mil spacing (5 to 6
V/mil)
IPC allows 15V / mil (IPC-2221A)
Electric Field (cont )
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Electric Field (cont.)
Prior studies have demonstrated varying effects of
electric field on ECM
DfR (seeding of IPC-B-25 coupons with NaCl) 6V / 6.25 mil = 1 V/mil (dendritic growth)
42V / 25 mil = 1.7 V/mil (no dendritic growth)
Undefined combination of electric field and spacing to
initiate ECM
Conductive anodic filament (CAF) growth models
have demonstrated conflicting behavior
V/d, V/d2, V/d3, and V/d4
E Field and Contamination
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E-Field and Contamination
0.0
1.0
1.5
2.0
0.5
6 10 14 18
Halide concentration (g/in2)
ElectricField(V/mi
l)
E Field and Dendritic Growth
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E-Field and Dendritic Growth
Immersion silver (Ag)plating
85C / 85%RH / 10VDC Observation
Migration only at tip ofcomb pattern
Dendrites stopped growing
Why? Maximum electric field
strength
Electric Field Strength (ImAg Migration)
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Electric Field Strength (ImAg Migration)
E-field (6.25 mil spacing) E-field (12.5 mil spacing)
Emax = 3.50 V/mil Emax = 3.02 V/mil
At 85C/85RH, the critical electric field strength to initiate
silver migration is between 3.0 to 3.5 V/mil
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Drivers
Contamination
Contamination
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Contamination
Two concerns
Hygroscopic contaminants (discussed)
Ionisable contaminants that are soluble in water (e.g., acids,
salts)
Ionic contaminants of greatest concern
Primarily anions; especially halides (chlorides and bromides)
Chemically aggressive due to chemical structure
Very common in electronics manufacturing process
Decreases pH; few metal ions found in dendrites are soluble at midto high pH. Cu dendrites require pH less than 5 to form.
Silver(I) ions are soluble at higher pH; reason it is one of easiest toform dendrites.
Cations primarily assist in the identifying the source of anions
Example: Cl with K suggests KCl (salt from human sweat)
Sources of Contaminants
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Sources of Contaminants
Printed board fabrication process
Insufficiently cured polymers
Rinse water
Fluxes
Handling
Storage and use environment
Sources of Contaminants (cont )
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Sources of Contaminants (cont.)
Rinse WaterNO4
Solder FluxWeak Organic Acids
Rinse Water, Air Pollution, Papers/ PlasticsSO4
Cleaners, Red PhosphorusPO4
Teflon, KaptonFl
Printed Board (flame retardants), HASL FluxBr
Board Fab, Solder Flux, Rinse Water, HandlingCl
Possible SourcesIon
Printed Board Fabrication Process
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Printed Board Fabrication Process
One of the most common source of
contaminants
Greatest use of active/aggressive chemicals Low margin business (reverse auctions)
Increasing use of no-clean assembly process (last
chance to clean)
PCB Contaminants (examples)
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PCB Contaminants (examples)
Etching Chloride-based: Alkaline ammonia (ammonium chloride), cupric chloride,
ferric chloride, persulfates (sometimes formulated with mercuric chloride)
Other: Peroxide-sulfuric acid
Neutralizer Hydrochloric acid
Cleaning and degreasing Hydrochloric acid, chlorinated solvents (rare)
Photoresist stripping methylene chloride as a solvent
Oxide Sodium chlorite
Electroless plating Sodium hypochlorite (in potassium permanganate)
Palladium chlorides (catalyst)
Printed Board Fab (other examples)
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b ( p )
Bromide sources
Surface processes Solder masks, marking inks, and fluxes
Flame retardant
FR-4 Epoxy has used a brominated bisphenol A (TBBA)
epoxy resin
IPC-TR-476A: Bromide in epoxy resin can diffuse to the
surface during a high temperature process such as
soldering
Source of Contaminants: Fluxes
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Chemicals used for preparing metal surfaces for soldering
High molecular weight chemistries
Slightly acidic
Optimum behavior
Maximum activity during reflow; minimum activity after reflow
Difficult balancing act
Flux nomenclature Rosin only (RO)
Rosin, midly activated (RMA)
Rosin activated
Water soluble Low residue (no-clean)
Flux Chemistry
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y
Two types: Rosin and Water-Soluble (WS)
No-clean can be rosin (Indium 3549-HF) or water-soluble
(Kester 977)
No-clean based on ability to pass surface-insulation
resistance (SIR) and electrochemical migration (ECM) tests
Initial classifications from military (Mil-F-14256E);primarily focused on rosin-based fluxes
Rosin/Resin (R), Rosin Mildly Activated (RMA), Rosin
Activated (RA), Rosin Super Activated (RSA)
Primarily R and RMA used for electronics assembly; someRA
J-STD-004 Flux Classification
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J
J-STD-004 (cont.)
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J ( )
Materials (Definition)
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( )
Rosin: Derived from tree sap. Mix of abietic acid and othercompounds
Resin: Natural and synthetic compounds designed to act likerosin
Organic (aka OA): Solution of organic acids and othercompounds (hydrohalides, amines, and amides). Tend to be
water soluble since they contain no rosin
Inorganic: Solution of inorganic acids. Tend to be watersoluble since they contain no rosin. Not as common asorganic (very aggressive)
Activity levels based on Copper Mirror Test (IPC TM-650)
Flux Classification Correlation
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L0 - All R, Some RMA, Some Low Solids "No-Clean"
L1 - Most RMA, Some RA
M0 - Some RA, Some Low Solids "No-Clean"
M1 - Most RA, Some RSA
H0 - Some Water Soluble
H1 - Some RSA, Most Water Soluble
Fluxes (cont.)
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5% of the market uses rosin
Preferred in the early days of electronics manufacturing
Insoluble residues encapsulated contaminants
Hydrophobic surface
Quasi-conformal coating
This is decreasing (concerns regarding environmental-friendliness of solvents required to clean)
25% uses water soluble fluxes
70% of the market uses no-clean
80-90% in consumer, computer, telecom markets
This is increasing
Fine pitch, low clearance, high density greatly decreases
cleaning effectiveness Electronics industry very cost sensitive (eliminate one process,
increase throughput)
No-Clean Flux Chemistry
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Activators (weak organic acids) -- 0.5 - 5%
Wetting agents (surfactants) -- 1%
Polyglycols (polyethylene glycol & polypropylene glycol),alkylenic ethoxylate, and fluorinated esters
Chealtors -- 1%
Reacts with green copper salts, formed from activators and
copper, to create clear or white copper products. Masking so operator will not think of this as a corrosion product
Solvents -- remainder (93 98%)
VOC-free is primarily water; others are IPA, methanol, etc.
Flux Residues
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Residues of no-clean soldering?
Water-soluble dicarboxylic acids
Hygroscopic polyethylene glycol ethers
List of potential weak organic acids (WOAs)
Benzoic, Butyric, Formic, Lactic, Malonic, Oxalic, Propionic,
Succinic, Citric, Glutaric, Adipic, Malic
Optimum flux
Acids are neutralized after soldering process
Residual wetting agents are minimized
Other Concerns
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Combinatorial effects are not well known
Anecdotal indications of unexpected deleterious
reactions between fluxes (poor understanding ofthe root-cause)
Reactions between fluxes and industrial pollutants
may also accelerate corrosive behavior
Excessive flux
Typically occur with wave, selective solder, and
hand soldering operations
Handling / Storage / Environment
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Handling Salts from human contact (KCl and NaCl)
Storage Cleaning chemicals
Outgassing
Polymeric materials
Use Environment Dust
Evaporated sea water
Industrial pollutants
Handling / Sweat
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Composition of dissolved salts in water
Can include other biological molecules.
Main constituents, after the solvent (water),
Chloride, sodium, potassium, calcium, magnesium, lactate,
and urea.
Chloride and sodium dominate.
To a lesser but highly variable extent, iron, copper,urocanate (and the parent molecule histidine), and other
metals, proteins, and enzymes are also present.
The main concern regarding sweat is as a source ofchloride
Handling / Sweat (cont.)
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0
20
40
60
80
100
120
140
160
Raw stock After Cleaning Handling (office) Handling
(exercise)
Handling (brow)
Type of Exposure
Con
taminationExtracted(
g)
Chloride
Sodium
Potassium
Calcium
Magnesium
Lactic acid
0.141.200.360.000.3946.610.00Handling (after wiping brow)5
0.090.920.410.000.3925.630.00Handling (after exercise)4
0.101.300.410.000.4914.350.00Handling (office environment)3
0.091.070.210.000.450.470.00After polish and clean2
0.071.000.260.000.432.140.00Raw stock aluminum1
SO4(g/in2)
PO4(g/in2)
NO3(g/in2)
Br
(g/in2)
NO2(g/in2)
Cl
(g/in2)
F
(g/in2)ID
0.141.200.360.000.3946.610.00Handling (after wiping brow)5
0.090.920.410.000.3925.630.00Handling (after exercise)4
0.101.300.410.000.4914.350.00Handling (office environment)3
0.091.070.210.000.450.470.00After polish and clean2
0.071.000.260.000.432.140.00Raw stock aluminum1
SO4(g/in2)
PO4(g/in2)
NO3(g/in2)
Br
(g/in2)
NO2(g/in2)
Cl
(g/in2)
F
(g/in2)ID
Influence of Pollutants: Creepage Corrosion
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Recent field issues with printed circuitboards (PCBs) plated with immersionsilver Sulfur-based creepage corrosion
Failures in customer locations withelevated levels of sulfur-based gases Rubber manufacturing
Sewage/waste-water treatment plants
Vehicle exhaust fumes (exit / entrance
ramps) Petroleum refineries
Coal-generation power plants
Paper mills
Landfills
Large-scale farms Automotive modeling studios
Swamps P. Mazurkiewicz , ISTFA 2006
Creepage Corrosion Failure of ImAg
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Corrosion product is semi-conductive (resistance of about 1Mohm).
Resistance decreases as humidity increases.
Traces sensitive to leakage current trigger the system failure.
Visual inspection required to identify failures (most are CNDs).
Initiation of
a short
Creepage Corrosion Mechanism
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Exposed Cu was consumed to form coppersulfide that could cause electrical shorts.
Creepage Corrosion: Observations
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Galvanic corrosion
Failures at locations with thin or
absent immersion silver
Edge of solder mask, deep in platedthrough hole barrel
Copper and silver in contact
Similar mechanism observedwith electroless nickel /
immersion gold (ENIG) plating
Corrosion of copper trace
at solder mask edge
Solder Mask
Laminate
Copper
Nickel
Creepage Corrosion: Observations
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Failure behavior not observed duringqualification testing [Mixed Flowing Gas(MFG) Testing Class III]
Reason 1: Lack of appropriate structures in
test vehicle
Reason 2: Test conditions may not beappropriate
Strong indication that creepagemechanism requires thatone or more MFG testparameters are exceeded
Especially %RH
Hillman: >75%RH
Cullen: 93%RH ----2005030520020402752IV
200502005020510020301702IIIA
----2005020510020302752III
1002020050103105301702IIA
----20050103105302702II
------------------------I
SO2
(ppb)
NO2
(ppb)
Cl2
(ppb)
H2S
(ppb)
Temp
(C)
RH
(%)
Class
MFG Test Conditions
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Are existing MFG test conditions still
relevant?
Different material system (silver, not copper) Changing environment (is there more / less
pollution?)
Industrial Pollutants (Sulfur-Based)
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SO2 MFG Test
100ppb, 200ppb
Average annual outdoor
2-20ppb (USA) 25-100ppb (Asia)
24 hour
~150ppb (NAAQS / Telcordia)
150-600ppb (Industrial-USA)
100-1500ppb (Asia)
May not be critical forsulfidation of silver
Rate independent of SO2
concentration
H2S MFG Test
10ppb, 100ppb, 200ppb
Average annual outdoor/indoor
0.05 to 0.8ppb 24 hour (outdoors)
8 to 100ppb (State Regs)
24 hour (indoors)
500 to 20,000 ppb
May be more critical
70000
28500
10000
4075
500
200
10
4
10
4
1.5
0.6H2S
40000
15300
10000
3800
1000
380
100
38
100
38
100 ug/m3
38 ppbSO2
Inside
industrial
Adjacent to
industrial
Urban with
heavy traffic
or industrial
RuralControlled
environment
Clean
room
IEC 60721-3-3
Test Conditions (cont.)
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Carbonyl Sulfide (COS) Ignored by MFG
Outdoor levels can be higher than H2S
Nominal: 0.5 0.8ppb
Elevated: 80 ppb
Can be as corrosive
as H2S
Test Conditions (Relative Humidity)
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Influence of %RH somewhat
contradictory
Vernon reported a critical %RH
(70-80%)
Graedel reported an increasing
corrosion rate with increasing %RH
Driven by monolayers (ml) of
moisture
ln (ml) = 2.73 p/p0 - 0.366
(p/p0 is %RH)
Rice reported no influence of %RH
Relative Humidity
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Validation of Rices observation
%RH levels in ceramic hybrid and thick film resistors coated withhydrophobic silicone likely low
Important differentiation by mechanism Most references investigate the tarnish aspect of sulfidation
Creepage behavior is likely very sensitive to %RH
The rough surface of a polymeric material becomesconducive to material transport once micro-condensationwithin occurs.
Filling-in of surface pores may greatly reduce the adhesion ofthe polymer surface
Allows forces created by volumetric expansion of corrosionproduct to push the growth out to an adjacent conductor
Discussion
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Modification of MFG test specs may be appropriate
Elimination of SO2 gases
Increase in H2S concentrations (>200 ppb)
Possible intro of COS Elimination or reduction of Cl2
Speculation that formation of
AgCl inhibits sulfidation of silver Elevated Cl2 displays parabolic
behavior
Elevated H2S displays unlimitedgrowth
Pollutants: Not Always in Industrial Settings
Drywall Sulfur Fumes Blamed for A.C. & Electrical Equipment failures
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Chinese Drywall Cited in Building Woes The drywall is emitting sulfur-based gases that are corroding
air-conditioner coils, computer wiring and metal picture frames.
Drywall blamed for A.C. failures Air-conditioning coils have turned black, along with wiring,
piping and even silver jewelry.
"We have definitely identified that a combination of sulfide
gases are the cause of the corrosion," said Robert P. DeMott,
managing principal of Environ.
"Foul odors reported by people living in the homes may also be
caused by the combination of sulfur gases being released from
the drywall,
Chinese drywall class action lawsuit LEE COUNTY, Fla. - The Lawsuit was filed against Knauf
Plasterboard Tianjin Co., LTD, The Knauf Group, Rothchilt
International Limited and the Banner Supply Company.
Known as "Chinese Drywall", it was manufactured oversees
and was made from waste materials. As a result, it emits
sulfur compounds that corrode copper wiring and other metals
found in homes. Copper Corroded by Sulfur flumes
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Drivers
Others
Insulation
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The influence of insulation (migration surface) onECM is poorly quantified
Hydrophobic surfaces superior Silicone
Solder mask / FR4 epoxy selection rarely based onability to resist ECM
Exposed epoxy glass is much more hydrophilic than mostsolder mask materials
Greater concern and investigation with CAF Degradation of insulation (epoxy/glass interface) results in
path formation
Conductor Material
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Conductors of concern SnPb (solder)
Sn alloy (Pb-free solder, lead plating)
Copper (traces, connectors, component leads)
Silver (conductive adhesives, thick film resistor)
Nickel (capacitor electrodes, lead plating)
Gold (connector plating) Harsanyi and Inzelt established a ranking of pure metals according to propensity
to migrate Ag > Pb > Cu > Sn
Ag will migrate under temperature/humidity/bias unless special prevention Alloying with an anodically stable metal such as palladium or platinum (MLCC) Using an organic coating (immersion silver plating)
Using a metallic coating (SMT resistors)
Cu and SnPb will alternately migrate
Propensity for Pb-free solder to migrate more or less than SnPb has not beendefinitively established Disagreement over which is more migratable (Sn or Pb)
Gold requires contamination to migrate (typically chloride)
Immersion Silver (ImAg)
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Effect of NaCl contamination level
Minimal effect at levels below 10 g/in2
Repressed dendritic growth at levels above 10 g/in2
AgCl is less soluble in water than Ag
0 g/in2 2 g/in2 10 g/in2
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Drivers
Time to Failure Models
Drivers of ECM
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Failure models can provide a more
quantitative understanding of the
primary drivers
Time to Failure (TtF) Models for ECM
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Relevant TtF models must include theprimary drivers of ECM
Electric field (voltage, distance)
Temperature
Relative humidity
Contamination
Other effects to consider
Conductor material
Insulator material
Potential TtF Models
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Fundamental chemical behavior Arrhenius
Eyring
Experimental observations Barton-Bockris
DiGiacomo
Similar corrosion-based mechanisms Peck
Howard
CAF
Arrhenius
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Widely used to describe variety of chemicalreactions
Limited to temperature effects
=kT
HAtf exp
A = scaling constant
H = activation energy (eV)k = Boltzmann constant (8.62 x10-5 eV/K)
T = temperature (K)
Arrhenius (Hornung)
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Based on dendritic growth of silver through borosilicate glass under an
electric field Activation energy determined to be 1.15 eV
Growth rate linear dependence on electric field.
No humidity effect
Not true ECM model (dry silver migration)
Proportionality constant must be empirically determined
= proportionality constantG = electrode spacing
V = voltage
H = activation energy (eV)k = Boltzmann constant
T = temperature (K)
= kT
H
V
G
tf exp
Eyring
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Extension of Arrhenius equation Takes into account multiple stresses and synergy with
temperature
Recommended by IPC SIR Handbook to determine accelerationfactors for ECM
Too comprehensive?
Number of stresses undefined
Number of unknowns increases twice as fast as the number ofstresses
Stress functions undefined (natural log, exponential, linear)
+
++
++
= 21exp ST
EDS
T
CB
kT
HATtf
Barton and Bockris
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Based on dendritic growth of silver in an electrolytic cell
h = conductor spacing
= interfacial energy (metal/solution)R = universal gas constant
T = temperatureF = Faraday constant
D = diffusion coefficient
c = metal ion concentration
= overpotential (function of dendrite rad.)i = current density at the dendrite tip
22
8
=DcF
RThtf
as i
Barton and Bockris
C t i t f i ti t l i t ti b t t f t i t
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Contains a term for migrating metal ion concentration, but not for contaminantconcentration
Dual effect of temperature
Linear dependence
D = D0exp
No humidity term
Good general model for dendritic growth
Requires dendritic growth to be rate limiting step
No terms for path formation, electrodissolution, and ion transport
Influence of ionic contamination on the various parameters, such as the
interfacial energy, would have to determined theoretically or measured throughexperimentation.
Zamanzadeh [11] reviewed this model
Concluded that [This] model does not give accurate growth predictions, but gives ageneral indication of the functional dependence of growth on a variety ofelectrochemical parameters.
DiGiacomo
r 2
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( )
=
v
HrkTerfce
kTVV
zFD
r
MCt
kTH
T
f
2
lnln
2
11
2
0
C = ionic concentration (Ag+)
zF= electric field,
D0
= diffusivity,
r = radius of curvature of the dendrite tip,
l/M = the critical amount of metal ions that must migrate to achieve dendritic growth,
= degree of oxidation or fractional active surface, changes from metal to metal,
VT
= determined by the critical current density for dendritic growth,
H = activation energy, k = Boltzmann's constant, T = absolute temperature,
= ln (r50/r16), sigma for a lognormal distribution
Based on migration of silver in encapsulated packages
Derived from use of the Butler-Volmer equation Relates electrode potential to current density
DiGiacomo
S i tifi i it l t d it
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Scientific curiosity unless current density canbe related to measurable physical factors
Current density is a function of conductance and
electric field Conductance is function of contamination and
relative humidity.
Could be the basis of describing time tofailure in terms of contamination, relativehumidity, and e-field.
Peck
( )kTEfRHA n )(
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tlife = time to failure,A0 = material constant
RH = relative humidity, n = empirical constant (2.66)
Ea = activation energy, k = Boltzmann constantT = temperature, f(v) = voltage function (power law, ~1.5)
( )kTEvfRHAt an
life = exp)(0
Galvanic corrosion of aluminum bond pads in encapsulated
microcircuits
Based on Eyring model
Review of previous research
Primary environments: 85/85, 110/85 Voltages: 5 to 70 VDC
Howard
w = conductor width
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Based on corrosion of conductors on ceramic substrates
Derived from Faradays Law
Maybe applicable to Huawei situation
tMV
wlhndFtf
=
w = conductor width
l = conductor lengthh = conductor thickness
d = density of conductor material
F = Faraday constantM = atomic weight of conductor
V = voltage bias
= resistivity of electrolyte
t = electrolyte thickness
Conductive Anodic Filament (CAF) Models
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( )( ) tt
m
n
eff
f MMMMV
Lfat >= ,
1000
ECM Time to Failure Behavior
Little interest in time to failure model for
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Little interest in time to failure model for
dendritic growth
Why? Under high humidity (>90%RH) testing, strong
experimental indications that rate limiting step
may be nucleation/initiation Most data suggests ECM will initiate within 72 to
120 hours under test conditions, or not at all
Additional indicators
Growth vs. Nucleation (1a)
Higher levels of stress result in failure mechanism shift
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Higher levels of stress result in failure mechanism shift
Prevents acceleration of mechanism
6.25 mils 6.25 mils
10 g/in2 Br + 2 g/in2 Cl 10 g/in2 Br + 50 g/in2 Cl
40C/93%RH / 10VDC / HASL Finish
Gro
un
d/A
no
de
Power
/Ca
tho
de
Growth vs. Nucleation (1b)
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40C/93%RH 85C/85%RH
No-Clean Rosin-Based Flux / Urethane Conformal Coating10VDC / HASL Finish
Growth vs. Nucleation (2a)
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Many drivers have critical stress levels
Relative humidity for deliquescence
Voltage for electrodissolution
Contaminant levels for ECM
Time to Failure
Stress
Wide variation in timeto failure over range of
stresses not observed
for ECM
Growth vs. Nucleation (2b)
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Direct evidence of critical
electric field strength
Recent study performed on
a dummy microcircuits of
copper on silicon
Dendrite formation instantaneous at right conditions
(presence of oxalic acid)
0.4V at 3m spacing
1.2V at 10m spacing
Growth vs. Nucleation (3)
D d iti h l t d d
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Dendritic morphology suggests dependence onnucleation
6.25 mils6.25 mils
Heterogenous nucleation Homogenous nucleation
Growth vs. Nucleation (4)
Where else are dendrites observed?
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Where else are dendrites observed?
Solidification of liquid metal
Analogy
Copper ions in solution nucleate on the walls ofthe cathode conductor (heterogeneous)
Solidification consist of nucleation and growth
At supercooled conditions, nucleation behaviordominates
Preventing ECM
If nucleation-dependent shift should be away
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If nucleation dependent, shift should be away
from predicting time to failure and towards
preventing failure
The most effective method is through tight
controls of contamination
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Contamination / Cleanliness
Detection, Measurement, and Requirements
How to Measure Cleanliness?
Standard ion chromatography (IC) testingIPC TM 650 M th d 2 3 28A
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IPC-TM-650, Method 2.3.28A
Submerge whole board; 75 IPA / 25 DI
Updated IC IPC-TM-650, Method 2.3.28.2
Submerge whole board; 10 IPA / 90 DI (Delphi requirements)
Modified IC Use of saponifiers or alternative solvent
Submerge whole board
Localized Testing C3 from Aqueous Technologies
PCB Cleanliness Control: Industry Specs
IPC-6012B, Qualification and PerformanceS ifi ti f Ri id P i t d B d S ti 3 9
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Specification for Rigid Printed Boards, Section 3.9 Requires confirmation of board cleanliness before solder
resist application
When specified, requires confirmation of board cleanlinessafter solder resist or solderability plating
Board cleanliness before solder resist shall not be
greater than 10 ug/in2
of NaCl equivalent (totalionics) Based on military specifications from >30 years ago
Board cleanliness after solder resist shall meet therequirements specified by the customer
Cleanliness Control: Test Procedures
IPC-6012B specifies a Resistance of Solvent Extract
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(ROSE) method Defined by IPC-TM-650 2.3.25
IPC-6012B specifies this measurement should beperformed on production boards every lot Class 1 boards: Sampling Plan 6.5
Class 2 and 3 boards: Sample Plan 4.0
Sampling plan (example) If a lot contains 500 panels of a Class 2 product, 11 panels
should be subjected to ROSE measurements forcleanliness testing
Test Procedures: Common Problems
ROSE is the least sensitive of ionic measurement techniques 5 ug/in2 detected by ROSE is equivalent to ~20 ug/in2 detected by ion
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g y q g ychromatography
Equipment is not calibrated
Insufficient volume of solution is used
Insufficient surface area Panels are preferred over single boards
Cut-outs are not considered when calculating surface area
Insufficient measurement time
7 to 10 minutes is preferred
Technique Technology Equivalency Factor
ROSE Static / Unheated 1
Omega-Meter Static / Heated ~1.5
Ionograph Dynamic / Heated ~2.0Modified-ROSE, Zero-Ion, etc. Varied ~4.0 (?)
Ion Chromatography 80C for 1 hr ~4.0
Test Procedures: Best Practice
Ion Chromatography (IC) is the gold standard
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Some, but very few, PCB manufacturers qualify lots
based on IC results
Larger group uses IC to baseline ROSE /
Omegameter / Ionograph (R/O/I) results
Perform lot qualification with R/O/I
Periodically recalibrate with IC (every week, month, or
quarter)
Cleanliness Control: Requirements
The majority of knowledgeable OEMs
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completely ignore IPC cleanliness requirements
Option 1: Requirements are based onR/O/I test results, but adjusted for lack of
sensitivity
Most companies now specify 2.5 to 7 ug/in2
Option 2: Requirements are based on IC test
results and then monitored using R/O/I
PCB Cleanliness: Moving Forward
Extensive effort to update PCB Cleanliness Standards
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IPC-5701: Users Guide for Cleanliness of Unpopulated
Printed Boards (2003) IPC-5702: Guidelines for OEMs in Determining Acceptable
Levels of Cleanliness of Unpopulated Printed Boards (2007)
IPC-5703: Guidelines for Printed Board Fabricators in
Determining Acceptable Levels of Cleanliness of UnpopulatedPrinted Boards (Draft)
IPC-5704: Cleanliness Requirements for Unpopulated
Printed Boards (2010)
PCB Cleanliness (cont.)
The old 10 ug/in2 (1.56 ug/cm2) being replaced
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Driven by Delphi requirements
Nominal Ionic Levels
Bare printed circuit boards (PCBs)
Chl id 0 2 t 1 /i h2 ( f 0 5 t 1)
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Chloride: 0.2 to 1 g/inch2 (average of 0.5 to 1)
Bromide: 1.0 to 5 g/inch2 (average of 3 to 4)
Assembled board (PCBA)
Chloride: 0.2 to 1 g/inch2 (average of 0.5 to 1)
Bromide: 2.5 to 7 g/inch2 (average of 5 to 7)
Weak organic acids: 50 to 150 g/inch2 (average of 120)
Higher levels
Corrosion/ECM issues at levels above 2 (typically 5 to 10)
Corrosion/ECM issues at levels above 10 (typically 15 to 25)
Corrosion/ECM issues at levels above 200 (typically 400)
General rule
Dependent upon board materials and complexity
Cleanliness Controls: Ion Chromatography
Contamination tends to be controlled through industrialspecifications (IPC-6012, J-STD-001)
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Primarily based on original military specification
10 g/in2 of NaCl equivalent Calculated to result in 2 megaohm surface insulation resistance (SIR)
Not necessarily best practice
Best practice is contamination controlled through ionchromatography (IC) testing
IPC-TM-650, Method 2.3.28A
*Based on R/O/I testing
PaulsGeneral
ElectricNDCEE DoD* IPC* ACI
Chloride (g/in2) 2 3.5 4.5 6.1 6.1 10
Bromide (g/in2) 20 10 15 7.8 7.8 15
Major Appliance Manufacturer (IC)
Incoming PCB Processed PCB
ContaminantMaximum Level
(ug/in2)
Maximum Level(ug/in
2)
Upper Control Limit(ug/in
2)
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Ammonium
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200150Total Weak Organic Acids
64Sulfate
64Phosphate
64Nitrite
64Nitrate
21Fluoride
42Chloride
1510Bromide
**Upper control limits may be considered maximum levels
for high reliability applications or products used in
uncontrolled environments
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Mitigation -- Cleaning
PCB Cleaning: Process Flow
At a minimum, PCB manufacturers should clean thePCB:
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PCB: Immediately before the application of solder resist
Immediately after the application of any solderability plating
HASL
Electroless Nickel and Immersion Gold
Immersion Tin
Immersion Silver
Some PCB manufacturers also perform a final clean Should not substitute cleaning after solderability plating
Residues from plating operations can become more difficultto remove with any time delay
PCB Cleaning Process: Requirements
Final rinse with deionized (DI) water 2-8 M is preferred; >10 M may be too aggressive
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p y gg
Distilled water is insufficient
City water is unacceptable
Potential options Use of saponifier during the cleaning process
Heated DI water is nice, but not absolutely necessary
Common problems DI water is only used if specified by the customer
DI water is turned off to reduce water and energy usage
Failure to monitor DI water at the source Failure to alarm the DI water on the manufacturing floor
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MitigationPCBA Cleaning
When to Clean?
Very high reliability applications Medical, Military, Avionics, Industrial, Telecom
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Sensitive circuitry High-impedance circuits High-frequency circuits
Operation in uncontrolled environments
Use of conformal coating Concern over compatibility
Note: Some high-reliability markets have moved
away from cleaning Automotive, Enterprise, etc.
Industry Standards on PCBA Cleaning
Requirements driven by J-STD-001 Mandates 10ug/in2 (1.56 ug/cm2) for ROL0 or ROL1 (others based on limit established
by user)
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Section 6.1
Assemblies should be cleaned after each soldering operation so that subsequentplacement and soldering operations are not impaired by contamination
Section 8.2.2
Cleanliness testing is not required (unless specified by the customer)
SPC not required (testing should be random, but sample plan not provided)
Minimum once every production shift
If any assembly fails, the entire lot shall be evaluated and re-cleaned and a randomsample of this lot and each lot cleaned since performing the last acceptable cleanlinesstest shall be tested
Some guidance provided by two handbooks
Guidelines for Cleaning of Printed Boards and Assemblies, IPC-CH-65A (1999)
Aqueous Post Solder Cleaning Handbook, IPC-AC-62A (1996)
Critical Aspects of PCBA Cleaning
Solder Paste / Flux Chemistry
Component Selection and Board Design
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Component Selection and Board Design
Equipment Batch vs. Inline
Critical equipment parameters
Cleaning Solution Includes solvent, chemistry, and temperature
Process Location When to clean?
Cleanliness Requirements and Assessment
J-STD-004 Flux Classification
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Whats Missing?
Minimal information on cleaning
Other than no-clean
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Information needed
How to clean (cleaning solution, cleaning process)
What to clean (amount and type of residues)
Why no cleaning information?
Partially driven by process temperature, board design, and
board materials
Missing Information (example)
Water Soluble is not equivalent to WaterWashable
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Rosin-based flux residues can be removed by
water in combination with a saponifier Non-water soluble flux residues can be removed
through impingement (residue is softened byheated water and knocked off by force ofsprayed water striking the residue)
No-Clean can sometimes be cleaned
Sometimes, it can not
Component Selection / Board Design
Challenges High density
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Low standoff
High density Number of components per area
Number of tall / wide components
Low standoff components Chip components (0402, 0603, etc.)
Area array components (BGA, CSP, etc.)
Bottom-terminated components (LGA, QFN, etc.)
Chip Components
Some debate about worst-case Larger components (e.g., 2512) have more surface area
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Smaller components (0402, 0603) tend to have lowerstandoffs (1-2 mil vs. 3-4 mil)
Potential design improvements for cleaning No traces under components (most common)
Smaller bond pads (lifts up the component)
Thinner solder mask (increases standoff)
Components in parallel (prevents blocking of cleaningsolution)
Board cutouts (primarily for larger chip components)
Area Array Components
Ball grid array (BGA) and chip scalepackages (CSP)
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p g ( )
Standoff and ball-to-ball spacing shrinking 1.0 mm pitch = 20-25 mil standoff
0.4 mm pitch = 10-12 mil standoff
Limited design improvements available
Solutions more process-driven
Bottom-Terminated Components
Components of greatest concern Standoff similar to chip components; surface area similar to BGA
Fast growing component packaging
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Fast growing component packaging
Current styles partially mitigated by thermal pad Limited area under package to clean
Future versions problematic
Dual-row QFN (current standard is single-row) Finer-pitch QFN (currently 0.4 mm)
Land grid array
Limited design improvements available
QFN and Dendritic Growth
Large area, multi-I/O and low standoff can trap fluxunder the QFN
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Processes using no-clean flux should be requalified Particular configuration could result in weak organic acid
concentrations above maximum (150 200 ug/in2)
Those processes not using no-clean flux will likelyexperience dendritic growth without modification ofcleaning process Changes in water temperature
Changes in saponifier Changes to impingement jets
Best Practices: PCBA Cleanliness
Confirm incoming PCB cleanliness
Clean after soldering operations
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g p
Control and measure
Water quality going into process
Assembly cleanliness
Cleaning Equipment
Two options In-line
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Batch (dishwasher)
Batch tends to dominate
In-line primarily justified based on volumes
High-volume manufacturing tends to not clean
Ultrasonic
Rarely used on PCBAs
Batch Cleaning Equipment
Motivations for batch cleaner Insufficient volume for a dedicated line
PCBA is reasonably sized
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Batch cleaner manufacturers EMC Global Technologies
Aqueous Technologies
Manncorp
Austin-American
The batch cleaner should be capable of monitoring outflow
Aka, built-in cleanliness tester or resistivity monitoring
Alarm tends to be set to 4 to 7 megaohms
Closed loop can be a plus in low-cost countries
Cleaning Process and Solution
Variables Pure DI or Saponifier (type and concentration)
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5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
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Temperature (120F to >175F)
Nozzle pressure
Agitation (spray in air)
Time
Cleaning Process (cont.)
The majority of water cleanable solder pasteshave their residues removed by means of
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137/172
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5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com 2004 - 2009 137
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