cleaning before coating - smta · deposition me men+ + ne* ne* + men+ me ... pu-coating ay-coating...
TRANSCRIPT
Influencing factors
Failure mechanisms
Coating failures
Cleaning before coating
Analytics and test methods
Cleaning Before Coating
Atmospheric change
Salt spray
Humidity Migration
Vibrations
Corrosion Corrosive gas
Temperature
Influencing Factors
Trend
Larger components
Low component densities
High standoff spaces
Simple component types
Miniaturized components
High component densities
Low standoff spaces <1mil
Complex component types
Industry Trends
Coating protects against failures due to
humidity:
Electrochemical migration
Leakage current
Signal distortion on high-frequency
circuits
Advantages of Coating
Manufacturing of
PCBs
Residues from acid
processes
Developer
chemistry
Alkaline cleaner
HAL-Residues
Residues of ENiG-/
CSN-Process
Rinse water quality
Manufacturing of
Components
Residues from
metallization baths
Rinse water quality
Deflashing Chemicals
Mold release agent
Flux residues
Assembly
Solder pastes
Wave soldering
Rinse water quality
Reflow soldering
Reflow condensates
Gas emission
Hand perspiration
etc.
Possible Contamination
Resin or rosin
Activator – organic or inorganic
Solvent (dilution)
Thixotropic agent
Fluxes/pastes
Contamination due to handling
i.e. Fingerprints, dust, oil, salts
Critical Residues
Influencing factors
Failure mechanisms
Coating failures
Cleaning before coating
Analytics and test methods
Cleaning Before Coating
Electrochemical Migration
Leakage current
Signal distortion on high-frequency circuits
Climatic Failure Mechanisms
Influence factor:
Moisture
Material factor:
Fluxes
Influence factor:
Humidity
Electrochemical Migration
Vapor/Humidity
+
Ionic contamination
(salts, flux activators)
=
Conductive electrolytes
+
Stress voltage
=
Electrochemical migration
Ground
VCC
Influence factor:
Condensation
Influence factor: Stress voltage
Material factor:
Fluxes
Influence factor:
Humidity
Electrochemical Migration
Reaction rate
Ele
ctr
ode pote
ntial E
SnO2 – H
2O
Passivity
Passivity
Immunity
Sn0
Sn02
Sn
Corrosion
SnH4
Corrosion
Corro-
sion
Sn4 +
Sn02 –
3
Sn2 +
– HSn0
2
pH-value
2,0
V
1,5
1,0
0,5
0
-0,5
-1,0
-1,5
-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
OH- alkalinity
Dissociation of Water and Tin Corrosion
Mechanisms of Dendrite Growth
Ion migration
MeMen+
+ ne*
V
ne* + Men+
Me
Anode PCB Cathode
Deposition
MeMen+
+ ne* ne* + Men+
Me
Anode PCB Cathode
Dissolving
MeMen+
+ ne*
Metal-ions
Anode PCB Cathode
Dendrite Growth
Lead free/Ag containing solder pastes
tend to build up short-lived dendrites
→ Electrochemical migration
→ Proven by continuous resistance
measurement
Climatic Failure Mechanisms
Electrochemical migration
Leakage current
Signal distortion on high frequency circuits
Leakage Current
Optically clean FR4 surface
Loading contrast of optical invisible,
conductive contaminations (SEM)
After climate change
Cracks
Activator Residues
Important: no humidity, only temperature change – day/night
→ Activators influence leakage current and climate safety
Coating Resin layer
After soldering
Flux indicator
Climatic Failure Mechanisms
Electrochemical migration
Leakage current
Signal distortion on high frequency circuits
Conventional contact connections
HF contact connection
Influence of Geometry on the Complex Resistance
Inductance
Capacity
1
0
Signal
Time
Signal
1
0
Time
Bit Failures in HF Connections
Dielectricity of the rosin/resin disturbs transmission
Cleaned PCB no transmission failure
Bit failure
Cleaning Before Coating
Influencing factors
Failure mechanisms
Coating failures
Cleaning before coating
Analytics and test methods
14000
12000
10000
8000
6000
4000
2000
0
20° C 30° C 40° C 50° C 60° C 70° C 80° C
68° F 86° F 104° F 122° F 140° F 158° F 176° F
PU-coating
AY-coating
Wax coating
Wate
r vapor perm
eability,
sta
ndardiz
ed [g / m
2 x h]
Temperature
Water Vapor Permeability
Contamination
Influences on Uncleaned Substrates
Contamination reduces adhesive forces and leads to delamination
Residues Consequences
Resin based residues as part of the flux
Subsequent delamination when exposed to
climatic changes
Cracks in coatings
Failures in HF connections
Flux activators
Hygroscopic – danger of failure
Corrosion on connections
Leakage currents
Organic tin-salts (reaction product from flux and
solder paste)
Insufficient adhesion and creeping underneath
coatings
Sulphur and ammonium compounds (can be
contained in fluxes)
Insufficient adhesion and creeping underneath
coatings
Chemical facilitators for component release Impacts wetting properties
Delamination possible
Oligomeric parts of substrates and solder mask
interaction
Contamination film
Similar effect as with chemical facilitators
Cleaning Before Coating
Contamination under coating is hygroscopic
Limited adhesion on un-cleaned PCB
Coating alone is inadequate because of vapor permeability (even worse
than cleaning without coating)
Coating is applied to increase climatic reliability
→ Reliable coatings require cleaning confirmed by coating suppliers
Cleaning Before Coating
Influencing factors
Failure mechanisms
Coating failures
Cleaning before coating
Analytics and test methods
Partially removed
residues
Partially attacked
residues
CRITICAL!!!
Almost removed residues
LESS CRITICAL!
Flux Residues completely
removed
TARGET!
Stages of Flux Removal
Aqueous
cleaning
Spray-in-air Ultrasonic Spray-under-
immersion
H2O
Semi-
aqueous
Cleaning
Cleaning Before Coating
Ultrasonic Spray-under-immersion
Aqueous Spray-in-Air High Pressure Inline Process
Dosage
Station
Pre-
Wash
Chemical
Isolation Rinse Final Rinse Dryer Dryer Wash
Treatment system Treatment system
2
1
3
4 5
6
7
Pre-
Wash
Chemical
Isolation
Rinse Final
Rinse Dryer Dryer Wash
Process characteristics:
Medium to high throughput
Spray pressure 70 – 140 psi
Aqueous Spray-in-Air Batch Process
Chamber
tank
Rinsing 1
with tap water
Rinsing 2
with DI-water
Tank for
cleaning agent
Heating
Process characteristics:
For low to medium throughput
Spray pressure < 50-70 psi
Small footprint
Cleaning Before Conformal Coating
Influencing factors
Failure mechanisms
Coating failures
Cleaning before coating
Analytics and test methods
Requirements for Coating
Evaluation of cleanliness (prior to coating):
Test Required value
Ionic contamination < 0.4 µg/cm2
Surface tension > 40 mN/m
ZESTRON® Flux Test Residue-free
ZESTRON® Resin Test Residue-free
Test Method – Ionic Contamination
Process schematic
VE - H 2 0 + IPA
PCB
Regeneration
Measuring cell
Pump
Bad wetting= unclean surface
Good wetting= clean surface
Wettability and Surface Cleanliness
Testing method – ink test
Resin residues do not become colored
Samples of Results after Flux Test
No encapsulation of activators in the resin
Resin Analysis
Approval of synthetic and natural resins
Shows local dispersion
Based on color reaction
Test Method – Tin Test
Evidence of stannous components
Based on color reaction
Tin test positive
Tin test negative
Climatic Reliability
Test according to IEC 68-2-3 Ca
40°C, 93% RF, 21 days
Ag-containing solder pastes have a tendency to illustrate
temporary dendrites
Advantages of the Coating Reliability Tests
Weak point analysis
(Coating Reliability Test)
Life expectancy test
(for example IEC 68-2 Standard)
+ During development - After development (type approval)
+ Fast (max. 10 hrs) - Time consuming (about 6 months)
+ Low costs - High costs
+ Identification of all weak points - Failures could remain undiscovered
- Pseudo failure rate +No pseudo failure rate
Conclusion
Coating neccessary due to moisture / condensation
Residues harm the coating
Simple test methods for Q-analysis available
Cleaning increases reliability of coating
Different cleaning processes are available