Voc-free wave soldering

Harry Trip, Manager Technical Support

Wave solder flux systems

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Alcohol based VOC-Free Low-VOC VOC-azeotrope

Global Composition Fluxes

Filmformers

Activators

Di-Water

Alcohols

Wave solder Fluxes

The perfect flux:

• Should solder everything• Should be environmental friendly (VOC-Free)• Should leave no residues• Should meet all safety standards (halide free?)• Should prevent all solder balls• Should prevent all shorts• Should not cause any ICT problems• Should make nice and shiny solder joints• Should have a pleasant or no odor• Should not pollute the wave solder machine• Should not be more expensive than current voc• Please add more……..

Main topics in new flux development

• VOC-free• Alternatively Low-Voc • Halide free (IPC L0)• Stronger fluxes to eliminate bridging, but still within

residue safety standards• Anti solder balling solution• Generally less visual residues on all resists• Special selective soldering fluxes

L1 Halides � < AcidNo Halides � > Acid

Residues, REL-0 or REL1

Visible residues do not necessarilymean corrosive residues

Acids in residue ����Hygroscopic effect ����Corrosion risk ����

Flux selection chart 5=Perfect for this purpose4=Generally good3=Might work , to be tested well2=Not advised for this purpose1=Wrong choice

21-January-2008

Applic

atio

n

327

390-

RX-HT

323-

ITV

396-

QM

X

396-

TNP

397-

BS

Alcohol based Voc-free Low-vocSelective sold. Process 5 5 5+ 3 2 3Sn100C alloy capability 5 5 3 5 5+ 5N2 around wave 4 5 5 5 4 5Very high thermal profiles 5 5 3 4 5 5Consumer electronics 5 3 5 4 5 4Med-Rel electronics 5 5 5 5 5 5Hi-Rel electronics 3 5 4 5 5 5

OSP compatible 5 4 5+ 4 5 4Ni/Au compatible 5 5 5 4 5 5Ag + Sn compatible 5 5 5 5 5 5Moderate preheat 5 5 5 3 3 3Reduces solder balling 5 4 4 5 4 4Reduces bridging 5 4 5 4 5 4No white residues 5 3 5 5 4 4No spreading of residues 4 2 4 5+ 5+ 5+ICCT compatible 3 5 5 5 5 5Conformal coating 3 5 4 3 3 4IPC-ANSI-J-STD-004 ROL1 REL0 REL0 REL1 REL0 REL1Acid number 29.00 15.80 21.93 15.59 27.70 14.87Solid Content % 4.94 2.2 2.27 1.47 3.00 1.50

Alcohol based Voc-free

What is a dendrite

A dendrite in metallurgy is a characteristictree-like structure of crystals

When a voltage is applied between an anode and cathode, a metal dendrite grows from the cathode through the ion conductor towards the anode

Dendrites

What makes corrosion or dendrites in soldering ?

A combination of:1. Metal2. Ionic contamination or ion conductivity (acids,

halides , minerals (also on finger prints!)3. Oxygen (air)4. Moisture (air)5. Polarity, voltage6. Temperature

What is VOC

• Volatile Organic Compound• Definition:

According to the American EPA (Environmental Protection Agency): “A solvent system with a vapor pressure of less than 0.1 mm Hg (Mercury)”.

Consequently this means that virtually no gas will escape out of the product at room temperature.

Why voc-free fluxes?

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60

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63

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17.2

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Global surface (combined land & ocean) temperatures

Voc-free fluxes eliminate:

• Process control problems due to solvent evaporation

• Thinner consumption

• Waste disposal problems

• Greenhouse effect considerations

• Fire risk

• Restrictions and extra costs for transport, storage and handling

So why are not all of us onvoc-free flux already ?

The contras of VOC-Free Fluxes

VOC-Free flux is build on water soluble = hygroscopic ingredients

• Risk for corrosion if flux is not properly neutralized by the heat of the wave (flux under pallets, on topside or just too much flux applied)

• Highest residue safety standards and widest process window still with alcohol based fluxes (alcohol or low-voc)

• Higher risk for solder balling (less choice in ingredients)

VOC-free versus alcohol based

• Acid in IPA is less active as same amount in H2O (water mix much more polar)

• All ingredients must be water soluble for stable solution• There for much smaller range of possible ingredients• Uncured residues are hygroscopic• H2O is more mobile than alcohol• IPA spreads better than H2O (surface tension)

Are there absolute safe flux residues

• Hard to say, flux can help growing dendrites but the milder the flux, the less the risk.

• Dendrites can even grow without flux• All depends of moisture level, temperature, pitch, voltage, pcb contamination and flux formulation

• Special high-end flux can greatly reduce risk.

Dendrite formation No flux. Pcb contaminated

Low-voc?

Maybe best of both worlds!70% emission reduction usually enough to meet demands of local or national authorities(Cobar 397-BS)

Solder balls Solder balls

Solder balls

Random or Systematic solder balls ???How are they formed ???What is accepted (acc. IPC-A-610 D) ???⇒Minimum electrical distance⇒Must be fixed in the flux residue

Solder balls

Solder balls

Parameters effecting solderballs

Atmosphere (Nitrogen)

4%

Storage conditions (humidity)

11%

Solder6%

Solder resist51%

Flux17%

Machine settings

11%

Resource:

The solderball problem. Research and results

H. Bell – Rehm / R. Zajitschek – Siemens VDO

Solder balls

Random solder balls

Solder will adhere to metalized parts

Gravity bigger as Surface Tension

Snapping action downwards

Micro ball propelled toward board

Pictures courtesy of: TKB-4u.com

Results in random solder balls,

No defined place, size differs from one ball to another

Pictures

Courtesey of TKB-4u.com

Solder balls (4)

Systematic solder balls

PCB

Solder

PCB

Solder

PCB

Solder

Soldering action Solder softens the solder resist

Longer contact time > solder adhesion to resist

Solder has low cohesion and sticks to resist

Results in Systematic solder balls,

Defined place, sizes on one component almost all equal.

PCB

Solder

PCB

Solder

Solder balls (5)

Solder resist (~51%)

Surface structure of solder resist

Tg (Glass transition temperature) of solder resistSolder resist gets soft above Tg and will

promoteattachment of solder and therefore solder

balls

Rough surface has smaller attach area as smooth surface, subsequently there will be less solder ball formation

Process is more critical

Important parameters:

• Flux volumes

• Preheat

• Design of solder pallets

• Wave set-up

Voc-free implementation

VOC-Free introduction Impacts

1. Application of flux by fluxing unit

2. Wetting of flux of non-metallic surfaces (solder resist)

3. Preheating of flux

4. Removal of oxides from metallic surfaces (pads & leads)

5. Post-solder residues

6. Maintenance of wave solder equipment

Low viscosity and the need for precise application of VOC-Free fluxes require nozzle spray fluxing

• General rule: Use as less as possible flux

• Optimize the spray pattern as much as possible

• Too much flux can cause:

- board contamination => corrosion risk

- formation of solder balls

- waste of flux

- equipment contamination

- skipped joints by flux accumulation in capillaries

Flux Application

Wicking in THT

Unsufficient Pressure or too

much flux

Parameters

Spray Pressure

Spray PatternSpray

Range

Spray Distance Spray

Angle

Note: Big difference between 45 and 60 degrees cap!

Board Transport in SolderingMachine

Finger conveyorPallets: design adjustments may be necessary!

Old pallets may accumulate water. This will come out violently when assembly is in the wave!

x

Preheating VOC-Free Fluxes

• FC may blow off the flux when applied in 1st preheat stage

• Water has a different IR-wave length than

IPA and Ethanol

• Water has a lower vapor pressure

• Water has a different boiling behavior (ST)

- of VOC-Free Flux

Absorbance of H2O

Preheat Requirements• IR-heat direct after fluxer

• Boiling water impacts efficiency of volatilization

• Efficient absorbance of IR by H2O in medium IR-wave length

• Peak temp. S-S pcb: 90-100 C, T-H pcb/L.F.: 115-130 C. topside (lead-free process)

medium-wave length IR +

emission temperature of

approx. 300 oC (max. 400)

1st zone

forced

convection

(or I.R.)

Other zone(s)

forced

convection

(or I.R.)

Preheat Requirements VOC-Free, Lead-Free Wave Soldering

Temperature: Topside AssemblyMinimum top-side temp: 120°C !!!! (only for through -hole pcb’s)

= IR

= IR/convection

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120

Time

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ture

130 C120 C

Components & VOC-Free Fluxes

Prevent water penetration and residues:

• VOC free ingredients (solids) are water soluble.

• Uncured residues stay acidic and are hygroscopic

• Water and acid = highly corrosive

• Using sealed components

• Proper fluxer setting

• Using temporary masks or tapes

Selective / Pallet Soldering issues

(Selective) Pallet Soldering (1)

Flux

PCB

Pallet

Flux residues between pallet and PCB

Flux Preheat phase Soldering phase

Wetting Wicking Drain phase

Cooling

Evaporate Activation Solidify

(Selective) Pallet Soldering (2)

Disturbance of the wave characteristics

PCB

Open jointInsufficient TH Filling

Flux Preheat phase Soldering phase

Wetting Wicking Drain phase

Cooling

Evaporate Activation Solidify

What happens with the flux next to the wave?

Selective & Pallet Soldering (4)

For these applications, the best advise is:Alcohol or low-voc flux

Better not voc-free

Thanks for your attentionBedankt voor uw aandachtVielen Dank für Ihre AufmerksamkeitDziekuje za uwageGrazie per l’attenzioneGracias por su amabilidadHorosho pa menya ......Tack för din uppmärksamhetGohairyo arigatou gozaimasu Xie xie ni di zhu yi