Lead Free Coating, Cleanliness and the Conformal Coating Process

Outline What is cleanliness? Considerations for lead free processing Why is cleanliness important for conformal coating? How can we measure cleanliness? How can we ensure cleanliness?

What is cleanliness?

Considerations for lead free processing

Why is cleanliness important for conformal coating?

How can we measure cleanliness?

How can we ensure cleanliness?

What is cleanliness? Cleanliness will determine: Application performance Conformal Coating finish Long term circuit reliability

Cleanliness will determine:

Application performance

Conformal Coating finish

Long term circuit reliability

What is Cleanliness? Imagine your bare printed circuit board Plating / Etch Salts Flux Residues Grease / finger salts Oils Soya Sauce

No-clean Flux Residues Grease / finger salts Hand cream Oils (eg. Wave, Silicone ) What is Cleanliness? Imagine your assembled printed circuit board

In a perfect world, substrates would be completely clean before coating Can be as many as 13 chemical processes in manufacture Interaction between residues? What is Cleanliness?

In a perfect world, substrates would be completely clean before coating

Can be as many as 13 chemical processes in manufacture

Interaction between residues?

Different residues have different effects: Ionic (flux-residues) – corrosion Non-ionic (greases, oils etc.) - coating issues such as de-wetting, delamination Why is Cleanliness Important?

Different residues have different effects:

Ionic (flux-residues) – corrosion

Non-ionic (greases, oils etc.) - coating issues such as de-wetting, delamination

E.g. No Clean flux residues, finger salts Osmosis leads to Blistering… leads to Corrosion! The Effect of Ionic Contamination

E.g. No Clean flux residues, finger salts

Osmosis leads to Blistering… leads to Corrosion!

What is Osmosis? “ pumping action” caused by: a contaminant solubalising and re-crystallising between the coating film and substrate, as moist air moves in and out. The contaminant is usually a salt and each induces different osmotic pressure which may be extreme. Conformal Coating

What is Osmosis?

“ pumping action” caused by:

a contaminant solubalising and re-crystallising between the coating film and substrate, as moist air moves in and out.

The contaminant is usually a salt and each induces different osmotic pressure which may be extreme.

Contaminant left on a coated board at 25 o C 50% RH

Board exposed to Humidity

Moisture has “found” contaminant, and a blister has formed through osmotic pressure

The soluble contaminate has now formed a conductive fluid with the moisture

A dendrite is forming between tracks

Dendrites are a problem! Grow and blow phenomenon Estimated root cause of 25% of returns Conformal Coating

Dendrites are a problem!

Grow and blow phenomenon

Estimated root cause of 25% of returns

Corrosion On Pin Delamination Of Coating Coating Dissolving & Cracking In Circled Area

Non-ionic contamination Major Problems seen during conformal coating application: Conformal Coating de-wetting Lack of adhesion of conformal coating Delamination of conformal coating

Major Problems seen during conformal coating application:

Conformal Coating de-wetting

Lack of adhesion of conformal coating

Delamination of conformal coating

Where do residues come from? Board fabrication Components Assembly equipment Soldering processes Operator handling Incorrect cleaning

Board fabrication

Components

Assembly equipment

Soldering processes

Operator handling

Incorrect cleaning

So you’re told to use no clean In the past to produce Hi-Reliability boards without cleaning would not have been considered. It can be now, if :- Incoming boards and components inspected for cleanliness. All operators doing in-process handling should wear gloves. “ low residue” no clean flux/paste is used.

In the past to produce Hi-Reliability boards without cleaning would not have been considered. It can be now, if :-

Incoming boards and components inspected for cleanliness.

All operators doing in-process handling should wear gloves.

“ low residue” no clean flux/paste is used.

So you’re told to use no clean The soldering window must be fine tuned and the system controlled. Low residue wire/flux used for rework. Process controls are put in place. The process is validated.

The soldering window must be fine tuned and the system controlled.

Low residue wire/flux used for rework.

Process controls are put in place.

The process is validated.

SMT with lead free solders Key variables summary Melting temperature of alloy Flux chemistry - activation, temperature effects Wetting and surface tension properties of the alloy Solder balling and bridging potential increases Component / board reliability Compatible rework / repair Compatible wave, selective soldering process

Key variables summary

Melting temperature of alloy

Flux chemistry - activation, temperature effects

Wetting and surface tension properties of the alloy

Solder balling and bridging potential increases

Component / board reliability

Compatible rework / repair

Compatible wave, selective soldering process

Compare SnPb and SnAgCu Tin-Lead 183°C Tin-Silver-Copper 217°- 220 ° C

Tin-Lead 183°C

Tin-Silver-Copper 217°- 220 ° C

New Solder Paste chemistries New activators New resins New gelling agents Better surfactants Additives to prevent oxidation Alloy specific fluxes Give rheological character Aid stencil release Hold components prior to reflow Remove oxides from the surfaces to be joined Protect against further oxidation Reduce surface tension in the soldering process

New activators

New resins

New gelling agents

Better surfactants

Additives to prevent oxidation

Alloy specific fluxes

Give rheological character

Aid stencil release

Hold components prior to reflow

Remove oxides from the surfaces to be joined

Protect against further oxidation

Reduce surface tension in the soldering process

Lead free and Tin Whiskers What are they ? Single crystal grow from electroplated tin coatings. Needle like, spiral, modules, mounds Diameter approximately 3-5um Various lengths, shape from micrometer up to mm’s A form of metallurgical stress release

What are they ?

Single crystal grow from electroplated tin coatings.

Needle like, spiral, modules, mounds

Diameter approximately 3-5um

Various lengths, shape from micrometer up to mm’s

A form of metallurgical stress release

SEM Photograph 150 and 900 magnification, 3 months aging Tin Whisker on Pure SnPb

Sn Plated Chip Caps

Lead free and Dendrites High Sn & Ag alloys promote dendritic growth

High Sn & Ag alloys promote dendritic growth

Lead free and Dendrites Field Failure due to dendritic growth

Field Failure due to dendritic growth

Summary Lead free and Reliability High Sn & Ag alloys are prone to dendritic and tin whisker growth. Conformal coating reduces the problem. All lead free fluxes are higher activation than Pb alloy solders. Higher flux residue volumes. Lead free & VOC free wave solder flux is 3 x solids content. Less compatible than leaded solder residues. More requirement to test compatibility. UV40 adds mechanical stability to lead free alloys and is our hardest coating.

High Sn & Ag alloys are prone to dendritic and tin whisker growth.

Conformal coating reduces the problem.

All lead free fluxes are higher activation than Pb alloy solders.

Higher flux residue volumes.

Lead free & VOC free wave solder flux is 3 x solids content.

Less compatible than leaded solder residues.

More requirement to test compatibility.

UV40 adds mechanical stability to lead free alloys and is our hardest coating.

Further Information Website www.conformalcoating.co.uk FAQs http://www.conformalcoating.co.uk/FAQs.php Technical Documents and bulletins http://www.conformalcoating.co.uk/technicalbulletins.php

Website

www.conformalcoating.co.uk

FAQs

http://www.conformalcoating.co.uk/FAQs.php

Technical Documents and bulletins

http://www.conformalcoating.co.uk/technicalbulletins.php