Dear Mary and Paul,
Interesting comments.
Also I have found de-wetting and adhesion to be two separate issues.
De-wetting, as you describe, is the effect seen when the coating is in a
liquid form, before cure. We see de-wetting issues mainly with silicone
contamination prior to coating.
Adhesion is when the material is cured. In this instance a low surface
energy of the coating is a bad thing and will lead to bad adhesion.
Thus optimum is a low surface tension before cure, and a high surface energy
after cure.
In this respect I don't agree with 'the lower the better' in terms of both
requirements (no de-wetting and good post cure adhesion).
As an example, a silicone conformal coating will wet just about anything,
but it does not adhere well to a PCB surface. It may pass a cross hatch
test, but only because the tape won't stick to it either! Silicone coatings
are often flexible enough to induce only very low stresses from TCE mismatch
throughout the life of the board, and the low adhesion therefore doesn't
manifest itself as delamination.
We have done several studies of adhesion and de-wetting, and the issues
involved range from free silicones (from heat transfer compounds, buffering
materials and even mould release agents from component case moulding) to
levelling agents in high gloss solder masks. Flux residues and reflow
profile can also impact on localised adhesion.
The result of our work is an adhesion modified coating material with no
degradation in terms of wetting ability.
In terms of thin edge coverage, application procedure is the key. Paul do
you dip or spray? I have notes for optimising your application to minimise
these effects if you wish to contact me off line I can mail them to you.
Best Regards.
Alan Brewin
Chief Chemist - Concoat Ltd.
-----Original Message-----
From: Davis, Mary <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 12 October 1998 17:54
Subject: Re: [TN] Confomal coating adhesion
>We found that success in wetting plastic packages very much depends on
>the conformal coating material. Assuming the packages are clean and
>free of oil and mold release, wetting is a function of the surface
>tension of the coating material and the critical free energy of the
>substrate. Some coating products have the appropriate surface tension
>for plastic substrates, some do not. Talk with your coating manufacturer
>and try various products. Do not assume that what worked elsewhere will
>work on plastic. Some plastics have very low critical surface free
>energies - in the range of 18-40 dyn/cm. Ask your coating manufacturers
>if they can tell you the surface tension of their coating products. The
>lower the better. If the surface tension of the coating is less than
>the critical free energy of the solid, complete wetting will occur; but
>if the surface tension is greater, partial non wetting can result.
>
>Call me and I can tell you the name of the product performed best in our
>application.
>
>
> Mary Davis
> Sr. Material & Process Engineer
> Product & Process Analysis Lab
>
> Raytheon Systems Company
> Naval and Maritime Systems
> KW 281, MK4C08
> 6500 Harbour Heights Parkway
> Mukilteo, WA 98275-4862
> 425.356.3311
> 425.356-3185 fax
>
>> -----Original Message-----
>> From: Paul Stolar [SMTP:[log in to unmask]]
>> Sent: Monday, October 12, 1998 7:09 AM
>> To: [log in to unmask]
>> Subject: [TN] Confomal coating adhesion
>>
>> Does anyone have any good experience at improving conformal
>> coating
>> adhesion to plastic packaging?
>>
>> Is it a cleanliness issue, or is it inherent in materials?
>>
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