Graham:
               Have you been successful in under filling QFP, BGAs and PGAs with CCs?
        Regards,
        Ramon
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Naisbitt
Sent: Friday, December 01, 2006 6:19 AM
To: [log in to unmask]
Subject: Re: [TN] conformal coatings as under fills

I cannot let Ed's message pass without comment in response.

First up, conformal coatings are to be at a thickness of 50 microns
±25 (0.002 ±0.001 inch (MILS if you must)) on a flat unencumbered surface. Of course it will be thicker around the component and thinner on the leads.

Dip coating by hand will NEVER be able to achieve such a thin film.  
Faster = thicker. Plus, you will have the problem of runs and the infamous wedge effect that are specifically regarded as "non- acceptable" under IPC-A610.

Having manufactured coating equipment for over 25 years, and run a coating service, I have never encountered such a phenomenon as Ed described unless the wrong solvent was employed.

Immersion should be slow enough to allow air that may be trapped under the components to be displaced by the coating. Of course the surface tension of the coating will have a controlling influence on the distance it is able to penetrate especially in low stand-off areas. The withdrawal speed should be ideally slower than the cascade effect of the coating material and is generally in the range of 100mm to 150mm (4 to 6 inches) per minute. Try doing that by hand! By this means you can minimise the wedge effect and, at least with air drying coatings, minimise runs and drips as the coating is drying as it is withdrawn.

Dip coating provides the benefit of applying the coating to both sides simultaneously.

Of course masking is the major drawback and can only be avoided if the components that should not be coated can be placed on 1 side of the assembly.

Ed is right to point everyone in the direction of the IPC-HDBK830, a lot of effort was put into that document from a significant number of aerospace engineers and coating specialists.

Graham Naisbitt

BTW Dewey, Planet Zog!

[log in to unmask]



On 24 Nov 2006, at 16:45, Edward Mines wrote:

> Mr. Naisbitt is correct that machine dipping PC boards in diluted 
> conformal coating will get the coating under every component.
>
>   The performance of the machines that he sells compared to dipping 
> boards in by hand surprised me and I had a lot of experience in 
> chemical engineering pilot plants prior to working with conformal 
> coatings. The slow speed of the machines (dipping inches per
> minute) causes many small bubbles to form that are not formed when 
> dipping by hand. Slow withdrawal keeps these tiny bubbles on the 
> surface of the liquid rather than the surface of the board.
>
>   As mentioned in an earlier message, solvent borne coatings (types 
> AR, UR & even some silicones) loose considerable volume as the solvent 
> evaporates so the cavity beneath BGAs will have a significant gas 
> space.
>
>   I have seen several instances where these "air" spaces blew out 
> between wires when the board was baked (the solvent comes out quickly 
> and the gas is expands). If the board is not baked solvent will slowly 
> diffuse through the cured coating until there is no more solvent.
>
>   Oxygen or moisture is necessary to cure one part polyurethane 
> conformal coatings and most silicone conformal coatings. Diffusion of 
> air through cured conformal coatings is slow so it's likely that it 
> will take a long time for these coatings to completely cure, if ever. 
> Some heat curing silicone coatings rely on the release of a reaction 
> inhibitor when heated; these may not cure completely or even cure at 
> all in thick cross section (the distance between the center of a BGA 
> and fresh air is much greater than the maximum conformal coating 
> thickness called out by IPC-CC-830).
>
>   These issues should be addressed in both IPC-CC-830 and the IPC 
> conformal coating handbook.
>
>   Ed Mines
>
>
> ---------------------------------
>
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Search the archives of previous posts at: http://listserv.ipc.org/archives
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