Residual moisture adsorbed on the board can cause bubbles.  Residual solvent on the board from a cleaning operation can cause bubbles - especially residual IPA with urethane coating.  It is best to bake the cleaned CCA prior to coating and apply coating without delay after the CCA has cooled to RT.

Brushing coating on with a bristled brush can work air into the coating and result in bubbles forming during heat cure.  This is most pronounced the higher the coating viscosity.

Definitely, oven curing a solvent containing coating without adequate flash off time can result in bubbles.

Mary Davis
Materials and Process Engineer, Retired
________________________________
From: TechNet <[log in to unmask]> on behalf of Steve Gregory <[log in to unmask]>
Sent: Thursday, July 16, 2020 10:04 AM
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] Bubbles in coating of ASIC leads

Hi Dean,

I've gotten them by trying to cure the coating too quickly, the coating
skins over before all the solvents have worked their way out. But here's a
good Humiseal link that gives some various causes and ways to eliminate
them:

https://blog.humiseal.com/simple-techniques-to-avoid-bubbles-in-conformal-coating#:~:text=For%20example%3A%20curing%20too%20fast,vapor%20under%20the%20coating%20surface.


Steve

On Thu, Jul 16, 2020 at 10:56 AM Stadem, Richard D <[log in to unmask]>
wrote:

> One of the client companies I work for builds extremely high-reliability
> propulsion control systems. For them, bubbles in the coating between
> fine-pitch ASIC leads are a defect condition. We are exploring options for
> elimination of the bubbles.
> The bubbles are not present in the sprayed on coating, and we have tried
> lightly brushing the leads to further "thin out" the coating before the
> CCAs go into the curing oven. No webbing or bubbles are seen at that point
> but manifest themselves after curing.
> I am not an expert at coating but I do know a bit about it. I was
> wondering just what might be the cause of the bubbles, and how to prevent
> them from happening? Any information is greatly appreciated!
> dean
>


--
Steve Gregory
Kimco Design and Manufacturing
Process Engineer
(208) 322-0500 Ext. -3133

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