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November 2006

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From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Wed, 22 Nov 2006 09:21:44 -0500
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        Hi Graham:
                    I have not made detailed experiments as you and Brian have. CC at 33% solids and 66% solvents when squirted under the BGA will adhere to all surfaces when wet. When it dries it shrinks significantly leaving plenty room for expansion. My question: Wouldn't it work based on the fact that the CC is shrunk?
        Best Regards,
        Ramon
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Naisbitt
Sent: Tuesday, November 21, 2006 12:32 PM
To: [log in to unmask]
Subject: Re: [TN] BGA and conformal coating

Vered

This is an old and thorny subject much debated on the TechNet in the past.

The TCE of apretty much all conformal coatings is >300ppm/DegC but the TCE of silicones are huge at ~300-350ppm/°C Solder being only ~16 ppm/°C this is a pretty significant TCE mismatch.

As for silicones as debated by Brian Ellis and myself some time ago, Brian said: They have a unique property that however soft they feel, they are rock-hard when subjected to mechanical shock (hence super- bouncing balls and bouncing putty) At only slightly elevated temperatures there would be tension on a BGA-ball-pad combination, that would increase by orders of magnitude should it be subjected to a small shock at the same time

If you must under-fill, then a soft purpose developed epoxy is the probable answer, check with Brian Toleno - wherever he is hiding these days?

Having said all of that, I have had aerospace customers who apply a very thin coating, diluting the coating and dipping at very slow speeds, that can ensure (as far as one can without removing the device to check!) coverage under the device. They then apply a "conventional" spray coat to complete the job.

I hope this helps

Regards,

Graham Naisbitt

Gen3 Systems - Engineering Reliability in Electronics

[log in to unmask]



On 20 Nov 2006, at 16:40, Vered BS wrote:

> looking for reviews/articles or case studies regarding conformal 
> coating on BGA for military/aerospace environment Appreciate any 
> comment or sharing your experiance Thanks Vered
>
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