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November 2006

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Subject:
From:
Graham Naisbitt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Graham Naisbitt <[log in to unmask]>
Date:
Wed, 22 Nov 2006 15:52:30 +0000
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Hi Ramon

Any coating at 33% solids or more is unlikely to penetrate fully  
beneath a BGA even when dip coating - with the exception of silicones  
as they have really low surface energy.

To rely on "natural product shrinkage" to avoid any fillets would be  
a little risky and how could you know for sure?

Kindest regards,

Graham Naisbitt

[log in to unmask]



On 22 Nov 2006, at 14:21, Dehoyos, Ramon wrote:

>         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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