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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