Hi Techies

This is a topic that has arisen previously and I herewith copy a rather useful input that is attributable to Brian Ellis:

Silcones and BGA: (However, not dissimilar conditions may be applicable to other types of coatings although their TCE's are normally lower) 

TCE of silicones are huge at ~300-350ppm/°C
solder is only ~16 ppm/°C
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


In short, it would be unadvisable to coat underneath such components if they are to be used in harsh environments.

Hope this helps

Graham Naisbitt
Gen3 Systems
www.gen3systems.com


On 30 Nov 2009, at 13:29, Douglas Pauls wrote:

> Good morning Juan,
> I suspect that Xilinx wants you do do a thermal cycle study on solder 
> joint reliability when conformal coating gets under the BGA.  The stresses 
> brought about by a mismatches in the coefficient of thermal expansion 
> (CTE) can rip a BGA off the board and can severely deform the balls. 
> Hillman and I did such a study here a few years ago.  I can share a few 
> pictures of what happens, if Steve Gregory would be kind enough to forward 
> his present e-mail address for the photos.  Not sure I have the right one.
> 
> Doug Pauls
> 
> 
> 
> "Juan T. Marugán" <[log in to unmask]> 
> Sent by: TechNet <[log in to unmask]>
> 11/26/2009 11:53 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> "Juan T. Marugán" <[log in to unmask]>
> 
> 
> To
> [log in to unmask]
> cc
> 
> Subject
> [TN] BGAs & Conformal Coating
> 
> 
> 
> 
> 
> 
> Hi,
> 
> I've read in a Xilinx guide that: "Xilinx has no experience or reliability 
> data on 
> flip-chip BGA packages on board after exposure to conformal coating. It is 
> 
> recommended that the end-user should characterize the board level 
> reliability 
> performance of Xilinx packages before production use."
> 
> Does anybody know any infomation or study related with this topic?
> 
> Thank you.
> 
> Juan T. Marugán
> Indra Sistemas SA
> España
> 
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