I'm thinking...suggest that the first temp rise causes the underfiller to
swell and elongate the solder ball. Done is done. The ball is deformed a
little and the crystals have changed their shape in accordance. Now, we let
the temperature change and go down. Will the underfiller force the solder
balls to contract and go through another deformation? If that is so, I can
fully understand what Dave demonstrated. Dave and Doug, sorry.
A dilatometer could easily tell you about the Z amplitude.
Inge
----- Original Message -----
From: "Inge" <[log in to unmask]>
To: "TechNet E-Mail Forum" <[log in to unmask]>; <[log in to unmask]>
Sent: Monday, November 30, 2009 10:26 PM
Subject: Re: [TN] BGAs & Conformal Coating
>I forgot to mention, that I have seen how the solder balls elongate during
>pull or prey test. They change from a flatted ball to a barrel. That means
>that the volume between the BGA and the board nearly doubles! I was very
>surprised. Didn't think solder was like chewing gum. Does the coating or
>underfiller really expand that much?
> I'm very interested in this, as we produce boards with a lot of largest
> available BGAs.
>
> Inge
>
> PS. Will see if I still have the amazing photos from the test.
>
> ----- Original Message -----
> From: "Inge" <[log in to unmask]>
> To: "TechNet E-Mail Forum" <[log in to unmask]>;
> <[log in to unmask]>
> Sent: Monday, November 30, 2009 10:15 PM
> Subject: Re: [TN] BGAs & Conformal Coating
>
>
>> Doug,
>>
>> how can that be possible? When temperature rises, the coating ought to
>> soften and should expand even horisontally, and when at lower
>> temperatures, the coating should crimp. The only way I can see is if you
>> have the situation of a closed in liquid, which one cannot be compressed.
>> BGAs use to be very hard to rip from the board. I measured some 350 kg
>> force necessary to pull off a very large BGA.
>>
>> Inge
>>
>>
>>
>> ----- Original Message -----
>> From: "Douglas Pauls" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Monday, November 30, 2009 2:29 PM
>> Subject: Re: [TN] BGAs & Conformal Coating
>>
>>
>> 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
>> Espańa
>>
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