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May 2001

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From:
"<Peter George Duncan>" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 4 May 2001 13:23:34 +0800
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Hi, Colin,

There has been considerable discussion about BGA's in this forum, even in
the short time since I first tuned in, so you could try picking through the
archives. I've collected quite a lot of the correspondence along the,
because, like you, I'm relatively new to BGA's, but have heard a lot about
the problems associated with them and determined to learn what I can. I can
give you some headlines to follow up on, otherwise this posting will become
a book, though not in the best-selling range of those produced by the TN
'gurus' .

Things to watch for:

   Flatness of boards to ensure good contact with BGA balls. We use FR4
   finished with 150u ins Nickel and 4.5u ins Gold. Uneven solder joints
   seem to lead to faster stress fractures in the joints themselves.
   The Above required the use of high melting point solder with a silver
   content.
   BGA joints are prone to cracking with temperature variations and
   vibration. This can be mitigated with the use of a special underfill
   material. Be careful which you choose though - some are reworkable and
   others are not; some can be applied before the BGA is fitted to the
   board (no-flow types) while others are applied after the BGA has been
   soldered (Flow types). I'm cautious of no-flow types because they have
   to act as a flux, and there are problems with out-gassing the volatiles
   that can lead to voids and short-circuits.
   Large BGA's (over 25 x 25mm) are more difficult to underfill than
   smaller ones.
   Achieving good quality, well-formed solder joints with BGA's seems to be
   something of an art, especially of you don't want to fry the components
   to death during the soldering process. Careful Thermal Profiling of the
   board for CR Soldering is essential.
   Be very aware of partially reflowing BGA solder joints once formed,
   if/when soldering the second side of the board and/or when wave
   soldering. It's difficult enough to get BGA joints up to correct
   soldering temperature without mucking them up again by partial reflowing
   later in the assembly process. We had some terrible joints caused by
   this - very coarse grained, poorly shaped and exhibiting all kinds of
   fissures and land-slip signs that left them peculiarly weak. Better to
   try and avoid the risk of reflowing these components in the first place,
   and there seem to be a few techniques for doing this that you can find
   in the TN archive or by surfing.
   Not least, match the CTE of the board material as closely as you can to
   the CTE of the BGA. This lowers the stresses imposed on the BGA joints
   and makes easier the job of any underfill material you may choose to
   use.

Contact me off-line if there is anything else I can help you with anything
else.

Pete Duncan




                    Colin Weber
                    <colin.weber@VARI        To:     [log in to unmask]
                    ANINC.COM>               cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
                    Sent by: TechNet         Subject:     [TN] BGA Design
                    <[log in to unmask]>


                    05/04/01 05:28 AM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to Colin
                    Weber






We are looking at using BGAs for the first time. In particular a 388 pin
AMD processor chip, 1mm pitch.

 I am looking for any tips and comments that can assist in my research into
this. From a first glance I
don't see that the use of such a device has to be difficult, but these
things have a habit of needing certain
precautions and design practices.

Any help at all would be appreciated.

PS: At this point in time it looks as though we will be forced to proceed
with the design using Protel99SE.
Anyone with experience with such designs and this version of Protel, I'd
love to hear from you.

Sincerely,

Colin Weber
Varian Australia Pty Ltd


Regards,

Colin Weber

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