Awesome information!
To add another aspect - Make sure whatever design you use, you also have the
materials, microstencils, and trained rework operators on hand BEFORE you go
into production. Proper inspection equipment is a must for qualifying your
process (we use a combination of X-Ray, HP Laminography, and ersascope
perimeter equipment - all depends on the access to the BGA).
Jan Pelchat
Benchmark Electronics
> -----Original Message-----
> From: <Peter George Duncan> [SMTP:[log in to unmask]]
> Sent: Friday, May 04, 2001 1:24 AM
> To: [log in to unmask]
> Subject: Re: [TN] BGA Design
>
> 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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