You need to do the math for the package size and ball size/style you're
working with.

 From applications where I've done the math it had to be tighter than .75%
(at most .5%).
However, don't put any stock in that SWAG -- I don't know the specifics of
your assembly and I'm not full time in SMT anymore.

For worst case, figure how much deflection there can be at max bow/twist
across your package footprint then look at paste height and see if all your
balls will even make contact with the solder paste.

For less than worst case,  consider the effect of bow/twist on the finished
solder column shape and solder joint reliability.
That is to say, even if the balls are all making contact with the solder
paste, give some thought to what your solder joints will look like at the
area of greatest separation between ball and board.
In those areas of greatest package to pwb separation, your BGA solder
column will be more elongated and may not be reliable.

cheers,
Dwight
Sr. Staff QE
"If I was any good at this engineering stuff how'd I wind up in QA?"


At 10:03 AM 6/27/2002 -0700, Mcmaster, Michael wrote:

>I'd consider a BGA to fall under the surface mount spec.
>----------  From:   Ong Seet Leng[SMTP:[log in to unmask]]  Reply
>To:       TechNet E-Mail Forum.;Ong Seet Leng  Sent:   Wednesday, June 26,
>2002 3:23 AM  To:     [log in to unmask]  Subject:        [TN] Wrappage
>Percentage for printed boards with BGA and micro BGA
>
>Hi, can someone guide me? In IPC-A-600F (2.11) it is stated as for printed
>boards using surface mount components, the bow and twist shall be 0.75% or
>less. For all other boards, bow and twist shall be 1.50% or less.
>
>1.    What I want to know is does the 0.75% include printed boards with
>BGAs and micro BGAs?
>
>2.    If not, what is the bow and twist specification for boards with BGA
>and micro BGA and from which IPC specifications that I will be able to
>find them.
>
>I had briefly gone through IPC-7095 for BGAs as well as IPC-A-600F but
>failed to locate any.
>
>NOTE:    My company is using 1% of board length for bow and twist for
>printed boards with surface mount components (BGAs and micro BGA included)
>& plated through-holes type.
>
>Regards
>
>Seet Leng