My opinion is that as long as you maintain symmetry of materials and
balanced copper, this number is attainable.  I generally find that balanced
product has warpage so low that it is essentially unmeasurable, certainly
less than 0.025" to 0.030" you are talking about.  An unscientific survey of
some boards at my desk with balanced construction confirms this.  The
problem is you'll be trying to buy product that is toleranced at 1/3rd the
IPC standard of 0.75%.  There won't be a lot of confidence in meeting
something that much tighter than what folks are used to.  I would have a
tough time selling my manufacturing people on this requirement until they
actually built it and measured it.  You're products will also have less
tolerance for material variations that would go unnoticed with a more
lenient warpage specification.  For that your supplier can expect lower
yields and you might have some supply interruptions should the "recipe" get
lost.

All that being said, I can't help but wonder how extensive was the testing
done to verify that 0.25% is "necessary"?  Are there other BGA manufacturers
producing similar packages?  If so, what is their warpage requirement?

I've seen a lot of device and connector footprints and designs that give
little to no consideration to the difficulty of building and assembling
product designed according to the guidelines.  It seems in too many cases
that the design allowances start out at the package/connector level then
down through assembly and whatever is "left" is generously given to the
board fabricator.

> ----------
> From:         Smith-BCD, Richard[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Smith-BCD, Richard
> Sent:         Tuesday, August 29, 2000 12:58 PM
> To:   [log in to unmask]
> Subject:      [TN] Warp and Twist spec for BGA's
>
> Hi Folks,
>
> Looking for some real world advice.
>
> We're using high pin count BGA's from a manufacturer who recommends a PWB
> flatness spec of .25%, The question is this: Is it reasonable to expect a
> fabricator to consistently build a 16 layer, .125" thick PWB meeting a
> .25%
> flatness requirement?
>
> The board is 12.25" x 10.25", of balanced construction and has even copper
> distribution.
>
> Thanks,
> Rick
>
> ##############################################################
> TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
> ##############################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with
> following text in
> the body:
> To subscribe:   SUBSCRIBE TECHNET <your full name>
> To unsubscribe:   SIGNOFF TECHNET
> ##############################################################
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> additional
> information.
> If you need assistance - contact Keach Sasamori at [log in to unmask] or
> 847-509-9700 ext.5315
> ##############################################################
>

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################