Michael's question is very similar to a situation we are experiencing here
also regarding pad size on a BGA. Our assembly house is complaining that our
current pad size on the PCB is too large (30 mils). The pin pitch on the BGA
is 39 mils and solder ball size ranges from 19 to 27 mils. The assembly
house is asking for 17 mil pads, but the manufacturer (Xilinx) recommends 30
mil pads with a 22 mil soldermask opening. Has anyone else encountered this
problem? What was your solution? Thanks in advance for any feedback.


-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: Wednesday, February 09, 2000 1:58 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Pad Size For BGA device


In a message dated 02/09/2000 3:19:18 PM Central Standard Time,
[log in to unmask] writes:

<< I have a questions about the pad size needed for a BGA device. We are
using a 25
 mil 10/90 balls on an organic substrate (FR4).
  We currently have 20 mil, non-solder mask defined, pads on both the device
and
 PCB.  What is the recommended  pad size?  We
 have control over the pad size on the device and PCB.  Thank you.

 Best Regards,

 Mike Forrester >>

Hi Mike!

Check out Daan Terstegge's new web page at: http://www.daan.tmfweb.nl/  and
then
click on the "Various PCB Assembly Links" button, and you'll find a link to
a
Motorola Application Note PDF file about BGA design and assembly.

According to that paper, Motorola changes the pad size according to the
pitch
of the balls on the device, and recommends that they both be the same size
(the pads on the BGA and on the PCB). They also recommend NSMD
(non-soldermask defined) pads.

For 1.5mm pitch balls, they use 25-mil diameter pads. For 1.27mm pitch they
use 23-mil diameter pads. In both cases the soldermask opening they use
31-mil diameter openings...

Daan's got a pretty neat web site...check it out!

-Steve Gregory-

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