Agree. I know the IPC standard says 80% but that I believe is a minimum.
Keeping the pads equal size to the BGA ball size means the solder wetting is
equal between BGA pad and PCB pad. The solder will be greater in volume
towards the side with the larger surface area. Equal means a cross-section
view will look like an hour glass.

Regards
TDK



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Greig
Sent: Friday, March 02, 2007 3:31 AM
To: [log in to unmask]
Subject: Re: [TN] BGA footprint

My preference is to find out the diameter of the solder mask defined pads on
the bga side and use that value for the NSMD pad on
the board. The goal is to have equal surface tension on either side when the
ball is in reflow. (ignoring differences due to
flux and intermetallics)


Best Regards
 
David Greig
______________________________
GigaDyne Ltd
5 Albany Business Centre
Gardeners Street
Dunfermline KY12 0RN
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
______________________________

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Perkins
Sent: 02 March 2007 00:36
To: [log in to unmask]
Subject: [TN] BGA footprint

Hi

I am working on a new board with a 0.5mm pitch BGA and I am wonder if
someone can advise me on the footprint.

1) the datasheet says the solder ball is 0.3mm +/-0.05mm
2) IPC recommends to use 80% of ball diameter for the pad which would be
0.24mm
3) Would it be OK to use a 0.2mm pad? I am hesitant to go against
recommended size without some corroborating documentation. The
reason I would want to use 0.2mm pad is to avoid using via-in-pad.

I am curious if IPC defines footprints based on what is good for
manufacturing or is it simply theoretical?

Thanks
Bob 

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