I'd best take back that bit about let the EMS dictate - I've gone with several in the past that have expressed a design
preference, but we were all in agreement anyhow. While the EMS has to stick the bit's down, they are not normally underwriting
product long term reliability.
Basically it boils down to following standards or any other expressed guidance of prestige. It's also useful, if time permits,
to keep up with publications and test reports relating to reliability, particularly if there's a lot of thermal stress. More so
at the moment with the impingent PB free solder transition.
The bottom line is that the designer is logically responsible at the end of the day.
Out of interest, is their an IPC rule relating to smaller than 0.5mm pitch BGA's? 80% of say a nominal 0.2mm(8mil) ball doesn't
leave much of a pad for any kind of joint.
-----Original Message-----
From: David Greig [mailto:[log in to unmask]]
Sent: 2005-May-10 18:50
To: 'TechNet E-Mail Forum'; 'John Guy'
Subject: RE: [TN] BGA Ball/Pad Size Ratio
Depends if you want to have solder mask defined pads or not.
If mask defined then a rule of thumb is to aim for the same mask aperture on the interposer.
Non mask defined ROT is to make the pad equal the minimum ball diameter so that solder joint forms around the pad sides
improving stress resistance.
Although, watch out for ball collapse, and consider what height is left if your needing to clean after reflow, and if there is
enough annulus for via in pad (if your forced down the HDI route).
Generally, the larger higher value device manufacturers will give some guidance. Altera and Xilinx have apps notes, some stuff
at Intel and IBM as well if you've got time to search.
If your using an EMS then they probably have their own preferences, best to go along with them because it's their job to stick
the bits down on the board with electric glue.
Some apps notes:
http://www.asat.com/products/library/fg1033.pdf
http://www.altera.com/literature/an/an114.pdf
The National Physical Lab in the UK also a wealth of data http://www.npl.co.uk/ on solder joint reliability including Pb free.
Here's a thought: With all this political correctness stuff prevailing (no naughty words etc), how do you folks on the sunny
side of the Atlantic get away with saying 'solder' and 'soldering' all of the time?
Best Regards
David Greig
______________________________
GigaDyne Ltd
Buchan House
Carnegie Campus
Dunfermline KY11 8PL
United Kingdom
t: +44 (0)1383 624 975
http://www.gigadyne.co.uk
______________________________
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Guy
Sent: 2005-May-10 17:59
To: [log in to unmask]
Subject: [TN] BGA Ball/Pad Size Ratio
Hello All,
I was wondering if there is some guideline for the recommended pad size for a BGA based on the component's ball size? I
have been told that it is recommended that the pad diameter be about 80% of the ball diameter, but I would like some
confirmation.
Thanks in advance,
John Guy
Surface Mount Process Engineer
diversifiedsystems, inc.
Custom Products Division
3939 W. 56th Street
Indianapolis, IN 46254-4961
Telephone: 317-299-9547, x336
Fax: 317-298-2055
E-mail: [log in to unmask]
Web: www.divsys.com
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