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Plane of highest stress during temp cycling is typically near the interface of the solder ball and the package substrate. So typical design rules specify this bond pad area should not be smaller than the pad on the PCB.
Best regards,
Leo
Leo M. Higgins III, Ph.D.
Sr. Vice President, Engg and Technology
ASAT, Inc.
13809 Research Blvd., Suite 635
Austin, Texas 78750
office phone 512-249-4758
mobile 512-423-2002
[log in to unmask]
www.asat.com
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Lester Subrattee
Sent: Thursday, May 17, 2007 11:09 AM
To: [log in to unmask]
Subject: Re: Asymmetric micro BGA joints ok?
Lothar,
I wouldn't say unreliable.
As Werner pointed out there are studies that show some "reduced
reliability", under those test conditions. If your end use conditions
are fairly benign and don't mandate or reflect the accelerated
conditions used in those studies then the "reduced reliability" is
likely OK for your product.
That being said, you should inquire why .3mm can't be printed
repeatedly. There are other manufacturers doing this right now.
Changing the pad layout may or may not help you but you probably would
not have addressed the root cause while possibly introducing "reduced
reliability".
Suggest checking stencil thickness, type of stencil (chem., laser,
electroform), stencil material and finish, paste particle size.
Depending on your pitch you may also be able to overprint a bit and the
square aperture will also increase volume.
So the pad is .3mm but maybe you print .35 mm.
Regards,
LS
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Lothar Thole
Sent: Thursday, May 17, 2007 4:04 AM
To: [log in to unmask]
Subject: Asymmetric micro BGA joints ok?
Our board loader advises that they cannot print 0.3mm (0.012in)=20
diameter paste reliably, and want us to increase the board pad to=20
0.4mm under a micro BGA. The package side pad is 0.3mm, so this will=20
create an asymmetric joint.=20
Is this ok, or will this be unreliable?
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