To all,
Based on the pictures, I would suspect that the reflow profile for BGA attachment needs refinement, especially the cool-down cycle. The shrinkage of the solder/copper places tremendous stress on the foil to laminate interface, as is shown by the lifted land with laminate attached to the bottom of the land. To shear the laminate up, the peel strength of the copper foil to laminate would not be suspect, as this shear force would be greater than the 5-9 PIW peel strengths required for foil lamination. The fact that the pictures do not seem to "fit back together" is not unusual, since the distortions from thermal stresses will displace the materials. Go back and look at the "hole-cracking" photos in IPC-TR-579 "ROUND ROBIN RELIABILITY EVALUATION OF SMALL DIAMETER PTHs in PWBs" done in the 1988 study. These were thermal cycling tests from -65ºC to +125ºC (or other cycling tests), which did not produce the type of thermal stresses typical of soldering at 250-288ºC, and yet these cycling test produced cracks in almost 100% of the copper barrels, and the pictures of the barrels after cracking do not "fit back together". Yes, I realize that BGA attachment and copper cracking is a different subject than barrel cracking from thermal cycling, but both of these phenomena are the results of the physics of thermodynamics. Rapid quenching of molten solder will create mechanical stresses beyond the strengths of the materials involved.
Dennis J. Cantwell
R & D Liaison
Printed Circuits, Inc.
1200 West 96th Street
Minneapolis, MN 55431-2699
952-888-7900
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-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
Sent: Tuesday, December 30, 2008 6:35 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Pad to trace interface on BGAs.
Chris -
Great pictures - my compliments.?? Open the Mahanna school of sectioning.
Question - slight mis-registration of balls to board seems to initiate trace cracking.? First image is well registered, but obviously cratered.? In image 3 - the "cracked copper" does not seem to come from the attachment pad. Is this a bit of the circuit trace to the pad that has mostly been ground away?? Image 7 looks like the broken copper might? have come from the pad on the board, but again does not quite fit "back together".???
Your sections are so precise that we can make these speculations.?
Denny Fritz
MacDermid, Inc.
-----Original Message-----
From: Chris Mahanna <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, 30 Dec 2008 11:21 am
Subject: Re: [IPC-600-6012] Pad to trace interface on BGAs.
Some cratering pics are attached. Note the cracked foil on C-3 and C-7.
Chris
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