I have also experienced this, but it has been about three years since it happened. At that time, it happened on three different slash types of Rogers high-temperature polyimide laminate from a particular supplier, but on three different unrelated programs.
What happened in the first two jobs was that the BGA pads that were unused (not connected electrically) were lifting off of the PWB during reflow. They would "float" up in the BGA solder ball and when the solder joint cooled, they were stuck in various positions on the outsides of the solder ball. When you looked at this BGA in the X-ray, you saw a whole bunch of "D"-shaped solder joints in amongst the normal round solder balls connected to pads with traces. The "D"-shaped solder joints were in various positions. The pads with electrically-connected traces did not float because the traces held them in place. At first I thought they had been abused (reworked) but then I realized that was not possible, as these were showing up immediately after the first reflow. So then I though perhaps someone performed some type of rework on the pads prior to assembly and reflow, but that also turned out to be not so. So then I checked the reflow profile thinking it was too hot, and it was the same as the benchmark profile used to qualify the process. The client had already built several hundred of the same assembly with no issues, and there were no baking changes or any other process changes. When one BGA was removed, what I saw in the X-ray was confirmed. We then checked the bare PWB and found that the foil lifted easily, with little or no effort you could barely touch the pad with a 600 deg. F. iron tip and the pads would lift off due to the wetting forces with the solder on the tip.
When we reported this back to the fabricator they told us that this particular laminate slash types were harder and less porous, and thus easier to pull off the copper, but they took all of the PWBs back from that lot and re-issued new boards, and they told us the pre-preg supplier had made a change in the bonding resin used to laminate the copper, and that fixed the issue.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Woolley, Mark D. (Mark)
Sent: Thursday, January 12, 2012 9:59 AM
To: [log in to unmask]
Subject: [TN] FW: New Question on PWB Analysis
I am in the middle of an issue concerning a high temperature Laminate
and low pull force copper pads. The copper can be pulled from the PWB
with low pull forces and are failing in the field.
I have examined multiple damaged PWBs and have found that in ALL cases
the damage is the failure of the copper to epoxy bond. A good PWB will
normally fail within the laminate material.
The units are built using Pb-free processes with Pb-free components. I
am trying to determine what can be done to show that the laminate is not
good or in the PWB manufacturer did not laminage the layers correctly.
I do not have easy access to bare PWBs that have not been processed.
Any help or direction would be welcomed.
Thanks,
Mark
mark
Mark Woolley |PTRL Laboratory | Avaya | 1300 West 120th Ave |
Westminster, CO 80234 USA |
Voice (Lab): (303) 538-2166 | email: [log in to unmask] |
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