We had issues a few years ago as we switched from SnPb to Pb-free
processing and from Dicy to other types of FR4 boards where the pull
strength was lower and failures occurred in hi stress pads. One vendor
suggested that an additional factor could be the amount of fill used in
the board in addition to the change from Dicy...
eric
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, January 12, 2012 3:29 PM
To: [log in to unmask]
Subject: Re: [TN] New Question on PWB Analysis
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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