Thank you Steve!
This is an excellent and informational document. Thank you for posting
it.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, October 17, 2005 11:05 AM
To: [log in to unmask]
Subject: Re: [TN] Voids in BGAs, again
Here's a link to a pretty good presentation about voids, I'm assuming it
is public information since I found it searching the internet:
http://www.smta.org/files/oregon_chapter_presentation0905.pdf
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
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| | "Kane, Joseph E |
| | (US SSA)" |
| | <joseph.kane@BAES|
| | YSTEMS.COM> |
| | Sent by: TechNet |
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| | 10/17/2005 10:35 |
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| | ; Please respond |
| | to "Kane, Joseph |
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| | <joseph.kane@BAES|
| | YSTEMS.COM> |
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| To: [log in to unmask]@SMTP@Exchange
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| cc: (bcc: Stephen R Gregory/LABARGE)
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| Subject: Re: [TN] Voids in BGAs, again
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Some BGA connections with voids fail.
Some connections without voids also fail.
And some solder joints with void area of 50% of the connection
cross-section last just fine.
This was much debated in committees developing the new revisions of
J-STD-001 and IPC-A-610 (and from what I hear, in the IPC-7095
committee). After many months of discussion, no one could point to any
rigorous data showing that voids in the bulk of the solder connection
cause any problems.
That being said, most agree that voiding at the interface can be a bad
thing, and should be avoided.
But seeing holes on an x-ray image does not, in itself, mean that the
thing will crack.
If there is any data out there that anyone can share, it would be much
appreciated, that much I know for sure.
Joe Kane
BAE Systems
Johnson City, NY
-----Original Message-----
From: Stadem, Richard [mailto:[log in to unmask]]
Sent: Monday, October 17, 2005 10:16 AM
To: [log in to unmask]
Subject: Re: [TN] Voids in BGAs, again
Hi, Ioan
I have had the "opportunity" to see several failures due to voiding in
previous jobs where I worked on military products. One of the most
memorable was a circuit board that was part of a device that was
launched; it was shot out of a gun. To qualify the device, several were
assembled, inspected, tested, etc. very carefully. Upon the gunshot, the
device failed. During failure analysis several cracked solder joints
were found on a particular SMT 1208 capacitor which was used several
times on the assembly. One of these that actually caused the electrical
failure came out of its solder joint on one end and caused an open
circuit condition. A very intensive analysis was undertaken to find the
reason why this particular capacitor part number's solder joints and no
other solder joints on the assembly cracked. There were many other
similar SMT chip caps and resistors, as well as much larger components.
None displayed any stress related to the gunshot. The failed capacitor
that caused the open circuit was at a 45 degree angle with the board.
The anode was still soldered to the pad, but the cathode was up in the
air, fully out of the solder joint. The top half of the solder joint was
still soldered to the end cap, the remainder of the solder joint was
still soldered to the pad. While much analysis was being done, X-rays,
SEM analysis to determine impurities, XRF to determine robustness of the
surface finish, etc. and other attempts at root-cause determination,
etc., a second unit was built up and was gunshot. This time all of the
processes had been double-checked. Not only did the second assembly fail
the same way, but the same capacitor fully fractured on one end and
caused the open circuit again, in the same location on the assembly, and
again it was sticking up in the air exactly as it had done on the first
assembly. Talk about perfect defect replication! At this point I was
called in to the factory to see if I could determine what the problem
was. Upon examining the X-rays of the two failed assemblies, I noted two
things. The first was that there was a small solder void on every single
one of the capacitor's set of solder joints, the second was that it was
always on the cathode end of the capacitor, never the anode. When
looking at the end cap's configuration I was puzzled, as both appeared
to be the same. So I assumed that some type of outgassing of the
component occurred on the end that was voiding. This was not the case,
however. When I was finally able to get some loose parts to examine, no
outgassing was observed up to 230 C. in a special chamber. After the
outgassing test, I looked for evidence of venting under a microscope.
What I found was a very small .015" hole on the bottom of the cathode
termination. This hole had been overlooked by those who had looked at
the parts before me, and I did not see it when working on the failed
boards. It was not shown or noted on the component print, as it was felt
to be "insignificant" by the component engineer. This hole provided a
means of trapping air and flux during assembly and was the cause of the
voids seen only on the cathode end. These voids were not large, perhaps
between 15 and 25% of the flat area of the solder joint formed between
the termination and the pad. However, it was enough to cause a solder
joint fracture, and because this particular failing component was near
the center of this board, the board flex was higher than those near the
edges. When the capacitor was replaced with a different vendor that had
no hole, the boards all passed gunshot testing.
I have also seen voiding cause solder fractures on interposed pin
components. When examining solder joints that underwent thermal cycling,
those solder joints with voids were much more likely to develop cracks
and fail electrically than those that had no voiding. Fixing the voiding
problem by changing to a different solder paste and or a slightly longer
time above liquidus temperature (TALT) fixed the cracks.
In a previous job, BGAs that had failed in the field and were returned
for failure analysis were found to have voids in the failed ball(s). In
nearly every case, if the BGA failed at all, it was noted that voids
were present. When we looked at good units returned from the field for
other problems, such as a cracked flex circuit, no voids were seen when
we examined the BGAs or they were extremely small. This assembly did not
see any abnormal physical stresses, and its normal service environment
was typically that of a laptop computer, it was essentially a field
computer for the military, so it saw occasional instances of extreme
heat and cold and some drop shock.
I know of several studies that have stated that voiding will help
prevent a crack from propagating completely through the solder joint as
it provides a stress relief mechanism, and several other papers that
take an opposing view. I am no Werner Engelmaier, but one thing I have
noted is that microsections of solder joints from assemblies where some
of the solder joints show voids typically have a much coarser grain
structure than those that do not, and in that case all of the
surrounding solder joints also display this coarser structure, even in
the solder joints without voids. Solder joints microsectioned from
assemblies where a different solder paste was used that was formulated
to resist voiding, and a slightly longer TAL during the reflow profile
was used, produced a much tighter grain structure and no voiding.
Whenever this was accomplished there were few, if any, fractures.
That much I know for sure.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Monday, October 17, 2005 8:03 AM
To: [log in to unmask]
Subject: [TN] Voids in BGAs, again
Hi Technos,
I know this has been debated at least once a month, but I will not ask
what is the acceptability.
What I want to know is if anybody had the ocasion to actually see
failures related to voiding. How did you realize they were due to
voiding? Also, do you have any dirty pictures of the issue?
Thanks,
Ioan
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