Hello Patricia,
These types of HOT failures are typically due to cracked PTH barrels
(although it can sometimes be interconnection separation). At room
temperature the 2 sides of the crack make mechanical contact with each other
and the circuit is closed. As the board heats up it expands, and the crack
separates until mechanical contact between the 2 sides of the crack are not
in contact and the circuit opens. Cooling the board back down reconnects
the 2 sides of the crack and the cycle starts again. This type of failure
is a frustrating one as it is difficult to pinpoint.
Since the board was in service for a period of time, the failure was likely
due to fatigue of the barrel from repeated expansion and contraction. That
is the easy part :-).... The big question you asked was why did the fatigue
failure happen ³before itıs time². PTH fatigue has many contributors, some
of which you listed in your e-mail. We can break the factors down into
three different parts: 1) Quality of the PCB (materials, manufacturing
quality); 2) Damage done to the PCB during Assembly (quantity of heat
cycles, duration of cycles, rework, etc.); 3) Cycling in the field (how hot
& how many times). Failures are usually some combination of the 3 factors
and each one of the three is complex in its own right.
When we (Microtek Labs) does this type of analysis we first try to confirm
the failure mechanism in the failed PCBA (usually by microsection). Once we
understand the mechanism of failure we try to eliminate the Quality of the
PCB. To do this, we have to have a Bare and unassembled PCB of the same
production lot. If this is not available then the only PCB analysis that
can be done on the PCBA is basic thickness of platings. Analysis of
material cure/Tg, quality of PCB plating & quality after thermal stress
(soldering simulation) can only be done on an unassembled PCB. If
reliability is in question, Thermal Shock on the PCB can be performed, but
the IPC requirement of 100 cycles is a very low bar in todayıs world....
If you would like further references, please do not hesitate to ask...
Best Regards,
Bob Neves
Chairman/CTO
Microtek Laboratories
[log in to unmask]
www.TheTestLab.com
Anaheim, CA
+1 (714) 999-1616
Changzhou, China
+86 (519) 8548-7805
On 10/26/09 10:59 PM, "Patricia Lui" <[log in to unmask]> wrote:
> Dear Friends,
>
> I have the following issue which is not able to determine the exact cause of
> the failure, the details are as follows:
>
> PCBA boards which is already 1 year old
> put to thermal aging cycle
> go through functional test (boards are run at low current)
> via hole at a certain location failed with open circuit when boards are hot
> when boards are cold via holes become connected and ok
>
> Checks done:
> Micro section on failed via holes
> 2pcs PCBA boards failed via holes found voids
> 3pcs PCBA boards failed via holes found no abnormility, hole walls are all
> within specs, all are ok
> so root cause is still unknown
>
> Suspected failure due to the folowing:
> moisture absorption in the PCBA
> material cannot withstand thermal cycle
> a wrong material is sellected, maybe a higher Tg / Td material should be
> selected
> customer design issue
>
> Currently we have the following boards on hand and is deciding sending to a
> lab for analysis but do not know what test is the most appropriate. We have:
> 10pcs of the same batch bareboards on hand
> 2pcs PCBA with identified via hole failure boards
>
> I am hoping that someone can give me a piece of advise on what test I should
> engage a PCB test lab to do inorder to be able to determine the cause of this
> issue.
>
> Hope to have your favourable advise.
>
> Thanks & Best Regards
> Patricia Lui
> Xuan Technologies
>
>
>
>
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