Some preliminary results back from SEM analysis:
After shear test and BGA device is removed, majority of solder balls
shear and remain on the PCB pads, an indication of good solder strength.
However, 3 pad locations on the pcb revealed de-wetting and min. solder
deposition. After inspecting the shape of the corresponding solder balls
on the BGA it was clear that there were contacts made between the solder
balls and pad during liquidous, but no proper wetting. This explains the
reason for intermitted "pass" after pressure was exerted on the
component.
It is still unclear why this de-wetting occur, surface integrity test
also showed that adequate amount of Cu/Ni/tin was presented at the BGA
pads. There was only a very thin layer of solder (mainly lead) found on
the BGA pads.
Rgds,
Peter
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Monday, September 23, 2002 4:50 AM
To: [log in to unmask]
Subject: Re: [TN] BGA failure and Rework
Tore,
There are always more QUESTIONS than ANSWERS.
You state that much efforts have been placed on this issue yet you have
not
specified what those actions are. Here some are things to consider
that
will eliminate what isn't the cause.
1). Is there more than one PCBA and/or BGA vendor? If, so
conduct matrix.
2). Is the failure VINTAGE related?
3). Is the failure PRODUCT RELATED?
4). What plating process does the PCB and/or BGA vendor
employs.
ie ENIG?
5). Can you duplicate the failure at POST Reliability Testing?
6). Have you conducted a MANUAL Shear/Peel Test?
7). Have you conducted fault isolation to a SPECIFIC BGA
location.
8), How did the CROSS SECTION of the BGA look? ie. nodules
free
and/or size of nodules?
9). What did SEM/EDX determine?
The list goes on and on. Very painful but necessary to determine
FAILING MECHANISM.
Victor,
-----Original Message-----
From: Tore Jakobsen [mailto:[log in to unmask]]
Sent: Monday, September 23, 2002 1:53 AM
To: [log in to unmask]
Subject: Re: [TN] BGA failure and Rework
Victor
We've put quite a bit of effort into this, but the fact remains.
After removing the suspect components (which fail during test of the
assembled product) we find that some pads on the component are
gray/brown.
Further analysis of these darkened pads, show that they probably never
gained a proper intermetallic connection. This kind of fault is a real
problem, as it's one of the faults we cannot detect by any other means
than
functional testing. And even then it's not guaranteed the product will
continue working.
We put focus on this problem, due to our customers finding products
failing
after thermal stress testing. (The boards were fully functional as they
left our site)
Upon removal, we found several black pads beneath the failing
components.
Rgds
Tore
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