Sounds like a combination of issues here, and honestly without a whole bunch
of photographs and samples I am not sure anyone will work this out by email.
I will attempt though to highlight a couple of the more interesting points
you have highlighted.
1 Some pads come off the board - no problem with the soldering there!!
2 Some solder joints just break ( no problem there either question is
how much force used and where did the joints break)
3 Some pads are still gold - This is the one that is giving me
indigestion
It looks on the face of it that there may (possibly) be some question on the
reflow profile.
Without seeing it, it is difficult to form an opinion but are there any
specific patterns to the "still gold" pads, for instance are they all in the
middle of the BGA field?
Also are the ones that "the pads came off" along the mid - edges of the
BGA?, and the ones that broke situated at the BGA corners?
Kind regards,
John Burke
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Happy Holden
Sent: Monday, July 31, 2006 11:19 AM
To: [log in to unmask]
Subject: [TN] SAC-BGAs on ENIG Using SnPb solder paste
Greetings Assembly Experts,
This is the first time I have asked a question of this robust group of
technologists, but I am way beyond my knowledge of what can happen.
We have many thousands of ENIG multilayers (8 and 10 layer-0.062") that an
assembler has returned as "Contaminated or Defective BGA pads". These
boards are smaller (3.0" x 4.25") but they have 3 to 5 fine-pitch BGAs
(0.8mm pitch with 0.3 mm lands) on the Primary side and some have an
additional FP BGA on the Secondary side.
The problem is that after reflow, if the BGAs are pulled, they come right
off the board. Some BGA pads are still gold, some pads come off from the
laminate, some solder joints just break. They returned the boards because
they said that the boards exhibited "non-wetting and de-wetting".
We have finally collected all the facts that we can about the assembly
process, and this is where I don't know what to expect, since the assembly
process seems to be a "Hybrid-RoHS Backward Compatible" assembly. Here is
what we know about the assembly process:
1. Components & FP BGAs - SAC Balls with other components fused tin, Imm
Palladium, Ni/Au and Pb-Free solder
2. ENIG PCB Finish (lab, SEM and EDX indicate no 'brittle fracture" of the
Ni, no C, O, or Br on the Au, a smooth crystalline surface, no unusually
high or low Phosphorous)
3. Assembler had reworked the bare boards by cutting traces and scratched
off the soldermask to make additional ground connections on the surface
before assembly
4. A tin-lead solder paste was used (R253 from Kester)
5. A higher temperature IR reflow profile was used, 3:30' to heat to 217C,
Peak: 230C, 45 sec above 217C, 120 sec. to cool down from 217C to 98C.
6. The assembly facility is on the West Coast of Mexico near the ocean.
7. The change to ENIG was sudden, same P/Ns were in the facility as HASL.
8. Some RoHS parts have come in mislabelled, when they were really Sn-Pb.
They also reported this soldering characteristic when some P/Ns were
soldered with a SAC305 solder paste (Alpha Metals-OM338) and a LF IR
Reflow: 3:00' to heat to 217C, Peak: 255C, 50 sec above 217C, 65 sec to
cool down from 217C to 98C.
What do you experts out there think about this situation?
Happy Holden
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