Looks like you have a whole series of compounding problems.....

1) BGA pads are still gold - This normally means no solder paste was put
on the pads...Even if you just put SAC 305 on the pads and wipe them off
later you should see a light solder covering

2) ...some pads come off from the laminate... This is a design/process
problem that occurs with all BGAs with small pads... a) The Cu-epoxy
adhesive is yielding and allowing the flux to become part of the
component mixture which weakens it, b) The Cu-epoxy adhesive is breaking
down at solder reflow temperatures or the reflow temperature shear
strength of the epoxy is less then the surface tension of solder which
will pull the pad away from the PCB 

3) ....some solder joints just break... -This depends on where they
break... If it breaks in the bulk solder then there is too much force
for the joint...If it breaks at the PCB pad interface then you can have
a porous Au layer that allows Ni oxidation...If it breaks at the BGA
substrate interface your PCB pads are too large...

Paul

Paul Edwards
Process/Quality Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48

-----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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