Charles Barker@IO-US
10/10/97 11:34 AM

We are about to get into processing boards with BGA devices.  We will more
than likely have products which will require a SMT reflow - wash - SMT
reflow - wash - through hole soldering (either selective wave or hand) -
wash, series of process steps.

If our design engineers don't change their minds, the first package we will
have to deal with will be a 432-ball plastic Xilinx critter with balls
1.27mm (0.050") apart. Once they are happy with this design, I am sure we
will be looking a micro-BGA, etc.  Our present board finish is LPI SMOBC
with hot air level. Bd material is regular FR-4.

The products we produce are used in the field all over the world in all
temps and climates. This first BGA product will be speced and temp cycled
from -10 deg C to +55 deg C.  Probable future units will have to handle
from -50 deg C to +85 deg C.

For those of you who have experience with BGA's, help!  We don't want to
totally re-invent the wheel!

1. Do you know of a source for dummy practice parts other than TopLine?
They don't have this particular size.

2. Comments about OSP pad finishes? Will they tolerate all the process
steps and still protect against oxidation at the latter stages? It could be
that the process may stretch out over several days. e.g., reflow and wash
day one, second reflow and wash on day two or three, and then final through
hole and wash on day four or five. Anything could stretch out the over-all
process time such as weekends and holidays.

3. What about other pad finishes? Palladium, Gold, Plain old hasl?

4. What stencil thickness for BGA? Do you find .006" sufficient? Have you
experienced bridging with .008"?

5. Did you find that your oven profile changed drastically from that used
for boards with, say, 200+ pin leaded devices?

6. What about solder mask defined pad areas as opposed to SM opening bigger
than the pads?

7. With this large of a device (40mm x 40mm) I am concerned about CTE
mismatch over the operating temp range specified as well as the process
temp range to which it will be exposed.  What board materials would you
recommend?  Would you spec one of the higher Tg materials?  Why?  I will
need to provide cost justification for wanting this more expensive and for
some shops, more difficult to process, material.

8. What are your responses to the questions above for Micro-BGA?

For those interested, I will pull together a summary of my findings.

TIA

Charlie B.

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