No, I haven't been ignoring you Moon-Man...
I'm on a consulting project that is will be using 2800 I/O's initially at
1.0, then going to 0.8mm. Not sure the layer count, but I know it's close to
yours.
Anyway, here's the probs I'm encountering:
1. Coplanarity- Just getting the paste & part to a decent level of
coplanarity is problematic. Seems like .15 is a good number, but the only
commit I get from the IC & assembly house is something that adds up to 0.3.
Vertical alignment should spec that total tolerances should always allow for
50%+ of the column to be in the paste. This stackup should include the
tolerances of the PCB fab and the chips.
2. Excessive/insufficient solder. The ball size in the paste, as well as
MOTS (materials other than solder) need to be minimized.
3. Plan on no-clean- unless you plan on baking these puppies post
assembly. 10 days at 50% RH and we still found water and saponifiers trapped
under the device
4. Eutectic is a must- near eutectic hasn't cut it (60-40 gets
intermittent)
5. Clean, clean, clean- make sure the board's OSP level is well
controlled and pure. Our best results came from boards that had nothing on
the copper (vacuum packed immediately after a argon cleaning etch- the
copper was very clean).
6. Thermal profile- longer at lower temps is better than shorter at
higher temps. There is a tendency to rush the profile in order to get more
thru-put- resist the temptation!
7. Do not repair in situ- take the CCGA/BGA totally off and
reball/recolumn; then put it back on.
8. Depending on the physical size (guessing you're somewhere around a
37.5X37.5), you may want to consider weighting the parts. When we got to a
40X40 device, we found that adding a temporary weight to the top of the
device during reflow improved the joint integrity. We used ground down
aluminum heatsinks; make sure you model the profile with these, otherwise
you'll be running too cold.
Let me look at what else we've uncovered & get back to you later.
-JAFO
-----Original Message-----
From: pod [mailto:[log in to unmask]]
Sent: Monday, October 11, 1999 4:15 PM
To: [log in to unmask]
Subject: [TN] Over My Head in CCGALAND
Folks,
We're doing, rather consistently, 1100 I/O CCGA's (rounded number). Going to
3,000 on same 1mm pitch and to .8mm. Any suggestions about concerns
regarding
column straightness, placement accuacy (sinks into mud and stays put), krud
between
columns, reflow and rework profiles, and all the usual stuff on 24+ layer
boards. Also,
going to duplicate current efforts using 24 parts per board. Just wondering
knowing
it might be a stretch.
Earl Moon
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