Hi Gaby
I've never done 20 layer OSP boards, so can't speak to that.

I can confirm that nitrogen during reflow will indeed result in a
higher occurrence of tombstones.  We can turn tombstones on and off for
both 0402s and 0603s by turning the nitrogen on and off.  Of course,
this also is related to the other parameters that impact tombstone
occurrence - such as uneven or oversize pads, and placement accuracy.
So the impact to these specific designs may be minor, or may be a
nightmare.
In our case, if we can't address the tombstones with tweaking the
placement and paste then we do in some cases go to an air reflow.  But
we aren't using OSP so we aren't as worried about oxidization of the
surface finish.

Good luck!


regards,

Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215

>>> [log in to unmask] 10/17/06 05:52PM >>>
Netters,
We recently started to use reflow under nitrogen on OSP coated boards.
These boards were previously immersion silver plated and we reflowed
them in air. Very few tombstoning was seen.
Our customer decided to change the finish and requires mandatory reflow
under nitrogen.
We are completing all the soldering operations within 2 days. We use no
clean solder paste and no clean flux.
Questions:
1. Did anybody experience increase of tombstoning of 0402 passives when
using nitrogen?
2. Is using nitrogen during reflow  beneficial for the next process-
selective wave soldering under nitrogen?
3. For thick multilayer boards with more than 20 layers, can complete
hole fill be expected during wave soldering of OSP boards after double
reflow?
If you had any similar experience, could you tell me how you solved
these problems?
Thank you,
Gaby

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