HI Gaby
The surface tension of solder is higher under nitrogen than air. This can
certainly increase the incidence of tombstoning. You don't really need
nitrogen if you have a decent paste. If you are soldering unleaded then this
is very much more the case. If you have an increase in 0402 tombstoning
then, assuming you can't optimise pad design for the new finish, it may be
you can mitigate with tighter placement and placement force accuracy, print
control and so on.
If you are soldering leaded, then you can use anti tombstoning alloy. The
right ones can be very effective. Unleaded: SAC305 can help, but you are
much more in the hands of the flux formulator here, unleaded is a question
of increased flux activity , but also speed. You need a slower flux as much
as one that can cope with increased oxide levels associated with High tin
solders processing.
Nitrogen during the first process will reduce the amount of re-oxidation on
component leads and boards. This may be beneficial to the second solder
process, depending on how marginal it is. (It can change a number of
marginal fails to marginal passes). A well setup process with good
solderability, proper flux etc, will not notice.
Nitrogen under the wave is virtually reverse of reflow: it is desirable for
leaded and very desirable indeed unleaded.
Heavy 20 layers OSP and hole fill on second go round? well that's heading
out to the edge of the performance envelope. As you probably guessed its an
it depends. Depends on the thermal load in the board, how much topside
preheat you can bring to bear, the performance and type of flux you have,
solderability of course. It can be done on a routine basis and so is a
reasonable expectation, but also it may be that you don't need to over
inspect. The hole filled requirement is at least as much about not wanting
inspectors to make discretionary judgements as it is
performance/reliability. 75% is enough for most apps per IPC.
It maybe the design allows PIHR with extras solder t ensure hole fill.
Preforms in tape and reel in other words. The cost of the preforms would be
offset by not having to run a second machine type/lack of nitrogen.
One final thought: did your customer specify the amount of nitrogen/ppm
oxygen? It could be argued that everyone solders in 80 something per cent
nitrogen for example. How much do you need to adjust the 100% figure by to
avoid tombstoning, I beg your pardon, optimise first time pass results?
Maybe you can get a cost saving as well?
Regards
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gabriela Bogdan
Sent: Tuesday, October 17, 2006 9:53 PM
To: [log in to unmask]
Subject: [TN] Tombstoning and nitrogen
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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