Thank you all for the quick reply.
For Graham:
Indeed, that's what I did today -switching from nitrogen to air for the same
assemblies on PS, I got tombstoning and no tombstoning.
For Mike:
2000ppm oxygen did not improve the situation-my boss tried to even exceed
the recommended amount without success. We are soldering leaded. The
placement accuracy was recalibrated today before the trial. The boards went
through 2 reflow cycles and the selective wave soldering will be done
tomorrow under nitrogen. It will be interesting to see the difference in
solder fill between the air reflowed and nitrogen reflowed boards.
For Scott:
A whole year of suffering for ENIG and HASL???
What advantages did you see with nitrogen? But thank you for letting me feel
belonging to the gang.
Gaby
----- Original Message -----
From: "Mike Fenner" <[log in to unmask]>
To: "'TechNet E-Mail Forum'" <[log in to unmask]>; "'Gabriela Bogdan'"
<[log in to unmask]>
Sent: Wednesday, October 18, 2006 7:01 PM
Subject: RE: [TN] Tombstoning and nitrogen


> 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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