Hey Rick,
As you know, the oxidation rate increases as temperature rises. It
certainly can become a benefit to reduce the amount of oxidation as product
complexity increases.
The flux that is within the solder paste is needed to clean (and remove
oxides and other contaminants) from the component lead or ball, the PCB pad,
as well as the solder powder within the solder paste. Typically there is
more than enough flux to clean these 3 areas. BGA's, small pasives, and
fine pitch technology can often benefit from nitrogen use.
BGA's are aided by nitrogen in many cases because the soldering surface area
on a ball is greater than on a typical QFP lead. Also many folks like to
bake the heck out of BGAs when they come in- and often bake them in an
oxygen rich environment- to increase the oxide level on the BGA ball. With
certain problematic BGA's we have found nitrogen levels tend to reduce
voiding (less oxides mean less flux outgassing, and lower chances of
voiding). If you use a slow wetting formula (as found with most no cleans,
you might not have to worry as much about trapping gas).
Often with small paste deposits (such as for 0201 technology), the ratio of
solder brick surface area to volume is quite high, and thus a high
percentage of the solder powder is exposed to the environment- often the
small volume of flux cannot overcome the additional oxides and the solder
powder will not fully wet together. Some fear that the improved
solderability of nitrogen will increase tombstoning. Tombstones are best
prevented by a tight screen print and placement process, so I favor nitrogen
for 0201 assembly.
QFP's that have questionable solderability might cause issues because many
folks reduce the ratio of solder paste per component pad to help eliminate
shorting on .020", .016" and .012" pitch components. Often the reduced
paste/flux volume will inhibit solderability.
I don't recommend nitrogen for everything, but it is nice to have when you
run into a jam with poor quality components or PCB's. Plus as I mentioned,
certain technology can benefit from nitrogen. Heck a shiny joint usually
means better wetting, and thus a better wetting angle that can improve
reliability of the solder joint.
If your OSP product is double sided mixed technology, using nitrogen will
reduce oxide formation on the PCB and give you a fighting chance for topside
solder in a no clean wave solder application.
Nitrogen is great to have if you do your own in shop dentistry (OK, that was
a joke). Good luck.
Steve A
> -----Original Message-----
> From: Richard Todd [SMTP:[log in to unmask]]
> Sent: Wednesday,February 06,2002 11:27 AM
> To: [log in to unmask]
> Subject: [TN] Nitrogen Reflow
>
> Tech-netters,
>
> We use both ENIG and OSP on 100% SMT products, no-clean, reflow in air.
> What are the advantages and disadvantages of converting to a nitrogen
> reflow? Are the significant benefits in quality or just in appearance?
> What other things would we need to change (i.e. paste chemistry, etc.)
>
> Any help would be greatly appreciated.
>
> Thanks,
>
> Rick
>
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