HI Bev
My usual advice on being presented with this question is summarised:
If you can't specify it out, pre-tin off line using a strong acid flux, wash
like ... crazy... and then solder on-line using normal process. Make sure
the flux is kept under lock and key when not in use - if it gets on the line
it will rot all your parts in a few months.
A longer answer and explanation would be
First let's clarify: this is a primarily a fluxing issue, alloy type is
secondary.
Now we have to differentiate between nickel the metal and nickel the
plating.
Nickel as a metal is beyond the capabilities of no clean/rosin based fluxes,
they don't have the poke to remove the passivation layer.
Some organic acid fluxes might be [marginally] OK, depending on the history
of the nickel part in question.
Really you need either to use an inorganic acid flux, and comprehensive
washing.
Or brighten the nickel mechanically or chemically (followed by comprehensive
washing) to allow a less active flux (fairly immediately) or apply some
protective coat.
[Or possibly you could use ultrasonics to break the passivation at time of
soldering and permitting use of no cleans etc.]
What I mean by 'history' is that if the part is made from nickel it is
unlikely that the metal bashers who made it were thinking about it being
soldered, why would they? So possibly it has had some heat treatment, stood
around a while, got machine oil or other contamination etc. You get my
drift.
If the part has been plated the story gets a little more complicated.
The plating could be a hard protective finish with brighteners to make it
look pretty, usually nickel plates have phosphorous and the amounts could be
wrong for soldering and so on. See above for how to proceed on soldering.
Or the plating could be optimised for soldering. There are proprietary mixes
which slow down passivation, render the surface solderable in some way.
These are effective (within limits) and are capable of being soldered using
resin materials. The semiconductor people use these finishes routinely for
example, in power die attach applications with Sn/Ag and resin/rosin fluxes.
However you gat the surface to a wettable condition, you have to allow for
the fact that nickel is slower to form intermetallics than usual surfaces
like copper, and your process will have to accommodate that. Some where in
the archives we have had this before, but from memory about 10x slower.
Hope this helps
Regards
Mike
Indium Corporation
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Thursday, May 07, 2009 12:41 AM
To: [log in to unmask]
Subject: [TN] Nickel - again
Technetters,
Have any of you had any experience with trying to solder tin-based solders
to uncoated nickel? What kind of flux did you use? Do you know if the
joints had long term reliability, not just appearing to be "strong" joints
right after formation? What about negative wetting angles causing possible
stress concentration points?
Bev
RIM
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