[sdiy] stupid solder

[Tony Clark]

   Hi Sowa,

> I have a lead-free solder alloy (Sn96%) that is real pain to use.
> It melts in higher temperature than leaded one, and it's almost
> impossible to achieve shining mirror finish with it. After many
> solder points I came to not bad results, i.e. solder surface is
> not matt, but it's not so shiny as Sn60 either.
> 
> Now my question goes - how bad is it when solder joint surface
> is matt, i.e. oxidized? Or is it those few % of silver that
> come out to the surface and make it look matt? Does it influence
> anything? Will it break sooner than shining one?

   I've been keeping up with the Lead-free issues (because I'll 
eventually have to deal with it here at work)... There is an article in 
the June 2002 issue of "Circuits Assembly" about the reliability of 
lead-free.  Although the article was written before all of the tests had 
been performed, it does seem to indicate that lead-free will outperform 
regular leaded solder, especially when bonding to lead-free parts/PCBs.

   Now this is of course that you are providing the correct environment 
for processing the lead-free solder.  Mostly the article is talking about 
reflow, which is a controlled temperature environment, something that's 
not achievable with a soldering-iron!  So it is quite possible that you 
will not achieve shiny solder joints.  Will it make a difference?  I 
don't know...are you subjecting your boards to lateral forces (ie bending 
the boards)?  Are your boards subject to long periods of vibration?  Are 
your boards subject to wide temperature operating environments?
   If not, then I wouldn't worry too much about the long-term failure of 
the solder joints.  While the matte surface of the joint would indicate a 
more fragile crystaline structure in the joint, the truth is that you 
probably won't stress the joints enough to cause failure.

   One trick to try would be to simulate the process of a reflow oven.  
The process is always a slow temperature ramp-up, followed by a very 
quick cool-down period.  Of course reflows are measured in minutes, not 
in seconds as with soldering irons!  So the most likely problem is that 
your solder joints are cooling too quickly.  You might try pre-heating 
the board and then soldering the joints while the board is still warm.  
The extra thermal energy stored in the PCB may prevent the joints from 
cooling too rapidly and causing the crystalization.
   Another thing to try would be to use an inert gas around the solder 
joint.  Nitrogen, I believe is what is traditionally used in PCB mfg to 
prevent dross formation in wave soldering, but I believe it's also used 
in some reflow processes as well.  Probably a bit overkill for most 
DIY'ers...

   Anyway, hope some of this has been of help!

   Cheers,

   Tony

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