[sdiy] RoHS

[clark at andrews.edu]

>>    We've been using lead-free parts (IC's mainly) in our production line
>>for the last 6 months or more and have had zero problems with
>>solder-joints.
>
> Isn't it kind of early to tell?

   Perhaps.  BUT, remember, we're only using some lead-free parts and
still using leaded solder, so the ultimate test is assembling a pure
lead-free part and THEN subjecting it to abuse....  That's the fun I
get to do in the very near future...

>>Are you sure that their 90% wetability isn't referring
>>to the lead-free solder process?  No one can guarantee 100% on that
>>score just yet...
>
> But you just said you had no problems.  Less than 100% wettability isn't a
> problem?  I've soldered some lead-free resistors with regular solder, and
> it is definitely more difficult to get these to wet.

   It largely depends on the type of flux used to wet the joints. 
Lead-free processes will pretty much dictate using highly active
organic flux.  Rosin core won't cut it.  Having said that though, I
routinely take parts out of the assembler machines reject bin and use
them to repair boards using good old Kester 44.  And these rejected
parts have sat around for months soaking in good old Michigan air and I
haven't had any issues.  Perhaps it depends on the manufacturer and
what finish has been put on.  I can't say.

>>    As I understand it, lead-free parts generally have a pure-tin finish
>>which poses no problem to traditional leaded (Pb) solder since it's
>>part tin anyway.
>
> Not according to the manufacturers that I have looked at.  They call it an
> alloy.

   Well yes, okay, almost pure tin...usually a small amount of copper and
or bismuth or antimony or other such things which are put in to retard
whisker growth and to provide for better flexibility and bonding.  But
this is also true for lead-free solder as well.

>>    This is why you shouldn't use lead-free solder with a part that has a
>>lead (Pb) finish.  The lead (Pb) on the surface of the part will mix
>>with the tin solder and create a very thin layer of tin/lead (Pb) which
>>is much weaker than the bulk of the tin solder surrounding the joint
>>leading to fracturing of the joint itself.
>
> Aren't you being inconsistent here?  If you can solder to lead-free with
> lead, then you should be able to do the opposite.

   Well it's not to say that you can't, but more that you shouldn't.  If
you solder to a part with a lead (Pb) finish, the point at which those
two alloys meet will create a weak intermetallic bond and since the
lead-free solder is much harder and less able to flex, you will have
the possibility of fracturing.  I'd have to look up the study that was
done on this to remember what the failure rate was on this, but it was
significant.  Since my company's products get routine abuse, this is a
very serious factor.
   The lead tin mixture of regular solder is a much more flexible material
and also creates a more stable bond to parts that have a pure or near
pure tin finish (alloy, call it what you will).

>>    Soon, within the next two weeks I'll get first hand experience in
>>lead-free production, and I'll be happy to share my results with the
>>group at that time (right now I'm in the process of getting lead-free
>>PCBs made).
>
> It will be very interesting to hear those results.  It would be nice to
> know more about the metallurgy here, but since no one is saying what their
> lead-free alloys are this may be difficult to winkle out.

   When I have the chance, I'll post all my findings and definately let
you know what alloys/fluxes/temps work (at least in a manufacturing
environment.  I also have wire solder to test out too, so I'll be sure
to add that in as well.

   Cheers,

   Tony

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