[sdiy] RoHS

[clark at andrews.edu]

>>    No, you do NOT need leaded components.  Lead-free components are 100%
>>compatible with leaded solder.
>
> Are you sure?  The manufacturer's literature I read on resistors only
> claims 90% wetability. Seems to work reasonably well, but I wouldn't call
> it 100%.

   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.  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...

   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.  So there is not major disruption in the metallic
composition, in particular the intermetallic layer where the solder
meets the component lead.  However, if your solder has no lead (Pb) in
it but is one of the tin/whatever(add your favorite mix here) the 
bonding surfaces have to be either tin or some other suitably receptive
metal in order for the solder to properly bond without forming a weak
intermetallic layer.
   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.

> And how are these new alloys going to behave with time?  What if you don't
> solder them right away?  What if you have to rework something?

   As for long-term parts storage, I haven't had any problems with this
either, however, it should be pointed out that normally lead-free
soldering requires a more aggressive flux, so I'm sure that this will
help alleviate some of the long-term storage issues that have come up.
   So far my PCB house claims better long-term storage of their PCBs with
a lead-free HAL finish than the standard HAL.  I really don't know if
this is true since we never keep more than 3 months stock of PCBs on
hand at any one time (typical storage life-expectancy is supposed to be
somewhere around 4-6 months).
   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).

   Cheers,

   Tony

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