On Sat, 6 Feb 2010, Jonathan Hallameyer wrote:

> I took a nasa soldering class, same thing  63/37 only, and no ultrasonic
> cleaning for stuffed boards.   As far as 63/37 only,  the other stuff may be
> alright, but none of it has been used for 100+ years like lead solder. There
> may be finally be coming good alternatives, but for a while, lead free meant
> higher temperatures for soldering.

It's not so much about temperature as the formation of tin wiskers. It's 
easy to make low temp solders without using lead. True they are much more 
expensive than standard tin / lead solder. One of the actions of lead in 
standard Sn/Pb solder is to inhibit the growth of tin wiskers. These are 
tiny little fillaments that grow from the surface of tin. They can cause 
trouble in circuits as they are conductive. Look up tin wiskers.

>  I'd say lead free was pushed before good
> alternatives were available, but if it werent pushed, who says they would
> have been developed... So catch .22 there
> 
> And you nailed it on the reasoning for the no ultrasonic cleaning.  Our
> instructor kept it simple.  Some dies are wirebonded using ultrasounds, so
> no ultrasonic cleaning to be extra sure...   Though I guess you could say
> some were bonded with just heat, so no heating the leads ;p
> 

I don't know why ultrasonic cleaning is banned from military or NASA 
circuits but I do know that untrasonic cleaning can cause microscopic 
pitting on surfaces. Maybe there is the risk of pitting of the surface of 
the chip buried inside the plastic package.

Regards
Sergio
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