On 1 February, Victor wrote:

> Hi,

> I am looking for info. or prices of the low temperature solder pastes?

> Are these solder pastes used in shop floor at all? What is your opinion 
> in using this ?

> Thank you very much.

> Best regards,

> Victor


Hi Victor!

     The last time I looked into using a low temperature solder paste was a few 
years ago on a dense double-sided board that had J-leaded parts on both sides. I
was worried about losing parts on the second-side, so I was thinking about doing
a "step-solder" process. I was going to do the second-side with a low melting 
temperature alloy so that I didn't have to worry about the first side becoming 
liquidous. That's until I started checking the prices out...whooo-doggies! The 
two metals that I was looking at to alloy with cost BIG BUCKS! It was Indium and
Bismuth. To give you an idea what it was going to cost, I weighed the board with
and without paste to get an idea of how many grams of paste I was going to print
on each board, and it worked out to around $20-30 per board using either Indium 
or Bismuth! MAN! 

     So needless to say, I didn't use it. I've heard a few negative things too 
about using bismuth or indium alloys...for one thing, the joints will be real 
dull, gray looking...not shiny at all, and I've also heard that the joints will 
be pretty brittle.

     One of the neatest things I've seen a bismuth alloy used for is with a 
little kit called "Chip-Qwik" (I think that's how it's spelled). It's basically 
a bismuth alloy that you can use to remove parts with. 

     You can use a single tipped soldering iron and remove say a 208-pin QFP! 
You take this special bismuth alloy and bridge all the leads together with it, 
and that mixes with the solder fillets that were already formed, which lowers 
the melting temperature so much that the residual heat left from bridging all 
the leads together keeps everything liquidous for a couple of minutes, and you 
can pick the part up with your fingers and set it in the palm of your hand 
without it burning you. The liquid solder can be put into your hand too...it 
kinda' acts like mercury until it cools...you could probably win some bar bets 
with this stuff (GRIN ;^D) "Why would someone want to use this alloy to remove 
parts?" you may ask. I think it might be useful if the PCB was very, very 
expensive and you don't want to risk heat damage to the PCB removing a defective
part, or vice versa...removing an expensive part without damage.


         
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