Re: The strengths of FORTH / OT: SMT soldering

"Brad Eckert" <nospaambrad1@[EMAIL PROTECTED]
> wrote in message 
news:f5737082-cb09-4f66-b602-c99ad3215be8@[EMAIL PROTECTED]
 soldered SMT parts for a few years with just a soldering iron,
I find that using solder paste makes things much easier.

Thanks Brad,

I had been wondering whether that might be a viable approach.  My concern 
was whether I could dispense a small enough amount of paste manually.
Based 
on what you've said, I'll probably give it a try. What size needle(s) do
you 
use? Have you had any shelf-life problems?  The Digikey catalogue says it 
has to be refrigerated and it has to be shipped  overnight or second day 
air. It sounds like this stuff might get very expensive when used in 
hobbiest quantities if it goes bad between projects.

I don't even use a stencil. Just dab a little blob on each land (the
paste comes in a syringe with a needle), or in the case of fine pitch
parts spread the paste across all of the lands together and hope for
relatively few solder bridges. A hot air rework station melts the
paste just fine, but I think a hot air gun would work. If it isn't hot
enough you could modify its fan to run slower. For small parts like
0402 (I've seen roach poop bigger than 0402) my tweezers have become
slightly magnetized so I can carry most parts to their locations by
letting them hang from the end of the tweezers. Touching the part to
the solder paste takes it off the tweezers and then it's easy enough
to nudge the part into position.

I have an Aoyue hot air rework station from Sparkfun.  I've used it to 
remove a couple of parts but haven't tried reflowing anything yet. 
Sparkfun 
have some good tutorials on the subject of SMT soldering but leave out key

details. I'm not too keen on the idea of using my prototype for 
experimenting with. I don't want to blister the board.  What temperature
do 
you run the hot air at?

At 63, I find the 0603 parts hard enough to deal with visually without the

microscope.  After this experience I wouldn't even think of using 0402 
parts.

Removing solder bridges from fine pitch parts is not a problem. Just
use lots of flux and use a soldering iron to pull away the excess
solder. If it doesn't come off you might need some solder wick, but
usually cleaning the tip will allow the iron to pull off excess
solder. If a fine pitch PCB footprint uses pads that are a lot longer
than needed, you could end up with bridges under the package that are
hard to reflow. QFN packages tend to be easy to reflow, don't have
much problem with solder bridges, and you can clean up excess solder
by running the iron around the perimeter of the package. Those are my
new favorite package.

I've had good success with 0.025" solder wick for removing solder bridges.

I also use a flux pen to soak the pads before I solder.  I can almost see 
the board (Express PCB lead-free) oxidizing before my eyes.  After a
couple 
of weeks exposure to the air, the pads are taking on a yellowish cast. 
I'm 
thinking of cleaning the pads up with steel wool before soldering.  Have
you 
any problems with oxide build-up?

I have four QFN packages to deal with and frankly have been quite worried 
about getting them soldered down correctly.  TI state that soldering the 
ground pad is necessary for heat sinking. Based on your comments, solder 
paste and hot air appear to be the best approach.


Bob Miller