[hpsdr] To kit or not to kit...redux.

[Ken Klein]

I know the issue to kit or not to kit is probably a dead one, but I feel
that I'd be remiss in not passing along a bit of information just learned
while building the TAPR Vector Network Analyzer.  

Frankly, I was scared to death to start the build process, but was really
interested in learning how to deal with SMT, particularly the really small
stuff.  I built the Atlas, of course, but that was easy, since the parts
were fairly large and I only needed a magnifying light.  For the VNA, I
opted for soldering the parts individually since I wanted to be able to
start and stop at will.  Solder paste dries up, and you need to do the whole
process at once, if you opt for the solder paste/hot plate/heat gun/toaster
oven.  Anyway, I got a 10X microscope; a very small tipped iron (the tiniest
I could find) with heat control, some very tiny gauge solid core solder and
some liquid solder flux.

I adjusted the iron to just melt the solder and a then some.  Sorry, but I
don't have any way of knowing the tip temperature, but I know it's not very
much above solder melting temp.  I don't think I'm in danger of damaging
parts from excess heat.

Anyway, the technique that worked the miracle is this:

1.	Wet the surface of the area with a very small amount of solder flux.
I kept thinking I was using too much, but it didn't seem to matter much.

2.	Place the part EXACTLY where you want it, nudging it with tweezers
until it is exactly in place.  The microscope works wonders here with my
old, old eyes.

3.	Now, again under the scope of course, touch the tip of the iron to
the PAD next to the leg of the device you are soldering being very careful
NOT to touch the part itself.  If you do, it'll displace the part from where
you want it to be.

4.	Add solder, again being careful not to touch the part itself with
either the iron or the solder.  When the pad is hot enough to melt the
solder, the solder will wick up the pad to the leg of the device then wick
up the leg and make a beautiful solder joint.  You won't ever touch the part
itself with either the iron or the solder.  The flowing solder does all the
work!

There's no need to hold the device in place anymore.  I don't need a third
hand. I don't have to load solder onto the tip and hope for the transfer.  I
don't have to presolder the pads. 

This technique works beautifully with discretes at least as small as 0605s,
20-mil PQFPs, SOICs, TSSOPs; in short anything with solderable legs or
exposed pads.  I haven't tried to do 0402 discretes yet, but I'm looking
forward to trying it.  

The TAPR VNA board took me about 4 hours to complete, and most of the time
was taken up sorting parts.  I thought it would take me weeks.  And it was
FUN!!!  Try this once and you'll be hooked too.

I hope I haven't bored everyone with the obvious, but this is new to me, and
I'm excited about acquiring a new skill so easily.  For those of you who
already know this technique, why didn't you tell me about it before now??
(I'm joking, of course, but I haven't read about this technique anywhere
else, and it SHOULD be well known.)

I'm off the soapbox now.  Thanks for listening.  Questions readily answered
if I can.

Ken  WR5H






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