[time-nuts] soldering QFN (was: GPS down converter question)

Attila Kinali attila at kinali.ch
Thu Dec 3 05:34:18 EST 2015


Salut,

On Wed, 2 Dec 2015 18:39:55 -0500
paul swed <paulswedb at gmail.com> wrote:

> Just adding that this is home brew so no real boards. At the IF level thats
> both large enough and easy enough to add to a ground plane. So its
> reasonable to build the basics and add stuff as needed.
> The LNA front end is cheap so will get that just to try. Have to look at
> the hot air suggested above to see what that might cost. 

The Leister one costs IIRC 300-400EUR. I.e. not the thing you'd buy
for a single project. There might be cheaper ones though. All you
need is something that is small enough that you can confortably
fit into your hand, has a small nozzle and temperature control between
200°C and >400°C.

But if you are not building a board anyways, there is an even "simpler"
way to go: Dead-Bug! :-)
For 0.5mm QFN/DFN i usually use AWG30 wirewrap wire or 0.08mm^2 tinned
copper wire. Enamelled wire should work too, but I am generally to lazy to
tin the tips and make sure that no blobs of enamel residue were left on
the wire. The way to ensure that you dont get any shorts is to spread the
wires out, such they gain enough distance. As you will be soldering RF chips,
which have a ground pad in the center that needs to be soldered, i recommend
using solder wick. Use 2 wide strips and solder them first (before any of
the pads) onto the ground pad, such that they point to opposide directions.
Make sure you do get solder on more of the wick than necessary. After
you have finished all the pads, solder the wicks to your groundplane at an
as short distance as possible. 

This is of course not optimal, but the inductance of the wick should be
small enough, thanks to the width and the braiding. And you have the advantage
that you can rework if something doesn't work out.

As for equpiment, you will need a soldering iron with an as fine tip as possible
(0.5mm is absolute maximum, 0.3mm is what you should get, if you can get a
smaller one, take that one). 0.3mm solder (use leaded, it's easier to work with).
I do not recommend using thinner solder. With those the core gets so tin that
you dont get enough flux. Oh.. and be carefull about stretching the solder wire.
The metal part can be stretched quite easily with such thin wires, but the core
doesn't. So if you get this speckled look, cut of that part with some fine
side cutting pliers. You will also need at least some kind of optics with an
magnification in the range of 10 to 20 to check the solder joints. It would
be best if you had access to a stereo microscope (with an 10-20 magnification)
but if you have good eyes you can do without and instead using some magnifing
glass (i recommend line tester/weaver's glass) to see whether the joint is
ok and doesn't have any shorts.

HTH

				Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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