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

paul swed paulswedb at gmail.com
Thu Dec 3 10:06:49 EST 2015


Atilla,
Thanks you have read what I have written and further your experience aligns
with mine for hacking these chips at least into the GHz range. I suspect it
really falls apart in the 2-3 GHz and above region. But I have built a lot
in the 1.296 GHz region so have that experience at least.
Regards
Paul
WB8TSL

On Thu, Dec 3, 2015 at 5:34 AM, Attila Kinali <attila at kinali.ch> wrote:

> 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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