[time-nuts] Linear Interpolator

[Chuck Harris]

Christopher Hoover wrote:
>> For me the catch seems to be that they are packaged in a QFN-32.
>> The pin spacing is 500 microns and the open space between the 
>> solder pads is only 200 microns.  Perhaps I am too timid but this
>> sounds like trouble for manual soldering, that's assuming the low
>> cost PCB suppliers could make the appropriate solder mask.  Any
>> comments from the experienced
> 
> They aren't too bad for manual rework.   Surface tension is your friend
> in SMT land.
> 
> Solder paste and hot air helps, but you can do it with it a good tip,
> steady hand, regular solder and lots of flux.

I used to do them with a fine tipped soldering iron, and some 0.015" diameter
solder, but it becomes awfully difficult when a solder bridge decides to
follow the pad up underneath the chip where you cannot reach it.  No amount
of solder wick will do the trick when this happens.

I found the best, and easiest way is solder paste, and a hot air rework
station.  The paste comes in a syringe and is used with a hypodermic needle.
Various sizes are available (difficult to get because needles are a controlled
substance in the USA), but I find the smallest sizes work the best.  Your bead should
be about 1/2-3/4 the diameter of a pad's width.  Don't even think about using lead free
solder paste!  It makes really ugly joints.  Kester R276 is great.  Get the
smallest size you can, as it has a short shelf life.  Techni-tool.com is a good
place to get it.   Run a bead perpendicular to the direction the pads go, and
plop your SMT part down.  Bump it this way and that so that it is close to
proper alignment.  Surfact tension will pull it into place.  Take a hot air rework
station with the proper nozzle, and start cooking.  Keep the air flow low, as you
don't want to blow the part away!  When the solder melts, the part will wiggle,
and align itself.  Perfect joints everytime!  If you are going into production,
you will probably want to use an oven type unit so the whole board flows at the
same time.

I imported a bunch of the 2738 units made by Aoyue (www.aoyue.com), and have
kept a couple for my own use, and sold a few to friends, etc.  They are a decent
unit easily equal to the quality of anything Hakko makes.   I still have some left
if anyone is interested.  $239 + shipping.  Shipping is typically $25 FedEx to
the USA.

-Chuck Harris