Re: SMD soldering (off topic)

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On Monday 19 January 2004 10:26, Tom Blough wrote:
> I know some of you out there are hardware guys and were wondering if anyone
> had any pointers on hand soldering surface mount devices.  I'm going to
> make a few boards to play with some of the 1-wire devices and have never
> done SMDs before.
>
> I'm thinking of supergluing (any problems with the superglue electrical
> wise?) the chip down, fluxing the leads, and then heating and applying
> solder to the pads (0.015 tip and solder).
>
> Will this work?  Any other methods for hand soldering?

Watch out, superglue is actually somewhat conductive.  Don't need it anyway.  
The hardest part about soldering fine pitch parts is just getting a look at 
things so you can see what you're doing.  Work lights with integral 
magnifying lenses are probably _not_ enough, get a loupe or some kind of 
stereo magnifiers to help out.  A very fine tip temperature controlled 
soldering iron is essential, plus the right flux.  Use absolutely the 
thinnest flux core solder you can get, something like the Kester 331 series 
in .015" diameter (low clean flux core).  Get a compatible flux pen like the 
Kester #951 for use with QFP packages and other fine pitch parts.  It's not 
needed for the discretes like resistors and caps.  If the board's already 
tinned that's usually good enough- leave the solder on the pads that came 
with the board.  Scrub the flux pen on the QFP pads and place the part (this 
is the _hard_ part) on the pads.  For the 331 solder, it works best if the 
tip temp is around 650 degrees.  Less than 630 or so and it won't flow 
quickly, more than 680 and it burns the flux and makes it harder to keep the 
soldering iron clean.  Once in position I start by grabbing the part with 
tweezers to stabilize it, then pick up a tiny blob of solder on the iron and 
just touch it to the outside edge of the pad at one corner of the package.  
That's usually enough to reflow that one pin.  Do an opposite corner, double 
check position, then finish it up by touching the iron to the outside edge of 
the pad, and applying a bit of solder under the tip from the side OPPOSITE 
the part.  This is the key, as it allows the pad to pull off just enough to 
wick up under each leg without bridging.  Usually you can do 3 or 4 legs 
before putting on a little more solder.  It's possible, but not always 
likely, you'll get all the legs soldered without bridging, so keep some fine 
solder wick around.  A little practice is all it takes.  Stay away from the 
caffine, though, you'll need your fine motor control in top shape...

Once completed, take a dental pick or equivalent tool and walk the pins all 
way around once looking for loose joints.  It happens, and you can't tell by 
looking, so may as well get in the habit.

Picking up a small amount of solder on the iron tip and sticking down the 
discretes by placing them and then touching one end and the pad is the 
fastest and easiest way to place things like 0603 resistors and caps.  Pick 
and place a few, then go back and solder the other ends in the usual way.  It 
really isn't that hard after you get a bit of practice.  Try to stay away 
from heat guns as that's a sure way to wreck a board if you don't have much 
experience with one of these things.  Also if something _is_ wrong with the 
board, provided you've placed all the parts correctly, it's most likely a bad 
solder joint.  Unless the board has been zapped somehow, it will NEVER be the 
qfp's or memory chips.

- -Jeff
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