Re: SMD soldering (off topic)

Jeff Sutherland wrote:

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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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I'm not really experienced in this area but here's what I found worked 
really well for a SSOP28 package.

In contradiction to what others say I used a very large (9mm) knife type 
soldering bit. First I tinned my homemade PCB using a soldering iron and 
SMFL. I then soldered down one pin, I accidently bridged the next with 
solder. A very small amount of solder was then applied to the knife tip 
and more SMFL to the board an chip pins. I then applied the knife tip to 
the pins and pads and slowly drew the tip across. I was amazed by the 
sucess of this, even the bridged pins cleared. This is the only SMD IC 
that I have ever soldered, it may be just luck!

SMFL is an rework solder flux spray and is non-corrosive.

Try on some spare chips, Maxim/Dallas free samples may be handy!

Good luck!

Andrew


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