Soldering high-density lead pitch SMD parts using a soldering iron

------------------------------------------------------------------

 by Jeff Frohwein, 2003-Feb-20th



 The key to successfully pulling this off is to use soldering

flux, a solder-coated pc board (silver colored finish, not raw

copper appearance), and not use any solder, unless really

necessary. The detailed steps are below.



 Note that a soldermasked board is not always necessary for

this type of work. Using a little bit of flux or solder wick

makes it easy to clean up solder mistakes on the board and

end up with professional looking results, even after making

several or many mistakes.





1) Clean your pc board with a dry cloth to help remove any dust

   or particles that might make life more difficult.



2) Obtain some rosin soldering flux (such as Radio Shack #64-021

   or similar) and using a resistor lead, or similar, apply a

   very small bit of rosin to all of the chip pads on the

   circuit board. You only need a very small amount. If the

   applied flux is twice as thick on the board as the chip

   lead thickness then that is plenty.



3) Do *NOT* attempt to skip step 2. Using flux is critical for

   this to work, otherwise you may end up with a big mess. You

   will then need to go out and buy some flux and proceed to

   step 12.



4) Carefully center the chip over the flux and set it in place.

   Using an Exact-o knife or other object, carefully slide each

   side of the chip until the pins of the chip perfectly align

   with the pc board pads. Once it is in place, you need to use

   one finger and *gently* apply a minimum amount of pressure

   to keep the chip from moving until you have a chance to solder

   it.



5) Using a fine-tipped soldering iron, clean the tip on a wet

   sponge and then touch one of the corner pins of the chip for

   about 1/4 of a second. If all goes well the minimal amount

   of solder on the pc board will be enough to solder the pin

   in place. You might consider putting some solder on the iron

   before doing this, but if you do, it should only be enough

   to coat the tip of the iron. If the tip has a "blob" of

   solder on it then you need to wipe that off, first, or else

   you will short 2 or more pins together, in which case you

   will need to proceed to step 12.



6) Align the other corner of the chip and then repeat step 5.



7) Carefully check that all IC pins are still aligned and that

   the chip is flat against the pc board. If so, go on to

   step 8. If not, reheat each corner and align it until it

   is right.



8) All you need to do now is lightly touch the soldering iron

   to each pin for about 1/4 second. Do not turn this into a

   long, drawn-out process because the flux is slowly drying

   on you. If needed, add a bit more flux if it is becoming dry.



9) Visually inspect all pins to make sure you soldered them all.



10) Using a sharp knife or extra resistor lead, go along each

   chip connection and lightly push on the foot of each chip

   leg at a 45 degree angle, from the side. If you can visually

   see any leg slightly move then you know that it did not solder

   to the board and needs to be soldered again. It really helps

   to have a powerful light and/or magnifying lens at this point.



11) If you develop a short between two legs then apply a very

   small amount of flux, if there is not some there already, to

   the short. You now have the choice of using a sharp, heat

   resistant instrument to scrape between the legs after you

   heat the joint, or try step 12. Most metal instruments are

   not a good idea for use here because they will stick to the

   solder. (A stainless-steel surgical instrument might work

   well though.)



12)

  A) I have found the following to always be effective for removing

   shorts between legs but it takes some practice. However, it

   should not be used on parts that are very sensitive to shock.

   The general idea is to heat the joint, and while the solder is

   still liquid, to basically slam the pc board against a hard

   surface along the edge of the board. However, the resulting

   solder that gets jarred loose can become a problem for other

   components of the board. So if you plan to do this, you need

   to cover the rest of the

   pc board with a paper towel or similar. Cut out a hole in the

   towel that is just large enough for using the soldering iron.

   Then use electrical tape or similar to secure the edges of the

   hole in the paper towel to the pc board. This is about the

   only way to prevent very tiny solder blobs from lodging in

   adjacent SMD chips.



  B) This suggestion comes from Artur Jasowicz: "As far as

   removing shorts from in-between pins, I've been successful

   with using Desoldering Braid like RadioShack's part 64-2090.

   Another technique I'd recommend is using sharp exacto knife

   to cut excess solder from in-between the pins and then

   re-flux the solder, which will suck extra solder from the

   edge to under the pin."



Have fun!