Click here for larger
picture (881 KB)
When I assemble a board, I squirt a very small glob of paste on each of the
pads. It takes a little practice to learn how much is the right amount.
On ICs, I just run a thin line of cream down the row of pads, rather than
trying to do each pin. Using tweezers, I position the part on the pads.
The picture above shows a board with the components placed, but, before
cooking. If you zoom in, you can see the solder cream around the component
pads.
Click here for
larger picture (859 KB)
Once all the parts are on the pads, I plug in my electric skillet and put the
board in the cold skillet. I then turn the skillet temperature control to
200 F. After a couple of minutes, you will see the cream turn a very dull
gray and start to spread out over the pads. This step is to warm the board and
components slowly, avoiding thermal shock. In the picture below, you can
see the solder cream spreading around the pads.
Click here for larger picture (1021 KB)
Once the all the paste has turned gray, I turn the temperature up to 350 F.
After a couple of minutes, you will see a little smoke come off the board.
Watch the board carefully, and you will see the cream start to turn shiny
and suck itself onto the pads. You may also see some of the parts pull
themselves into alignment with the pads. In the picture below, you can see
the solder cream has melted and turned shiny around the components on the left
side of the board, while it is still gray around the components on the right
side.
Click here for larger picture (984 KB)
My skillet seems to have some areas that are warmer than others. I see
some sections starting to melt before others. Be patient and keep watching
the board. When you see all of the solder has melted, turn the skillet
off. Do not remove the board yet! Wait a few minutes until the
solder cools enough to harden. If you
are really impatient, you can poke at a joint with a dental pick and see if it
is hard.
Once the board cools enough to handle it, remove and let it cool to room
temperature.
Finished!
Click here for larger picture (1324 KB)
You will notice small, stray solder balls scattered around the board. You can
brush these off with a stiff-bristled brush, such as toothbrush.
The flux is water soluble and Kester says it does not need to be removed
from the board. I do not like the sticky residue, so I wash the board
under running warm water and dishwashing liquid.
Look the board over carefully, looking for any joints you missed or solder
bridges between IC pins. Use solder wick to remove any bridges. Use
your soldering iron and regular solder to fix any joints you may have missed.
One problem you may see is a thing called "Tombstoning." This is
where uneven surface tension of the melted solder causes a component to stand up
on end. This is usually caused by improper pad design and happens most
often to chip resistors and capacitors. Again, use your soldering iron to
fix these. I have only seen this when I tried to mount 0603 parts on 0805
pads.
I bought a new electric skillet at my local Black and Decker Outlet store for
$15. It is Black and Decker Model #SKG100. Obviously, once you have used a
skillet for soldering, do not use it for cooking food.
One last note, this only works on boards that have parts one one side of the
board. If your board has parts on both sides, you may want to consider the
approaches illustrated in the websites below.
There are a couple of other websites about homebrew SMT soldering you should
also look at:
Cash Olsen
Cash uses a low cost, hobby shop hot air gun for a heat source.
http://www.zianet.com/erg/SMT_Soldering.html
Cecil Bayona
Cecil is using a toaster oven to do his SMT soldering
http://www.hpsdr.com/Public/Projects/SMT/SMT.html
Spark Fun Electronics
These guys have tutorials on many subjects of interest to homebrewers.
Scroll down the page to "Surface Mount Soldering Tutorials"
http://www.sparkfun.com/commerce/hdr.php?p=tutorials
Some more good SMT soldering links:
http://www.geocities.com/vk3em/smtguide/websmt.html
http://www.seattlerobotics.org/encoder/200006/oven_art.htm
Happy homebrewing!
You can reach me at:
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Copyright Paul Alexander WB9IPA 2006
