Drag soldering is virtually rather simple when
you get the feel of it. It certainly is the preferred method at our location
and we have tried most all methods available. Drag soldering lends itself
more freely to the practice of replacing fine pitch IC's, which have been
very difficult by other methods.
The proper way to initiate drag soldering
is to start with perfectly clean pads on the PC board. This can be
accomplished with a good desoldering tool like the JBC DIS-1A or any good
with desoldering tips designed for cleaning pads, or desoldering braid. When
using desoldering braid, care must be taken not to apply to much heat to the
pads of the PC board as they can be lifted very easy causing the board to be
destroyed or virtually unrepairable.
Next, you will want to add a spot of
No-clean gel flux to the opposing corners of the IC pads. Then place the IC
on the pads and use a stereo zoom microscope,
LCD Video Zoom Inspection
System, or whatever
is necessary to obtain perfect alignment of the IC to the pads. Then place a
very small amount of solder on your soldering tip and solder tack one lead
on opposing corners of the IC, making sure that the IC is placed correctly
and is in perfect alignment.
Now you prepare the leads to accept the
drag soldering process by applying
gel flux (or a gel flux of your choice) to all of the leads of the IC.
If you have not already, you will want to install the drag soldering tip of
your choice into the soldering iron of your choice. Our preference for this
job is the JBC CD-1BC or
JBC DDTP245 which happens to be the finest soldering stations in the world. It
is our preference because it has such a fast response time, which allows you
to work faster and more efficiently. There are also a number of drag
soldering tips available for these two instruments.
Now we are going to start to work on the
side of the IC that we haven't solder tacked (considering we are working on
a Quad Flat Pack or QFP). That way we will not disturb the one lead that was
soldered to hold the IC in perfect alignment. Now you will apply a liberal
amount of wire solder to the soldering tip and wipe it on a wet cleaning
sponge or soldering tip cleaner to properly tin the drag soldering tip.
Next we apply a very small amount of solder
to the cup or hoof of the drag soldering tip and place the cup side on the
leads farthest away from you and drag it towards you and perpendicular to
the leads of the IC. If you have the correct amount of solder in the cup and
you have allowed time for the leads and pads to heat properly while dragging
across the leads, you should have one very good looking side of the IC
soldered. Now all you have to do is three more sides just like it.
You will note that sometimes you may get in
a hurry and drag a little to fast. When this happens, it may look like you
have done a great job, but upon closer inspection you find that the solder
only is on the leads and not on the pads. The best way that I have found to
check this is to take a dental pick or even a tooth pick and drag it across
the leads while monitoring the leads with some type of magnification. If any
lead moves even the slightest, then you need to do the process again and
this time a little slower. If you find that you need to do the process
again, be sure to add more flux.
Sometimes you may add a little too much
solder in the cup and you end up with a few bridges. These can be removed by
simply cleaning your drag soldering tip to remove any excess solder, adding
a spot of flux to the bridge, and touching the drag soldering tip to the
bridge. This will usually be enough to get rid of the bridge, but if it does
not, just clean your tip again and lay it on the bridge and brush away from
the IC or parallel to the leads at the bridge. Also, sometimes you can
simply clean any excess solder from your tip, flux the IC leads again, and
do the same drag soldering process again over all of the leads. The clean
tip will wick up the excess solder causing the bridge.
If you have done a good job with your drag
soldering, the new IC will look like it had never been replaced. To get it
to look real good, you may have to clean the flux with a good flux cleaner
or denatured alcohol, but remember you are using
no-clean flux which can be left on with
no detrimental effects.
You may want
to inspect your masterpiece with one of our many microscopes or video
inspection stations which can be seen here.