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Hot Air or Thermal
Contact
The purpose of this article
is whether to use hot air or thermal contact (soldering iron) for
removing and replacing surface mount IC's. I am here to tell you in my
own words and from my own experience, (plus talking to a lot of
professional people) about the subject. My conclusion on this subject is
to use hot air to remove the IC and use a good quality soldering iron
and magnification to replace the IC and the following is my reason for
this conclusion.
I can remember when most
manufacturers would not let you perform warranty on their boards unless
you had the tools to remove and replace components with hot air. Some of
the more expensive tools on the market simply used thermal contact for
removal and replacement, but some of the manufacturers would not allow
the use of those tools on their board.
Times have changed
I must say that
fortunately times have changed from the thinking of the old days when
surface mount was in it's infancy. I can remember when the proper way to
change a microprocessor (according to the manufacturer of the board) was
to take a razor blade and cut the leads off. Then take a soldering iron
and wipe them off of the pads. This worked OK until the technician got a
little careless and also cut the traces that ran under the
microprocessor. Also, if the failed microprocessor ended up not being
the problem and you wanted to reuse it, that was out of the question
(As was usually the case). In those days the microprocessor was usually
the first to blame and the last to fail so this system did not work well
in that respect.
Butane Torch
The next process that came
from a very respected manufacturer was to use a butane torch to remove
and replace the surface mount component. Of course the manufacturer was
trying find a way to change their components economically because most
of their warranty stations could not afford the expensive equipment that
is required. Also the manufacturer could not afford to pay the warranty
claims that would justify purchasing that expensive equipment. If a
technician is extremely careful, it is possible to change a surface
mount with a butane torch, but very impractical. With some of the torch
temperatures going up to 1300°C, you can see why it is very easy to burn
the board and everything in the vicinity if you are not extremely
careful and attentive to business.
Solder Paste
This brings us to another
facet of using hot air for replacement. You need to use solder paste to
replace the component and solder paste is not very easy to use. Sure, it
is exactly what is used in the original manufacturing process and works
very well for that process. That does not mean that it will work as well
for the repair process. In the manufacturing process, the exact amount
of solder paste is screen printed on the circuit board pads via a laser
cut stencil. Then the components are accurately placed by a pick and
place machine costing several tens or hundreds of thousands of dollars.
Then the circuit boards are conveyed through an oven with very accurate
temperature control for a very accurate period of time. Another very
expensive item, that is not acceptable in SMD rework.
The main reason that
solder paste is so hard to use is that it is very hard to accurately
dispense the correct amount on the pads. Even with a costly air
dispenser, it still comes down to how accurate the operator can place
the solder paste on the pads and how accurately they can dispense the
right amount. Anyway that you try it, it is still very hard. Now the
real fun begins...trying to accurately place the component without
smearing the solder paste. Again, there are some operators that can
perform that operation, but as a general rule, most can not do it
successfully often enough for me to recommend.
Another process that I
have tried with moderate success is to solder tack the opposing corners
of the IC while using some type of magnification to accurately place the
IC (preferably a
Stereo Zoom Microscope). Then take your syringe of
solder paste and lay down a bead on the end of the IC leads. The
diameter of the bead will depend on the lead spacing of the IC. The
larger the leads and spacing, the more solder paste you can use. This
becomes very critical on fine pitch IC's as to large of a diameter of
bead will lead to several bridges that will need to be removed later and
of course to fine of a bead will mean not enough solder to form a
quality joint. The former is usually the case. It really does not take
very much solder to make a quality joint.
Solder Paste Cycle
Another reason that solder
paste is very hard to use is that it must go through a somewhat precise
cycle before it will effectively turn into a good solder joint. To make
solder paste, you must take many small solder spheres, mix with paste
flux and add solvent. I am sure it is more complicated than that, but
for this discussion, that is enough information. The cycle that I am
talking about is called the Pre-heat Cycle (where the circuit board is
slowly brought up to temperature), the Soak Cycle (where the solder
paste is between 125°C and 175°C to extract the solvents), the Dwell
Cycle (where the solder paste is brought above solder melt temperature,
typically 220°C), the Cool Cycle (where the solder paste is allowed to
cool to room temperature. This cycle usually takes a period of 8 minutes
or 500 seconds. It needs to be noted here that the Soak Cycle is very
important as this is where the solvents are extracted from the solder
paste. When doing this process visually, you can watch the solder paste
turn from a dark gray to a light gray. If you try to speed up this
process by heating the solder paste to fast, you will spatter little
solder balls all over the board. This is caused by the solvent gases
trying to escape prematurely and will make an ugly situation when
looking at it with an inspection microscope.
I must say that I have
been trying for years to find a way that the average rework technician
can use solder paste and be effective 100% of the time. So far it has
not happened, so I can not recommend using solder paste for rework. I
know there are rework technicians that have figured out how to use it
effectively and for those technicians, I applaud them, but for most of
the technicians, I have to recommend soldering the IC's in.
Working with two leaded chip
components
Some technicians have found
that they can remove the two leaded chip components with hot air and
stainless steel tweezers and install the new component back in with the
same solder that was left on the board. Just add a little flux and
install the new chip just like you removed the old one.
A much faster way of
changing chips (if speed is your cup of tea) is with the
Micro Hot Tweezers from JBC. You
simply squeeze the chip to pick it up, while melting the solder at the
same time. Then you pick up the new chip with the micro hot tweezers and
set it down on the pads while melting the old solder on the pads. After
you get solder melt (which is almost instantaneous) you simply let go of
the chip and the molten solder will hold the chip in place until the
solder solidifies. This technique also works well for the extremely small
chips (0201). That is the chip that looks like a speck of pepper when
there is no solder on the ends of the leads.
Seen the Light
Thank goodness the
soldering iron manufacturers seen the light and came up with some good
soldering tips for soldering IC's to their respective pads. Fortunately
it is something that everybody can do with a little practice and
patience. Yes, the better iron that you purchase, the faster your job
will be, but you can be effective with most any manufacturers iron that
has a good selection of surface mount tips. All of the
soldering irons that we sell have a good
selection of Surface Mount Tips.
Chip Quik
At this time I have to
talk about a very effective way of removing IC's from the printed
circuit board with just a simple soldering iron. I only recommend this
for the technician who is only replacing one or two IC's per week as it
will not replace the speed of hot air, plus it is a consumable item that
you have to replace. It can be a very effective tool for removing IC's
and some manufacturers have recommended using it because you can rework
at a much lower temperature. I am talking about the
SMD1 by Chip Quik®.
Chip Quik is a very low temperature alloy that is in the shape of
solder, but it melts at 136°F. You can stick it in a cup of McDonalds
coffee and it will melt. The way to apply it is to take you soldering
iron and melt it on to the leads of the IC that you want to remove. You
must apply it quite liberally as it must mix in with the existing solder
to bring the melting temperature down to around 200°F. At this time you
should be able to remove the IC with a vacuum pick or stainless steel
tweezers. Now you just follow instructions for cleaning up the board and
prepare the board for installation of the new IC according the following
instructions.
Reinstalling the IC with a
soldering iron
Now to solder the IC back
in. This is the most effective and easiest way that I have found and
when I am in trouble, I always resort back to this way as it always
works.
Step one: Clean the solder
pads with a desoldering tool or solder braid without disturbing the
pads. In other words, watch your desoldering temperature and the time on
the pads. I have found that a circular motion with a good desoldering
tool is the most effective. Make sure that your desoldering tip is well
tinned.
Step two: Place the IC
with a vacuum pick or high quality stainless steel tweezers.
Step three: Solder tack
the IC on two opposing corners while using some form of magnification to
get perfect alignment of the IC. I recommend a
Stereo Zoom Microscope
package that we have put together since the soldering and inspection can
both be performed under the microscope.
Step four: Apply a good
quality gel flux to all of the leads of the IC. The flux can be seen at
http://www.howardelectronics.com/amtech/products.html.
Step five: Using a good
quality soldering iron and tip designed for soldering in surface mount
IC's, add a little solder to the tip. A good rule of thumb would be to
add just enough solder to complete one side of the IC. Starting at the
corner of the IC, drag the tip perpendicular to the leads of the IC,
while maintaining contact with the leads and pads at the same time. On
some IC's, you can use a small angled tip and simply lay it on the pads
where they extend beyond the leads, while still remaining contact with
the leads. You will note that after a few tries, you will be able to
make it look like the IC was never removed.
Stereo Zoom Microscope
I feel that I can not
stress enough the importance of a good Stereo Zoom Microscope. The
microscope package that we have put together is perfect for this type of
inspection. It gives 7 inches of working distance, while giving a 3.4x
-22.5x magnification. It is perfect for soldering and inspecting. You
can never believe how easy it is to solder surface mounts until you have
done it under a quality
Stereo Zoom Microscope.
My final recommendation
when soldering under a microscope is to use a good fume extractor. The
solder fumes and smoke will rise right into your nose when using a
microscope. You do not have to worry about the expense as we have very
good fume extractors at very moderate prices. Even if the solder fumes
were not considered harmful, you would still want to extract the smoke
away from your nose when using a microscope. Solder fumes are no fun to
breathe.
In Conclusion
In conclusion, I would like to
say that my favorite way of extracting surface mount components is with
Hot Air (unless they are two leaded components), and my favorite way of
installing surface mount components is with a good quality soldering
iron. When I say good quality, that is what I mean. I must say that in
my travels to different working service centers, I have seen a lot of
good technicians that were using some pretty poor soldering and
desoldering equipment. I won't elaborate on it at this time except just
to say that their equipment was pretty good (Thirty Years Ago), and they
were working on camcorders with this equipment. I would just say that it
is time to upgrade, you are not working on five tube radios any longer.
Good tools will always pay for themselves, but sometimes it is very hard
to make that first investment. Any shop that has a need to change
Surface Mount Components would do well to update their old equipment.
I hope that this article
has helped you in some way. If you want to call and talk about it, please
call me a 1-800-394-1984 or 316-321-2800. I would be happy to
hear your input and/or give some more myself if needed.
Jerry Howard
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