STONES SOUND STUDIO
Soldering of Digital Amplifiers
courtesy to jan 41hz
you know how, it is perhaps simpler for Diyers to build with surface mount
components than with traditional trough-hole components. When you get used to
soldering these, it is probably even faster than building with through-hole
There are several reasons to switch to surface mounts:
The components are
smaller, more stuff on the same area.
No bending legs,
fitting lengths of components, no cutting.
Many modern components
are not available in through-hole versions
the other hand, there are complications when soldering SMT: s. One thing is the
sheer size of parts, another is that you can usually not hold the components and
solder at the same time. Therefore, working with SMTs requires a bit more
planning than when building with traditional components.
So how is the
First, you need some equipment and material
Strong light helps a lot
glass. I prefer the type you wear like a pair of glasses or with a strap
around your head. These can be bought for around €20. You can also use a low
magnification stereo microscope, but I find these slower to work with and
they are a lot more expensive.
A support to bring
your PCB to the right work height. A comfortable work position with support
for your arms is crucial for getting a good result. It is very difficult to
solder small components without a good support for your arms.
A good solder iron.
Our PCBs are in double weight, double sided copper. These require a
substantial amount of heat. While some people have used smaller irons
successfully, a 60W soldering iron is the minimum recommended.
The solder tip should
be thin and in good condition. Tips with long life coating should be used
and are much easier to keep clean.
When soldering parts
that are connected to the ground plane, a substantial amount of heat is
required. However, do NOT turn up the solder iron temperature as this may
damage the underlying FRP. It may be required to preheat the PCBs. I use a
tabletop pot heater on my workbench, the type that is used to keep food hot.
These usually have a surface temperature of around 100 C which is just fine.
The boards will have a temperature of about 80C when lying flat on the
heater. In this way, not so much heat needs to be added, which decreases
risk of overheating the FRP and making the copper leads lift.
Thin solder wire. I
recommend 0.5 mm (20 mil) or thinner, as it is easier to apply in small
quantities to the iron.
Solder flux. I use a
flux pen of the "no clean" type. These are felt pens with solder flux
liquid. Flux is helpful for getting the solder to flow to where it should be
and also removes oxides.
Thin solder wick for
removing access solder. It works as a strip of tissue when heated on excess
And now to the
SMT components are too small to hold and solder at the same time.
you are just soldering a few components, here is a good method
Melt a small
amount of solder on one of the solder pads
Hold the component in
place with a tweezers
Melt the pad with the
solder. Push the component lightly down on the PCB so it mounts flat. Let
the pad cool.
Solder the second (and
Melt the first pad
again and let it cool down
"large" components like SMA, SMB and SMC size diodes, I usually move the solder
iron sideways relative to the component, soldering along the pad, from one
corner to the other. This usually distributes the solder evenly. A similar
method can be used for 0805 size components.
small multi connector packages, like the main chip of the AMP3, or the chips of
the AMP2 it is important to be careful, as it can be difficult to repair
mistakes. I recommend the following procedure.
Make sure the solder
tip is very clean and in good condition: wipe the hot iron clean on a wet
sponge. This removes oxides that otherwise mess up the soldering.
Put the chip in place,
so the legs match the PCB pads. You can use a small clip to keep it in place
until the first pads have been soldered.
Put a VERY small
droplet of solder on the solder iron tip. If it is not small and does not
flow out well, wipe it of on the sponge and try again. IMO the number one
problem when soldering small components, is using too much solder. With the
right amount of heat and flux, very little solder is needed, it is hardly
visible. If you look the PCB of a machine welded commercial product like
a peripheral card in a PC, you should get an idea of how little is actually
needed. These are soldered by basically dipping the whole card into a bath
with molten solder. Yet no bridges are formed! This is due to the use of the
right combination of temperature, solder, flux and time. However, when hand
soldering small components, the number one caution is to use little solder.
If the temperature/solder/flux/time is right, the capillary flow will make
the solder go where it should be. You can even use a "wide" (1.5mm) chisel
shaped iron tip, soldering two or three chip legs at a time. Again, if the
temperature/solder/flux/time is right, the solder will not form bridges.
Solder one pad,
preferably a corner pad. Then solder the diagonal corner and the remaining
corner pads so the chip is securely in place.
Soak the legs with
flux. This helps a lot in making the solder flow to the right right place.
Flux also helps remove oxides, giving clean joints.
The pads are often
more difficult to heat that the component legs; Place the iron tip on the
outer end of a pad and move the solder iron tip along the pad towards
the chip. This way, the pad will the be heated when the solder iron touches
the chip leg. You should see a change in the surface of the leg as the
solder flows up the surface when the iron touches it.
If you happen to get a
solder bridge join two legs, remove as much solder as you can with the
solder wick. Add some flux to the wick. Heat the wick with the solder iron
and the wick should soak up excess solder. You can also use a very sharp,
thin knife like a break off knife, to cut away access. Be careful not to cut
any tracks. But cutting with a knife should not be needed if you have a
Previously I was using a very thin needle point iron tip, working one
component leg at a time. But recently I have found that with the right
conditions, a flat chisel shaped iron also works very well, even with very
small, tightly spaced chip legs. The right combination of temperature,
solder and flux will make solder flow, by capillary force and surface
tension, to where it should be, so multiple legs can be soldered at the same
time, without bridges forming. But I would not try this without having
solder wick near by, that can soak up excess solder in case messing up.
When all legs have
been soldered, test all the pins with an ohm meter. You need test pins with
thin tips. Some test pins supplied with budget testers are too thick to be
used. I usually set it in "beep mode". Place one pin at the outer end of the
exposed pad on the PCB. Set the other pin on the top of the chip leg, where
it enters the chip. In this way, you do avoid pressing the leg towards the
PCB, which could indicate a connection, even if there is none. I move the
pins one by one. In this way, you can quickly test both the connection
between the component leg and the PCB and at the same time check that there
is not connection between adjacent legs /pads.
of soldering SMT components
you are soldering a quantity of components, here is a rational way. Use epoxy
glue to fix the components to the board before soldering. You should use SMT
type epoxy like Loctite 3609. It is expensive but good. I usually apply the
epoxy to the PCB with a small syringe. Only a small spot is needed for each
component. The epoxies suited for this is are of a one component, heat-curing
type. They do not harden until you heat them to around 100C for about one
minute. Therefore, you have plenty of time. I use the following procedure:
If there are
components to be soldered in an oven, do this first. If not, you can skip to
Spread solder paste to
all PCB pads that should be soldered in the oven
Place the components
on the PCB
Pace the PCB in the
Preheat to about 140C
for two minutes
temperature to get a peak temperature of about 220C. The time above 200C
should be 30-40 seconds
Let the PCB cool
slowly. Don not force cooling.
Bring out all the SMT
components required for a board
Place epoxy dots on
With a small pincer,
place the components on the board
When all SMT
components are in place, bake the board in an oven at 100 C for about two
Take the board out and
place it on the table heater
Solder the SMT
procedure above may seem complicated, but once you get the hang of it you will
probably work faster this way, than with traditional through-hole components.
Further, down on this page you can find more about the details of how to solder
finally add any through-hole components To the PCB
Soldering in an
Components that have connectors that are not accessible, like bottom side
cooling pads, need to be soldered in an oven. For the 41hz audio amps this only
applies to one of the chips of the AMP2. A small toaster oven or other domestic
oven will do. Do NOT use the same oven for food unless you are prepared to do a
thorough cleaning. You do not want lead in your food and you do not want grease
in your PCBs. For repeated use, I recommend you get a separate little oven
toaster for the PCBs. You could even make a simple oven by using an ordinary
kitchen pot on your stove. Preferably, use a stainless steel pot with a lid and
a good thermometer. Most solder melts at around 180 C. When soldering, you have
to go a bit higher the melting point temperature, to get a good wetting and
ensure proper melting. On the other hand, components can be damaged if heated to
Hot air soldering
Yet another possibility is to use a hot air gun. I have not tried this and would
use it with precaution so that components are not damaged. Is there anyone out
there who has tried this or has comments or ideas?
usually solder the heat slug of the heat dissipating AMP2 chip in an oven, or
skip the soldering and use heat transfer paste between chip and PCB and put a
small heat sink on top. Of course the legs of the chip have to be soldered. The
chip is rated to be soldered at 270C for 10s (typical solder specification for
chips). Solder melts at 180C and is very fluid at 190-200C. I would say it is
impossible to heat both PCB and chip in a controlled manner with an iron.
you can also solder the "normal, way" but then add heat transfer paste between
chip and PCB for better cooling. Or add a heat sink on top of the chip.
If I would solder the chip, or have it done, I would like to test it before
delivery but that would be difficult, without building the rest of the board.