STONES SOUND STUDIO
surface_mount_soldering _of_digital_amplifiers article from jan 41hz
Once
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 components.
There are several reasons to switch to surface mounts:
On
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
soldering done?
First, you need some equipment and material
Magnifying 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.
And now to the
actual work.
Most SMT components are too small to hold and solder at the same time.
If you are just soldering a few components, here is a good method
For "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.
For 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.
Alternative ways
of soldering SMT components
If
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:
(A)
(B)
The 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 SMT components.
(C)
Now finally add any through-hole components To the PCB
Soldering in an
oven
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 much.
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?
I 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.
Well
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.