STONES SOUND STUDIO

Surface Mount Soldering of  Digital Amplifiers                         courtesy to 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


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

  1. Melt a small amount of solder on one of the solder pads
  2. Hold the component in place with a tweezers
  3. Melt the pad with the solder. Push the component lightly down on the PCB so it mounts flat. Let the pad cool.
  4. Solder the second (and other) pads
  5. Melt the first pad again and let it cool down

 

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)

  1. If there are components to be soldered in an oven, do this first. If not, you can skip to (B) below...
  2. Spread solder paste to all PCB pads that should be soldered in the oven
  3. Place the components on the PCB
  4. Pace the PCB in the oven
  5. Preheat to about 140C for two minutes
  6. Increase the temperature to get a peak temperature of about 220C. The time above 200C should be 30-40 seconds
  7. Let the PCB cool slowly. Don not force cooling.

(B)

  1. Bring out all the SMT components required for a board
  2. Place epoxy dots on the PCB
  3. With a small pincer, place the components on the board
  4. When all SMT
    components are in place, bake the board in an oven at 100 C for about two minutes
  5. Take the board out and place it on the table heater
  6. Solder the SMT components.

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.