STONES SOUND STUDIO

Digital Amplifiers

by

 Russell Storey

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First, you need some equipment and material

• Good visibility. Strong light helps a lot
• 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.
• 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 solder.

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.

• 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 solder wick.
  
  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.

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.