Making a Homemade SMT PCB Agustin Acosta, BSEE

8 December 2005

This paper is written to show how to make a homemade, single-sided, Surface Mount Technology PCB quickly and easily. The goal of using this technique is to provide a working SMT PCB quickly, without any computer CAD or other complex programs. The resulting product will work, but it will not be the prettiest PCB available, but it WILL work. Working with SMT components can be extremely difficult for the hobbyist due to several reasons. Component size can be a stumbling block to most. I have elected to use the following rules when making a home-made PCB:

• Use 1206 for resistors and ceramic caps wherever possible. • Use 1206 size zero Ohm jumpers to bridge over lines that need to

be crossed. • Use large pads for the I/O connections. • No holes at all, if possible.

The circuit to be built will be a simple Audio Amplifier using the SMT version of the LM386 with an Electret Microphone input.

The above is the circuit to build, but it does not have any component size data, such as whether to use a 1206 or an 0805 size component.

C5 is not currently available in 1206 or 0805 ceramic and so it will be an SMT Tantalum with a case size “D”. Such important info as this needs to be somewhere on the actual schematic and so the following was placed on the schematic:

It is critical to have the actual parts in hand because we are going to “free-hand” draw them in on the PCB. But before we do that, we need to do a rough layout of where the parts are going to fit on the PCB. You must have a PCB with enough surface area to place all of your parts comfortably. A machine such as a Pick-and-Place can put the parts in very close and tight, but we can’t do that with our “free-hand” style of SMT component placement. Since our circuit is not too sensitive to component placement, we can spread them out so that working with them will be easy. It is important to be able to have enough space between components so that servicing the circuit (i.e. change part values) will be easy when we are done. The PCB I’ve selected is a single-sided, 1-ounce copper, FR-4 epoxy glass, with 0.032” thickness. The “standard” thickness PCB that the PCB houses have is 0.062” and will usually charge extra if you want anything thicker or thinner. I’ve selected 0.032” because I can cut it much easier than the 0.062”. Thinner is better in this case. I also have done this with 0.020” PCBs. The PCB is 1.56” x 1.1”. It was cut to size on a paper cutter. The steps involved are as such: Step One: Clean the PCB. This can be done several ways. I used regular household rubbing alcohol. Scrub the copper side with an alcohol-soaked washcloth so that it is as shiny as you can get it. The purpose is to clean the PCB so that the Permanent Ink will adhere well and there will be no oily smudges or such. There are better ways to clean a PCB, both chemically and abrasively, but I chose the simple, cheap way. Step Two: Lay your parts on the PCB so that you will know where the PCB lands (also called traces) will go. Place the components so there will be a minimum of lands that have to cross each other. Remember, we are avoiding any

vias (also known as thru-holes) on the PCB. Each time that you must cross a land, use a 1206 Zero-Ohm Jumper as the bridge. The 1206 is wide enough to allow a good size trace to fit between the 1206 Jumper’s ends. You can also use wire as a jumper, but using the 1206 jumper keeps the PCB more “SMT” than with wire jumpers. Here is what my layout looks like:

Make sure that the proper polarities are observed. The white lines on the caps shown are the Positive ends. Pin 1 of the LM386 is in the top-left corner. Since there is no silkscreening on the PCB, there are no reference designators (R1, C1, etc), which can be a little tricky. If the circuit is more complex than the circuit I have selected, do a rough sketch on paper to keep track of which part is which. Do not mark on the PCB itself. Do not contaminate the PCB with any other writing. Allow plenty of space between the components. You do not have to crowd them together unless you want to make a very small PCB. Keep in mind where the Power, Ground, Speaker Lines, and MIC lines will be. The pads for them need to be large enough to hold the size wire they will be soldered to. The pads for them do NOT have to be on the edge of the PCB, though they usually are. In this case, I have to run Ground between the Power In pad and the edge of the PCB. Doing it this way avoided one jumper. If you have to use a jumper, it is better to use it for a signal line instead of power or Ground lines.

To make is easy to follow, I’ve labeled the parts:

Step Three: Using a Permanent Ink pen (I used a new Sharpie) trace out the wires. The marker pen you use is critical. I have found the Sharpie Fine Point is the best to use. It has just the right ink width and ink flow to deposit a good trace with good definition. It also works well when going back over traces to widen them. The Sharpie Ultra Fine Point is more difficult to use because the ink deposited is very thin. The thin lines often do not have enough ink on them to give a good definition when the etching is complete. This can cause opens (breaks in the traces), which are not good. However, if carefully done, they can still be used to draw small, thin traces where small, thin traces are required. Start with the IC first. Draw as cleanly and straight as you can. I hold the IC with a pair of tweezers on the PCB and draw a short trace to each pin. What I am doing is defining the “footprint” of the IC. I draw the traces so that the IC will sit on top of the traces with a short amount of trace under the IC pins. The footprints are NOT critical in the sense that a certain specific footprint size HAS to be conformed to. You do want to have enough copper trace for the component to sit on. Do NOT draw your traces so that they just meet up to the component, the traces have to go under the component’s soldering pad. Remember that you will be soldering by hand and so larger soldering pads are required. Do the other parts. As you lay out the circuit, keep in mind that GROUND has to go everywhere. If you look at the bottom center, next to the Tantalum Cap, I had

to add a 1206 Jumper so that pin 4 of the IC would connect to ground. This extra 1206 jumper is not in the original layout picture of above.

Look at your PCB carefully. There will be pin holes in places where the ink is applied thinly. After the ink dries, which is in seconds, go back and put a little ink in places where the copper shows through. It is better to have wide traces than thin traces in most circumstances. Especially with Power and Ground. Use an eye loupe to inspect your drawing. Try to use up as much ground as possible. Use thick lines for ground and power. The Ferric Chloride will eat away any copper that is not covered in ink and so the less copper to “eat”, the quicker the chemical process will be completed. The less copper that the Ferric Chloride has to absorb, the more PCBs it can do. The amount of PCBs that any given amount of Ferric Chloride can eat will depend on how much copper is exposed. Try to cover as much copper as possible and this can be accomplished by using as much Ground as possible. The I/O pads required for this circuit are the following:

• Power In • Power Ground • Mic Power • Mic Ground • Speaker Positive • Speaker Negative

If you need to redo a trace, simply use a cotton swab (Q-Tip) and very little rubbing alcohol. Gently remove the ink you just placed. The permanent ink will easily come off with rubbing alcohol. In the process of laying out the traces, you will probably do this once or twice. Make sure the ink is removed as much as possible. Rubbing alcohol will immediately wipe out your permanent ink traces so be careful when using it. Confirm that all the parts are placed correctly and that the circuit traces connect to where they are supposed to be. It is easy to forget to connect ground to all of the areas that need it. Verify that there are no shorts or traces too close together that could cause a solder short later. Once you are satisfied that the layout is good, the next step is to etch the PCB. Step Four: This part can be messy so be careful. Ferric Chloride will stain your skin and your clothes. Read the Material Safety Data Sheet (MSDS) before using the Ferric Chloride. Use latex gloves to be on the safe side. Carefully open the plastic mini zip-lock bag and carefully put the PCB in it. Do NOT puncture the bag with the PCB or you will have a mess. Do not handle the PCB too much; you do not want to wipe out any of the traces you just put on.

Carefully open the Ferric Chloride container and pore the contents into the bag. Close the zip-lock bag and make sure there is a good “zip-lock”.

From here, hold the bag in your hand with the ends of the PCB being pinched by your fingers, and shake it back and forth. The Ferric Chloride will “eat” the copper traces that are not covered by the permanent ink. The process will take several minutes and will go even faster if the Ferric Chloride is warm, such as holding the bag under very warm water until it heats up a bit. Hold the bag in your hand while running warm water over it so that you do not overheat the Ferric Chloride.

The Ferric Chloride will darken in color as the copper is etched away. Also, you can see that the Ferric Chloride is yellow in color and will try to leak out of the bag. The yellow discoloration is actually inside of the bag in the pix above and two pictures below. The zip-lock bag MUST be properly sealed or the Ferric Chloride will leak out and cause a mess.

Step Four: Remove the PCB from the bag with a pair of plastic tweezers or similar item. Do not use your fingers because they will stain. Also, do not use anything metallic; the Ferric Chloride will try to eat that also. If you have to use a metallic tweezers, rinse it out right away with clean water. Rinse the PCB with clean, warm water for a minute or two. You want to make sure the Ferric Chloride is all gone so it does not continue to dissolve the remaining copper.

As you can see from the picture above, the yellow stain will go everywhere. You must rinse the PCB well, as shown below. Also notice that the permanent ink is still on the traces. The Ferric Chloride will not bother the ink.

Step Five: Remove the permanent ink from the PCB so that the copper traces are exposed. Do this with rubbing alcohol. Put some alcohol on a washcloth and scrub the ink off. The ink on the washcloth is permanent ink so be careful on how you dispose or clean the washcloth. Carefully inspect the PCB to make sure the copper is only where you want it to be. Use an Exacto knife or similar item to trim any copper away that may be too close to another trace.

You can place the components on the bare copper PCB to see what it will look like. Notice that the traces are nice and wide.

Step Six: Now you are ready to solder the parts in place. Use a small tipped iron. When soldering SMT components, the smaller the iron the better. When soldering in the components, make sure that you tin one trace first. If you tin both ends, the component will raise above the PCB. It will be tilted because you are soldering in one end while the other end has a “solder “bump” on it and will lift the part up off the PCB. This will not harm the part, but it will look “bad”. Try not to do it. If you have liquid flux, use it. If not, make sure you have a good RMA flux in your solder. Most common solder does have flux in the center core. Flux will help the

solder flow easily and quickly. It can be messy as you work it, but it washes off easily with alcohol when you are done. My personal experience is the more flux the better. Solder the IC first. I actually tin all 8 pins before I place the IC. I do this by applying a solder bump to the pins and then removing the excess solder with some solder wick. This leaves a thin layer of solder on the pads which helps tremendously when soldering.

Now solder the rest of the parts. Add solder bumps to the I/O points so that the wires will solder on nicely. The PCB is now complete and will need to be cleaned with alcohol or flux remover when you are done. This is to make the PCB cleaner and is not absolutely necessary, but should be done anyways. Here is the finished product minus the I/O. The whole process can be done in less than an hour.