Intro: How to Assemble a Printed Circuit Board (PCB)
How to assemble a PCB by hand intro: Printed circuit boards or PCBs are at the heart of modern electronic equipment. Without them many of the electronics we use would be impossible to build. Computers, cell phones, TVs, Blu-ray players and such would not exist. The first PCBs, actually called printed wire boards were made by hand using Masonite or other such product. Prior to PCBs, electronic devices such as televisions and radios were wired by hand. Imagine something as complicated as a television being completely hand wired (Fig. 1). Modern PCBs arrived in the 1950s. They were made from phenolic materials and were single-sided (Fig 2). Modern PCBs are made from several different materials such as fiberglass and its variants, Teflon, and woven glass polyester. However, this is not a complete list. New materials are being invented all the time. Printed circuit boards can be one layer or many layers depending on the complexity of the device being made. Single-sided circuit boards use through-hole components whereas multi-layered boards are likely to use surface mounted components. Multi-layered PCBs using surface mounted devices can be populated much more densely. This is imperative for modern electronic devices (Fig 3). Most hobbyists are likely to use a single or two-sided PCB and through-hole components for ease of assembly. Thanks to improving PCB fabrication processes together with healthy competition, printed circuit boards are affordable for most hobbyists and companies seeking prototype boards alike. One board shop that I have had the best experience with so far is Custom Circuit Boards. Try dropping them a line if you are need of more sophisticated boards that you can fabricate yourself. OK, back to the tutorial on how to hand assemble a Printed Circuit Board.
Step 1: Tools Required
There is a minimum set of tools required to successfully assemble a printed circuit board. There’s the obvious need for a small wire cutter, small pliers, and a soldering iron but it is nice to have other tools such as a dental probe and a volt/ohm meter. Also, I find having a pair of magnifiers handy. You may need a work light. A must have is an anti-static mat with wrist strap. Although most semiconductor devices have internal static protection, they are limited in how much energy they can absorb. It is always best to error on the side of caution. Another necessity are solder removal tools should rework become necessary. Finally a damp sponge comes in handy to clean the tip of your iron between solder connections.
Step 2: Soldering Equipment
There are several choices when it comes to soldering devices. The most inexpensive and simple soldering iron plugs right into the wall and have no way to adjust the temperature. For electronics work a 15 to 30 watt iron is recommended. Next are the thermostatically controlled irons. Various means are used to regulate the temperature. Some are set by adjusting a dial to the desired temperature. Other soldering irons use magnetized tips at a set temperature. These irons use the Currie effect (as the temperature increases the magnetism decreases). When the magnetism is high current flows to the heating element, as the magnetism decreases the heating element it turned off. Whatever type of soldering iron you buy make sure that the tips can be replaced and there are several sizes available. Another soldering device uses hot air to melt the solder. These are rather expensive but are very useful if you plan on soldering surface mount devices. Hot air soldering stations have the advantage of having attachments that come in various sizes and shapes to fit standard SMD packages (Fig 4) (Fig 5) (Fig 5.1).
Step 3: Types of Solder
There are basically two categories of solder for electronics work; lead/tin alloy and lead free. There are two lead/tin alloys commonly used; 63% tin, 37% lead and 60% tin, 40% lead. The melting temperature of either is around 360 degrees Fahrenheit so either of these is suitable for electronics work. Most electronics manufacturers use lead free solder due to environmental concerns. Lead free solders are made from tin, silver, and copper. This alloy has a melting temperature of about 423 degrees Fahrenheit. My personal opinion is that leaded solder is cheaper and easier to use and since so little solder is used by the hobbyist, it doesn’t present a pollution or health problem.
Step 4: Proper Soldering Technique
Proper soldering consists of heating the surfaces being soldered beyond the melting point of the solder being used so that the solder flows freely among the surfaces. Use only enough solder to bond the surfaces being soldered. Watch out for cold solder joints. Cold solder joints happen when too little heat is applied and surfaces are not hot enough for the solder to flow properly. Proper soldering like anything else is a learned skill. I would suggest that first timers use something to practice on before attempting to assemble your circuit board. One more thing; avoid breathing the smoke and fumes produced by the flux in the solder.
Step 5: Soldering Through-hole Components
On a one layer PCB insert the component and bend the leads slightly, just enough to keep it in place while you solder it in place (Fig 6). While keeping your soldering iron at a 45 degree angle to your work, let the tip of your iron contact both the lead of your component and PCB at the same time. Apply the solder from the opposite side from the tip. Heat the surfaces until solder flows freely to the tip side of the work (Video). Be careful not to overheat the pad as this can damage the PCB by lifting the pad from the board.
Step 6: Soldering Two Sided Boards
Step 7: Soldering Two Sided Boards
So far I have covered placement and soldering through-hole components. Because electronic manufacturers are assembling PBCs with higher and higher component density, it is getting harder to find many semiconductors in through-hole packages. You will, at some point have to deal with SMDs. This requires a very fine tip and a steady hand. First, melt a small amount of solder on one of the pads of the SMD you’re trying to place. Next, place the SMD on its pads, making sure that the device is properly oriented, and while holding the device in place (this is where a dental probe or some other suitable tool comes in handy), heat the lead of the device covering the pad that you pre-soldered or tinned. This will tack the device in place while you solder the other leads of the device. Make sure that all other leads are centered on their pads. Remember to re-solder the lead you used to hold the device in place because chances are that lead is not soldered well. Some SMD circuits such as microprocessors and memory chips have such a high pin count and fine lead pitch that it is impossible to solder them by hand. For these you need a hot air soldering station with the proper adaptor and solder paste (Fig 8) (Fig 8.1).
Step 8: Different Types of Electronic Components
There are basically four types of electronic components that you are likely to run into; through-hole passive components (resistors, capacitors, inductors, etc.), through-hole active components (diodes, transistors, integrated circuits, etc.) and surface mount passives and actives. Passive components do not require any special handling except for observing polarity of electrolytic capacitors. Digi-key is a good place to buy components. They have a large selection and don’t mind selling small quantities. It’s a good idea, unless the component is expensive, to buy a spare in case something happens to the original.
Step 9: Printed Circuit Board Design
Designing your own circuit board can be fun and rewarding. Custom PCBs allows the hobbyist to design a circuit tailored to his or her needs. Most PCBs are designed using computer aided design (CAD) programs. These programs can be very costly and overkill for the average hobbyist. Many PCB manufacturers actually give away this software to promote their service. One such program is called Dip Trace. Dip Trace is a fairly simple program offering an extensive pattern library that should satisfy most PCB design requirements. If you have no PCB design experience, I would suggest that you research the subject before attempting your first design. An alternative method to designing a PCB is to make your own board. Kits are available that enable the hobbyist to design and make your own PCB. The problem with these kits is they are limited in the complexity of a board you can make. For example there is no way to plate holes to connect the top and bottom of a two sided PCB. Furthermore you have to come up with your own component patterns. Thirdly, your finished PCB will not have a solder mask layer which among other things helps prevent short circuits when soldering. Finally your board won’t have a text layer which identifies component placement. If you are using a CAD program it is a good idea to print out a 1x copy of the board to make sure that the patterns match up to the actual components that you’ll be using in your project. One common mistake is not checking your hole sizing. Too small and your through-hole components won’t fit.
Step 10: Printed Circuit Assembly
Handle your circuit board by the edges to avoid contaminates that may make soldering more difficult. Inspect the board for any obvious errors. If your board is not too complex you may want to use an ohmmeter to check for continuity and shorts. Arrange all of your components in such a fashion that you will be able to find them easily during assembly. How you arrange your parts is up to you, whatever works best. If you use some sort of tray or bins, make sure that they are static proof to avoid damaging semiconductors. Now assembly can begin. I generally start placing the passive components first but that’s only a personal preference. A good place to start is with the resistors. For through-hole resistors, bend the leads at the body 90 degrees (Fig 6). Now insert the resistor through the board tight up against the board and bend the leads slightly to hold the resistor in place while soldering. Note: Any resistor that may become hot during normal operation should be mounted up off the board a quarter inch or so to allow air circulation around the resistor and prevent failure of the resistor and burning the PCB (Fig 10). Once the resistor is in place, turn the board over solder it in place, and then trim the leads as close to the board as possible. Once you are done with the resistors, place the capacitors in a similar manner observing polarity of electrolytic capacitors. When finished with capacitors, place any diodes and transistors your circuit may have. Be careful not to overheat semiconductor devices as this could damage them. Now it is time to mount any ICs your project calls for. ICs in dip packages come with the leads bent slightly outward. You may have to bend them slightly inward to fit the hole pattern. Make sure the device is oriented in the right direction. As I mentioned before, surface mount devices can be held in place by tacking one lead in place then soldering the others. Because of the small pin pitch, you should use a very fine soldering tip to avoid creating solder bridges between leads.
Side note: As you know, hand assembling smaller components is not only hard, but it is very time consuming. Multiply that by 10-20 boards and you need to quit your day job. That being said, since posting this instructable, I have had quite a few people ask me about SMT printed circuit board assembly companies and which one I would recommend. I have only used a few PCB assembly companies for quick turn purposes, but I had the best experience with Quick PCB Assembly. They have two facilities and were great to work with. Check them out if you are looking for SMT quick turn PCB assembly services. Ok, back to the tutorial... again.
Step 11: Rework
Unfortunately rework can be a part of assembling a PCB. A resistor in the wrong place, a diode in backwards, or a trace wrongly connected. There are several methods for dealing with these problems. Solder wick, braided copper tape that when heated will act like a sponge to remove solder. While solder wick is good for single sided boards, it is not terribly effective for two sided boards with plated through holes. There are two ways to deal with plated holes. The best way is with a solder sucker. As its name implies, a plunger is released creating a vacuum that sucks the solder out of the hole. A cheaper method is to cut the component out and then heat the connection and pull the lead out through the board. However this method may still leave you with a hole full of solder that has to be dealt with. Again, a solder sucker is the best way to deal with this. Alternatively, you can heat the connection and push the lead of the new component through the hole. After doing so you will want to re-solder the leads of the device to assure a good connection. In the event you find a trace going to the wrong place, you will need to cut a segment of the trace out. Cut the trace with an X-acto knife in two places about ¼” apart. Using your soldering iron, heat the cut segment applying firm pressure and keep heating until the segment becomes lose from the board. If the trace needs to be routed to another place, sand the edge of the cut trace until the copper is exposed (this is assuming that your board has a solder mask). Using wire wrap wire (30ga, single strand, insolated, tinned copper wire), make connections to correct the error.