This page offers advice on testing and fault finding circuits built by students and the problems that commonly occur when constructing circuits and PCB's. They are a helpful guide for teachers identifying faults that students tend to make and not aimed at the experienced constructor.
PCB PRODUCTION
When using photo-etching as the process for making circuit boards, check for bridging of tracks, especially when many thin tracks run along side one another. This can happen if the PCB hasn't etched the gaps between tracks properly.
Hairline cracks in tracks are difficult to see and can cause a circuit to behave strangely. Use a magnifying glass to inspect the board. A multimeter set on a low ohms range can help test continuity but be sure that you always get a reading of less than 1-2 ohm. A higher resistance would indicate a break in the track. A common causes for this are scratches on the acetate (if using photo etching), poor printing quality or over exposure in the UV box.
SOLDERING
By far the most common error made by students is the bridging of tracks with solder. Usually this occurs by pads with large balls of solder touching solder on other pads. Try using a desolder pump or copper braiding to remove excess solder. Take care with desolder pumps as they are quite aggressive and can literally pull the pads and tracks off the board if heated for a long time.
Dry joints can be caused by moving the PCB/component before the solder has cooled or by using am iron that is not hot enough. Sometimes the joint will appear dull and speckled rather than shiny. Remove solder and re-apply with fresh fluxed solder.
The secret to good soldering is clean copper and a well tipped soldering iron at the right temperature. When using photo-etching process, I have found fine grade wet and dry paper works really well to clean off the etch resist, but it is essential you use it wet, soaking the abrasive and PCB under the tap to begin with and then frequently while cleaning. Usually 800 grit is a good choice. Try to complete soldering in one session so the copper can be cleaned and soldered as quickly as possible. Avoid fingermarks on the copper tracks as this will rapidly tarnish the copper.
The choice of solder can make quite a difference. If you are using an 18w Antex iron which I have found to be most suitable then be sure that the solder you order and use has a melting point suitable. I recommend Hydro-X solder which has a water soluble flux. The flux which discolours with excess heat can be washed off using a toothbrush under a running tap. This process also cleans the tracks again and gives a good finish. A good tip when the circuit has been tested is to spray the copper track side with a laquer to keep them shiny and prevent corrosion over time.
Use a bulldog clip attached to a firm base to allow the PCB to be held fairly securly while the solder and soldering iron are applied. You will have to ensure the components are secure in place so they don't fall out when the PCB is upside down.
COMPONENTS
A common mistake is the orientation of certain components. Diodes and transistors are the main culprits and can cause damage to themselves and other components if incorrectly connected. Transistor connections can be tricky with all the different case types, in particular TO92 types. For example, BC183 and BC183L have different connections. Click here for transistor pinouts.
When using diodes for protecting transistors that are driving inductive loads (such as relays), be sure to check orientation carefully. They need to be reverse biased, if not they will conduct when the transistor switches on and usualy destroy itself and the transistor due to the large surge of current.
When inserting components into the PCB, do not force them if the holes are not large enough. Re-drill them to the required size. Most holes will be large enough at 0.9-1mm. For certain components like presets and PCB terminal connectors, a 1.2mm hole is required. Other components like PCB mounted switches and sockets, hole sizes can be up to 2.5mm.
Electrolytic capacitors can be dangerous when connected the wrong way round. If the power supply has the current capability, the electrolyte heats very quickly and the gas produced has enough pressure to propell the metal case many feet accompanied by a bad smell and a mess on the circuit board. This can be dangerous if inspecting the board at the time. TAKE CARE and always check the orientation of electrolytics, in particular the 100uF types (they are the largest values without pressure release grooves in the casing).
Integrated citcuits are easy for students to insert incorrectly. There is a dot on the face of the IC that identifies pin 1 and a divot in the case that identifies the top. Many IC's will survive a reverse connection for a few seconds or more, but the ones that will not tolerate it, are the CMOS 4000 series and the TTL. Their power connections are usually opposite each other so reverse polarity power connection can occur. These can heat up and crack the case if inserted incorrectly and are almost certainly destroyed if not. It is always good advice to use IC holders.
Not so common, but students can make mistakes with the resistor colour code and thus soldering the wrong values in place. This is not usually a problem if the value is higher than required and the component will just need replacing. If they are a lower value than intended, then damage to other components is a possibility. Check whatever is in series with the offending component and test/replace if circuit still refuses to work correctly.
If LED's fail to light correctly this can be due to the series resistor value too high or the LED incorrectly connected. Some of the leg lengths and flat faces that identify polarity can be difficult to see, (Long leg is anode, flay section on rim is cathode side). Test the LED separately with a 330ohm resistor and a 5 volt supply. If an LED is connected without a current limiting resistor, it will almost certainly destroy itself within seconds. Use V=IR to calculate series resistor values, most LEDs have a forward voltage of about 2 to 3 volts.