It is not an uncommon experience for engineers to find that their newly assembled printed circuit board has a short. Not only does a short prevent the board from functioning as intended, but it may also lead to an uncontrolled current consumption, leading to damage to a few components or to the tracks on the board. According to Rush PCB Ltd, it is necessary to have an understanding of the cause of shorts in PCBs to know how to detect them. Shorts may appear in bare boards and in assembled boards.
Cause of Shorts in Bare Printed Circuit Boards
Lack of proper reviews and inspection methods is the major cause of shorts in bare printed circuit boards. This usually happens due to:
While laying out the traces during the design phase, a designer may overlook maintaining the minimum distance between two traces, a trace and a pad, and between two pads. This may be due to inadequate applications of Design Rules and subsequent slip-ups in DFM reviews.
Inadequate etching controls
Inadequate etching may result in copper slivers remaining between adjacent copper instances on the PCB. A proper inspection regime should be enough to detect such deficiencies and adjust the process controls to overcome them.
Cause of Shorts in Printed Circuit Board Assemblies
Shorts may appear in a PCB assembly, even when the bare board did not have any. There may be several reasons for such shorts to occur:
Presence of excess solder is the most common reason for shorts. Extra solder bridging two neighboring pads can cause a short. Manual soldering is one of the major reasons for deposition of excess solder leading to shorting between adjacent pins. Another reason is excess solder paste deposition through a thick stencil.
Neighboring components with metallic bodies may touch each other to cause a short. This is common in boards with through hole components, where components standing out on board is a regular affair, but it is rare with surface mount components.
A damaged component may also be the source of a short. If the component has developed an internal short, it may not be visible externally. However, once the board assembly is powered up, the components acts like a short.
Detecting Shorts in Bare Printed Circuit Boards
Using a multimeter is an adequate arrangement for detecting shorts in bare PCBs manually. The inspector requires a knowledge of the various individual nets on the board. Test pads provided by the designer makes the testing easier.
The multimeter must show high resistance between any two nets. Any indication of low resistance is due to an unwanted short.
For quick detection during high volumes of production, automated methods are preferable. Test pins or flying leads touch test pads and assess the resistance between them. A computer compares the readings collected with reference readings from a known good board, highlighting the differences.
Detecting Shorts in Printed Circuit Board Assemblies
Detecting shorts in assembled PCBs is more complicated, and requires the inspector to have experience and ingenuity. Testing for shorts in assembled circuits requires powering the assembly, typically through a current limited power supply, to avoid damaging components.
Depending on the nature of the short, it may be possible to detect it using visual methods or through instruments.
If a short circuit is causing a rise in temperature somewhere, it may be easy to spot visually or by touch. The temperature of a trace going up due to a short may discolor the trace, making it easy to locate. A solder short on two adjacent pins of a component may cause a rise in the temperature of the component. Touching components with a finger may help in detecting the hot component.
A thermal image of an assembled board after power-up can give a clue to the presence of a short. Automated devices can compare the thermal image of the test board to that of a reference board, thereby flagging any discrepancies.
A component with an internal damage may deteriorate when the board is powered up. This may result in visible signs like a bulging capacitor can, a discolored resistor, or flaking colors giving an indication of the fault.
There are various ways a short may manifest itself in a printed circuit board, depending on whether the board is bare or assembled. Detecting the short or shorts quickly depends on the nature of the short and the methods used by the inspector. Rush PCB Ltd recommends understanding the effects of the short to lead to a quick detection.