in whiskers are not fanciful or imaginative items, but are real and pose a serious problem for all types of electronic manufacturing. Pure tin is often used as a finish material on printed circuit boards (PCBs) to protect the exposed copper pads from tarnishing. However, pure tin spontaneously grows conductive whiskers, thin wire like growth that can form electrical paths and affect the operation of the PCB assembly.
Understanding Tin Whiskers and their Effects
First reported in the 1940s, tin whiskers are mostly invisible to the naked eye as they can be ten to hundred times thinner than a human hair. They grow to considerable lengths bridging fairly long distances between tracks and pads on the PCB. Once bridged, the whisker can short the conductors. There is no set timetable for the whiskers to commence growing. Their incubation may be fairly rapid, ranging from days, or slow, taking years.
These needle-like tin whiskers can create a short circuit between two conductors. As they are very thin, most whisker growths usually fuse or burn out when current flows through them, creating a momentary short circuit. However, in rare circumstances, rather than vanishing like a fuse link does, the whisker can form a path capable of conducting several hundred amperes. The conductive path created by whiskers generates false signals at incorrect locations, which can cause the device to operate improperly.
Sometimes, whiskers break away and fall across other traces on the PCB or between neighboring conductive components, where they can disrupt or interfere with local electrical signals. For instance, falling on MEMS, whiskers may interfere with intended mechanical functions, or diminish the transmitted light if they fall into optical systems.
As more and more electronic systems form the backbone of our manufacturing and transportation systems, our communications and financial systems, and our conventional and nuclear power plants, the problem of whisker growth in pure tin-plated electronic PCBs becomes increasingly ubiquitous.
Impact of Tin Whiskers on PCB Assembly Reliability
Manufacturers utilize tin for coating several different components used on PC board assemblies. One popular way to stabilize the tin finish is by introduction of lead. However, this method is contrary to the concept of Restriction of Hazardous Substances (RoHS), which most governments follow, as lead is a dangerous substance affecting human health. Instead of using lead, most companies now use special alloys.
Whiskers can form in different ways, some of which are:
- From stresses on poorly formed components that do not fit together very well
- From intermetallic formation
- From different outside sources of stress
- From external or internal problems causing scratches, stretching, or bending of the assembled PCB
Whiskers are not to be confused with dendrites or other such shapes in PCBs and components, as they are considerably different in both nature and function. Unless they are found and identified correctly, whiskers can pose a serious problem for a circuit board assembly. These structures of crystalline formation, whiskers most commonly occur in electroplated tin used as a finish on components and PCB traces.