Tag Archives: Whisker Growth

Whisker Growth in Printed Circuit Boards

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.

Preventing or Mitigating Whisker Growth in PCBs

Growth of whiskers puts PCB assembly at considerable risk, since whiskers interfere with components, and this automatically qualifies a good product as a defective one. Although a growing tin whisker may seem harmless, it can pose a very real threat to both the product as well as to the human operator. In PCB assembly, one of the most common problems that whiskers create is a short circuit or arcing. This can cause breakdown of electrical equipment, as well as harm people from the arcing. Either way, it ultimately leads to a loss in time and money.

The impact of whisker growth on global PCB assembly results in ruined circuitry, broken equipment, and overall shoddy artisanship. Therefore, it is very important to address the issue of whisker growth. For mitigating or preventing whisker growth, the following precautions may help.

As pure tin coating is the basic reason for the growth of whiskers, avoiding the use of pure tin plating on PCBs and other components is the most obvious method. However, as this action falls in the realm of manufacturing, it is not always possible to implement at the PCB assembly level. Most manufacturing companies do utilize alloys to help stabilize the tin coating to mitigate tin whisker growth, but it is better to be cautious.

If there is a high risk of whisker growth, it may be possible to outsource the PCB/component to a contract manufacturing company to re-plate the area. To avoid tin whiskers, it is highly advisable to let the external manufacturing company strip away the current plating, and reapply newer plating.

Application of a coating or housing foam encapsulation on the whisker prone area can help to prevent problems of growth in the future. However, this method depends on several factors, including type of foam encapsulating coating used, the amount applied, and the intensity of infection of the whisker prone area. In actual practice, the foam encapsulating coating normally helps to prevent short circuits.

An alternate method is to relieve the stress on the area by using hot oil reflow, or by conducting a new reflow soldering job.

Most reliable assembly manufacturers are aware of tin whiskers, and are willing to help with any whisker growth problem. Several turnkey assembly manufacturers are also certified and make sure they use alloys in place of pure tin components for mitigating whisker formation. Of course, faulty and counterfeit components do raise the risk of causing tin whiskers, but working with US-based manufacturing and assembly companies normally ensures an overall higher standard of quality.

New Research on Preventing Tin Whiskers

New research in preventing growth of tin whiskers points to the use of an additional metal coating on the tin layer. Depositing a thin layer of nickel as an electroless metal deposition seems to be the most practical method. Although tin whiskers can penetrate most metal in days, a hundred-atom thick, about 35 ┬Ám, of nickel forms a virtually impenetrable layer. A thicker layer of nickel not only retards the growth of tin whiskers, it truly prevents their formation permanently. However, this requires a two pass electroplating process, one for depositing the tin layer, and the next for depositing the nickel layer on it.