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 µm, 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.
