Tin whiskers are electrically conductive, crystalline structures that grow from surfaces where tin is used as a final finish and from lead-free solder joints. Because these whisker structures are conductive, they often lead to transient or stable short circuits. In high voltage environments, metal vapor arcs can occur, which can be very destructive.
Tin whiskers can grow to be as long as 10 millimeters, and have diameters of several microns. There are many factors that can impact tin whisker growth and growth rates, including temperature, humidity, pressure, and applied electric field. Lead (Pb) is known to have a mitigating effect on tin whisker formation and growth, and for that reason tin/lead alloys have historically been employed in electronics packaging.
Lead-Free Electronics
Lead has been found to be hazardous to the environment and poses occupational human health threats when exposed during manufacturing and recycling. To reduce exposure, the electronics manufacturing industry has been moving to lead-free electronics. This effort to reduce lead was initiated in 2002, when the European Union passed the Reduction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, which require all electronics sold in Europe to meet strict standards. Without the use of lead in electronics, tin whiskers are more prevalent, and cause electronics to be less reliable. U.S. military and aerospace communities require long service life and high reliability and as a 2013 AIA Issue Paper illustrates, the consequences of failure can be catastrophic.
SERDP/ESTCP Projects
In an effort to understand the mechanisms of tin whisker formation and growth and in order to develop strategies to mitigate whisker formation and the damage that can occur, several SERDP and ESTCP efforts are underway.
Tin Whiskers Inorganic Coatings Evaluation (TWICE) is investigating alkali silicate glass materials as conformal coatings for circuit boards.
Novel Whisker Mitigating Composite Conformal Coat Assessment is studying organic polymers and nano composite coatings for whisker mitigation.
Tin Whisker Testing and Modeling is an effort to understand the mechanisms of whisker growth and provide a risk assessment framework to the design engineer.
Contributions of Stress and Oxidation on the Formation of Whiskers in Lead-Free Solders is investigating the role of stress and oxide formation on whisker growth in lead-free systems.
