Perhaps the greatest reliability issue to come out of the Restriction of Hazardous Substances (RoHS) Directive is tin whiskers. This residual effect of RoHS is nothing new to electronics manufacturing. Tin whiskers were first been reported in the 1940s and credited with taking down satellites and military planes, causing critical failures to pacemakers and space shuttle systems and even the shutdown of a nuclear power facility.

Tin whiskers cause their fair share of headaches, but they can’t be solved by keeping them out of water and not feeding them after dark. Tin whiskers, their causes and how to prevent them, typically bring more questions than answers. These are the answers Dr. Werner Hügel, Robert Bosch GmbH, has been seeking for years and will discuss during his presentation, “Whisker Prevention and the Relevance of Plating Conditions” during the CALCE Annual Tin Whiskers Symposium at IPC TechSummit, October 28-30, 2014 in Raleigh, N.C.

Dr. Hügel is responsible for the backend technology, galvanic surface finishes and solderability requirements for the automotive electronics division at Robert Bosch. The electronic packages his group is responsible for are found in such areas as: airbag, engine management and transmission control systems, all of which ensure driver and passenger safety. High reliability and low failure rates—especially those caused by tin whiskers—are essential. An electronic short to one of these systems could result in potential harm to people or costly recalls.
Hügel will discuss: four basic whisker growth mechanisms, the relevance of the properties of the tin-finish on whisker formation and whisker mitigation. He will also share insights and data on the influence of plating conditions and plating parameters on whisker performance. Attendees will also learn an approach for effective whisker prevention supported by a modified testing procedure.

The following is a preview of the data, findings and recommendations Hügel will share.

Why whiskers grow

Why tin whiskers grow is a question that has boggled minds for decades. Hügel, however, does have his own theories—on tin whisker formation. According to him, whiskers will grow if there is an overall compressive stress (close to the yield limit of tin) in combination with a stress gradient (- 50 MPa/µm) toward the surface. This situation can be achieved by four basic mechanisms:

•       Growth of irregular intermetallic compounds (IMC) between Cu and Sn
•       Mismatch of coefficient of thermal expansion (CTE) between the base material and tin plating (e.g., delta CTE > 8 ppm)
•       External mechanical force on the tin layer over a long period of time (in press-fit, clamping) not bending
•       Corrosion-induced whisker growth (humid environment)
Why tin finish properties are important
Whisker growth can compromise the functionality of applications; therefore, the risk due to whisker growth has to be minimized, so it’s understandable why tin finish properties themselves are so important. Hügel indicates that tin finish properties are highly relevant to tin whisker growth in terms of:
•       Tin plating process (preparation)
•       Process parameters (process window) (temperature, concentration, agitation, current density, etc.)
•       Electrolyte (type)
•       Maintenance (process conditions such as agitation and current density)
•       Texture of the plating and the robustness of the texture against variations of current density, agitation, etc. The texture itself is the relevant factor regarding whisker growth of the tin plating (impact on CTE, chemical potential, etc.)
Minimizing risk
To minimize the risk of whisker growth, the root cause of whisker growth has to be identified and countermeasures have to be implemented.
According Hügel, “Conformal coating is not seen as the mitigation strategy because of the different side effects and the effectiveness of the coating in case of whisker growth. Third-generation electrolytes can provide excellent performance if the process is well defined and maintained.”
This only scratches the surface of the many parameters that go into whisker formation and prevention. Hügel will delve even deeper into these topics and others.
He will explain why maintenance is a major critical point for tin plating and the necessity for tin plating layer systems to fulfill minimum requirements (thickness, plating system setup, type of plating). He will also share how, by arming yourself with the knowledge of the robustness of the plating, a no-whisker, tin plating can be achieved.
For more information on the CALCE Annual Tin Whiskers Symposium at IPC TechSummit 2014, and to register, go to www.ipc.org/TechSummit.