The use of “commercial-off-the-shelf” (COTS) lead-free electronics assemblies in DoD systems has resulted in increased tin whisker short circuit failure risk. The objective of this effort was to evaluate the whisker risk and coating whisker mitigation on lead-free soldered assemblies to ensure reliable low cost systems with the most current technology for the warfighter. Tin whisker testing was targeted to evaluate key part, manufacturing, and environmental variable combinations hypothesized to contribute to whisker growth, with particular emphasis on areas unique to the DoD such as corrosion, rework, and long term storage. In order to utilize lead-free solders in diverse military and aerospace systems environments, the whisker risk factors posed by these material combinations must be understood and mitigated. By understanding the interaction of these variables and how conformal coating behaves in the long term it will be possible to put appropriate controls in place to manage the whisker phenomena.
Systematic tin whisker testing was performed on lead-free soldered assemblies over a range of environments applicable to military systems. Whisker growth from high stress short term test environments were compared to longer term lower stress environments. Two thermal cycle ranges and two isothermal high humidity conditions were evaluated. The longest test was a low temperature/high humidity storage test performed over three years. In the testing, several key design (lead materials, bias voltage and part types), manufacturing (solder alloy, flux, cleanliness level) and environmental (temperature and humidity) variable combinations are analyzed with a focus on the combination of factors that result in significant whisker growth. Particular attention was paid to the cleanliness and metallurgical details before and after environmental exposure in an effort to gain insight on the stress relaxation mechanisms associated with whisker nucleation and growth. In addition, a tin whisker risk model was created to statistically evaluate electronic assembly short circuits. A unique modeling approach was developed to rapidly evaluate different whisker length distributions for a given set of parts. The model also considers conformal coating mitigation and circuit voltage to determine the probability of a short circuit on a single part or group of parts. The short circuit failure risk of any system functional group could be obtained by combining the whisker statistics, the part geometry spacing distribution, and circuit details.
The results from two screening experiments are presented first to establish some key parameters for the more extensive set of primary experiments used to obtain the whisker growth statistics. The primary environmental tests were designed to obtain whisker growth on Sn3Ag0.5Cu (SAC305) soldered assemblies with respect to contamination level, lead material, voltage bias, rare earth alloying, and conformal coating.
The tin whisker short circuit modeling portion of the project leveraged the experimental whisker growth results and published literature. A unique modeling approach was developed to rapidly evaluate different whisker length distributions for a given set of parts. Monte Carlo analysis was used to determine whisker bridging spacing distributions for various piece parts. The spacing distributions were then convolved with whisker length probability data and electrical conduction probabilities to determine the short circuit risk. The model also considers conformal coating mitigation and circuit voltage to determine the probability of a short circuit on a single part or group of parts.
For access to the open-source modeling tool and resources: https://web.calce.umd.edu/tin-whiskers/spreadsheet/
With the advent of lead-free electronics resulting from the European Union RoHS (Reduction of Hazardous Substances) legislation, new lead-free materials and processes have replaced heritage tin-lead materials and processes. The warfighter benefits by having the most current technology by the adaptation of low cost COTS electronics to weapon systems as long as reliability is maintained. The lead-free materials have changed piece parts, printed wiring board (PWB) materials, component and PWB finishes, solder alloys, solder processes, solder flux chemistries, and cleaning chemistries. Regardless of the how the materials and processes have changed, DoD assemblies are still expected to perform in a diverse set of environments and mechanical loading conditions. These environments range from the high humidity of the jungle, the extreme cold of the arctic, the heat of the desert and the corrosive conditions of the oceans while being subjected to an equally diverse set of mechanical loading conditions, which in the most extreme cases includes gun/cannon fire, aircraft carrier landings, parachute drops, depth charges, rocket launches, and jet engine vibration.
Progress is being made toward DoD focused tin whisker risk assessment and understanding whisker growth mechanisms (long term testing, corrosion/oxidation in humidity, and thermal cycling). The current research is addressing several priority knowledge gaps from the Lead-free Manhattan Project’s whisker mitigation topic. The Strategic Environmental Research and Development Program (SERDP) sponsorship continues to help the DoD advance toward closure of the Lead-free Manhattan Project’s tin whisker knowledge gaps.