A review of lead-free solders for electronics applications

Spontaneous Growth of Metal Whiskers on Surfaces of Solids: A Review

The effect of electroplating parameters and substrate material on tin whisker formation

Enhanced mechanical properties of polyurethane composite coatings through nanosilica addition

Properties and interfacial microstructure of Sn–Zn–Ga solder joint with rare earth Pr addition

The application of a conformal coat has been considered as a mitigation strategy to prevent unintended shorting events induced by tin whisker formation in electronic products. While various conformal coatings have been shown to be effective at containing tin whiskers on treated coupons, the effectiveness of conformal coating on actual assembled hardware has not been adequately examined. In this study, the ability of six types of conformal coatings to contain tin whiskers was examined through their application to assembled gull-wing lead quad flat package test specimens. Nonuniform coverage of conformal coating on the gull-wing leads was found to be a primary concern. Quantitative image analysis using scanning electron microscopy in backscattered electron mode was developed to aid in quantifying coating coverage. The ability of applied coatings to contain tin whiskers was examined after specimens were subjected to sequential temperature cycling and elevated temperature/humidity conditions as well as exposure to corrosive gases. For all but one coating, tin whiskers were observed to escape areas of relatively thin coating. Parylene C coating was found to be the most effective coating in providing uniform coverage and thickness, and containing whiskers.

Evaluation of Effectiveness of Conformal Coatings as Tin Whisker Mitigation

Reliability of conformal coatings used to mitigate tin whisker growth depends on their ability to contain tin whiskers. Two key material properties required to assess the reliability of a polyurethane coating are documented experimentally: adhesion strength and puncture strength. A modified blister test using a predefined blister area is employed to assess the adhesion strength and a puncture test is employed to evaluate the puncture strength of the coating. After measuring the properties at time zero, the coatings are subjected to accelerated testing conditions (high temperature/humidity storage and temperature cycling) and the degradations of the coating properties are documented. [DOI: 10.1115/1.4026922]

http://terpconnect.umd.edu/~bthan/paper/Adhesion/Conformal%20coating%20adh.pdf

Adhesion and Puncture Strength of Polyurethane Coating Used to Mitigate Tin Whisker Growth

This study evaluated the growth of tin whiskers on tin plated alloy 42 lead-frames and copper lead-frames stored in ambient conditions for 4 years, samples stored in ambient conditions after a post-bake treatment, and stored at 55plusmn1degC/85plusmn3% conditions for 3000 hours. Analysis was conducted to investigate the propensity and the mechanisms of growth.

Observations of the spontaneous growth of tin whiskers in various reliability conditions

When a tin whisker bridges two differently biased conductors, an electrical short is not guaranteed. In many instances, the voltage must exceed a threshold level in order to produce current flow due to weak physical contact and the presence of a non-conductive film such as an oxide layer. This paper presents a study that examines the breakdown voltage of tin whiskers and its relation to contact force. Whisker contact force studies were conducted using gold- and tin-coated tungsten probes, and the breakdown voltage was measured using a semiconductor parameter analyzer. It was verified that contact force is a critical factor in determining the type of current-voltage transition and level of breakdown voltage. Lower contact force between the probe and the whiskers caused the multiple transitions in current-voltage characteristics. The tin oxide layers on whiskers were analyzed using field emission transmission electron microscopy (FE-TEM).

Electrical Shorting Propensity of Tin Whiskers

Tin whisker growth has become a reliability concern during the course of Pb-free solder transition. A common mitigation strategy is to use conformal coatings to prevent electrical shorts by tin whiskers. However, recent research indicates that under elevated temperature and humidity, or in areas of thin covering, whiskers can grow and penetrate conformal coatings. Additionally, for long environmental exposure, the effectiveness of conformal coatings may be compromised.

Sungwon Han

Papers

Evaluation of Effectiveness of Conformal Coatings as Tin Whisker Mitigation

Adhesion and Puncture Strength of Polyurethane Coating Used to Mitigate Tin Whisker Growth

Observations of the spontaneous growth of tin whiskers in various reliability conditions

Electrical Shorting Propensity of Tin Whiskers

Reliability Evaluation of Conformal Coatings for Tin Whisker Failure Mitigation in Accelerated Testing Conditions