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Fighting Tin Whiskers in Surface Mount Assembly

22nd May 2015

Tin Whiskers

If there is anything that makes the calmest and most even-tempered designers of surface mount assembly circuit boards scream, pull out his hair and sky-rocket his blood pressure, it is just a phrase of two words: tin whiskers. It doesn’t help that these can also be called zinc whiskers.

These whiskers are not so much walrus moustaches as micro-strands of tin and sometimes zinc metal that grow out of surface and solders made of the pure tin or zinc. These hairs grow between the separate solder pads and can cause short circuits and electrical failures.

Whiskers grow out of the pure, or nearly pure, metal that is needed for advanced surface mount assembly. They have been recognised as a problem in electronic design for nearly a century. The traditional method of dealing with this problem has been to use a lead alloy in the sold to slow down whisker growth.

Commercial manufacturers of electronic circuitry started to move towards using either tin or zinc finishes and solders on circuit boards from 2006. This was the year that the European Union issued its Reduction of Hazardous Substances Directive that banned lead use.

One knock-on effect has been to create the tin whisker problem that has affected the defence and aerospace industries in particular, as these are the spearhead industries for advanced electronics.

One solution to tin whisker growth in surface mount assembly is to coat the components and so mitigate future growth. This has spawned more problems. These coatings are usually some form of unfilled polymers. These materials range from acrylics to polyethylenes, polypropylenes, nylons, phenols, epoxies and silicone resins. Their advantage is that they have a relatively low electrical and thermal conductivity and so isolate the components on the circuit board. Polymers can be filed with anything from talc to carbon fibre to increase their conductivity.

However, tin whiskers have managed to penetrate the unfilled polymer coatings, causing an even greater risk of critical electronic system failures. The challenge now is to develop more powerful polymer coatings that have a high strength, toughness, stiffness and adhesive properties.

One method is to incorporate nanoparticles of silica and alumina in a polyurethane coating. However, problems occur if the distribution of the nanoparticles on the applied coating is not uniform.

If you have any questions, please feel free to contact us.