Before the introduction of the Restriction of Hazardous Substances Directive (RoHS), electronic components were plated with an alloy composed of 80 per cent tin and 20 per cent lead. However, the use of the lead in the alloy was deemed to be environmentally unsound and the directive banned its use.
The lead, however, did serve one crucial purpose, by preventing the growth of what are known as ‘tin whiskers’ from the surface of the compound.
These are caused by a metallurgical phenomenon and involve the spontaneous growth of tiny, hair-like extensions from the surface of the metal. Although some researchers have reported whisker growth from tin-lead alloys, such whiskers are dramatically smaller than those from pure tin-plated surfaces and are believed to be sufficiently small so as not to pose a significant risk for the geometries of today’s microelectronics.
Although the mechanism behind the metal-whisker growth is not well understood, it seems to be encouraged by compressive mechanical stresses, including residual stresses caused by electroplating, mechanically induced stresses, stresses induced by diffusion of different metals and thermally induced stresses.
‘A general understanding is that stress builds up inside the plating, which is about eight-microns thick, and at some point the stress is relieved by the growth of the tin whisker. They don’t start to grow straight away because there isn’t any great stress on the coating. That happens over time,’ said Dr Chris Hunt from the National Physical Laboratory (NPL).
With the directive now effectively removing the use of lead from electronic components, the issue of tin-whisker growth, and more importantly how to prevent it, has become an important issue. That is because the use of pure tin-component termination finishes could affect the safety and functionality of electronic products used across many manufacturing sectors, since these are prone to the spontaneous growth of tin whiskers, which can cause catastrophic failures in electronic circuits.
‘Unfortunately, tin whiskers can grow up to 20mm in length, and with the average separation of components on an integrated circuit being in the order of 200 to 300 microns, or less than 0.5mm, it is easy to understand how these whiskers could short out components. It is a serious problem for engineers developing high-reliability systems,’ added Hunt.
According to Hunt, reported failures include the loss of at least two communications satellites and the unplanned shut down of a nuclear reactor, but a solution may now be in sight.
The use of conformal polymer coatings (materials applied in thin layers, often by dipping, spraying or flow coating) applied to the tin may be able to impede, or even stop, the tin-whisker growth. And many manufacturers are working to develop such coatings.
National Instruments, for example, has identified two finishes for RoHS applications that would limit tin whiskers: using immersion silver, and/or electro-less nickel/immersion gold (ENIG), which offers the most flexibility in its manufacturing reflow cycles and suitable storage life, while providing solder-joint reliability and quality.
‘Tin whiskers can still grow under these conditions but National Instruments has taken the best possible route to avoid creating the environment for these conditions to exist,’ said John Zukowsky, global value engineering manager at the company.
For his part, to allow engineers to assess the ability of different conformal coatings to meet these goals, Hunt and his colleagues at NPL developed a new measurement system and test method. The method can also help conformal-coating developers refine or modify their coatings to further inhibit whisker initiation, growth and penetration.
Because the plated metal terminations on an integrated circuit are very small, and the cross-sectional area between the terminations is just fractions of a square millimetre, observing tin-whisker growth on actual components in situ could be a complicated and lengthy procedure.
The NPL test set-up circumvents these issues. It comprises two parallel plates of around 40 square-centimetres that are first plated then coated before being placed in close proximity to one another. By simply measuring the change in impedance between the two plates over time, the researchers can then observe whether any tin whiskers have grown out of their surface.
According to Hunt, the test vehicle provides conformal-coating developers with the ability to discover the effectiveness of their coating within a couple of months or so. While that may seem like a long time, it’s a lot better than the incubation time of several years that might pass before such whiskers grew in any real-life scenario.
That’s because rather than coating the plates at the eight-microns typically found in most electronic components, Hunt and his team coat their larger plates at around a three-micron thickness. ‘It turns out that if you thin down the coating, that makes it more sensitive to stress and more likely for whiskers to form, and that allows us to accelerate the process of whisker formation,’ said Hunt.
Hunt and his team believe that the new test set-up will help coating manufacturers that wish to assess the potential effectiveness of their new formations to mitigate against whisker growth.
‘It provides a highly accurate measurement technique to analyse what could be very damaging problems for the electronics industry,’ claimed Hunt.
(NASA has developed a specific website covering tin-whisker growth, at http://nepp.nasa.gov/whisker/)
A new method helps to assess the ability of conformal coatings to inhibit the growth of tin whiskers, the microscopic hair-like extensions that grow on the surface of electronic components