Technical Specialist (Electron Microscopy)
School of Engineering, Computing and Mathematics (Faculty of Science and Engineering)
Tin whiskers are growths of pure tin that form spontaneously from tin electroplated surfaces. Tin whiskers can grow up to a few millimetres in length (but typically only a few 100 μm) and may be straight, kinked, curved or in the form of odd-shaped eruptions. Tin whiskers can cause problems in electronics by growing long enough to bridge between surfaces causing the circuit to short out, a situation worsened by the continuing trend for miniaturisation in electronics and move to Pb-free. The growth of these whiskers can begin very soon after the electroplating of a sample, however the initiation of growth may take years to occur; it is this unpredictability of whiskers that causes concern to reliable operation. At the current time, there is no one widely accepted mechanism for why and how whiskers grow; though there are number of theories and some commonly agreed factors that affect the growth of tin whiskers.
Conversion coatings will be produced to form a significantly thicker oxide layer compared with electrochemical oxidation.
Whisker growth was evaluated using optical microscopy; three randomly selected samples for the electrochemically oxidised and untreated groups of samples were analysed at each analysis point and the average whisker density calculated. The first graph shows that, for both sets of samples, the whisker density increases progressively with time, however, the presence of the electrochemically formed oxide significantly reduces whisker growth compared with that from the untreated samples that were left to develop a native air-formed oxide.
The second graph shows that the thickness of the oxide on the untreated samples steadily increases over time and becomes similar in thickness to that on the electrochemically oxidised samples after ~84 days of storage; this graph also shows that the thickness of the oxide on the electrochemically oxidised samples remains relatively consistent over time with very little change over a period of 204 days. The increase in oxide thickness for the untreated samples may contribute to the reduced rate of whisker initiation observed after ~84 days. However, the reduced rate of whisker growth may be due to other factors such as reduced rate of intermetallic compound (IMC) growth.
The longest apparent length of whisker in each frame used to determine whisker density was measured at each time interval and is plotted in the top graph. For the untreated samples, the results indicate that the whisker length increases gradually over the 220 days of storage but that the rate at which the whiskers grow progressively reduces with time. Compared with the untreated samples, the length of the longest whisker is significantly reduced for the electrochemically oxidised samples throughout the period analysed, with little apparent increase in whisker length between ~80 and ~220 days. The bottom 2 graphs shows the change in the distribution of the longest whisker per frame for both electrochemically oxidised and untreated samples, between 14 days and 119 days of storage.
After 14 days all the measured whiskers were less than 100 µm in length for both the untreated samples and the electrochemical oxidised samples. After 119 days the whisker distribution for the untreated samples has shifted to much greater lengths and each frame analysed contained a whisker greater than 20 µm in length. The whisker length distribution for the untreated samples shows that the majority of the whiskers were greater than 100 µm in length after 119 days, whereas after 14 days, no whiskers were greater than 100 µm in length. In comparison, for the electrochemically oxidised samples, most of the whiskers were still less than 40 µm in length, although the maximum whisker length has increased compared with after 14 days of storage.
The bottom left graph shows that the whisker lengths on the
electrochemically oxidised samples are greatly reduced compared with the untreated
samples, thereby significantly reducing the risk of the whiskers bridging a gap
between components or connections and causing a short circuit. The reduction in
the number of long filament whiskers may be due to the increased incubation
period for the electrochemically oxidised samples.
Alternatively, it may suggest that the electrochemical oxide influences the rate at the whiskers grow once the whisker has penetrated the oxide layer. It might be expected that when a whisker penetrates through the oxide film, the whisker would have a similar rate of growth to the untreated samples; therefore, the electrochemical oxide also appears to be having an influence on the driving force for whisker growth as well as inhibiting whisker initiation. It was shown previously that for Sn-Cu on Cu, IMC growth didn’t appear to be influenced by the presence of the electrochemical oxide [1]. In the absence of an oxide film, the surface of the tin serves as a source for vacancies to facilitate tin diffusion through the coating, to support the growth of whiskers [2]. However, the presence of a sufficiently thick oxide may impede diffusion of tin atoms by reducing the number of available surface vacancies [3], therefore slowing the rate at which whiskers may grow. This may suggest that whisker growth on the electrochemically oxidised samples is diffusion limited.
The set of the images to the left are photographs of tungstate conversion coatings on electroplated pure Sn. The photographs show that for increased cycles, the quality of the coating decreases. This was due to gas being liberated at the service, causing non-uniformity of coating thickness. This can be overcome by reducing the current density, as shown in the bottom row of photographs. These photographs show that lowering the current density while passing an equivalent charge will improve coating quality; additionally an increased amount of charge can be passed using a lower current density, therefore enabling the formation of thicker coatings.
It was found that the tungstate conversion coatings had much finer cracking compared with the molybdate conversion coatings. However, it was observed that for thicker tungstate conversion coatings there were areas that had either flaked off or were thinner. It was also observed that all of the whiskers were growing from these areas. The micrographs here show a) 10 mA cm-2 at 30 cycles, b) 5 mA cm-2 at 60 cycles, c) 10 mA cm-2 at 90 cycles, d) 10 mA cm-2 at 150 cycles and e) 10 mA cm-2 at 90 cycles. They show that for fewer cycles and lower current density there is no cracking, however for more cycles and increased current density, cracking becomes apparent.
The images to right are optical micrographs of four different molybdate coatings after ~2 months of room temperature storage; the top left was produced by simple immersion for 5 min, the top right was produced at -0.45 V vs. Ag/AgCl for 5 min, the bottom left was produced at -0.6 V vs. Ag/AgCl for 5 min, and the bottom right was produced at -0.75 V vs. Ag/AgCl. It was observed no cracks were present in either of the top two samples, whereas the bottom two samples had a high density of cracks; with the samples produced at -0.75 V had the largest amount of cracking. This cracking may act as weak points for whiskers to grow through.
A high density of cracking was observed for molybdate conversion coatings produced at increased cathodic potentials. It was also observed that all the whiskers observed grew from the cracks, as shown in the micrograph. The cracks act as weak points and allow the whiskers to grow, relatively unimpeded compared to trying to grow through the bulk of the coating. However, due to the high density of cracks, there was a high density of whiskers compared with the lower cathodic potenital of -0.4 V vs. Ag/AgCl even though the coating was thicker.
The first and second graphs show the whisker growth studies for the molybdate conversion coatings and tungstate conversion coatings, respectively. It can be seen from both graphs that both of the conversion coatings will significantly reduce whisker growth compared with a native air-formed oxide. However, the graphs show that the tungstate conversion coating produced at 5 mA cm^-2 for 90 cycles will reduce whisker growth the most. The least effective tungstate coating had a whisker reduction ratio of ~36:1, whereas the least effective molybdate conversion coating had a whisker reuction ration of ~8:1. It should be noted, however, that this is still comparable to the reduction ratio of an electrochemically formed oxide. The reason for the higher whisker density for the -0.6 V molybdate conversion coating, was the fact that it contained a high density of cracks where whiskers were observed to grow from.
The following conclusions can be drawn from the results obtained in the current study:
For further information and/or ideas for further work please contact Dan Haspel at PEMC.
This work was part of a paper submitted to the Transactions of the IMF journal which can be found at the following DOI: 10.1080/00202967.2019.1587263