The figure 4.1 above shown the indentation crater after the 10 days indented by 200 g weigh at ambient condition. After 240 hours indentation, there are several grain boundaries appeared changed position around the indentation crater and a few pores formed on the surface of pure tin solder. Each of the indented sample have clear formed carter. Meanwhile, the figure 4.2 show the SEM images of the formation of hillock on the pure tin solder deposited onto the copper plate after the indentation of 400 g load after the 24-hour period in the room temperature.
Figure 4.2: The back scattered electron images of hillock on the surface of pure Tin solder deposit on the copper plate after the indentation of 400 g load for 24 hours.
As shown in the figure 4.2 of the surface morphology of the 1mm steel ball indentation with load of 400 g onto the pure tin solder deposited on the copper plate was under observation by using SEM. In the figure 4.2(i), shown the formation of hillock about 5 µm. Meanwhile, in the figure 4.1(ii), shown the initial peak of rounded hillock. The mean average of the formation of the hillock observed are about 209 µm. This prove that within 24-hour period with the applied stress of 12.6 kPa can formed about less than 5µm. The other pure tin solder sample is indented with the 200 g for the 240 hours.
Aside from the presence of the stress created from the indentation of load, it can be deduced that the presence of the copper-based substrate can also act as the precursor of the formation of pure tin growth. Both samples of different load are stored together in the same ambient temperature but within different period of storage. Based on the several current studies, the formation of pure tin whisker involved the grain boundary sliding movement which start with the critical pressure about -15 MPa 32. Eventually, the stress gradient is created to drive out the pure tin atoms from the surrounding undeformed areas to the base of hillock and push up the hillock outwards.
The thin layer of pure tin solder plating need less time for the movement of intermetallic compound towards the pure tin grain boundaries plating to generate the compressive stress 58. According to Horváth et. al, the manipulation of the size of the pure tin grain will cause the alteration in the rate of grain boundary diffusion and the level of exerted compressive stress.
4.1.1. Effect of Temperature on the Pure Tin solder.
There are two sample of pure Tin solder dip are placed under indentation of the same load about 200 g are placed at room temperature and at 150 for the period of 240 hours. For the first sample that indented under the 400 g within the room temperature, only several nodules are formed in comparison with the pure tin sample under indentation of 200 g in the 150 oC