RESULTS AND DISCUSSION
Properties and Characterization of the Pure Tin Solder.
There are several characterization methods that used on the indented pure tin solder such as Energy Dispersive X-ray Spectroscopy (EDS) which analyze the elements of the pure Tin solder samples. The Scanning Electron Microscope (SEM) is used to view the formation of the pure tin whisker which formed under the controlled parameter of external stress and high temperature surrounding.
The Surface Morphology
After the constant indentation of the steel ball, the surface morphology of the pure tin whiskers formed near the indentation of steel ball are viewed via SEM are classified into the nodules and hillocks formations. Besides that, the formation of the intermetallic compound of Cu6Sn5 through the annealing process at 150 oC for 240 hours also observed.
Effect of Stress on the Pure Tin Solder.
The parameters of stress which set up on the sample is manipulated by the different value of weight used, which are 200 g and 400 g. Both indentation of took placed at ambient condition with different duration of indentation period which are 24 hours and 240 hours. While, the third pure tin solder sample is indented at the elevated temperature of 150 oC for 240 hours
Figure 4.1: The SEM images of the indentation crater of 1 mm steel ball on the pure tin solder plating
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.
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
Figure 4.3: The secondary electron image of nodules and hillocks on the pure tin solder after 240 hours indentation
The Formation of the Pure Tin Whisker
The formation of whiskers was examined at various time interval which set up as in 24 hour and 240 hours by using scanning electron microscope (SEM) and energy dispersive x-ray analysis (EDX). The measurement of pure tin whiskers is according to the JESD201 standard which measure the axial length of whisker where the distance in between the pure tin plating surface towards the tip of the pure tin whiskers.
Figure 4.4: The back scattered electron images of the nodules and hillocks
Figure 4.5: The OM image of pure Sn microstructures bulk with magnification of 5x, 10x, 20x, 50x and 100x.
Based on the figure 4.5 above, there are several micropores exist among the microstructure of pure tin. Even though, the smaller grain size of pure tin enhanced the high rate of grain boundary diffusion, the presence of multiple micropores on the pure tin solder plating will become one of the factor which degenerate the formation of pure tin solder whiskers 57-58.
Figure 4.6: The Vickers Microhardness testing on the pure tin bulk
The figure 4.6 shown the average value indentation of the Vickers microhardness on the pure tin bulk. The hardness of pure tin gradually decreasing through the indentation for each sample. The presence of the micropores will reduce the toughness and the hardness of the pure tin which also enhance the stress relaxation mechanism among the grains structures