7884079557

NPL Report MATC(A) 164

The following steps were carried out:

• The substratc was cut into sections suitable for the shear tester holding jig, using a water-cooled, Iow speed diamond saw, which produces a clean edge with minimum stress to the board.

• The sections were cleaned (e.g. with iso-propyl alcohol) to remove any contaminant residues from the cutting stage, and dried using compressed air.

• The stand-off height was set at h/2 (typically 80 pm) between the bottom of the shear tool and board surface (h is the stand-off height between the component and the board surface). This is the most important test condition.

• During each test, the shear tool was moved forward at a defmed speed (200 pm/s) against the test component, and the applied force inereased until the attachment was broken.

The data obtained in the test were analysed in terms of the ultimate shear force reąuired to rupture the solder joint, and then plotted as a function of the number of thermal cycles to which the assembly had been subjected. An average value was calculated from the 26 indiyidual measurements and plotted together with the first and third quartile of a box-whisker diagram shown in Figures 10.

The box-whisker diagrams reveal the distributions of ultimate shear forces measured after reaching a certain number of cycles. Although the testing was performed after a defmed number of cycles, the locations of boxes plotted in the diagram have been moved sideways around the nominał number of cycles to avoid overlaying of data. The top and bottom sides of the boxes indicate first and third ąuartile (Ql, Q3) values of the population sample. The linę drawn across a box is an indication of the median (ąuartile Q2). The vertical lines (whiskers) from these boxes extend to the last data point within the rangę of the limits. The upper and lower limits are calculated according to the relationships:

Lower Limit: Q1 -1.5 (Q3 - Ql), Upper Limit: Q3 + 1.5 (Q3 - Ql)

The circle represents the population mean, and the asterisks (*) are outliers (data points above the fourth ąuartile or below the first ąuartile).