November_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 19 NO. 4 6 DESTRUCTIVE ANALYSIS To inspect the solder material, a cross section was performed with optical (Fig. 7) and scanning electron microscopy (SEM) imaging (Fig. 8). A crack was confirmed under one pin. The other side of the component was not affected. The solder material thickness between the pin and the pad was found to be very low. The SEM observation showed that phase segregation has occurred. The intermetallic at the interfaces with the pin and the pad is continuous and seems correctly formed. Other functional DC/DC converters were also studied to control the soldermaterial. Recrystallizationwas found onmany of them(Fig. 9). Recrystallization is an early stage of crack formation. The strain that is stored in thematerial is released by rearranging the atoms in smaller grains. UNDERSTANDING THE ROOT CAUSE The hypothesis is that thermomechanical stresses generated by the different coefficients of thermal expan- sion (CTEs) are the cause of the failure. No clue was found to incriminate the fabrication process. The thermome- chanical stresses that apply to the solder joint may stem from various causes: • The CTE of the molding resin itself • The different CTEs between the component and the PCB • The position of the device on the PCB (far from the center) Because a redesign of the PCBwould have implied too many industrial consequences, the focus was on minor modifications, such as a change of resinor soldermaterial. IMPACT OF RESIN CTE The resin CTE was measured with thermomechanical analysis (TMA) in the compression mode (Fig. 10). The postreticulation energy also was controlled with differ- ential scanning calorimetry. The results show that the resin is correctly reticulated, but the glass transition temperature ( T g ) is low compared to the application temperature range. Indeed, the T g was measured at approximately 80 to 100 °C, but the DC/DC converter itself is supposed to self-heat at approximately 120 °C when operated. If the operating temperature of the device exceeds the T g , the CTE increases quickly. This phenomenon implies that the stress applied on the solder joints increases until recrystallization and a crack appear. IMPACT OF SOLDER JOINT THICKNESS The plastic strain stored in the solder joints was con- trolled with the electron backscatter diffraction (EBSD) technique. This typeof imaging isperformed insideanSEM chamber. It is useful to acquire the orientation of the crys- talline network for every pixel of an SEM image. Because strain induces local changes in the crystalline network (continued on page 8) Fig. 6 X-ray image of the SO8 defective device Fig. 7 Optical images of the pins and solder material Fig. 8 SEM images of the crack