All the test samples were assembled in Electronics Packaging Lab at Advanced Process Lab in Universal Instruments Corporation (UIC). Once the design was finalized and accepted, two stencils with different diameters of openings were designed for assembly. The first stencil is used for solder paste/flux application and another is set for solder sphere placement. The machine used in stencil printing was a DEK Galaxy Figure 3.2. The flux (Kester TF-6522) was first applied on the board, followed by general quality inspection using a microscope and automatic optical inspection (AOI) device to check any defects. Once the manufacturing requirement is met, solder spheres were populated with a brush to fill all the apertures using the second stencil, then boards were ready for reflow process.
The reflow process was accomplished using an 8 zone Pyramax 100N reflow oven within a nitrogen gas environment, as depicted in figure 3.3. The reflow thermal profile was identified so that it would be representative of typical manufacturing processes. Based on the main constituents of the solder joint, 2 reflow profiles were conducted. Detail reflow profiles are shown in figure 3.4. The preheat time (35 °C -150 °C) was 200 seconds. The maximum temperature above the liquidus for both profiles was 237°C. The peak temperature rising rate is about 0.75 degrees per second. The cooling rate was 1.38. On the other hand, the maximum time above the liquidus was 20 seconds for SAC305 and 32 seconds for SAC-3Bi and SAC-6Bi. A cross-section view of a typical solder joint before testing is shown in figure 3.5.
After the reflow process, x-ray inspection was performed to check for quality issues in the solder joints in the assemblies (e.g. voids).
