Achieving Non-underfill SMT Process for Large Size Packages by Creative Package Pin Map and PCB Pad Designs

In the present work, extra 56 solder balls were added to the outer ring of UFS based on the traditional pad design, where the solder balls increased to 209. Moreover, some non-functional balls were combined to improve mechanical reliability, thereby achieving the goal of UFS without glue dispensing. Subsequently, the solder joint stress, microstructures and warpage of optimized UFS were investigated. The reliability of optimized UFS was analyzed by using a drop test, thermal shock test and four-point bending test. The stress simulation analysis showed that the addition of extras 56 non-functional solder balls significantly reduced the stress level of the inner ring solder joints, where the maximum stress was reduced by 79.9%. The warpage test results showed that the optimized UFS met the processing requirements of +/- 30μm warpage at the temperature of 220°C to 240°C, meanwhile, optical microstructure (OM) observation indicated that the combined solder balls of the optimized UFS were full after solder bonding. The pad craters and via fractures occurred in the traditional UFS after the drop test at 1500g/1ms and 50 cycles, whereas the pad craters and via fractures only formed in non-functional solder joints of the outer ring for the optimized UFS, where the inner functional solder balls were undamaged at 1500g/1ms and 500 cycles. Additionally, the outer and inner solder joints of the optimized UFS were normal after a four-point bending at the strain condition of 3800ue. During the thermal shock test, all solder joints of the optimized UFS were normal after 200 cycles. Although fine cracks (<10%) occurred in the outer solder joints after 500 cycles, the functionality was intact.