Development of Solder Resist with Improved Adhesion at HTSL (175 deg C for 3000 Hours) and Crack Resistance at TST for Automotive IC Package

Innovations like virtual cockpit and autonomous car have changed the application landscape of the automotive industry, and two key changes have derived: the footprints of electronic devices in cars increased and the industry demands even higher-density and even higher-performance ICs with higher count I/O for smarter vehicles in the coming future. BGA (Ball Grid Array) is one of the key technologies expected to support these growing and diversifying automotive IC applications, including under-hood and other harsher use cases, which require higher heat resistance and durability. For example, the automotive industry's standard "AEC-Q100 Grade 0" now requires BGA packages heat resistance of storage temperature at 175 deg C, even higher than the conventional marking point of 150 deg C. Development of new packaging materials is the pressing need to support these even more stringent requirements. More reliable solder resists will play the critical role to provide reliable insulation for the BGA technology, but delamination and/or TST (Thermal Shock test) cracks are reported with storage test at 175 deg C and/or shorter high temperature cycling. Delamination is caused mainly by insufficient heat resistance of the resins and degraded adhesion between the SR and base material or Cu layers due to stress changes caused by temperature at the higher range. For the cause of TST cracks, we have checked and determined, by the series of simulation and tests, that they are caused largely because changes in complex modulus derive from crosslink density changes at high temperatures and leads to increase in stress at lower temperatures. These problems need to be solved in order to offer really reliable insulation for smarter automotive ICs. In order to solve the above problems, we first obtained higher Tg by optimizing the filler/resin bond in order to raise the inorganic filler/resin ratio and by engineering a better matrix resin composition which enabled higher thermal crosslink densities. We established a technology that effectively suppresses the heat degradation under high temperature by adopting this higher Tg, which we demonstrated provided excellent dielectric properties. We also developed a method to suppress crosslink density change associated with prolonged exposure to heat and thus to minimize thermal-mechanical changes (i.e. changes in complex modulus) and changes in stress caused by high temperature storage. Furthermore, we fabricated a nanophase separation technique for the elastomer which improved the stress relaxation during thermal cycling without sacrificing the mechanical properties and which provided the internal stress relief due to high temperature storage in the HTSL. We fabricated test coupons using prototype SR accordingly and conducted a high-temperature storage test at 175 deg C for 3000 hrs. We observed neither delamination nor cracks in the test coupons during and after the HTSL. The dissipation factor of this material is 0.008, which is a one-thirds of that of conventional materials. Therefore, this methodology is effective for reducing a loss of high-frequency signals. We also made a 5 cm-square test vehicle with 2.5 cm-square die and verified that our prototype maintained excellent crack resistance even after 2000 cycles of Thermal Shock test. Thus we conclude that our new SR offers suitable properties required for automotive BGAs, including robustness maintained under 175 deg C.