Siuli Sarkar

Development of low-temperature drop shock resistant solder alloys for handheld devices

As handheld devices become increasingly smaller and complex, there is a shift in reliability requirements of solder pastes. Considering that thermal management and drop resistance of such devices become more challenging, improved thermal fatigue and mechanical shock properties grow into must have requirements. Additionally, multi-step assembly process and a surge in use of temperature sensitive components bring additional challenges that necessitate the use of low temperature alloys. Here we present the findings of our Alloy Development Program on the next generation of low temperature alloys that can be used in reflow soldering temperatures from 170 to 200°C. By using micro-additives we have created low temperature alloys with superior mechanical properties, higher drop shock resistance and improved fatigue life.

Wafer level CSP with ultra-high thermal reliability lead-free alloys

Wafer-Level Chip-Scale Packaging (WLCSP) has become increasingly popular in portable electronics. One of its main characteristics is its reduced scale and that the solder balls are attached directly to the device. One of the main challenges in WLCSP is how to overcome the effects of thermal mismatch between the silicon die and the printed circuit board that arise from these characteristics. Use of new solder alloys is one of the ways to mitigate thermal fatigue stresses resulting from coefficients of thermal expansion mismatch. Tensile tests and high temperature creep tests were used for initial screening of the alloys and understanding the potential impact of each addition on the reliability of the solder in the final application. Here improvements in thermal, mechanical and metallurgical properties of the new alloy Maxrel Plus are discussed and compared to SAC405. Based on drop shock test, single ball shear test (high temperature storage, PCT and MSL1), thermal cycling test and intermetallics measurement results, we conclude that Maxrel Plus is specially recommended for use in WLCSP.

Thermal and mechanical reliability of low-temperature solder alloys for handheld devices

Low temperature alloys are used to achieve peak reflow temperatures from 170 to 200°C. Sn-Bi alloy stands as logic choices due to its low melting point, higher strength and low cost. However, use of Sn42-Bi58 alloy as soldering material is limited by a series of drawbacks such as low ductility, and poor thermal and mechanical reliability. Here we show how the use of micro-additives in eutectic Sn-Bi alloys improves thermal fatigue and mechanical shock properties. The basic properties of the new alloys shown here were fully characterized and their use in SMT applications evaluated, especially in drop shock and temperature cycling tests. Among the new alloys proposed, alloy B demonstrates superior mechanical properties, thermal cycling, drop shock and creep resistance, against benchmarks. Use of this new alloy in applications such as portable devices, PV ribbons and high efficiency LEDs will be highly beneficial due to its superior performance.

Introduction of Advance Lining System for Tundish at Durgapur Steel Plant

Developments in the continuous casting of steel have put increased demands on the refractories used in the tundish. Tundish working lining systems are required to provide the steel maker with a clean steel practice over a reasonably longer sequence, proper insulation, wear resistance, flow control and easy deskulling. In order to achieve these objectives, the tundish lining material also advanced significantly. The present practice of using silica board as cold tundish working lining material in Durgapur Steel Plant has some limitations in the aforesaid requirements. This paper deals with a “dry tundish working lining system” which allows the steel maker to produce cleaner steel. This material contains around 80% MgO and resinous bond. This bond is activated at relatively low temperature (250°-300°C) to generate sufficient strength. The thickness of the working lining varies between 40 and 60 mm depending on tundish size and sequence length. Dry tundish working lining can be used replacing SiO2 or MgO boards in cold tundish practice. In addition, the other components of the tundish working lining, e.g. impact pads/turbo-inhibitors, metering nozzles, dams are also looked into to match the higher sequence length. The system has been proven as a cost effective refractory solution for long multiple sequence casts.