Reliability of nano-silver soldering paste with high thermal conductivity

Abstract

As electronic devices are becoming faster and incorporating more functions, they are simultaneously shrinking in size and weight. These factors suggest significant increases in the packaging densities and heat fluxes for the integrated circuits. Effective thermal management will be the key factor to ensure that these devices perform well with high efficiency and reliability. As the heat dissipation concentrates on tiny gate fingers, the operation of the GaN high-electron mobility transistor (HEMT) posts a huge challenge on thermal management. In order to enhance the heat dissipation capability of GaN power device, it is mostly soldered on the heat spreader with the AuSn solder. However, the thermal conductivity of the AuSn solder is only 57 W•K−1•m−1, which cannot fulfil the reliability requirements of future power electronic devices. New interconnection technologies have to be developed and one of them is a low-temperature pressure-less silver sintering paste with nano silver technology.The joining strength, thermal conduction, electric conductivity and long-term reliability of nano-silver soldering paste under low temperature without pressure are studied, and then compared with AuSn bonder. It forms a strong, highly electrically and thermally conductive bond. Chip shear tests show that 200 °C is already sufficient to generate bonds comparable to solder and high-strength welding interface if the remaining parameters (r, t and T, respectively) are set correctly. However, the strength of the welding interface is only a necessary criterion as chip performance comes into play. Therefore, reliability performance of thermal shock test, high temperature storage test and low temperature storage test are run, which return superior reliability of the sintered samples.In addition, the electrical performance of the GaN power chip is tested. The nano-silver bonded sample is compared with the AuSn sintered sample. The saturated signal output power of these two packaging processes can reach 41dBm, and the leakage current is in the normal range of 0.2A~0.3A. Through reliability test, there is no significant change in these samples, and they all achieve normal electrical performance of the power amplifier component.Besides, thermal performance is one of the important parameters affecting the reliability of high-power devices. In the field of electronic assembly, it is of vital importance to reduce the junction temperature. Ordinary conductive adhesives have lower thermal conductivity which generally below 10 W•K−1•m−1 compared with the AuSn solder. Considering that, it is not suitable to join GaN power devices with the AuSn solder and conductive adhesives. In order to verify the heat dissipation performance of the nano-sintered silver solder samples, 20 samples are prepared for thermal imaging experiments. By observing and comparing the temperature distribution on the surface of nano-silver solder paste sintered chips and the AuSn sintered chips. There are some fluctuations in the thermo-resistance of nano-silver solder paste sintered chips and the AuSn sintered chips. Although the thermo-resistance of both samples are concentrated in 1.00 °C•W−1to 1.20 °C•W−1 and heated by the same power, the nano-sintered silver solder achieves around 16.7% decrease in the maximum temperature of the test chip. With the attached heat spreader, 50 W power can be dissipated.