| In recent years, with the rapid development of the electronics industry, the performance of electronic products has gradually increased and evolved into miniaturization and integration. Smaller chip size means higher current density and higher heat dissipation. The third generation SiC chip which is capable of high temperature services has put into production. Meanwhile, the packaging reliability issues have become increasingly prominent because of the lack of compatible high temperature solder.Since the promulgation of RoHS, most of leaded solder has been replaced by other materials. At present, the main methods of die attachment are nano-silver paste sintering, transient liquid phase bonding and high temperature alloy soldering, etc, but each of them has its drawbacks. Nano-silver sintering method has good thermal and electrical properties, but the cost of nano-silver paste is extremely high which means it can not be used in large-scale production. Transient liquid phase bonding is low-cost, but needs much longer reaction time and tends to incomplete reaction which can cause reliability problems. The solder alloy for high temperature(higher than 250?) are mostly Au-based alloys like Au-Sn(Tm=280?) and Au-Ge(Tm=356?). They are all very expensive and exhibit higher hardness as well as the reflow temperature which are not suitable for soldering. These limit the wide range applications of the high temperature solder alloy. Therefore, this study aimed to explore a possible new solder material which is low-cost but can achieve interconnection under low temperature(232?) and work at high temperatures(above 250?).The presented paper proposed a kind of core-shell structure metal powder as the soldering material. This kind of powder has the structure of Sn outer layer and Cu core. When reflowed at 250?, outer Sn are melted and react with the Cu core to form the intermetallic bonding. The solder joint with a structure of Cu particles dispersed in IMC has high melting point, leading to the capability of high temperature service.The core-shell metal powder mentioned in this paper was prepared using the chemical method. This paper used spherical Cu particles with 1?m, 5?m, 30?m particle size as raw material. To control the amount of coated Sn(one, two, three, four times of the standard amount), different solutions with Sn2Cl2·2H2O concentration of 4.2g/L, 8.4g/L, 12.6g/L, 16.8g/L were used. Due to the reaction between Cu and complexing agent, the electrode potential of Cu2+/Cu was reduced. Thus, the nobler Cu can replace Sn2+.Two methods for preparing the mentioned metal powder for soldering to achieve interconnection were used: the reflow of solder paste and the reflow of preform. The metal powder and commercially available rosin-based flux were uniformly mixed and the mass ratio of the metal powder ranged from 76% to 92%. When a large amount of flux was added, the liquidity of the solder was better, but much more pores were found in the joint. Solder paste with higher metal ratio exhibits higher joint quality. The soldering joints using core-shell solder paste with different particle parameters were also compared and characterized. By using preform as the solder material, the inevitable pores in the solder paste joints were eliminated. Thus, a joint structure which is much closer to the expected result was achieved. The solder joints formed by preforms with different particle sizes and coated Sn amount were compared and characterized. |
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