- Die attach

Die attach, also known as die bonding, is the process of connecting or bonding a die or chip to a substrate, package or another die. This process can happen in many forms and can be applied in many ways, and differences between them are entirely dependent on the user's desire. Various options are discussed in [20], including high lead solder alloys, lead-free solders, high-temperature reflow soldering, nano-silver paste sintering, and transient liquid-phase bonding. Transient liquid-phase bonding is capable of bonding at low temperature and servicing at high temperature, which is a significant trend in the field of high-temperature packaging for SiC power modules. Due to the disadvantages of time-consuming nature and required special equipment, some research on high-throughput solutions is under way [35].

Solder is required to have characteristics of a high melting point, strong bonding strength, excellent thermal conductivity, and electrical conductivity. It can efficiently conduct the heat generated by switching losses to a radiator in time. Thus, it dramatically improves the capabilities of SiC power modules to operate reliably under the condition of high frequency and high-temperature [36]. Some readily available lead-free alloys fall within the category of the high melting point, with melting points well above 220 ◦C, which are listed in Table 5. The lead-free alloys are a good option for high-temperature die attach. As can be seen, Au20Sn80 with the highest melting point of 280 ◦C is suitable for high-temperature soldering and packaging. Gold-based solders such as AuSn, AuGe, and AuSi, have a good reputation because of their melting points of 280 ◦C, 365 ◦C, 363 ◦C or so, respectively, but the gold-based solders are costly and not suitable for mass production. Other alloys such as zinc and bismuth alloys have been studied for their potential as well [37,38]. Silver sintering has better long-term reliability compared to leaded-free solders, but the challenge of microstructure evolving or coarsening at elevated temperature is a hindrance.


**Table 5.** The thermal mechanical characteristics of materials for die attach.
