Power semiconductors rely on good electrical and thermal interconnections to minimize their operating temperature. For high power applications the thermal impedance of the die attach layer can play a significant role in the ultimate operating temperature. Therefore, one would like to use the highest thermal conductivity material s consistent with the overall package manufacturing process. Package manufacturing processes are typically below 350°C. This has precluded the use of high conductivity silver or gold metal pastes for standard die attachment because they typically require heating the paste to at least 500 - 600°C in order to obtain the high thermal conductivities typical of these metal foils. Under a US Navy ManTech program, a silver paste was developed that can be processed at temperatures as low as 275°C, and still exhibit a high thermal conductivity (2.4 W/°K-cm).

All semiconductor chips or devices have to be packaged to perform their intended functions. In a typical electronic packaging process, chips are attached to substrates and electrically connected before they are encapsulated or sealed for protection. The attachment and electrical interconnections provide the chip an infrastructure for the flow of electrical signals, supply of electrical power, mechanical support, and heat removal. For many semiconductor devices used in power electronics circuits, the chip attachment or die-attach must also serve as an electrical interconnection such that the die-attach material has to be a good electrical conductor as well as a good thermal conductor for heat removal. The two types of die-attach materials that are widely used today in electronic packages are soft or hard solder alloys and polymer-matrix or glassmatrix composites. Soft solders (e.g. lead-tin or indium-based alloys) and polymer-matrix composites (e.g. silver-filled epoxies) have low processing temperatures and low elastic moduli for low mechanical stresses on devices, but they have relatively low thermal and electrical conductivities. Soft solders are alsosusceptible to fatigue failure under thermal cycling conditions. On the other hand, hard solders (e.g. gold-based eutectic alloys) and glassmatrix composites (e.g. silver-filled glasses) are used to enable devices to run at higher junction temperatures, but their higher elastic moduli and processing temperatures can generate high mechanical stresses in devices, and these materials also have relatively low thermal and electrical conductivities. In this article, we introduce a low-temperature (as low as 275°C), pressureless (i.e. no externally applied pressure) sintering technology that utilizes nanosilver paste to provide superior electrical, thermal and mechanical properties, and high-temperature capability to device attachments and interconnections. Properties of the Low Temperature Sintering of Nanosilver Paste for High-Temperature Device Interconnection (Continued from page 1) sintered silver joints along with some typical solder and epoxy joints are listed in Table 1.

The pressureless sintering technology for device attachment and interconnection relies on a nanosilver paste material, which is a mixture of metal powder, mostly nanoscale silver particles, an organic dispersant and binder system. Figure 1 illustrates the basic processing steps of making and using the material to attach devices. The dispersant helps prevent the particles from agglomerating, a major issue with extremely small particle size. The binder system, a solution of organic binder and solvents, imparts the paste-like consistency and provides the green strength to the dried paste so the assembly will not come apart during handling prior to firing. Using nanoparticles, as opposed to the micron-size powder used in commercial thick-film pastes and inks, lowers the effective sintering temperature of the powder down to as low as 275°C by increasing surface energy that drives the densification process. There are nuances to the process that must be appreciated when considering the use of a sintered metal joint as opposed to soldering. A typical firing cycle will take the paste through a drying stage to remove the solvents, a binder burnout stage to eliminate the binder and any remaining organics, and finally a densification stage wherein the assembly of particles is transformed into a dense polycrystalline structure with the necessary mechanical and physical properties. The extent of densification depends on factors such as uniformity of dispersion, particle size and particle size distribution, drying time and rate, heating rate, temperature, heating time and the effect of constraint by the substrate and die sandwiching the paste. These parameters can have a strong effect on the green microstructure that in turn influences the final sintered structure. The mechanism(s) for densification occurs by solid-state diffusion processes, which is considerably slower than straight melting, as in solder reflow. Proper drying is also important to prevent the formation of large cracks and delamination from the bonding surfaces. The timing of the densification, attained through the temperature range and heating rate, must be done right in order to obtain the microstructure with sufficient strength. Because of the inherent instability of the nanoparticles, it is necessary that the desired sintering temperature range is reached by the time the organic molecules (which keep the particles apart and hinder densification) are burned off to obtain a dense microstructure.

Because it has a low sintering temperature, does not require external pressure to bond, and has viscosity and flow characteristics favorable for printing or dispensing, the nanosilver paste can be used as aone-to-one replacement for solders and epoxies that are commonly used in today’s manufacturing processes. Furthermore, once the material is sintered at 275°C it will not melt unless it is heated to 961°C (the melting point of silver), such that the paste can be used for subsequent die-attachments as well as other interconnections in a packaging process that involves multiple joining steps. This eliminates the need to design a hierarchy of process temperatures that is required to accommodate different reflow schedules for a series of solder alloys. Figure 2 shows examples of interconnected devices that were sintered on substrates metallized with gold or silver; one shows both the top and bottom terminals of devices connected by sintered joints. Since the sintered joints are made of pure silver, they have substantially higher (a factor of five, as shown in Table 1) electrical and thermal conductivities than those of the other joints. The relative properties of sintered silver joints are expected to be even higher at device junction temperatures as the soldered joints are operated much closer to their melting points than the sintered silver. The apparent elastic modulus of the sintered silver was found to be around 9 GPa due to a relative density of approximately 80%. Such a low elastic modulus in the sintered joint, about 12% of that of bulk silver, significantly reduces the amount of mechanical stresses transferred to the chip. While a soldered die-attach layer is prone to formation of large voids, residual pores in the sintered silver are very small and distributed uniformly (Figure 3), thus eliminating hot spots underneath the device. Formation of brittle intermetallic compounds in soldered joints has always been a concern for joint reliability. As the sintered joint is an elemental metal, formation of intermetallics in the bulk of the sintered joint is not an issue. With carefully selected device and substrate metallization, the concern of intermetallics at the interfaces of a sintered joint may also be eliminated. Finally, by designing silver electromigration paths out of the package, we have shown in our preliminary tests that devices interconnected by the sintered silver joints are capable of continuous operation at 300°C. The pressureless sintering technology for device attachment and interconnection was developed through a research effort aimed at high-performance and high-temperature packaging of wide bandgap semiconductor devices, such as SiC power diodes and transistors. Advances made recently in SiC material and device technologies are opening up opportunities for engineers to design and implement unique applications, such as power management under extreme environment on a navy warship. With SiC devices, substantial increases in power density could be achieved because SiC has higher thermal conductivity, higher breakdown voltage, and higher saturated carrier velocity than silicon. Furthermore, SiC’s larger bandgap can allow higher junction temperatures, thus reducing the need for bulky cooling systems. Based on the materials listed in Table 1, the nanosilver paste, with its ability to sinter at low temperatures under no external pressure has a clear edge in meeting the requirements of high-temperature packaging of SiC devices. Given the superior properties and ease of processing offered by sintered joints, we expect that the sintered interconnect technology described here will soon be adopted to address interconnection needs in other applications like automotive electronics, LED solid-state lighting, laser diode or RF communication modules.