Low-temperature Silver Sintering Research Articles

Pressureless Silver Sintering on Nickel for Power Module Packaging

Die attach by low-temperature silver sintering has been widely used in power electronics packaging. Most of the reported work was done on direct-bond-copper (DBC) substrates metallized with silver or gold. There is a lack of studies of sintered-silver bonding on nickel (Ni), a low-cost metallization on DBC. In this study, we fabricated a power module using pressureless in-air sintered-silver die attach on a Ni-plated DBC substrate. Strong die-shear strength of over 40 MPa was achieved. It was also found the static and switching characteristics, and the transient and steady-state thermal performance of the modules using Ni metallization was as good as that using Au metallization. Cross-sectional microstructure and chemistry analyses of the sintered-Ag/Ni interface revealed limited Ni oxidation. We believed that the high-density packing of silver particles and outgassing of the organic molecules from the paste during the bonding process helped to lower the partial pressure of oxygen in the bondline, which, in turn, prevented rapid Ni oxidation and gave rise to extensive formation of strong Ag–Ni metallic bonds at the interface. The findings of this study show the low-cost potential of the die-attach technology for power module packaging since the bonding process can be done pressureless in air on low-cost Ni metallization.

Bonding of Large Substrates by Silver Sintering and Characterization of the Interface Thermal Resistance

Low-temperature silver sintering technology, which has been proven to be a promising die-attach solution, was extended to bonding large substrates. Strong bonding strengths for substrates with a dimension of greater than 25 mm × 50 mm were achieved by sintering a nanosilver paste at temperatures below 270 °C under a pressure of less than 5 MPa. To characterize the thermal performance of the substrate-attach interface, we applied a transient thermal technique with cumulative structure function analysis. Using self-heating and the temperature-sensitive threshold voltage of a power device, we measured the transient thermal responses of the device placed at various locations on the bonded structures. Each transient thermal response was used to determine the cumulative structure function, which represents the relationship between cumulative thermal capacitance and cumulative thermal resistance from the device junction to the ambient environment. Two-dimensional maps corresponding to interface thermal resistance were obtained from structure function plots. We found that for well-bonded substrates, the average specific thermal resistance contributed by the sintered silver interface was between 5.20 and 5.78 mm2K/W with a variation of 4.7%–6.0%.

Towards High Temperature Electronic Modules in which all Components would be attached using Silver Sintering

Abstract This article points out the fact that conventional low temperature silver sintering processes do not seem to satisfy the needs for reliability when they are used to assemble passive components (such as multilayers ceramic capacitors) in electronic modules undergoing wide amplitude thermal cycles (−65°C / +200°C). These processes indeed lead to very porous microstructures of silver which favors rapid fatigue cracks propagation [1], [2], [3]. Even if these types of porous microstructures may be suitable for die attach, tests presented in this paper show they cannot resist to the levels of thermomechanical stress reached in attachment joints of multilayers ceramic capacitors (MLCCs). To find process parameters minimizing porosity in attachment joints of capacitors, measurements of the shrinkage of silver during sintering have been carried out by dilatometry. Then the impact of densification of attachment joints on the ageing behavior and on the mechanical behavior of assemblies has been assessed in this study.

Evaluation of Thermal Cycling Reliability of Sintered Nanosilver Versus Soldered Joints by Curvature Measurement

Low-temperature silver sintering technology is emerging as a lead-free die-attach solution to significantly improve the heat dissipation and reliability packaging of power devices and modules die attached by solder alloys. With the recent introduction of nanosilver materials, which dramatically simplify the bonding process by lowering the required pressure down to a few megapascals, the silver sintering die-attach solution is poised for wide use in the manufacture of electronic products. In this paper, the thermomechanical reliability of sintered-silver joints was studied in comparison with soldered joints of two lead-free solders, SN100C and SAC305. Die-attach samples were fabricated by bonding 10 mm × 10 mm silicon mechanical chips to silver-metalized copper blocks and direct-bond copper (DBC) substrates according to the respective heating profiles of a nanosilver paste and the two solders. The die-attach samples were thermally cycled between -40 °C and +125 °C. The bonding reliability was evaluated by measuring the bending curvatures of the cycled samples and examining the cross sections of the samples under an electron microscope. Bending of the bonded structures, which is the result of mismatched coefficients of thermal expansion between silicon and copper or DBC, offered a nondestructive method for monitoring the integrity of the bond line. The bending curvatures of all of the die-attach samples decreased rapidly after they were thermally cycled. Most of the drop in curvature can be attributed to stress relaxation in the bonding materials without bond-line cracking. However, in the samples on copper blocks, after 800 cycles, the curvatures of the soldered samples decreased to near 0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , but those of the silver-sintered samples still had about 30% of the original curvatures. Scanning electron microscopy images showed that the joints of the soldered samples with near 0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> curvature had been cracked almost all the way through, but the joints of the sintered samples were still intact. These results demonstrate that the sintered-silver joints are more reliable than the soldered joints.

High-Temperature Operation of SiC Power Devices by Low-Temperature Sintered Silver Die-Attachment

In this paper, we present the realization of high-temperature operation of SiC power semiconductor devices by low-temperature sintering of nanoscale silver paste as a novel die-attachment solution. The silver paste was prepared by mixing nanoscale silver particles with carefully selected organic components which can burn out within the low-temperature firing range. SiC Schottky diodes were placed onto stencil-printed layers of the nanoscale silver paste on Au or Ag metallized direct bonded copper (DBC) substrates for the die-attachment. After heating up to 300degC and dwell for 40 min in air to burn out the organic components in the paste and to sinter the nanoscale silver, the paste consolidated into a strong and uniform die-attach bonding layer with purity >99% and density >80%. Then the die-attached SiC devices were cooled down to room temperature and their top terminals were wire-bonded to achieve the high-temperature power packages. Then the power packages were heated up from room temperature to 300degC for high-temperature operation and characterization. Results of the measurement demonstrate the low-temperature silver sintering as an effective die-attach method for high-temperature electronic packaging. An advanced packaging structure for future SiC transistors with several potential advantages was also proposed based on the low-temperature sintering technology.