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Microstrucuture and mechanical properties of pressureless sintered silver die-attach materials
Keywords: sintering, silver, reliability
Sintered silver materials are now the most promising die-attach material for power electronics operated at high temperatures over 200 C [1,2]. It is thought that mechanical properties of the joint between die and substrate are directly related to reliability of power devices. This work proved out why the nanoparticle-based sintered silver material showed a good reliability under high temperature operation although it was sintered without pressure. Wide gap semiconductors are expected to be operated at high temperatures higher than 200 C in power device because they have a great merit in high energy efficiency. The operation under high temperatures has required some challenges for die-attach material. Sintered silver requires sintering process after die-mount process instead of reflow for conventional solders. Sintering is usually carried out at 150 to 300 C where silver particles can be sintered or fused each other to solidify to a dense silver layer. Sintering process is essential to ensure good performance of sintered silver materials, while it is generally divided into two types, pressure-assist sintering and pressureless sintering. It is mandatory for micro-sized silver particles to be sintered under external applied pressures of 1030 MPa, while nano-sized silver particles can be well sintered without pressure due to the size effect. In case of pressureless sintering, the material properties of sintered silver directly affect the mechanical properties of sintered silver layer because sintering among silver particles is not assisted by applied pressure, but originates in only the nature of the material. Therefore, mechamical properties and reliability of pressureless sintered silver depends on the characteristics of sintered silver material. Watanabe et al. showed that a pressureless sintered silver joint had almost the same reliability as a pressure-assisted silver joint in a SBD package after 2,000 cycles of a thermal cycling test conducted at a temperature range between 40 and 200 C [3,4]. It means that pressureless sintered silver can have almost the same mechanical properties as pressure-assisted silver against heat stress. In this work, stress-strain properties of sintered silver materials were examined to study the reliability of pressureless sintered silver. Nano-based silver particles having various particle size distributions were synthesized. Sintered silver pastes were prepared by adequately mixing the nano-based silver particles with organic solvents. Typically, the silver pastes were printed on a bare Cu lead frame by stencil printing. Si die were mounted on the bare Cu lead frame, and then sintered in an electric oven. Typical sintering temperature was 275 C. For comparison, pressure-assisted sintering was carried out at 5 to 30 MPa. Thermal cycling test was carried out mainly at a temperature range between 40 and 200 C. High temperature exposure test was conducted at 250 C. Mecanical properties of sintered silver are examined by a tensile testing machine and a die shear tester. Microstructure of sintered silver layer was evaluated by scanning electron microscopy. Void and delamination are examined by scanning acoustic microscopy. Mechanical properties of sintered silver depend on particle size. The smaller size of silver particles, the higher tensile strength was given. The higest tensile strength at break point was 140 MPa in this work. The value surpassed many pressure-assisted sintered silver materials in previous literature [5]. It means that pressureless sintering has enough mechanical properties for a practical application. We are now examining to find out the reason why the nano-based silver shows a good performance from the point of view of microstructure of sintered silver. Die shear test showed that broken faces of the sintered silver gave a so-called edimplef structure. It means that the sintered silver is not broken just at the fused interface among silver particles, but it is well fused to elongate to some degree before getting to the break point. Therefore, it proved out that pressureless sintering made silver particles sufficiently sintered. Further experiments of the sintered silvers are now underway. This work shows that pressureless sintered silver has a good mechanical performance. It can be considered that the good performance of the sintered silver is derived from the well fused structure among ilver nanoparticles.
Masafumi Takesue,
Bando Chemical Industries
Kobe, Hyogo

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