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Considerations for Sintering in High Temperature Electronics
Keywords: Sintering , High Temperature Electronics, interconnect materials
High temperature electronics are used in a wide range of applications especially in extreme environments. There is a clear trend in aircrafts to have electrical controls mounted closer to the engine [1]. In cars more and more mechanical and hydraulic systems are replaced by electromechanical or mechatronic systems [2]. They are getting closer to high temperature environments like the engine or brakes. To its nature, avionic and automotive applications require predictable, highly reliable systems. Because elevated temperatures will increase the speed of material aging, the combination of high operation temperatures and high reliability is quite challenging. This applies in particular to interconnect materials such as solders or bonding wires. In order to fulfil both needs new temperature robust interconnect technologies are required. There are different materials and related processes available like transient liquid phase soldering, high temperature solders (e.g. high lead or zinc), conductive adhesives etc. [3]. This presentation will be focused on pressure assisted silver sintering processes and the use of low temperature sintering will be discussed. Epoxy filled solutions are not part of this presentation. The sinter silver (Ag) interconnect consists of 100% Ag. The melting temperature of Ag is 962°C. In addition, bulk Ag provides excellent electrical (62 x 106 S/m [4]) and thermal conductivity (429 W/mK [5]). The in-package values will be lower, but still very high compared to conventional solder [6]. Because of the high melting point and its thermo-mechanical properties, Ag sinter joints providing long life time even at high temperatures [7]. Pressure assisted sintering is mainly separated in following steps [6]: 1. Paste application on substrate (by printing) 2. Pre-drying of sinter paste to evaporate solvents 3. Die attach 4. Sintering Pressures of 5-20 MPa and temperatures of 200-300°C are commonly used. Pressure assisted sintering is working in air-atmosphere [6] and protective atmospheres. Applying pressure to thin dies (≤ 100μm) is requesting homogeneous pressure distribution to the die. This is crucial in order to avoid damages to the semiconductor. The behaving of pressure tools is almost as a liquid to create hydrostatic pressure during the sintering process. They have already shown their suitability for this process successfully [8]. To maximize UPH it is required to sinter several substrates at the same time. More force has to be applied if the sintered area will increase. To guarantee stable and reliable processes it is further requested to control, adjust and monitor the applied force carefully. As sintering in air requires noble metals as contact surfaces, Au or Ag finishes are state of the art finish materials on substrates and dies. Particular on substrates Au or Ag are not standard for ceramic based substrates. Their use will create additional costs. Sintering on bare Cu substrates is more economic. However, oxidation of the Cu during sintering has to be prevented, as Cu oxides would inhibit sintering. Furthermore, oxide free surfaces are required to ensure a successful and reliable wire bonding process as a next step. Sintering in protective atmospheres as N2 is at least required to minimize oxidation during sintering. Company PINK GmbH Thermosysteme is offering a System called SIN200, which ensures a robust and controlled pressure assisted sintering process. The soft tool used for sintering is nearly product layout independent. It shows kind of a liquid behavior during the process, providing hydrostatic pressure on the product. The substrate temperature, the atmospheric pressure and gas composition during the sintering process within its vacuum chamber is adjustable. Besides of prevention of oxidation during sintering it enables the in-situ reduction of the products surfaces before and after the sintering process if needed. The system provides a force of 200 tons resulting in a maximum pressure of > 30 MPa at the given tool size. In combination with the adaptability of the pressing force and record of process parameters the system provides sufficient process control to ensure stable production of high temperature electronics.
Thomas Krebs,
PINK GmbH Thermosysteme
Wertheim, D
Germany


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