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March 2014 • SMT Magazine 67 section after N=3000 doesn't show that effect (Figure 9). No delamination showed in the HT1 alloy after all cycles. Figure 10 shows the results after 4,000 cycles. As a summary of this investigation, for stan- dard applications SnAg3.5 is used, but when an improvement is needed for higher tempera- tures, the HT1 is the selected alloy. Sintering Soft solder alloys with or without additives does have a limit of electrical and thermal con- ductivity. Devices which need more of those physical properties or operate at higher tem- peratures (200°C), alternatives for the assembly technology are needed, including materials. One of these possibilities is sinter technology. A comparison between soft solder alloy and sinter technology is described in Figure 13. On one hand, silver has a higher melting temperature (961°C) than Sn-based soft solder al- loys; there are no aging effects when operating at higher temperatures (e.g., 200°C). On the other hand, the thermal conductivity is much higher than soft solder alloys. That guarantees a higher level of thermal management, and junction tem- perature is transformed and more effective. State-of-the-art is sinter materials with nano silver particles inside (NTV) 6, 7 . This technology needs operation pressure by >30MPa for die placing. The challenge for this technology is re- duction of pressure and process temperature 8 . By using nano silver 9 it is possible to come to technology without any pressure 10 . The limit- ing factor is thickness of silver layers up to 10 µm. With more layers, the shrinking makes an inhomogeneous structure that limits the ad- feaTure aSSEmBLY maTErIaLS FOr hIGh-TEmPEraTUrE aPPLIcaTIONS continues Table 2: Table showing the most important properties. figure 10: cross-section hT1 after n=4000 (source: Faunhofer enaS).