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March 2014 • SMT Magazine 63 Soft Solder alloy The table in Figure 2 shows a list of soft sol- der alloys that are already in use for electron- ics. All alloys are based on tin (Sn) and thus not low-temperature alloys like SnBi. SAC alloys have been in use for standard ap- plications on organic PCB substrates in electron- ics for years. The eutectic SnAg3.5 are mostly used for thick film applications (TF), as well as power electronics based on direct copper bond- ing (DCB) for temperatures up to 125°C, and in some applications even higher. The applica- tion field for Innolot solder paste on PCBs is up to 150°C, and can withstand a higher number of cycles (1–3) on organic PCB substrates. But Innolot will not work for power or TF appli- cations; the HT1 alloy has been developed for such applications. metallurgical aspect for hT1 Based on an investigation for BGA balls, an alloy has been developed for increasing the re- sistance to thermal cycling with higher temper- ature differences. Due to the fact that BGA balls require a high quantity of Ag inside, it wasn't possible to realize a powder production for soft soldering alloys, because Ag3Sn intermetallic had been created during the process (Figure 2 and Figure 3a-3e). The first modification for changing the Ag- 3Sn structure was developed by using a crystal modification. Figures 4a and 4b compare the structure with and without that modification (KM). One major advantage is that the structure of the critical Ag3Sn intermetallic has changed to a 3D structure (star structure). One of the causes of this phenomenon is the critical nucleus ra- dius due to the fact that Ag3Sn and NdSn 2 have nearly the same radius 5 . Nevertheless, anoth- er factor is the content of silver. Different Ag contents of 2%, 2.5%, 3% and 3.75% have been invested. Different alloys were soldered onto feaTure aSSEmBLY maTErIaLS FOr hIGh-TEmPEraTUrE aPPLIcaTIONS continues figure 2: cross-section after powder production (source: Müller, nMb 160804). figure 3a-3e: Mapping of the powder. figure 3b is ag3Sn (source: Müller, nMb 160804).