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26 SMT007 MAGAZINE I MAY 2020 thermal cycling performance. This new LTS alloy combines an optimal level of Bi and the right combination of alloying additions to improve its thermal and mechanical reliability, as shown herein. LTS Paste and Assembly The selected alloys were processed into IPC type 4 powders and mixed into a paste using an appropriate paste flux for further solder joint thermal and mechanical reliability evaluation. The reflow profile used for assembling the test vehicles with HRL1 solder paste is shown in Figure 1. Soaking was done at 100–120°C for 60–90 seconds. The time above liquidus (TAL) was 35–40 seconds, while the peak reflow tem- perature was 185–190°C. All the BGAs evalu- ated had SAC305 balls. Bulk Alloy Properties A combination of solid solution strengthen- ing and precipitation/dispersion hardening can be used to improve the mechanical strength of metallic Sn. Elements such as Bi, In, and Sb have relatively high solubility in Sn, forming a solid solution, whereas other elements such as Ag and Cu have little solubility in Sn-Bi and, among other elements, can be used in minor additions for their strengthening. Bulk alloy properties can provide insightful information about solder joint mechanical and thermal fatigue resistance, going beyond the microstructure observations. Table 1 shows some key physical properties of eutectic 42Sn58Bi, HRL1, and SAC305 alloys. Solidus and liquidus temperatures of high purity 42Sn58Bi alloys are identi- cal (i.e., eutectic) and about 138°C. As per the Sn-Bi phase diagram, reducing the Bi con- tent below the 58 wt.% corre- sponding to the eutectic point, results in higher liquidus tem- perature, depending on micro- alloying additions. In the case of alloy HRL1, the solidus and liquidus temperatures are 138°C and 151°C, respectively. Next-generation LTS Alloys It is important to note that just reducing the melting point of an alloy is not enough for delivering high-reliability solutions for such technical challenges. For example, the eutec- tic 42Sn58Bi alloy would be a logical choice, given its melting point at 138°C, but its lower ductility and poor thermal fatigue life do not come close to the currently used SAC305 alloy. Its Bi-rich phase causes embrittlement, mak- ing eutectic 42Sn58Bi prone to brittle frac- ture under high strain rate conditions [4] . Mate- rials suppliers and industry consortia, such as iNEMI, have been extensively working on developing and testing new low-temperature alloys to fulfill such requirements [5–11] . The addition of silver has been one of the most common ways to modify the microstruc- ture and resulting properties of the eutec- tic SnBi alloy. MacDermid Alpha Electronics Solutions's extensive research on solder alloys went beyond that and worked on developing a comprehensive family of low-temperature solders with improved thermal and mechani- cal reliability. SBX02 solder (an Ag-free eutec- tic SnBi with micro-additives) has been shown to have significantly higher mechanical shock and thermal cycling properties than commonly known 42Sn58Bi and 42Sn57.6Bi0.4Ag alloys [8] . More recently, HRL1 (a non-eutectic SnBi solder with about 2 wt.% performance addi- tives) has shown superior drop shock and Figure 1: Reflow profile for the HRL1 low-temperature solder.

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