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12 SMT007 MAGAZINE I MAY 2019 using Bi-absent solder paste for SMT assembly, depends on the: • Type of component • Configuration and dimensions of the component lead • Surface area of component leads embedded in the solder joint • Thickness of the coating • Resulting solder joint volume (including the solder paste volume) • Substrate surface metal (e.g., Cu vs. Ni) As an illustration, one study focused on 20-mil (0.5-mm) pitch QFP208 with Cu-lead or Alloy 42 lead-coated with SnPb or SnBi, using SAC305 or SnPb solder pastes reflowed at 245°C (for SAC305) or 220°C (for SnPb). Test results indicated that SnBi coating/SAC sol- der paste performed better than SnBi coating/ SnPb solder paste, which was better than SnPb coating/SnPb solder paste under accelerated temperature cycling (-40–125°C, 10 minutes dwell). However, with the same system except for the component lead material (replacing Cu leads by Alloy 42 leads), both SnBi coating/ SAC solder paste and SnPb coating/SnPb sol- der paste performed better than SnBi coating/ SnPb solder paste [1–3] . For BGA components, a Bi-containing sol- der ball is expected to contribute to the result- ing solder composition in a much significant proportion compared to leaded-components. Nonetheless, the resulting solder joint compo- sition will contain less Bi than in a BGA sol- der ball composition when a Bi-absent solder paste is used during SMT assembly. Bi contri- bution from BGA component to the composi- tion of the solder joint depends on the: • Diameter of BGA solder ball • Resulting solder joint volume (including the solder paste volume) • Substrate surface metal (e.g., Cu vs. Ni) Regarding surface finish, one study examined the effect of Bi-coated PCB pads on the solder joint integrity using SnPb eutectic solder paste [4] . The PCBs were deposited with 4–6 micro- inches of pure Bi and assembled using LCC and QFP components under surface mount processes. In comparing the Bi finish with the SnPb HASL finish, the fatigue data exhibited that two surface finishes essentially imparted similar thermal fatigue results in terms of the failure percentage at given temperature cycles. Visual inspection also revealed that the solder joints have the same general appearance after temperature cycling. Thus, Bi contribution from the PCB surface finish to the composition of the solder joint depends on the: • Thickness of the surface coating • Dimensions of pad • Resulting solder joint volume Bi is a unique metal that can offer multiple positive effects on solder joint performance (outlined in Part 2, Part 3, and Part 4 of this col- umn series). In Sn-based binary solder alloy, its two-phase phase diagram possesses multiple strengthening mechanisms (Figure 1). There are opportunities to maneuver the microstruc- ture through compositional tailoring and pro- cess condition variations. However, Bi is a brittle metal and has a finite solid solubility in an Sn matrix. The Bi pre- cipitation process is expected to be additive to other strengthening phenomena. There is a natural breakdown in the relationship between Figure 1: Schematic of Sn-Cu phase diagram.

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