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The Role of Bismuth (Bi) in Electronics, Part 6

In this installment of my column series on the role of bismuth (Bi) in electronic products, I’ll look at the effects of Bi on the properties and performance of solder interconnections in electronic products when Bi is not contained in the solder alloy for the SMT assembly process (Bi-absent solder alloy composition of solder paste). The effects of Bi in solder joints are created by an unintentional path through the supply chain, which introduces Bi into the resulting solder alloy of solder joints. The performance and reliability of the resulting solder joint impacted by the presence of Bi can vary; it can be beneficial or detrimental or no-detectable-effect.

From the supply chain in electronics assembly, Bi can come from component lead coating, passive component termination coating, and PCB surface finishing that are Bi-containing material (albeit, this has not been a common PCB surface finish in recent years). Accordingly, even in a Bi-absent assembly process (e.g., using SAC solder paste or SnPb solder paste), the changes in the properties and performance of solder joints due to the introduction of Bi into the solder joint could occur. Similarly, for BGA components, the Bi-containing solder ball will make Bi-containing solder joint.

In SMT assembly after reflow, the composition of solder joints is expected to deviate from the composition of the solder alloy used in solder paste. The compositional change in solder joints as a result of Bi contributions from one or both of component and PCB surface finish should not be dismissed. Bi contribution from component leads (component surface coating) to the composition of the solder joint, while 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 solder 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 solder paste performed better than SnBi coating/ SnPb solder paste^[1–3].

For BGA components, a Bi-containing solder ball is expected to contribute to the resulting solder composition in a much significant proportion compared to leaded-components. Nonetheless, the resulting solder joint composition will contain less Bi than in a BGA solder ball composition when a Bi-absent solder paste is used during SMT assembly. Bi contribution from BGA component to the composition 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 microinches 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 column series). In Sn-based binary solder alloy, its two-phase phase diagram possesses multiple strengthening mechanisms (Figure 1). There are opportunities to maneuver the microstructure through compositional tailoring and process condition variations.

Figure 1: Schematic of Sn-Cu phase diagram.

However, Bi is a brittle metal and has a finite solid solubility in an Sn matrix. The Bi precipitation process is expected to be additive to other strengthening phenomena. There is a natural breakdown in the relationship between yield strength and Bi volume fraction as a result of the transition of the strengthening mechanism. Bi must be used properly to eschew any likely adverse effects in the reliability of solder joint, which may lead to likely product failure.

To utilize the benefits that Bi can offer in forming electronic solder interconnections, a specified dosage in a specific alloy composition system is required; the knowledge of its intricate interplay with other constituents in an alloy composition is indispensable. An understanding of the design-for-performance demands as well as application constraints is also a prerequisite.

Overall, the concentration of Bi in solder joint has to be carefully designed. Unfortunately, many studies and testing programs with an intent to compare Bi-containing alloys with a Bi-absent solder alloy have often selected a Bi dosage apart from what is required or desired. This lack of proper composition (a proper Bi dosage in a specific system) has contributed to highly publicized negative test results. Thus, this has impeded the application of Bi in Sn-based solder alloy systems during the first decade of deployment of lead-free solder interconnecting materials for producing electronic products.

References

1. H-Technologies Group Inc. “Internal Reports,” 1990–1999.

2. Hwang, J. “Lead-free Implementation Series, Part 6—The Role of Bismuth,” IPC Professional Development Courses, 1999–2010.

3. Hwang, J. Environment-friendly Electronics—Lead-free Technology, “Chapter 28: The Role of Bismuth,” Electrochemical Publications Ltd., Great Britain, 2001.

4. Hwang, J. Environment-friendly Electronics—Lead-free Technology, Electrochemical Publications Ltd., Great Britain, 2001, p. 758.

Dr. Jennie S. Hwang is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions.