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The Role of Bismuth (Bi) in Electronics, Part 5

Editor’s Note: This is the fifth article in a series. Click here to read: Part 1, Part 2, Part 3, and Part 4.

Part 5 of the series will address the most interesting, yet intricate, aspect of the subject—plausible underlying operating mechanisms among the four elements (Sn, Ag, Cu, Bi) in a SnAgCuBi system. I will provide some illustrations on relative elemental dosages in relation to relevant properties and performance. In an Sn-based system, the starting point is to consider the metallurgical interactions of three elements—Ag, Cu, and Bi—with Sn as the matrix.

How do these three elements affect physical and mechanical properties individually and collectively as well as the importance of the specific dosage of each element in the system to the resulting properties and performance of a specific alloy composition?

First, to be in line with SnPb eutectic reflow peak temperature (SnPb eutectic continues to be a viable reference point), the target is to make the melting temperature as close to that of SnPb eutectic as feasible since the required reflow process peak temperature is directly determined by the alloy’s melting temperature.

And the reflow temperature is crucial to the integrity of the circuit boards produced, primarily to avoid encountering the risks of any detectable or undetectable thermal damages while not to marginalize the required process window.

Accordingly, a challenging goal has been to lower the melting temperature of the lead-free Sn-based alloy without entering into the territory of low-temperature alloys (e.g., melting temperature below 175°C). Thus, research, development, and manufacturability efforts have been so directed in our studies. Also, in this writing, for practicality, the liquidus temperature of a non-eutectic alloy composition is expressed as melting temperature, and all dosage percentages are expressed in weight percent.

Melting Temperature

Figure 1: Melting temperature vs. Cu dosage wt% [2].

In an SnAgCuBi system, all three elements (Ag, Cu, Bi) affect the melting temperature of the resulting solder alloy. More practically, they can lower the melting temperature of the Sn matrix when their respective dosages in the system are properly constituted. With an objective to lower the required reflow temperature, the identification of the optimal dosage of each element in this quaternary system to lower the melting temperature of the resulting alloy while maintaining the desired level of physical and mechanical properties is an intricate endeavor as well as most scientifically appealing.

Within practical dosage ranges, the following is a capsule view of experimental findings [1] in the relationship between the melting temperature and respective dosages (all dosages are expressed in weight percent; percentage bears approximately ±10% variations):

• The melting temperature dropped with the addition of Cu and reached a minimum at 0.5%. Beyond 0.5% Cu, the melting temperature remained almost constant with further increase of Cu up to 5.0% (Figure 1).

Figure 2: Melting temperature vs. Ag dosage wt% [2].

• Similarly, the melting temperature decreased with increasing Ag and reached a minimum of about 3.0%. When Ag content increased from 3% to 4.7%, any further reduction in the alloy melting temperatures is negligible. However, when the Cu content is in the range of 0.5–3.0 % and the Ag content is less than 3%, the liquidus temperature of the melting range increases notably with the decreasing Ag content (Figure 2).
• Bi in this alloy system plays a major role in further reducing the alloy melting temperature. The alloy’s melting temperature near-linearly decreased with increasing dosage of Bi, but increasing the dosage of Bi to lower melting temperature is not a panacea, which will be highlighted later (Figure 3).

Experimental results are consistent with the indication of binary or ternary phase diagrams where available.

Figure 3: Melting temperature vs. Bi dosage wt% [2].

Metallurgical Science vs. Properties

Metallurgically, Ag forms a second phase—primarily e (Ag3Sn) in the Sn matrix. The morphology of Ag3Sn particles, intermetallic in nature, can range from nodular to long-needle shapes; Cu interacts with Sn to form intermetallic compound h (Cu6Sn5) in essentially nodular shapes. The element Bi works differently in the Sn matrix. The solid solubility of Bi solute in Sn solvent can reach about 21 wt% at the eutectic point (138°C). However, the solid solubility of Bi solute in Sn solvent can be dramatically reduced with temperature, which is about 1.0 wt% at room or ambient temperature. Additionally, the three elements (Ag, Cu, Bi) are expected to interact mutually in a thermodynamically competitive manner.

In mechanical behavior, the yield strength of the SnAgCuBi quaternary system generally follows an approximation of the linear rule of mixture in the volume fraction of the second phases (Ag3Sn), the volume fraction of intermetallic compound (Cu6Sn5) and the volume fraction of Bi precipitates in the Sn matrix. In accordance with Mott and Nabarro’s strain field theory, the strengthening effect of Ag3Sn particles is interpreted as the result of the long-range internal stress built by the elastic modulus and volume differences between the second phase and the Sn matrix. The mobile dislocations in the soft Sn matrix can largely pass by the Cu6Sn5 particles in a largely free manner due to the relatively larger interparticle spacing. The strengthening mechanism of the elastic Cu6Sn5 particles is attributed to the building of an elastic internal stress field in the Sn matrix, giving a backstress for dislocation movement.

In fatigue life, like other alloy systems, the underlying operating mechanism is more complex, engaging in multiple events and varying with strain amplitudes. At the relatively large strain amplitudes, fatigue crack propagation is a dominating event throughout the fatigue lifetime. At the small strain amplitudes, fatigue crack initiation is a dominating event throughout the fatigue lifetime.

In terms of the mechanisms identified, at the large strain amplitudes, Ag3Sn particles are a much more effective block for the fatigue crack propagation than the Pb-rich second phase in 63Sn37Pb. The Cu6Sn5 particles in the Sn matrix are expected not to be fatigue-fractured at the fatigue conditions. Like the Ag3Sn particles, the Cu6Sn5 particles serve as effective barricades for fatigue crack propagation. The formation of Cu6Sn5 particles also can partition Sn grains, resulting in finer grains, which contributes to the extension of fatigue lifetime by enhancing grain boundary gliding mechanisms.

At the small strain amplitudes, fatigue crack initiation is a dominating event throughout the fatigue lifetime. Since the cyclic deformation in the process of fatigue crack initiation almost entirely takes place in the Sn matrix, the Ag3Sn particles and Cu6Sn5 particles are expected to play little role in retarding the cyclic deformation damage or fatigue crack initiation. With the presence of Ag and Cu in the Sn matrix, the overall fatigue life is controlled by the fatigue fracture capacity of either the Sn matrix or interphase bonding. In this regard, Bi, on the other hand, offers additive-enhancing events in the Sn matrix, which compensates the deficiencies of Ag and Cu. However, its dosage is an overriding factor.

As mentioned earlier, alloy’s melting temperature near-linearly decreased with an increasing dosage of Bi. Does this imply that to achieve a lower melting temperature and thus the process temperature, we can build a high dosage of Bi into the SnAgCuBi? The answer is a resounding no.

The attainment of a delicate and tricky balance among the dosages of all three elements is the key to delivering the desired performance; simply put, this means the balanced strength and fatigue resistance as well as the desirable manufacturability. In electronics, to serve as sound solder joints that connect semiconductor packages to the outside world, it is safe to say that the mechanical strength can readily be obtained (within the scope of the electronic circuit board). The ability of resistance to thermal fatigue is the top priority in the performance of solder material to produce reliable products (barring other extraneous failure modes).

All in all, when Ag, Cu, and Bi can “comfortably” reconcile together within the Sn matrix, and when each element is at its optimal dosage, the system delivers wonders.

References

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

2. Jennie S. Hwang, Environment-Friendly Electronics: Lead-Free Technology (Chapter 10), Electrochemical Publications LTD, Great Britain, 2001.