Estimated reading time: 5 minutes

The Role of Bismuth (Bi) in Electronics, Part 4

To link science and technology with commercial applications, Part 4 of this series continues to address two pivotal questions: Why SAC is not able to be a universal interconnecting material for electronic circuits, and why a quaternary alloy system offers a more wholesome approach (note: a quaternary system referred herein does not include SAC incorporated with one or more doping elements).

The baseline for designing a viable solder joint material for applications in advanced electronic circuit boards that require increasingly higher functionalities and higher power in a small form factor is to deliver reliable physical properties and mechanical performance that are not lower than 63Sn37Pb alloy.

Another baseline characteristic is to provide an alloy that can adapt to the established electronics manufacturing infrastructure including production flow, process parameters and compatibility with the thermal stability of a PCB and a variety of components. One of the critical process parameters is the required minimum peak temperature in the reflow process that enables mass production capable of delivering high throughput without introducing production defects and/or insidious thermal damage to the internal structure of a PCB and components. To this end, the alloy shall possess "agile" wetting ability and a compatible melting (liquidus) temperature. In this regard, wetting ability controls not only interfacial metallurgical interaction but also the rate of interaction that needs to be in sync with the inherent characteristics of the reflow process.

With these baselines in mind, a viable alloy should possess an adequate thermal fatigue resistance to withstand the increasingly adverse and harsh conditions in microelectronics and electronic applications while providing a moderate melting temperature (170°C–215°C, more desirably 175°C–213°C) suitable for sound manufacturability without causing undue thermal damages.

This was the genesis of designing quaternary alloys, as stated in many of my professional development courses, workshops, webinars and publications since the late 1980s, including the U.S. Patent 6,176,947 (1999).

The ability to deliver a higher performance level in thermal fatigue resistance is particularly critical to connecting those high-power, large-size IC components onto the circuit board, as these components impose a larger amount of thermal stress on solder joints during power-on/power-off and/or elevated temperature excursion. In mobile electronics, these thermal excursion-related stresses may be compounded with machinal-shock related stresses that could be occurring during the product’s useful life.

SnAgCuBi is one of the quaternary systems studied. Again, it is important to accentuate that the scientific base to design the SnAg-CuBi system was not to add an element (in this case, Bi) to an SAC alloy. Rather, it was a material innovation holistically using the underlying science and engineering of metallurgical principles by taking the commonly-occurring solder joint failure mechanisms into consideration. In other words, the objective was to mitigate those likely failure mechanisms so that solder joints can reliably connect the ever-powerful semiconductor chips to the outside world by serving as electrical, thermal and physical conduits at chip level, package level and on circuit boards.

How do the proper compositions of the SnAgCuBi alloy system (containing 2.5–3.5% Ag, 0.2–2.5% Cu, 0.5–4.0% Bi, balance Sn) perform in comparison with the standard alloys? (Note: All compositions expressed herein are in weight percent.)

Comparison with SnPb Eutectic—63Sn37Pb

As an example, take Sn3.0Ag0.5Cu2.0Bi as the composition. It offers higher strength as well as more than 200% higher fatigue life than 63Sn37Pb in accordance to ASTM Standard E606-92 (Standard Practice for Strain-Controlled Fatigue Testing).

Comparison with SnAg Eutectic—96.5Sn3.5Ag

The composition of Sn3.0Ag0.5Cu3.0Bi has a melting temperature 209-212°C that is 9°C lower than the eutectic 96.5Sn3.5Ag (221°C). When comparing the basic mechanical properties with 96.5Sn3.5Ag, Sn3.0Ag0.5Cu3.0Bi composition performs better in strength and fatigue life—more than 150% higher in fatigue life.

Comparison with SnCu Eutectic—99.3Sn0.7Cu

Sn3.0Ag0.5Cu3.0Bi demonstrates significantly better performance in strength and fatigue, but lower plasticity than 99.3Sn0.7Cu. Its melting temperature is 15°C lower than 99.3Sn0.7Cu.

Comparison with SnAgCu Near-eutectic—Sn3.0Ag0.5Cu (SAC305)

Sn3.0Ag0.5Cu2.0Bi exhibits high strength (both yield and tensile strengths and higher thermal fatigue life). Another important advantage of SnAgCuBi over SnAgCu is the ability to offer lower liquidus temperature. The composition of Sn3.0Ag0.5Cu2.0Bi offers 7°C lower than SAC305. Further, the intrinsic wetting ability of SAC system does not measure up to that of SnPb or SnCu. With SAC305’s high liqiudus temperature, the tendency to use a process peak temperature below the optimal temperature often leads to a marginal process, which further aggravates the SAC305’s lower wetting ability, thus increasing potential production defects.

Focusing on the integrity of a printed circuit board assembly, the liquidus temperature of the interconnecting solder alloy plays an important role in alleviating any potential defects or thermal damages to components or PCB, which can be detectable or undetectable on the production floor or during quality control verification. Concentrating on solder joint reliability, the thermal fatigue resistance sits front and center to the performance and reliability of a circuit board.

Overall, the SnAgCuBi system offers more robust performance than any of practical binary alloys, such as 63Sn37Pb, 96.5Sn3.5Ag, or 99.3Sn0.7Cu, and ternary alloys, such as SnAgBi and SnAgCu. In comparison with SAC305, Sn3.0Ag0.5Cu2.0Bi exhibits higher strength (both yield and tensile strengths). More importantly, its thermal fatigue life is higher under harsh conditions (e.g., a large temperature swing, high-temperature excursion). Further important advantages of SnAg-CuBi over SnAgCu are its lower melting temperature and superior intrinsic wetting ability. The practical compositions can offer as much as 9°C lower in melting temperature than SAC305.

Melting at a few degrees lower, SnAgCuBi compositions are advantageously positioned for circuit board manufacturing. Embracing diverse PCB assemblies and process window requirements to achieve high yield, low-production defect rates, an alloy having a melting temperature below 215°C is considered necessary to deliver robust manufacturability.

Part 5 in this series will outline the underlying operating mechanism among the four elements (Sn, Ag, Cu, Bi) of SnAgCuBi system and elemental dosages in relation to desirable performance properties.