The headlines coming out of today’s automotive world tell a story of progress. The momentum is increasing for vehicle electrification, and benchmarks for higher levels of autonomous driving are starting to appear on the road ahead. Yet just beyond the headlines are the many smaller stories of challenges faced by engineers developing next-generation automotive technologies — not just for more obvious elements like sensors and semiconductors, but also for under-the-radar components. One example: the solder used to enable critical interconnections between the electronic and the electromechanical.

Figure 1. Essential to the journey into tomorrow’s automotive world are sophisticated design methods, materials and tools that result in reliable and cost-effective packaging. Source: Tayeb MEZAHDIA/CCO via Pixabay

Thanks to the expanded use of electronics in the automotive sector, there’s a need for new solder alloys able to withstand higher operating temperatures and meet extended life requirements. The combination of harsh operating conditions, increased power densities and miniaturization has added to the complexity of assembly designs — making traditional surface mount solders unsuitable.

MacDermid Alpha Electronics Solutions, a provider of specialty chemicals and materials for electronics manufacturing, recently reported on its work developing and testing a next-generation high-reliability solder alloy for automotive electronics. The alloy, referred to here as Alloy 10, makes use of complex metallurgy techniques including solid-solution strengthening, precipitation strengthening, grain refinement and diffusion modifiers. MacDermid Alpha found that using micro-additives can enhance performance over traditional tin-silver-copper (Sn-Ag-Cu) alloys. Those micro-additives contribute to intermetallic compound (IMC) formation and strength retention at high operational temperatures.

Developing a new alloy

Changes in alloy composition need to be carefully designed to avoid drastically reducing solder joint thermal reliability. Putting its focus on retaining strength at high operational temperatures, the developers at MacDermid Alpha worked with proportions of bismuth (Bi), indium (In) and antimony (Sb) in the course of developing their novel alloy. Each of these elements possesses high solubility in a traditional tin matrix, with variations by degrees dependent on temperature. The developers also considered how silver and copper possess a limited tin-matrix solubility, leading to the formation of intermetallic compounds (Ag₃Sn and Cu₆Sn₅). Uniform distribution of these compounds suppresses plastic deformation and propagation of cracks formed during thermal cycling. Other micro-additives (up to a maximum of 0.5 wt.%) can also form fine distribution of intermetallic compounds to enhance alloy strength.

Armed with these tools and extensive materials knowledge, the MacDermid Alpha developers arrived at their new alloy (Alloy 10) — a Sn-Ag-Cu based alloy with additions of Bi and Sb. The Bi addition is between 20% and 50% of the added antimony; other additives make up less than 0.5 wt.% of the alloy composition.

Testing the alloy

Solder alloy suitability and selection for electronics assembly is primarily defined by thermo-mechanical reliability, which can be evaluated using various tests. The developers of the new alloy tested selected board components for thermal fatigue, thermal aging and mechanical fatigue. A traditional high-reliability alloy (Sn-Ag-Bi-Sb-Cu-Ni) was selected as a reference for comparison testing of the new alloy, and benchmarks for failure were established based on IPC and JEDEC standards.

Table 1 illustrates physical and mechanical properties of the traditional alloy as compared to those of the new alloy.

Table 1. Physical and mechanical properties of the traditional high-reliability alloy are compared to those of the new alloy developed by MacDermid Alpha. Data source: Proceedings of SMTA International

Thermal fatigue

The thermal fatigue test was performed in an air-to-air thermal cycling chamber using two harsh testing profiles with large temperature transitions. During testing, each component’s electrical resistance was measured with a data logger.

Melting behavior is an important consideration for high-reliability solder selection. Compared to the traditional alloy, the new alloy’s higher Sb content conferred a higher liquidus temperature; the MacDermid Alpha developers were able to counterbalance this with other alloy components. At the same time, the lower modulus of the new alloy made it less stiff — a characteristic likely to help improve solder joint thermal fatigue resistance.

Figure 2. Thermal fatigue testing shows variation of tensile strength and yield strength with temperature. Retention of high strength at higher operating temperatures is expected to translate into higher thermo-mechanical fatigue resistance. Source: Proceedings of SMTA International

The variation of tensile strength and yield strength with temperature can be seen in Figure 2, where a maximum increase of 36% tensile strength was achieved in the new alloy at 125° C. The new alloy also showed at least 77% higher yield strength at elevated temperatures, enabling it to better resist plastic deformation. The retention of high strength at higher operating temperatures is expected to translate into higher thermo-mechanical fatigue resistance.

Compared to the traditional alloy, the new alloy also showed an increase in creep strength greater than 150%. This is an important defense against creep fatigue resulting from cyclic stresses — eventually, if the solder alloy is unable to accommodate creep damage, the solder joints will deform or fracture.

Testing on one of the board’s ball grid array components yielded sufficient data to plot Weibull curve failure patterns. Compared to the traditional alloy, the new alloy showed a characteristic life improvement ranging from 50% to nearly 60% depending upon the testing profile used. The plots also revealed a much slower failure rate for the new alloy. For the traditional alloy, the maximum number of failures occurred earlier in the process.

Figure 3. Solder joint cross-sections taken at various intervals in the thermal cycling process, illustrate better joint quality in the new alloy. Data source: Proceedings of SMTA International

An examination of solder joint cross-sections (see Figure 3) for the complete outer row of ball grid arrays at various intervals showed better joint quality in the new alloy, underscoring the developers’ working theory that high temperature tensile and creep properties can be used as an indicator of an alloy’s long-term thermo-mechanical reliability.

Thermal aging

Thermal aging testing was performed at 150° C for a total duration of 2,000 hours, and measurements of intermetallic compound thickness were made at two different stages. The developers observed that the micro-additives in the new alloy helped to restrict IMC growth to less than 6 µm. This restricted growth helps to achieve higher thermo-mechanical reliability, as required by the automotive industry.

Mechanical fatigue

Drop shock testing was performed until all on-board components failed, and the electrical continuity of each component was monitored with a high-speed event detector. As in the thermal fatigue test, the higher yield strength of the new alloy (see Figure 3) again played a role in yielding superior performance — in this case, a characteristic life improvement of 30%. Because plastic deformation sets in much later for Alloy 10, recovery of the solder joint occurs for a larger number of drops.

Next-generation solder

The MacDermid Alpha developers report that performance results for their novel high-reliability solder alloy are very encouraging, demonstrating its potential and suitability for surface mount applications with challenging reliability requirements. These characteristics certainly apply to automotive electronics, especially as the industry turns the corner into the next generation of progress and innovation.

New solder alloys will be a key component of a changing automotive landscape that includes expanding vehicle electrification and increasing reliance on sensors. These alloys call for improved cycles to failure and operation under increasing harsh environments — including higher operating temperatures, cycling through large temperature ranges, and a combination of vibration and shock loading. The need for efficient power dissipation of complex semiconductor packages in both hybrid and plug-in vehicles will further push the demand for high-temperature electronics.

Essential to the journey into tomorrow’s automotive world are sophisticated design methods, materials and tools that result in reliable and cost-effective packaging. Each of these elements is evidenced in MacDermid Alpha’s approach to developing and testing a next-generation, high-reliability solder for enabling thermo-mechanical performance in automotive applications.

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