Ultrasound and faster cooling could make lead-free solders more reliable

New manufacturing techniques could overcome reliability problems associated with lead-free solders, a university researcher has suggested. Ultrasound, rapid cooling and new alloys are all possible routes, but none will work on its own, according to the University of Leicester engineer.

"Traditional lead-based alloys have a 50-year history and there has been extensive investigation of their micro-structural stability and reliability," said Sergey Belyakov, a Russian postgraduate working in the materials research group of Leicester's Department of Engineering, who presented his research this week.

He added: "New solder materials have been proposed to replace the traditional alloys, but there may be a deterioration in the reliability of solder joints. The objective of my research is to bridge the technical gaps and meet the challenges of lead-free solder application in the electronics industry through the fundamental understanding of lead-free assembly and reliability issues."

Belyakov said that the biggest problems he had uncovered in lead-free solders - typically tin-silver-copper alloys - were intermetallic phase formation, where areas of different alloy composition form within the solder, and crystallographically-faceted voids, which manifest as tiny tetragonal holes.

He said that all three of the factors he investigated - cooling rates, alloy composition and ultrasound - had their pluses and minuses. For instance, faster cooling improved the microstructural integrity of solder joints, but would add considerable stress to the item being soldered because all its system components have different coefficients of thermal expansion.

As for ultrasound, he said this has several positive effects.

"First, high-intensity ultrasound of 30kHz promotes fine and equiaxed structure formation, which results in solder microstructure anisotropy increase and zonal segregation inhibition. Second, ultrasound introduction into a crystallising lead-free alloy produces microstructure with a lower amount of eutectic phase, which is a desirable effect in terms of solder thermal-fatigue life, and promotes more uniform distribution of intermetallic particles within bulk solder.

"And third, ultrasonic vibration affects the amount of undercooling of solidifying lead-free alloys. It has been noted that this effect is a function of alloy composition. Small additions of minor elements, such as antimony, in combination with ultrasound can lower the amount of undercooling drastically.

"However, part of the positive effect vanishes when the system solder-substrate is considered, and moreover, ultrasound even deteriorates solder joint mechanical properties. It has also been observed that ultrasound intensifies the diffusion processes at the liquid alloy-substrate interface, causing more intensive substrate material (copper) dissolution and consequently, resulting in more copious intermetallic phase formation."

He added: "Adjusting the alloy composition seems to be more effective in terms of lead-free solder joints quality improvement, but again, there are many variables are involved."

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