Detrimental voids discovered in Pb-free solder joints - University of Leicester

Detrimental voids have been discovered in Pb-free solder joints, according to a University of Leicester researcher.

The researcher, post-graduate Sergey Belyakov, is taking a deep look at Pb-free soldering for the University's materials research group.

"I have discovered a process which has not been described yet: the formation of crystallographically-faceted voids in the bulk of lead-free solders," Belyakov told Electronics Weekly.

The results are new and Belyakov only has preliminary results.

"They are constantly observed. This is very undesirable effect which needs to be terminated," he said. "At the moment I am trying to investigate the nature of this phenomenon to propose possible ways of avoiding it."

Traditional lead-based alloys have a long history and there has been extensive investigation of their micro-structural stability and reliability.

"New solder materials have been proposed to replace the traditional alloys but there may be a deterioration in the reliability of solder-joints," said Belyakov. "The objective of the 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."

Like other research, the Leicester programme has not found a clear reliability winner between Pb-inclusive and Pb-free solder joints.

"The picture is mixed: in some conditions lead-containing solder joints last longer, in some lead-free ones," said Belyakov.

For example: "Pb-free solders better sustain high-temperature operational conditions, whereas Pb-containing ones could be plastically deformed and re-crystallise under temperatures higher than 125 C," he said. "However, lead-free solders' ability to wet solder pads is poorer compared to lead-containing ones."

Belyakov provided some detailed examples of differences between the two solder groups:

"Due to considerable undercooling before crystallisation, up to 30C, lead-free solder joints produce lower amount of the [beta]-Sn grains upon solidification. Further it was shown that due to anisotropy of the coefficient of thermal expansion of the [beta]-Sn phase, during thermal cycling a fracture occurs along the grain boundaries. Since lead-containing solder joints consisted of a vast amount crystallographically disoriented grains, they failed considerably earlier compared to the solder joints assembled with use of SAC405 solder."

On the other hand

"Large amount of undercooling in lead-free solders promotes pro-eutectic intermetallic phase development to a big scale, up to 50microns, during solidification, which could deteriorate mechanical properties of a solder joint, since physical and mechanical properties of the intermetallic phases differ from those of matrix material."

Crystallographically faceted voids (CFV) in the microstructure of lead-free solder joints. The CFV are always correlated with [beta]-Sn grain crystallographic orientation.