Most articles discussing the copper-tin intermetallics that form during soldering refer to them as a necessary evil. The evil being the perception that intermetallics are brittle and can lead to failures in thermal cycling or drop shock.

I view the situation differently. From my perspective, the formation of copper-tin intermetallics is the miracle of soldering. Look at it this way, to assemble electronics, bonding copper to copper (the leads on the components to the pads on the PWB) in the presence of polymers (the PWB epoxies and the component cases) is required. These polymer materials can only take about 250°C for a few minutes. Copper melts at 1083°C, so bonding copper to copper in the presence of polymers would appear to be quite a challenge. Enter tin-based solder.

Lead-free (tin-based) solder, say SAC305, melts at about 219°C. So, with a peak temperature of about 245°C, in the reflow oven, solder can be melted and form an electrical and mechanical bond with the copper in the leads and pads. At 245°C, the many polymer materials are unharmed for the 90 seconds or so that soldering requires at this temperature.

But, what about the material properties of the intermetallics that are formed? Aren’t they too brittle? Lee et al* performed analyses that suggest that the intermetallics formed in soldering are not brittle. Their work also suggests that the failure modes are not in the intermetallics, but in the interfaces between the intermetallics and the solder, copper, or the different intermetallic compounds, Cu3Sn and Cu6Sn5. These two intermetallic compounds are shown in the figure below.

Copper tin intermetallics from Roubaud et al, “Impact of IM Growth on the Mech. Strength of Pb-Free Assemblies,” APEX 2001.

It has long been assumed that the thicker the intermetallics, the greater the risk of failure due to the intermetallic thickness. Lee’s work would appear to bring this concern into question.

Stay tuned for a continued discussion on intermetallics and their effect on reliability.

*Lee, C. C. et al, “Are Intermetallics Really Brittle,” IEEE Electronics Components and Technology Conference, 2007, pp. 648.

Cheers,
Dr. Ron