Meaning
An intermetallic compound morphology forms at the interface between copper pads and tin-based solder alloys during the soldering process. These Cu6Sn5 scallops emerge as solder melts and reacts with the underlying copper substrate. This morphology represents the initial stage of intermetallic layer development before subsequent aging.
The rounded shape of the structures influences the mechanical integrity of the solder joint.
Morphological Evolution
Growth of these hemispherical structures is driven by a diffusion-controlled reaction and capillary effects. During the liquid-state reaction, copper dissolves into the molten tin, leading to the rapid precipitation of the intermetallic compound. Over time, particularly during subsequent thermal aging, these Cu6Sn5 scallops coalesce into a more continuous and planar layer.
This planarization changes the stress distribution along the copper-solder interface when the joint is subjected to mechanical loading.
Mechanical Vulnerability
Interfacial regions between the scallops and the solder matrix can become a locus of stress concentration. When the intermetallic layer is excessively thick, the brittle nature of the compound increases the risk of interfacial fracture under shock or vibration. High drop-test performance is achieved with thin, uniform intermetallic layers, whereas thick, rough interfaces are prone to failure.
Adding small amounts of nickel or cobalt to the solder can refine the microstructure and limit the growth of the scallop-like features.
Growth Mitigation
Managing the reflow profile and the peak temperature is the primary method to control the size of these structures. Faster cooling rates after reflow can restrict the growth and result in a finer scallop morphology.
