Reliability / Microstructure of Lead-Free Joints

Francesca Scaltro, Mohammad H. Biglari, Alexander Kodentsov
Olga Yakovleva, Erik Brom
Mat Tech BV, Son, The Netherlands
Department of Materials and Interface Chemistry
University of Technology of Eindhoven, The Netherlands

The reliability of lead-free (LF) solder joints in surface-mounted device components (SMD) has been investigated after thermo-cycle testing. Kirkendall voids have been observed at the interface component/solder together with the formation of fractures. The evolution, the morphology and the elemental analysis of the intermetallic layer have been evaluated before and after the thermal treatment. Voids produced by the release of volatile species during the soldering process due to the application of flux were present. If compared with SnPb soldered systems, lead-free joints are characterized by larger and a higher amount of voids.

In several electronic joints (ball grid arrays (BGA), surface-mounted device components (SMD), etc.) fractures developed after the thermal stresses generated during the accelerated thermal aging. Warpage of the PCB has also been observed. Backward and forward compatibility of SnPb and lead-free BGA connections has been performed on pads with an ENIG finish. The effect of the reflow peak temperature on the structure of the intermetallic layer has been assessed.

The microstructure of the IMC layer of different electronic components has been investigated for lead-containing and lead-free alloys. The following conclusions can be formulated:

A difference of peak temperature of ten degrees in the soldering process (225°C or 235°C) does not greatly influence the structure and thickness of the intermetallic layer in the lead-containing and lead-free solder systems that have been examined. In all cases, the segregation of lead at the interface solder/component, in lead-containing alloy joints, limits the growth of the intermetallic layer.

An ENIG finish might negatively affect the quality of a soldered joint. The formation of a phosphorous rich Ni layer at the interface solder/component facilitates the formation of a brittle layer and decreases the reliability of the joint.

Voids produced by the release of volatile species during the soldering process due to the application of no-clean flux were present. If compared with lead-containing alloy systems, lead-free alloy joints have larger and a higher amount of voids.

After accelerated thermal cycling tests on lead-free alloy systems, cracks and warpage of BGAs have been observed. Those defects were caused by the stresses generated during the soldering process due to the different expansion coefficients of the alloys in contact. In addition, the coalescence of the micrometric Kirkendall voids facilitates the development of fractures at the solder/component interface.