The EMPF Helpline received a call from a manufacturer who had experienced plating issues with a lead-free solder base. The call led to a request for SEM testing of samples in order to help establish the cause.

The customer requested failure analysis of gold (Au) plated nickel (Ni) barrier atop tin silver (Sn96Ag4) soldered samples. Gold embrittlement was the concern. The lead-free samples included failed solder joints in a connector. The method of assembly included the joints (nickel-gold plated pins soldered to pre-tinned pads) being hand soldered, using a tin-silver based solder, and encapsulated in an aluminum housing. The customer supplied two samples for analysis. The EMPF was tasked with identifying possible causes of failure.

The samples were examined using optical and scanning electron microscopy (SEM). Figure 3-1 shows a fracture of a soldered pin. The solder joints were cross-sectioned in accordance with IPC TM-650 2.2.1. Energy dispersive X-ray spectroscopy (EDS) was used for elemental identification.

Results
Elemental analysis of the pin fracture surfaces showed the presence of tin (Sn), lead (Pb), gold (Au), copper (Cu), nickel (Ni), and aluminum (Al), as expected.

Cross-sections of the fractured solder joints showed that the failure occurred at the solder joint’s pin interface. Elemental analysis of the fractured joint showed the presence of lead and gold in the solder. The lead contaminant was dispersed throughout the solder with high concentrations at the center of the joint where the pin fractured. Large SnAuCu intermetallic regions were observed at the solder-pad interface.

Figure 3-2 shows a cross-section of one of the soldered pins at low and high magnifications. The image on the bottom shows the solder-pin and solder-pad interface at the bottom of the solder joint. SnAuCu intermetallic layers were observed at both interfaces. Voids and degradation were observed in the solder adjacent to these intermetallic layers.

Conclusions
The separation of solder from the pin was caused by contamination of the solder joint. Gold and lead contamination caused intermetallic formations near the pin interface that weakened the solder and caused premature joint failure.

Possible sources of lead contamination include an uncleaned soldering iron tip or cross-contamination of the solder materials (i.e., during rework). The solder materials should be analyzed for content and soldering tips must be cleaned and kept free of lead prior to soldering with lead-free solders. The plating on the pin is the likely source of the gold contamination. Small pins used in surface mount applications should be “tinned” (pre-soldered) prior to soldering operations.