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A study of how two finishes held up under thermal and vibration testing.

When analyzing the reliability of different Pb-free surface finishes, it is common to evaluate a set of first-run or prototype assemblies. These assemblies are usually the first experience an assembler has with Pb-free solders, components and boards. Often the assembler will experiment with variables such as profile, alloys or surface finishes to determine which parameters produce the best set of boards.

In one such case, the customer was looking at two finishes: electroless nickel immersion gold (ENIG) and immersion silver (ImAg). The customer supplied assembled boards for thermal cycling, vibration testing and cross-section analysis. The boards were soldered with a SnNiCu solder paste. The Pb-free finished boards were then assessed at different stages during thermal cycling and after vibration testing.

Test methods. Thermal cycling was performed in accordance with IPC-SM-785 Class 2 recommendations. The temperature alternated between 0° and 100°C with a 10 min. dwell time at each temperature extreme. Each board endured 1000 thermal cycles. Samples were removed at intervals of 250, 750 and 1000 cycles, and continuity testing was performed on all samples at these thermal cycling intervals. Next, z-axis vibration testing was performed on samples that had endured 1000 thermal cycles. Vibration frequencies ranged from 20 to 2000 Hz. Finally, optical imaging, cross-sectioning and scanning electron microscopy (SEM) were performed on random locations of the boards.

Results. At 250 thermal cycles, no fractures or separations were observed in either of the Pb-free finishes. As expected, the different Pb-free finishes produced different intermetallic layers.

The number of voids observed in BGAs depended on the type of Pb-free finish. Boards finished with ImAg showed significantly more voids than those finished with ENIG. Many of these voids were observed at the solder-component interface (Figure 1). Still, the number of voids did not exceed the IPC-recommended limit of 25%.

After 750 cycles, a fatigue crack was observed at the component interface of one of the boards. No other fractures or cracks were observed in the surface-mounted solder joints that underwent 750 cycles. Both finishes experienced intermetallic layers that grew nonuniformly into the solder on boards. Small voids were observed in the solder adjacent to the intermetallic layers (Figure 2).

Large voids observed at the component-solder interface of the BGA components likely resulted from trapped flux gasses caused by improper solder reflow. Smaller voids observed along the intermetallic layers at the interfaces possibly resulted from the Kirkendall effect, which is voiding caused by differences in diffusion rates.

After 1000 thermal cycles, small cracks were observed at the component-solder interface on the corner BGAs for boards containing both lead-free finishes. The crack sizes ranged from 10 to 20 µm.

Boards were examined after vibration testing. Again, the number of voids depended on the type Pb-free finish on the board as the ENIG finish yielded smaller and fewer voids than the ImAg finish.

The boards containing different Pb-free finishes passed environmental testing. Nevertheless, for tests conducted on this customer's sample, the ENIG finish displayed significantly less voiding and greater solderability than did ImAg. Also, for this customer's sample, fewer and smaller voids were observed at the component-solder interface for boards containing the ENIG finish.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.