A customer called the EMPF helpline requesting that printed circuit boards be analyzed to determine the reliability of two different lead-free surface finishes, electroless nickel immersion gold (ENIG) and immersion silver (ImAg).

ver the past few years, the EMPF Helpline has been helping OEM's and contract manufacturers convert from leaded to lead-free products. One common request is to evaluate a set of first run or prototype assemblies. These first run assemblies are usually the first experience an assembler has with lead-free solders, components, and boards. Often the assembler will experiment with variables such as profile, alloys, or surface finishes, to determine what parameters produce the best set of boards. The customer supplied assembled boards for thermal cycling, vibration testing, and cross-section analysis. The boards were soldered with a tin-nickel-copper solder paste (SNC). The EMPF staff then examined the lead-free finishes by assessing the boards 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 0ºC and 100ºC with a 10 minute 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 20Hz to 2000Hz. Finally, optical imaging, cross-sectioning, and scanning electron microscopy were performed on random locations of the printed circuit boards.

Results
250 Thermal Cycles
No fractures or separations were observed in either of the lead-free finishes after 250 cycles. As expected, the different lead-free finishes produced different intermetallic layers.

The number of voids observed in ball grid arrays (BGAs) was dependent on the type of lead-free finish. Specifically, boards finished with Immersion Silver (ImAg) showed significantly more voids than those finished with Electroless Nickel Immersion Gold (ENIG). Many of these voids were observed at the solder-component interface. Still, the number of voids did not exceed the IPC recommended limit of 25 %. Voids are displayed in the figure 3-1.

750 Thermal Cycles
A fatigue crack was observed at the component interface of one of the boards after 750 cycles. No other fractures or cracks were observed in the 750 thermal cycle surface mount solder joints. Intermetallic layers grew non-uniformly into the solder on boards with both lead-free finishes. Small voids were observed in the solder adjacent to the intermetallic layers, as shown in Figure 3-2.

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.

1000 Thermal Cycles and Vibration Testing
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 micrometers to 20 micrometers.

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

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

For more information, please contact the EMPF helpline at (610)362-1200 or via email at helpline@empf.org.