Pb-free beats SnPb on lead pull, but not shear force.

An RoHS-compliant assembly must meet the same rigid performance and reliability standards and specifications of leaded-solder assemblies. Because Pb-free solder pastes and solders are typically used and processed in a different manner (higher melting temperatures, longer dwell times, etc.), accelerated aging tests become paramount to meet standards. ^1-3

This study used SnPb solder for the control and Pb-free solder for the test group. Various components (six inductors, eight chip resistors and two headers - compliant and noncompliant) were hand-soldered to a compliant test board (surface finish: matte immersion tin). Next, the assembled boards were subjected to an array of accelerated aging tests (thermal shock, cycling temperature/humidity, vibration and a combination of all three).⁴ Finally, the solder joints were analyzed for defects, joint integrity, tin whiskers, lead pull strength and shear force. Table 1 lists specific parameters for this study. Figure 1 shows the assembled test board, and Figure 2 the lead pull test board.

Prior to hand soldering, the test boards were labeled A-1 through A-8 for standard SnPb and B-1 through B-8 for Pb-free. During and after hand soldering, both technicians were questioned as to differences observed between the solders. Comments included:

• Longer time to heat Pb-free before it melted.

• Pb-free does not flow as well as SnPb.

• Pb-free does not "wet out" as well as SnPb.

• More difficult to solder the leads to the pads with Pb-free compared to SnPb.

• The solder joints appeared duller and more grainy with Pb-free.

• General concern about poor solder joints with Pb-free.

• More solder used with Pb-free to give confidence of good solder joints.

• Much greater preference to use standard SnPb.

Following assembly of the test boards, they were segregated into two groups for accelerated aging tests, Phase I testing (moderate) and Phase II (more severe). The specifics for these two Phases are listed in Tables 2 and 3.

Figures 3 and 5 show microscope photos (45X) of a solder joint taken on one of the chip resistors of both test boards (A-7 and B-7), prior to subjecting the test boards to accelerated aging. Figures 4 and 6 are photos of the same solder joints of boards A-7 and B-7 after accelerated aging (thermal shock, temperature/humidity and vibration).

Figures 7 and 8 show high magnification photos of boards A-7 and A-8 after Phase II aging. Also prior to aging, boards A-7 and B-7 were subjected to shear force testing on the inductors and boards A-8 and B-8 were tested for lead pull strength. Table 4 lists lead pull and shear force data, including those obtained after aging tests (thermal shock, temperature/humidity and vibration). Figure 9 shows a diagram of the shear force test applied to the inductors. Table 5 lists the accelerated aging equipment and instruments used for this study.

After the accelerated aging tests, plus specific test and magnification examination, the data were analyzed. Findings included:

• Both hand-soldering operators had a strong preference for SnPb solder and disliked the Pb-free solder. Part of this can be attributed to the unfamiliarity in using Pb-free solders and would likely diminish with experience. One operator suggested that higher wattage and hotter tip soldering irons would probably help the Pb-free solders.

• Resistance measurements across five resistors were taken on boards A-7 and B-7 both prior to accelerated aging and after Phase II accelerated aging. In all cases, the resistors showed 10V, +/- 1%. There were no fractures in the solder joints (with separation) after accelerated aging, which would cause opens, and there was no formation of high resistance components within the solder joints (intermetallics, oxides, etc.) as a result of the accelerated aging.

• Table 4 shows that the average lead pull strength of the Pb-free solder joints prior to accelerated aging was higher (by about 30%) than that of the SnPb. After accelerated aging, the average lead pull strengths of both solders actually rose about 10%. Therefore, Phase II accelerated aging did not deteriorate lead pull strength.

• Conversely, Table 4 showed that the average shear force strength of the SnPb solder joints prior to accelerated aging was higher (by about 50%) than that of Pb-free. After accelerated aging (Phase I for boards A-3 and B-3 and Phase II for boards A-7 and B-7), the average shear force strengths of both SnPb and Pb-free decreased (about 25% for SnPb and nearly 75% for Pb-free). Furthermore, after Phase II accelerated aging (boards A-7 and B-7), the average shear force strengths of SnPb was nearly double compared to Pb-free.

• Figures 3, 4 and 7 (100X magnification) show microscope photographs of a SnPb solder joint before and after Phase II accelerated aging. Even though the solder joint became dull and discolored after the aging, there was no evidence of tin whisker formation or other failure.

• Figures 5, 6 and 8 (100X magnification) show microscope photographs of a Pb-free solder joint before and after Phase II accelerated aging. The accelerated aging did not cause the joint to become duller (in fact, it looked a little shinier); however, there was evidence of striation (Figure 8), which would indicate some fatigue in the joint. There was no evidence of tin whisker formation or other failure.

The results show clear differences between SnPb and Pb-free solders; however, no real failures were noted. Pb-free solder appeared superior to SnPb in terms of lead pull strength. Conversely, SnPb was significantly stronger than Pb-free in terms of shear force strength and both SnPb and Pb-free shear force strengths degraded after accelerated age conditioning. Further work may be necessary to determine the affects of shear force reduction on long-term reliability.

References

1. Ray Prasad, "Part 1: Pb-free Reflow Profile Developments," SMT, February 2006.

2. Cookson Electronics Assembly Materials, Alpha OM-5100 Fine Pitch Solder Paste technical bulletin, revision 7-13-04.

3. Metallic Resources Inc., Metapaste NC-500 LF No Clean Solder Paste product bulletin.

4. J. S. Hwang, Implementing Pb-free Electronics, McGraw-Hill, 2005, chapter 10.

Acknowledgments

The author would like to acknowledge the technical staff of Global Testing Services Inc. for their assistance in implementation of this study. Thanks to Diversified Systems Inc. for the test boards, and Foresite Inc. for the high magnification photos.

 

Larry Fisher is product/sales manager at Global Testing Services Inc. (globaltesting.net); lfisher@globaltesting.net.