Environmental and mechanical stress testing of Pb-free and SnPb solder: 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.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

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.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

* 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.

[FIGURE 7 OMITTED]

* 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.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

References

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

2. Cookson Electronics Assembly Materials, Alpha 0M-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); [e-mail].

Table 1. Study Parameters

Material             Specifics

Laminate             Polyclad #370 (Tg 170[degrees]C), 0.031" surface
                       mount test board; compliant
Surface finish       Uyemura PRESA-RMK-20 matte, immersion tin;
                       compliant
Leaded solder        Kester SnPb40, #44 rosin core wire, noncompliant
Pb-free solder       Kester SAC-305 (SnAg3.0Cu0.5), Pb-free wire,
                       compliant
Chip inductor        Cooper Bussman, #UP 0.4 SA 682, 6.85 millihenrys,
                       compliant
Header               FCI Electronics, #54201, noncompliant
Thick film resistor  Multicomp, #MC-0603-WCF, 100 [ohm], compliant
Soldering iron       Weller TC 201T, standard tip
Lead pull wires      Alpha Wire Co, 20 AWG, Cu-coated with tin wire

Table 2. Phase I Testing Conditions

Test Boards  Conditions

A-1 and B-1  Thermal shock, MIL-STD-202, Method 107, -40[degrees] to
             +105[degrees]C, 24 hr. test, cycled with 30 min. dwell
             times
A-2 and B-2  Vibration, MIL-STD-202, Method 204, 2 hr. each axis, 10 to
             57 Hz--0.06" displacement; 57 to 500 Hz--10g; sweep
             rate--0.37 octaves/min.
A-3 and B-3  Combination of thermal shock (above), followed by vibration
             (above)

Table 3. Phase II Testing Conditions

Test Boards    Conditions

A-4 and B-4    Thermal shock, MIL-STD-202, Method 107, -40[degrees] to
               +125[degrees]C, 96 hr. test, cycled with 30 min. dwell
               times
A-5 and B-5    Temperature/Humidity, MIL-STD-202, Method 103, 96 hr. at
               85[degrees]C/85% RH
A-6 and B-6    Vibration, MIL-STD-202, Method 204, 4 hr. each axis, 10
               to 57 Hz--0.06" displacement; 57 to 500 Hz--10g; sweep
               rate--0.37 octaves/min.
A-7, A-8, B-7  Thermal shock (above), followed by temperature/humidity
and B-8        (above), followed by vibration (above)

Table 4. Lead Pull and Shear Force Testing

                                                   Break-Point Results
                                Break-Point        after Phase II
Test                            Results Prior to   (Thermal Shock and
Board  Test Performed           Accelerated Aging  Temperature/Humidity)

A-8    Lead pull, right side    8 Newtons          12 Newtons
A-8    Lead pull, center        Invalid            Invalid
A-8    Lead pull, left side     16 Newtons         16 Newtons
A-8    Average                  12 Newtons         14 Newtons
B-8    Lead pull, right side    20 Newtons,        Invalid (wire fell
                                                     off)
B-8    Lead pull, center        21 Newtons         14 Newtons
B-8    Lead pull, left side      9 Newtons         28 Newtons
B-8    Average                  17 Newtons         20 Newtons

                                                   Break-Point Results
                                Break-Point        after Phase I
Test                            Results Prior to   (Thermal Shock and
Board  Test Performed           Accelerated Aging  Vibration Tests)

A-3    Shear force, right side  47 Newtons         40 Newtons, part
                                                     broke in half, but
                                                     joints held
A-3    Shear force, center      56 Newtons         58 Newtons
A-3    Shear force, left side   55 Newtons         18 Newtons
A-3    Average                  53 Newtons         39 Newtons
B-3    Shear force, right side  Invalid            20 Newtons
B-3    Shear force, center      35 Newtons         18 Newtons
B-3    Shear force, left side   34 Newtons         26 Newtons
B-3    Average                  35 Newtons         21 Newtons

                                                   Break-Point Results
                                Break-Point        after Phase II
Test                            Results Prior to   (Thermal Shock,
Board  Test Performed           Accelerated Aging  Temperature/Humidity)
                                                     and Vibration
                                                     Tests)

A-7    Shear force, right side  47 Newtons         50 Newtons
A-7    Shear force, center      56 Newtons         39 Newtons
A-7    Shear force, left side   55 Newtons         40 Newtons
A-7    Average                  53 Newtons         43 Newtons
B-7    Shear force, right side  Invalid            25 Newtons
B-7    Shear force, center      35 Newtons         15 Newtons
B-7    Shear force, left side   34 Newtons         21 Newtons
B-7    Average                  35 Newtons         20 Newtons

Table 5. Equipment and Instrument List

Test           Equipment/Instrument

Phase I        Ransco thermal shock chamber
thermal shock
Phase II       Tenney thermal shock chamber
thermal shock
Phase I        Ling Electronics vibration system
vibration
Phase II       Lab Line, Environ-Cab temperature/humidity chamber
temp/humidity
Phase II       Ling Electronics vibration system
vibration
Lead pull      Mark 10 Force gage and Chatillon motorized force
strength         measurement table
Shear force    Mark 100 Force gage and Chatillon motorized force
                 measurement table
Magnification  Amber Depot stereo microscope, up to 45X magnification
analysis
Resistance     Fluke True RMS Multimeter
High           Leica stereo microscope
Magnification
analysis