Acceptance Testing Of Low-Ag Reflow Solder Alloys (Part 2)
In Part 2 of this two-part article series, the authors discussed the test results for low-silver alloys using these solder paste alloy assessment protocols for BGAs and leaded components, and the impact of the alloys on printed circuit assembly process windows.
Test Results and Discussion
Manufacturing DoE Results
Measured on BGA Manufacturing DoE Test Boards (see Figure 1 in Part 1) that were reflowed 3 times with a peak reflow temperature of 250 °C and 120 seconds TAL.
Two BGA body sizes were evaluated [14 mm and 23 mm]. IMC thicknesses for alloys A and B were normally distributed with no significant difference between the large and small BGA. For alloys C and D, greater variability in the large BGA IMC thickness resulted in values significantly different than smaller BGA (as seen in Figure 1 and Table 1).
Figure 1: IMC thickness distribution by Alloy (A-D) and BGA size (large/small)
Table 1: Summary of IMC thicknesses (microns)
This was measured on the BGA Manufacturing DoE Test Boards (see Figure 1 in Part 1) that were reflowed 3 times with a peak reflow temperature of 250°C and 120 seconds TAL.
Cu thickness measurements were made at each solder joint, in the middle of the joint and underneath the soldermask adjacent to the joint. Cu removed was the difference in copper thickness at these two locations. Cu dissolved was determined by subtracting Epsilon (thickness of Cu removed during soldermask etch in the PCB fab process) from the Cu removed values.
Figure 2: Cu dissolution distribution by alloy
Table 2: Summary of Cu dissolved (microns)
The amount of Cu dissolved after soldering was less for the low silver alloys than that for SAC305. The Cu dissolution results were normally distributed for the evaluated alloys but varied with alloy and BGA size (as seen in Figure 2 and Table 2). The Cu dissolution performance varied by BGA size, though not in a consistent fashion. While alloy B showed comparable Cu dissolved values between the 2 BGA sizes, alloy C and D both had more Cu dissolved for the larger BGA while alloy A had greater Cu dissolution values for the small BGA (this could be attributable to bare board Cu characteristics between the different vendors).
Acceptability of Solder Joints
Solder joints of the leaded DoE test board (SMTA Saber) assemblies using the respective low-Ag paste alloys were evaluated to the IPC-A-610E criteria. A range of lead and joint types such as J-leads, gull-wing leads and passive components were inspected. However, BGA joints were not inspected due to lack of access for visual inspection.
Solder defects such as cold solder and lack of fully reflowed paste were found across all low-Ag alloys and all process conditions of 230°C and 240°C. Cold solder is defined per IPC-A-610 as ‘‘a solder connection that exhibits poor wetting, and that is characterized by a grayish, porous appearance. (This is due to excessive impurities in the solder, inadequate cleaning prior to soldering, and/or the insufficient application of heat during the soldering process.) Defects (see Figures 3-8) appeared randomly distributed across the PCAs. Acceptable solder joints were observed on boards processed at 250°Cand times above liquidus of at least 60 seconds (see Table 3).
Table 3: Visual inspection summary of soldering acceptability criteria per IPC-A-610E
Examples of the observed defects:
Figure 3: Low-Ag Alloy A - Peak 240°C at 60 sec TAL (Uncoalesced/Unreflowed solder paste)
Figure 4: Alloy A - Peak of 230°C at 120 sec TAL (Uncoalesced/Unreflowed solder paste)
Figure 5: Alloy A – Peak of 240°C at 60 sec TAL (Uncoalesced/Unreflowed solder paste)
Figure 6: Alloy A – Peak of 240°C at 60 sec TAL (Cold Solder)
Figure 7: Alloy B – Peak of 240°C at 60 TAL (Cold Solder)
Figure 8: Alloy B – Peak of 240°C at 30 sec TAL (Cold Solder)
X-ray evaluation of low-Ag SMT solder joints also showed several instances of unusually high levels of voiding as shown in Figure 9. While voiding at this level is not considered a defect, it was found to be generally higher than seen in typical SAC305 SMT joints.
Figure 9: Example of voiding observed
Alloy A (0.3% Ag) had comparable drop/shock performance to Sn-37Pb and better performance than SAC405 in both electrolytic Ni/Au and Cu OSP surface finished boards. The solder joint failure mode during mechanical shock testing was predominately cracking in the intermetallic layer/bulk solder near the PCB side for both Alloy A and SAC405, while Sn-37Pb solder joints failed on both the component and PCB side in the bulk of the solder, as well as at the PCB IMC layer.
Alloy B (0.3% Ag) had similar drop/shock performance to Sn-37Pb and SAC405 on both electrolytic Ni/Au and Cu OSP boards. The solder joint failure mode during mechanical shock testing varied by alloy but not by surface finish. Alloy B (0.3% Ag) solder joint failures were in the solder near the PCB intermetallic compound (IMC) layer.
The drop/shock performance of alloys C (0.3% Ag) and D (1.0% Ag) were similar for both electrolytic Ni/Au and Cu OSP boards. They both performed worse than Sn-37Pb but better than SAC405. The solder joint failure modes for alloys C and D were also similar - predominately cracking in the intermetallic layer on the PCB side.
Table 4: Relative mechanical shock performance of Alloys A-D compared to their controls, on Cu OSP and Electrolytic Ni/Au board finishes
Figure 10: Post Mechanical Shock solder joint cross-sections for paste alloys
Accelerated Temperature Cycling:
In accelerated thermal cycling (0-100 °C, 10 minute ramps and dwells, to 6000 cycles), all four low-Ag alloys performed better than Sn-37Pb. Alloy A (0.3% Ag) and alloy D (1.0% Ag) did not have any electrical failures nor was any damage seen in cross-sections after 6000 ATC cycles. Alloy B (0.3% Ag) had 100% failures by 4857 ATC cycles and its performance was still better than Sn-37Pb. Alloy C (0.3% Ag) only had 4 parts out of 32 fail at 6000 thermal cycles, when the test was terminated.
Table 5: Relative accelerated thermal cycle performance of Alloys A-D compared to their controls
The solder joint failure mode in accelerated thermal cycling was in the bulk solder near the component interface for the alloy B (full opens), alloy C (full opens), SAC305 (partial opens) and Sn-37Pb (full opens).
Figure 11: Post ATC solder joint cross-sections for paste alloys
Prior studies of low-Ag alloys primarily investigated BGA ball alloys and had not fully explored the implications of increased liquidus temperature on reflow paste alloys. This work found the peak reflow temperature has to be increased by 10-15°C over the liquidus temperature when using low-Ag paste alloys.
For the alloys studied, this implies that a minimum reflow peak temperature of 240°C is required. When combined with the maximum package temperature of 245°C, this results in an effective process window of 0-5°C, when accounting for temperature deltas across the board. However, if solder joints are properly formed, reliability (thermal fatigue, mechanical shock) of low-Ag alloys is comparable to SAC305. The drivers for whether a low-Ag reflow alloy is acceptable are board complexity and thermal mass. The challenge for assemblers is in developing reflow programs that minimize the temperature delta between the coolest and hottest locations on the board. Indeed, it is unlikely that an acceptable process window is feasible for general-use company PCAs using low-Ag SMT alloys (≤1%Ag). Low-Ag alloys with liquidus temperatures closer to SAC305 (
 Elizabeth Benedetto et al, “Acceptance Testing of Pb-free Bar Solder Alloys,” SMTAI 2013.
 Helen Holder et al, “Test Data Requirements for Assessment of Alternative Pb-Free Solder Alloys,” SMTAI 2008.
 Aileen Allen et al, “Acceptance Testing of BGA Ball Alloys,” ECTC 2010.
 Gregory Henshall et al, “Progress in Developing Industry Standard Test Requirements for Pb-Free Solder Alloys,” IPC Printed Circuits Expo®, APEX® and the Designers Summit 2010.
 Pb-free Assembly, Rework and Reliability Analysis of IPC Class 2 Assemblies, Jerry Gleason, et al, ECTC, 2005.
 iNEMI Pb-Free Alloy Alternatives Project Report: State of the Industry, Greg Henshall, et al, SMTAI, 2009.
 IPC/JEDEC J-STD-020, Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices.
 IPC-A-610E, Acceptability of Electronic Assemblies.
 IPC J-STD-003, Solderability Tests for Printed Boards.
 IPC-9701, Performance Test Methods and Qualification Requirements for Surface Mount Solder Attachments.
 JEDEC JESD22-B111, Board Level Drop Test Method of Components for Handheld Electronic Components.
 NIST Special Publication 960-15, NIST Recommended Practice Guide: DTA and Heat-Flux DSC Measurements of Alloy Melting and Freezing.
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 ASTM 1875-00, Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Sonic Resonance.
 ASTM E831-06, Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis.
 ASTM E92-82, Standard Test Method for Vickers Hardness of Metallic Materials.
 ASTM B193-02, Standard Test Method for Resistivity of Electrical Conductor Materials.
If you missed Part 1, click here.
Editor's Note: This paper has been published in the technical proceedings of IPC APEX EXPO.