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Thermal Cycling Profile on Thermal Fatigue Performance of a 192-Pin Chip Array BGA
Analysis Lab |
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Authored By:Dan Burkholder Intel Corporation, Chandler, AZ, USA Russ Brown, Jagadeesh Radhakrishnan Intel Corporation, Folsom, CA, USA Pubudu Goonetilleke, Raiyo Aspandiar, Yunfei Wang Intel Corporation, Hillsboro, OR, USA Richard Coyle Nokia, Murray Hill, NJ, USA Faramarz Hadian, Babak Arfaei Physics Dept., SUNY-Binghamton, Binghamton, NY, USA Vasu Vasudevan Dell Technologies, Round Rock, TX, USA Aileen Allen HP, Inc., Palo Alto, CA, USA Keith Howell Nihon Superior Co., Ltd., Osaka, Japan Qin Chen Eunow, Suzhou, China Derek Daily Senju Comtek Corp, Santa Clara, CA Haley Fu iNEMI, Shanghai, China Carol Handwerker Purdue University, West Lafayette, IN, USA Ralph Lauwaert, Daniel Werkhoven Interflux Electronic nv, Belgium Kei Murayama Shinko Electric Industries Co. LTD., Nagano, Japan Hongwen Zhang, Francis Mutuku, Huaguang Wang Indium Corporation, Clinton, NY, USA Morgana Ribas MacDermid Alpha Electronics Solutions, Bengaluru, India Murali Sarangapani Heraeus Materials Singapore Pte Ltd, Singapore SummaryThere is an increasing interest in many market segments to use solder alloys with lower melting temperatures for electronics assembly. Low temperature solders can provide manufacturing, economic, and environmental benefits. Since 2015, the International Electronics Manufacturing Initiative (iNEMI) Low Temperature Solder Process and Reliability (LTSPR) Project has been evaluating Low Temperature Solder (LTS) paste formulations based on the Bi-Sn system. This paper summarizes the findings from a thermal cycling test to evaluate the effect of thermal cycling profile on thermal fatigue performance of low temperature solders. The study uses a daisy chained printed circuit board and two daisy chained ball grid array (BGA) test vehicles, a 192-pin chip array BGA (CABGA192) and an 84-pin thin core BGA (CTBGA84). The test matrix includes multiple LTS solder alloys designated by code names. The alloys are down selected from the larger project alloy matrix based on assembly effectiveness and mechanical test performance. There are two types of solder alloys, so-called ductile metallurgies that employ alloy modifications to improve the properties of the basic Bi-Sn alloy, and joint reinforced pastes (JRP) that employ resin additions that generate in situ filets during reflow to provide joint support. Components manufactured with the established SAC305 (Sn3.0Ag0.5Cu) composition are used as the baseline for the study. Three types of LTS solder joints are evaluated, homogeneous, hybrid (heterogeneous), and hybrid formed with joint reinforced pastes (JRP). Hybrid joints have a SAC BGA soldered with a ductile metallurgy LTS solder paste. The resultant solder joint consists of an unmelted SAC region at the package side of the joint and a melted region at the PCB side containing Bi from the solder paste. A hybrid joint also may be described as heterogeneous because it contains two regions with clearly distinct microstructures, compositions, and properties. Homogeneous joints are created when a BGA manufactured with LTS solder spheres is soldered to the PCB using a LTS solder paste with matching composition. A JRP joint is a special type of hybrid joint that has resin filets that form during reflow to enhance joint support. The test plan includes two distinct Accelerated Temperature Cycling (ATC) profiles, 0/100 °C (IPC-9701B, TC1) and -15/85 °C (selected due to the homologous temperature comparison between Sn-Ag-Cu and Bi-Sn solder), using the CABGA192 BGA component. This report includes the temperature cycling results for 7 experimental legs of the CABGA192 BGA component (Table 2) tested with the -15/85 °C profile compared to the results of the 0/100 °C profile (presented in a previous publication [37]). Weibull statistics, microstructural characterization, and failure mode analysis are used to compare the differences in alloy performance and to compare the performance of hybrid and homogeneous solder joint configurations. Failure analysis from both ATC profiles showed that most of the solder joints failed due to fatigue cracking within the bulk solder near the package side, regardless of alloy composition, joint construction (hybrid, homogeneous, or hybrid JRP), and package type. The CABGA192 component was affected differently across the 0/100 °C and -15/85 °C thermal cycling profiles. From a characteristic lifetime analysis, all of the low temperature solder test legs, across both ATC profiles, were shown to have a characteristic lifetime performance equivalent to the SAC baseline, with one exception; the homogeneous Red Flesh leg as run in the 0/100 °C profile. However, this performance delta went away when this leg was run in the less aggressive -15/85 °C profile. When looking at the thermal cycle profile ratio comparisons for characteristic lifetime, all test legs were extended beyond the SAC baseline in the less aggressive -15/85 °C profile, except for the Hybrid Sultan 2 and the Golden Pillow 2 JRP, which showed a decrease in the ratio. From a 1% cumulative failure analysis, the homogeneous Red Flesh and homogeneous Sultan 2 were shown to have relatively lower performance as compared with the SAC305 baseline in the 0/100 °C profile. However, this performance delta went away when these legs were run in the less aggressive -15/85 °C profile. It was also observed that for both the Beserah JRP and Golden Pillow 2 JRP as run in the less aggressive -15/85 °C profile, there was relatively lower 1% cumulative failure as compared to both the 0/100 °C profile and to the SAC305 baseline. This finding was also observed when looking at the thermal cycle profile ratio comparisons for 1% cumulative failure, which suggests that JRP resins may have a sensitivity to the colder accelerated temperatures as were present in the -15/85 °C profile. ConclusionsThe thermal fatigue resistance of four Low Temperature Solder (LTS) alloys was assessed using a CABGA192 ball grid array test vehicle and accelerated thermal cycling profiles of 0 to 100 °C (IPC-9701B, TC1) and -15 to 85 °C. The original concern with the accelerated 0/100 °C thermal profile was that the Bi-Sn might not perform as well with the higher acceleration provided at an upper temperature extreme of 100 °C. As 100 °C is close to the onset of the low temperature solder melting temperature, it may result in a decrease in the strength of the solder, possibly lowering the characteristic lifetime. Additionally, most computer products, used by consumers, do not operate continuously at 100 °C. For both reasons, another profile with a lower upper temperature extreme was also used for this study. Thus the -15/85 °C accelerated profile was selected (after considering the homologous temperature comparison between Sn-Ag-Cu and Bi-Sn solder). Based on the results of this study, even with failures occurring faster with the 0/100 °C profile, the low temperature solders do perform well at 100 °C. The alloys with code names Red Flesh and Sultan 2 were evaluated using a hybrid assembly process (SAC BGA with LTS solder paste) and a homogeneous assembly process (matching LTS BGA and LTS paste). The alloys with code names Beserah and Golden Pillow 2 were evaluated as hybrid assemblies with joint reinforced pastes (JRP). JRP solder pastes form a resin fillet that gels around individual solder joints as the solder solidifies. In all cases, SAC305 BGA assemblies were used as the performance baseline. The thermal fatigue resistance of four Low Temperature Solder (LTS) alloys was assessed using a CABGA192 ball grid array test vehicle and accelerated thermal cycling profiles of 0 to 100 °C (IPC-9701B, TC1) and -15 to 85 °C. The original concern with the accelerated 0/100 °C thermal profile was that the Bi-Sn might not perform as well with the higher acceleration provided at an upper temperature extreme of 100 °C. As 100 °C is close to the onset of the low temperature solder melting temperature, it may result in a decrease in the strength of the solder, possibly lowering the characteristic lifetime. Additionally, most computer products, used by consumers, do not operate continuously at 100 °C. For both reasons, another profile with a lower upper temperature extreme was also used for this study. Thus the -15/85 °C accelerated profile was selected (after considering the homologous temperature comparison between Sn-Ag-Cu and Bi-Sn solder). Based on the results of this study, even with failures occurring faster with the 0/100 °C profile, the low temperature solders do perform well at 100 °C. The alloys with code names Red Flesh and Sultan 2 were evaluated using a hybrid assembly process (SAC BGA with LTS solder paste) and a homogeneous assembly process (matching LTS BGA and LTS paste). The alloys with code names Beserah and Golden Pillow 2 were evaluated as hybrid assemblies with joint reinforced pastes (JRP). JRP solder pastes form a resin fillet that gels around individual solder joints as the solder solidifies. In all cases, SAC305 BGA assemblies were used as the performance baseline. characteristic lifetime analysis, all the low temperature solder test legs, across both ATC profiles, were shown to have a characteristic lifetime performance equivalent to the SAC baseline, with one exception; the homogeneous Red Flesh leg as run in the 0/100 °C profile. However, this performance delta went away when this leg was run in the less aggressive -15/85 °C profile. When looking at the thermal cycle profile ratio comparisons for characteristic lifetime, all test legs were extended beyond the SAC baseline in the less aggressive -15/85 °C profile, except for the Hybrid Sultan 2 and the Hybrid Golden Pillow 2 JRP, which showed a decrease in the ratio. One hypothesis for the lower hybrid solder joint performance difference versus SAC could be the occurrence of the ball drift mechanism (solderballs shifting out of their position during thermal cycling, potentially increasing the solder joint strain within the unmelted SAC region with increasing number of thermal cycles). Additionally, the Golden Pillow 2 JRP performance may be influenced by HoP or partial HoP defects generated during the reflow soldering process (i.e., at Time Zero), which might also account for the Golden Pillow 2 JRP performance difference versus SAC. From a 1% cumulative failure analysis, the homogeneous Red Flesh and homogeneous Sultan 2 were shown to have relatively lower performance as compared with the SAC305 baseline in the 0/100 °C profile. However, this performance delta went away when these legs were run in the less aggressive -15/85 °C profile. It was also observed that for both the Beserah JRP and Golden Pillow 2 JRP as run in the less aggressive -15/85 °C profile, there was relatively lower 1% cumulative failure as compared to both the 0/100 °C profile and to the SAC305 baseline. This finding was also observed when looking at the thermal cycle profile ratio comparisons for 1% cumulative failure. As this JRP behavior was not observed with the 0/100 °C profile, it suggests that earlier 1% cumulative failure values for resin reinforced material properties may be more sensitive to the colder temperatures found in the -15 °C to 0 °C temperature range, or it may be possible that the resins are not adhering as firmly to the solder mask or solder joint at -15 °C as they do at 0 °C. To validate these hypotheses, additional JRP experiments (including but not limited to nano-hardness evaluations) should be conducted to determine the impact of thermal cycling temperatures below 0 °C on resin reinforcement materials. As resin reinforcement is shown to be effective in resisting fatigue damage for greater characteristic life performance, the potential effects of other variables should be considered. It should be noted that other factors may also influence the reliability performance of resin reinforcement, such as the differences in the JRP test legs of alloy composition, reflow profile, Bi mixing level (Figure 8), microstructure, and resin chemistry and properties. As this JRP behavior was not observed with the 0/100 °C profile, it suggests that earlier 1% cumulative failure values for resin reinforced material properties may be more sensitive to the colder temperatures found in the -15 °C to 0 °C temperature range, or it may be possible that the resins are not adhering as firmly to the solder mask or solder joint at -15 °C as they do at 0 °C. To validate these hypotheses, additional JRP experiments (including but not limited to nano-hardness evaluations) should be conducted to determine the impact of thermal cycling temperatures below 0 °C on resin reinforcement materials. As resin reinforcement is shown to be effective in resisting fatigue damage for greater characteristic life performance, the potential effects of other variables should be considered. It should be noted that other factors may also influence the reliability performance of resin reinforcement, such as the differences in the JRP test legs of alloy composition, reflow profile, Bi mixing level (Figure 8), microstructure, and resin chemistry and properties. Initially Published in the SMTA Proceedings |
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