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NASA DoD Environments Testing Results
Analysis Lab |
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Authored By:Cynthia Garcia Raytheon Company McKinney, TX, USA SummaryAs part of the NASA-DoD Lead-Free Electronics project, combined environments testing was performed to validate and demonstrate lead-free solders as potential replacements for conventional tin-lead solders against aerospace and military electronics industry requirements for circuit card assemblies. Solder alloys Sn3.0Ag0.5Cu, Sn0.7Cu0.05Ni (≤0.01Ge) and Sn37Pb were used to assemble components on two different printed wiring board test vehicles: manufactured and re-work. The rework test vehicles included BGA-225, CSP-100, PDIP-20, and TSOP-50 components that were removed and replaced. The test vehicles were subjected to thermal cycling from -55 to 125 degrees Celsius, a ramp rate of 20 degrees Celsius per minute, and dwelling at each temperature extreme for 15 minutes in a HALT (highly accelerated life test) chamber. Pseudorandom vibration was applied continuously through-out the life test beginning at 10 grms and increased by 5 grms after 50 cycles until a maximum of 55 grms was reached. The test vehicles were electrically monitored for 650 cycles using event detectors. Solder joint failure data of a given component type, component finish and solder alloy were evaluated using 2-parameter Weibull analysis. The reliability of each lead-free solder alloy tested was compared to the baseline Sn37Pb solder alloy. ConclusionsOverall, component type has the greatest effect on solder joint reliability performance. The plated-through-hole components proved to be more reliable than the surface mount technology components. The plated-through-holes, PDIP-20, TQFP-144 and QFN-20 components performed the best. The BGA-225 components performed the worst. Solder alloy had a secondary effect on solder joint reliability. In general, tin-lead finished components soldered with tin-lead solder paste were the most reliable. In general, tin-silver-copper soldered components were less reliable than tin-lead soldered controls. Though, the lower reliability of the tin-silver-copper 305 solder joints does not necessarily rule out the use of tin-silver copper solder alloy on military electronics. In several cases, tin-silver-copper 305 solder performed statistically as good as or equal to the baseline, tin-lead solder. The effect of tin-lead contamination on BGA-225 components degrades early life performance of tin-copper solder paste, but it can also degrade early life performance of tin-silver-copper 305 solder paste. The effect of tin-lead contamination on BGA-225 components soldered with tin-silver-copper 305 solder paste was less than the effect on tin-lead contamination on tin-copper solder. CSP-100 components are the exception, where tin-lead CSP-100 components soldered with tin-silver-copper 305 solder paste performed better than or equal to tin-lead CSP-100 components soldered with tin-lead solder paste. The chip scale package components were not drafted correctly during the design stage, therefore CSP-100 component results can only be used to compare within the chip scale package type. The probability plots of soldering tin-lead and tin-silver-copper 305 solder components onto electroless nickel immersion gold (ENIG) finished test vehicles were compared using BGA-225 and CLCC-20 components. In general, tin-lead components soldered with tin-silver-copper 305 solder paste onto immersion gold surface finish performs better than tin-silver-copper 305 components soldered onto ENIG surface finish test vehicles. One exception is the performance of tin-lead CLCC-20 components soldered with tin-silver-copper 305 solder paste onto an ENIG surface finished test vehicle which performed better than the immersion gold test vehicle. Keep in mind, the ENIG sample size consisted of two. In general, reworked components are less reliable than unreworked components. This is especially true with reworked lead-free CSP-100, reworked lead-free BGA-225 and unreworked lead-free TQFP-144 components; these components did not survive beyond 200 cycles. About 40-percent of the outliers were early life failures from lead-free BAG-225 components reworked with flux only. Another 30-percent of early life failures came from lead-free TQFP-144 components that were not reworked but were adjacent to rework sites. The exceptions were the immersion gold plated-through-hole components, nickel-palladium-gold TQFP-144, matte tin and tin-lead QFN-20, and tin PDIP-20 components, where a majority of these components were soldered with tin-lead solder and did not fail. Approximately, 37-percent of rework test vehicle components soldered with tin-lead solder paste failed, whereas, 53-percent of rework test vehicle components soldered with tin-silver-copper 305 solder paste failed. This suggests that reworking surface mount technology components with lead-free solder continues to pose processing challenges. When comparing the performance of components on manufactured and rework test vehicles, the immersion silver sur-face finish of the manufactured test vehicles appears to enhance the reliability of the solder joints. Initially Published in the SMTA Proceedings |
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