Investigating pure tin as an alternative finish
“The challenge facing the electronics industry with regard to environmentally friendly packaging is to make a switch to materials and processes that have comparable reliability, manufacturability, price and availability.”
BY RAHAMAT BIDIN,
ROSEMARIE TAGAPULOT,
CARL NICHELLE D. LAO AND RODEL MAÑALAC
Increasing awareness of the environmental and health impact of various industrial materials has brought about changes to the way these are being used and processed. In the electronics industry, the elimination of lead (Pb) used in surface finishing, and the brominated and antimony-containing materials used as flame retardants in molding compounds, has been the subject of research and discussion because of their detrimental effects on the human body and the environment.
 Figure 1. General reflow profile (260°C peak temperature). |
This article aims to determine the suitability of pure tin and tin-copper as an alternative finish for component terminations, and the reliability performance of molding compounds without the bromine and antimony constituents. In addition, an assessment is done on the packages assembled with these new materials to verify the effects of the higher reflow peak temperature (260°C). The ultimate goal is to achieve environmentally friendly packaging while maintaining the equivalent or better moisture sensitivity level when reflow temperature is raised from 245 to 260°C.
 Figure 2. Comparison of tin-lead, pure tin and tin-copper surface appearance. |
The challenge facing the electronics industry with regard to environmentally friendly packaging is to make a switch to materials and processes that have comparable reliability, manufacturability, price and availability. For the component maker, the substitute for tin-lead in lead finishing should offer easy process control during electroplating, and an equivalent solderability performance and visual and mechanical quality. Tin and tin-copper plating (both matte finish) are possible substitutes for tin-lead. Pure tin finish, though widely used in ceramic and other similar devices, has continuously encountered resistance in usage for fine-pitch devices because of the tendency of the deposit to develop whiskers over time. In recent breakthroughs on the whiskering phenomenon, electroplating chemical companies have developed a pure tin deposit that has a larger grain size with a pyramidal/polygonal structure to inhibit such growth. Pure tin holds much appeal as a finish because of its easy availability and lower cost compared to plating tin-silver or tin-bismuth or to using palladium pre-plated leadframe. Also, its application is more understood than the rest of the alloys being considered for lead-free processing (Table 1). The tin-copper finish addresses the whiskering problem in pure tin by adding a small amount (less than one percent) of copper, an element that likewise is in sufficient supply and has a lower cost than tin-silver, tin-bismuth and palladium. The emergence of these two alloy compositions, however, is accompanied by the need for an increase in the reflow process temperature. This is a result of the higher melting points of the lead-free alloys, which for pure tin is 232°C, and tin-copper, 227°C. The major contributor to the rise in reflow temperature is the use of high-temperature solder pastes, such as tin-silver-copper.
 Figure 3. SEM of pure tin surface structure at normal and high current densities. |
Molding is another area of the packaging process that presents an opportunity for switching to non-hazardous materials. The compounds used have to pass the UL Standard UL94-Vo test, thus the need for flame-retardant constituents. For this purpose, halogen complexes and antimony oxides are usually employed. However, these materials have the possibility of generating the highly toxic dioxin and furan when burnt. Based on development activities by mold compound makers, the non-halogen, non-antimony substitutes identified are mainly hydroxide-based. Those with the most potential are hydroxides of aluminum and magnesium, or transition metal-magnesium hydroxide complex (TMMHC). These retardants were chosen after passing tests on flammability, thermo gravity analysis (TGA) characteristics, warpage and shrinkage, glass transition temperature (Tg), viscosity and effect of reliability testing on mold compound properties.4
Methodology
The following sequence of activities was conducted during the evaluations.
- Preparation of test dice and leadframes
- Normal processing at front-of-line operations
- Molding using green compound
- Post-mold curing
- Dambar cut
- Lead-free plating using pure tin or tin-copper
- Marking
- Trim and form
- Whisker testing.
 Figure 4. EDX showing peak of tin in the deposit. |
During electroplating, the pre-treatment (deflashing and etching) and post-treatment (neutralization) chemicals and conditions employed were the same as those used for tin-lead plating. Pure tin metal balls were used as anodes. The representative packages MQFP 28 x 28, MQFP 14 x 20, LQFP 14 x 14 and PLCC 68L were the test vehicles. The matrix in Table 4 shows the packages that were evaluated to determine the moisture sensitivity level at a higher reflow temperature. These packages represent the different package families, size and thickness. Biphenyl compounds are used for thinner packages, like LQFP, for higher reliability, while OCN compounds are used for larger packages, such as MQFP and PLCC. The general reflow profile applied in the preconditioning step at the reliability testing stage is shown in Figure 1. During the preconditioning for the moisture sensitivity, the peak temperature used for reflowing is 245°C. The major difference for the lead-free application profile is that the reflow temperature was set higher at 260°C. Scanning was performed before and after exposure to check whether the package was able to withstand the higher heat application.
Results and Discussion
Plating quality: Plating quality checks normally performed on tin-lead plated parts were made on the tin and tin-copper samples to determine the suitability of these alternative materials. The pure tin matte plating has a comparable performance to the tin-lead finish (Table 2) with respect to thickness, visual/mechanical quality (including inspection for whiskers) and solderability (Table 3). The solderability testing was performed as per Mil-Std-883D (8 hours steam aging, 5 to 10s dwell in Rosin-based flux, 5 ±0.5s dwell in tin-lead [eutectic 63/37] solder bath), but the aging condition was tightened to dry bake for 48 hours at 155°C and steam for 16 hours. The ionic contamination levels were also measured by ion chromatography, and they were all below the maximum allowable limit of 2 µ/in2, with the highest value measured at only 0.0023 µg/in2. Carbon content in the deposit, measured with trace analysis by thermal conductivity, was also very low with values less than 0.01 percent (specification limit: 0.05 percent). The tin-copper plating quality and solderability results were likewise comparable to those of tin-lead, using the same testing conditions as indicated above. The results are summarized in Table 5. The ionic contamination level was at a maximum value of 0.0022 µg/in2, and carbon content was below 0.01 percent. Figure 2 shows the comparison between the visual appearance of tin-lead, pure tin and tin-copper on as-plated parts.
 Figure 5. Pure tin sample after 18 months of storage, 50°C, no stress. |
SEM/EDX analysis: Pure tin samples were plated both at normal high-speed application current density, and at a very high density to check on the change in the deposition pattern. The coarser appearance at higher current density is expected, but it can be seen that the pattern did not change, and that burning did not occur. Pure tin-plated parts were also subjected to storage at 50°C for 18 months, and no whisker growth was observed upon 100-percent 30X-magnification scope inspection. This is true for both the non-stressed (straight) and stressed (bent) areas on the leads, detailed photos of which are shown in Figures 5 and 6, respectively. In these photos, the left images is of the areas where a SEM was taken to check for whisker growth, the middle images (SEMs) show no whiskers after 18 months of storage, and the right images illustrate the appearance of the cross-section and the plating coverage on the leadframe.
 Figure 6. Pure tin sample after 18 months storage, 50°C, stressed (bent). |
SEM and cross-section analysis of the as-plated tin-copper shows an even structure comparable to that of tin-lead (Figure 7). The cross-sections for tin (Figures 5 and 6) were taken along lead length to emphasize bent and straight areas; the cross-section for tin-copper (Figure 7) was taken along the lead width.
Plating processability: Based on the actual evaluations done on the high-speed plating line, the pure tin process is easier to control than the alloy systems of tin-lead and tin-copper. As compared with tin-lead, pure tin contains only three components to monitor and control, and alloy composition control problems are eliminated with the single-metal finishing. The tin and acid concentrations are both easily determined by titration, and the additive concentration by UV-visible spectrometry. It was likewise confirmed that this particular pure tin chemistry is adaptable to the high-speed plating process, thus eliminating the need for a separate capital investment should the use of tin-lead plating be stopped in the future.
 Figure 7. Tin-copper plated surface structure and lead cross section. |
Chemical control for the tin-copper process also employs titration and UV-visible spectrometry methods. However, the alloy composition is difficult to control. The copper in the plating layer has to be kept at less than one percent, and there is presently no real-time method of taking composition measurements, which for tin-lead and tin are normally done through an X-ray fluorescence (XRF) machine. This method isn't used for tin-copper because the detection of copper in tin-copper plating over copper leadframe is difficult; copper is present in both the plated layer and the leadframe. There is, therefore, an interference of the base material when a reading is being done on the sample. The vendor recommendation is to do a destructive test by dissolving the deposit and analyzing the content through atomic absorption spectrometry (AAS).
Moisture sensitivity level: The results obtained after subjecting the packages to JEDEC JESD22-A113-A (Preconditioning of Plastic Surface Mount Devices Prior to Reliability Testing) Preconditioning Levels 3 and 4 are summarized in Table 6. The focus here is on whether the package itself can retain its integrity (i.e., no delamination between compound and dietop) after subjecting to preconditioning with 3X reflow at 260°C. The biphenyl green compound for LQFP packages is capable of passing Level 3 preconditioning. However, the OCN green compounds for MQFP/PLCC are good only for Level 4 preconditioning.
Conclusion
The results clearly show that pure tin is a workable solution to the challenge of lead-free packaging. Whisker growth was not observed after storage at 85°C/85 percent relative humidity (672 hours), 50°C/85 percent relative humidity, and 50°C /85 percent relative humidity (6 to 18 months). Further reliability testing on temperature cycle, pressure cooker, humidity and high temperature storage tests are ongoing. Further work will be carried out to establish a lower peak reflow temperature, as well as board level reliability.
AP
References
- AC Tan, Tin and Solder Plating in the Semiconductor Industry, Chapman & Hall, 1993.
- Manfred Jordan, The Electrodeposition of Tin and its Alloys, Eugen G. Leuze Publishers, Germany, 1995.
- Manolo G. Mena, Materials Science for Semiconductor Engineers, College of Engineering, University of the Philippines, 1996.
- Yamaguchi, et al, Non-Halogen/Antimony Flame Retardant Systems for High End IC Package, Electronic Components and Technology Conference, IEEE, 1997.
Acknowledgments
The authors would like to thank the front- and end-of-line engineering team and the failure analysis group for supporting this project. Special recognition goes to the mold and plating process engineering teams for their valuable contributions.
RAHAMAT BIDIN is director for assembly engineering for leaded packages; CARL NICHELLE D. LAO is section manager for the marking and lead finishing department; RODEL MAÑALAC is section manager for end-of-line process engineering; and ROSEMARIE TAGAPULOT is process engineer for lead finishing at ST Assembly Test Services. For more information, contact Carl Nichelle D. Lao at No. 5 Yishun St 23, Singapore, 768442; 65-751-1276; Fax: 65-755-3137; E-mail: [email protected].
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Table 1. Comparison of lead-free finishes.
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Table 2. Solder plate critical responses summary.
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Table 3. Pure tin test results summary.
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Table 4. Evaluation Matrix.
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Table 5. Tin-copper test results summary.
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Table 6. Preconditioning results (reflow at 260°C).