THE INFLUENCE OF THE PWB FABRICATION ELECTRODEPOSITION PROCESS ON COPPER EROSION DURING WAVE SOLDERING SECOND REPORT, 4 LEAD-FREE ALLOYS

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1 THE INFLUENCE OF THE PWB FABRICATION ELECTRODEPOSITION PROCESS ON COPPER EROSION DURING WAVE SOLDERING SECOND REPORT, 4 LEAD-FREE ALLOYS Chrys Shea Jim Kenny Jean Rasmussen Cookson Electronics Jersey City, NJ, USA Girish Wable Quyen Chu Jabil St. Petersburg, FL, USA Shiang Teng San Jose State University San Jose, CA, USA Keith Sweatman Nihon Superior Osaka, Japan Kazuhiro Nogita, Ph.D. University of Queensland Australia ABSTRACT Lead-free solder alloys, particularly those with higher silver and lower copper contents, have been shown to react with the copper on circuit boards to dissolve the copper much more quickly than tin-lead solders. Rapid dissolution of copper into tin-rich alloys can create hidden defects and reliability issues. As copper is washed off the PWB, the exposed traces, annular rings, and barrel knees can become thinner or even nonexistent. A series of studies have been developed to gauge the effect of copper erosion on PWBs when exposed to various combinations of time, temperature and alloy in both standard and selective soldering processes. The study was broken in to phases, the first of which aimed to isolate the influence of the copper s manufacturing process on its likelihood to erode during subsequent soldering processes. The results of phase 1, using SAC305 and three alternative solder alloys, are presented. Key words: Lead-free, wave soldering, copper erosion, copper dissolution INTRODUCTION Solid copper reacts readily with molten tin for form a layer of intermetallic compound, Cu 6 Sn 5 at the contact surface. That is what makes tin an essential ingredient in most solders because it is that intermetallic layer that provides the metallurgical connection to ensure good electrical and thermal conductivity through the solder joint. However, that intermetallic itself dissolves quickly in molten tin. As a result of these sequential dissolution processes the underlying copper can be quickly eroded. In eutectic 63/37 SnPb solder, the inert lead diluent slows the rate of dissolution of the intermetallic. The effect of the lead s presence in the eutectic SnPb is enough to slow the copper dissolution rate such that extreme copper erosion has not been a significant problem. However, the high tin content (typically >95%), coupled with the absence of lead in modern lead-free solders, raises the rate of dissolution to the extent that the copper erosion which can occur in normal soldering and rework processes may be sufficient to compromise the integrity of the circuit. Under certain conditions, the copper can dissolve too fast, resulting in the removal of most - or even all - of it from the circuit board features. Traces and plated through holes that are exposed to wave soldering and rework processes are at a 255

2 high risk for excess dissolution. Knees, the area where the barrel of the PTH meets the annular ring, are particularly susceptible to erosion problems. BACKGROUND Different lead-free alloys have demonstrated different rates of dissolution. Previously published studies 1, 2, 3 have shown that alloys with high silver and low copper content - SAC305 & erode copper more quickly than alloys containing low or no silver content and higher copper contents like SACX0307 and Sn100C (0.7% Cu). It is believed that silver acts as an erosion accelerator, while certain additives in the alternative alloys act as erosion inhibitors 4. The level of copper in the melt also factors into erosion rates. The plated copper surface itself is suspected to play a part in the erosion process. This is indicated by the fact that two identical circuit boards from different suppliers can demonstrate vastly different dissolution properties in the same soldering process. It is believed that the dissolution rate may be affected by the grain structure of the copper. Loose, open grain structures that are typically associated with columnar grain morphology are perceived to dissolve more readily than tighter, fine, equiaxed grain structures. Generally, finer grain structures exhibit higher bond strength between the grains, and thus require more energy to break loose or dissolve. coupon material from the same bath. The laminate sheets were plated to 1 mil nominal thickness; the copper foils for tensile testing were plated to 2 mil nominal thickness. Ten fabricators returned laminates; 8 of them also returned foils. Upon receipt of the plated materials from the fabricators, the specimens were labeled A though K. They were then divided into groups for the different tests, which were performed in different laboratories. The grain structure analysis took place at the Department of Materials Engineering, University of Queensland, Australia. The tensile testing was performed in the laboratories of Enthone in Lagenfeld, Germany. The soldering tests were performed at Jabil s Advanced Manufacturing Technology Laboratory in San Jose, California, USA. Metallographic Analysis Samples of both the laminates and foils in their as-received conditions were excised from the larger sheets for crosssection analysis. Handling was minimized, and no cleaning or additional processing treatments were applied. Upon receipt at the laboratory, two specimens of each product were cut and mounted for metallographic examination as shown in figures 1 and 2. Tensile elongation at break is an indicator of grain structure that is used as a quality assurance measure by PWB fabricators. Elongation of 16% or higher is considered to indicate a good grain structure, whereas elongation in the 12-14% range is taken to indicate that there could be potential quality issues with the copper plating. Elongation of less than 12% is considered unacceptable and can be a criterion for rejection. If elongation at break is an indicator of grain structure, and grain structure is a factor in dissolvability, then a relationship may exist between elongation and erodability of the electrodeposited copper. Figure 1. Specimen mounting setup EXPERIMENTAL DESIGN Separate tests were undertaken to assess three key characteristics: grain structure, tensile elongation at break, and copper loss behavior. Grain structure was assessed through both optical and SEM metallographic analysis. Elongation was assessed by tensile tests in accordance with method IPC-TM Copper loss was assessed by partially exposing samples to flowing solder and then crosssectioning them to compare copper thicknesses in areas that were and were not exposed. SAMPLE PREPARATION Sheets of 18x 24 inch laminate were secured from a single production lot. The laminates were lead-free capable Polyclad 370-HR. Ten different fabricators agreed to plate the laminates in their production baths, and provide tensile Figure 2. Example of mounting and cross sections 256

3 Samples were ground with #240, #500 and #1200 SiC, and polished with 6 and 1 micron diamond paste. They were etched for 30 seconds in an ethanol/hydrochloric acid/ferric chloride solution. They were then subjected to optical and SEM/EDX analysis. Additional pieces of each sample were reserved for analysis of the morphologies of their solderable surfaces. Tensile Tests The foils were conditioned at 125C for 1 hour and tested in accordance with test method IPC-TM-650. Four specimens were tested from each electrodeposited sample. The requirements for acceptability of electrodeposited copper, as described by IPC-6012B, are minimum tensile strength of 248 MPa and minimum elongation of 12%. 6 Copper Loss The 18 x 24 inch laminate sheets used in the soldering tests were cut into quarters, 9 x 12 inches each, for ease of handling. To negate the effect of copper oxidation on solderability from the time they were each plated, they were all treated with a high-temperature OSP process. The Entek Plus HT process removes surface oxides with an acid microetch, then applies a solderability preservative to the copper. Following the OSP process, the laminates were cut to their final sample sizes of approximately 2 x 3 inches. A mechanical shear was used to cut the coupons to their final size. Upon receipt at their testing laboratory, the edges of the coupons that would be used in the tests were ground to remove any edge effects of the shearing operation on the copper. An Air-Vac PCBRM15 Selective Soldering & Rework system was used to expose the samples to flowing solder. To limit sample-to-sample variation, a fixture was built for the copper substrates to insure repeatable immersion depths and positions relative to the solder flow well. The fixture is shown in figure 3, both before and after installation on the Air-Vac system. most active alloy with respect to dissolution and erosion. The alternative alloys were labeled 1, 2, and 3. Alloy 1 was low silver SAC, Alloy 2 was SnCu with Ni, and Alloy 3 was low silver SAC with Ni. The solder pot temperature for the SAC305 was 265C; the solder pot temperature for the alternative alloys was 270C. All samples were processed with a solder flow setting of 6.5. Each specimen was immersed until the copper laminate made contact with the solder nozzle drainage walls. The mechanical setup is depicted in schematic and photographic formats in figure 4. Solder Wave Flow Well Pictorial Diagram of Setup Copper Laminate Fixture Actual Setup Figure 4. Solder exposure test setup with test sample. Samples were dipped in a strong organic acid flux immediately before exposure to the solder. No preheat was applied. Contact times for each alloy were determined in prescreening experiments. SAC305 and Alloy 1 used dwell times of 20, 30, 40 and 50 seconds. Alloys 2 and 3 used dwell times of 40, 60, 80 and 100 seconds. Two replicates at each of the contact times were performed. The laminate was plated on both sides, offering 4 data points per contact time. After the laminate samples were soldered, they were mounted and cross sectioned. Copper thicknesses in areas that were and were not exposed to the flowing solder were measured. The locations where the copper thickness was measured are shown in figure 5. Non-exposed Area Soldered Area 2mm from Edge Solder Cu Laminate Cross-sectional View Figure 5. Copper loss measurement locations Aluminum Fixture Fixture as Mounted in Air-Vac System Figure 3. Fixture used to position samples during solder exposure. SAC305 and three alternative alloys were tested. SAC305 was the first alloy tested and initially reported on in It was used as a benchmark for comparison purposes, as it had been established through previous testing 1, 2 that it is the In order to obtain accurate copper thickness measurements, the specimens were ground with #320, #600, #800, and #1200 grit SiC papers, then polished with 1um Alumina, 1 um diamond, and a colloidal silica suspension. The copper thicknesses were measured using a metallurgical microscope at 200X magnification. FINDINGS Metallographic Analysis The cross sections were first examined optically at 100X magnification. Dark areas were noticed, as shown in fig

4 process. These cavities are created in porous regions of the copper where the etchant acts more quickly on the copper than it does in less porous areas. When cavities were observed, there were three main areas where they were noted: Area 1, the electrodeposited copper. This is the surface of the PWB that is exposed to the solder. Area 2, the discontinuity between the laminate copper and the electroplated copper Area 3, the interface between the laminate s resin/glass system and its copper layer. Figure 6. Optical Micrograph of laminate sample. Notice the dark area running laterally across the specimen. The samples were then inspected with scanning electron microscopy at 1000X, as shown in figure 7. Not all samples showed comparable levels of porosity. Figure 9 provides a close look at two cavities: one between the laminate copper layer and the electrodeposited layer, and one within the electrodeposited layer. It is assumed that the discontinuity within the electrodeposited layer is due to an interruption to the plating process. Figure 7. SEM Micrograph of same sample. The dark areas are more apparent. Energy Dispersive X-ray Analysis (EDX) was used to determine the nature of the dark areas. The three areas that were tested are shown in figure 8. Figure 9. Cavities produced by etchant in regions of high porosity. The surfaces were also analyzed at magnifications up to 10,000X. The surface morphology varied over a wide range from smooth and featureless with no obvious porosity to highly featured with deep porosity between protruding columnar grains. Figures 10 through 12 depict some examples of the variation that was observed. These surfaces had not been etched. Figure 8. Areas analyzed via EDX. Copper was the only element detected in the darkened areas; no contamination was indicated. It was concluded that the darkened areas were cavities produced by the etching 258

5 Figure 10. Smooth, low porosity solderable surface Table 1. Tensile Test Results. Suppliers C and E did not submit foils. Copper Loss Tests Total copper loss at each contact time was measured on both sides of the test coupons and averaged. To isolate the effect of the electordeposited copper from the laminate copper, the total loss was compared to estimates of the copper thicknesses shown on the SEM cross sections in Appendix A. Isolation of the electrodeposited layer is important in this study because the comparison focuses on the effects of different electroplating baths. Figure 11. Fine grain structure observed on solderable surface Figure 12. Topographical characteristics of columnar grain structure. Tensile Tests For each supplier, four tensile specimens were prepared and tested; their results are averaged. The tensile tests revealed a wide range of elongation results, ranging from 6.14 to percent. The results for all samples are shown in table 1. For SAC305 at 20 second dwell times, most of the samples demonstrated erosion in the electrodeposited copper range; several had broken through to the laminate copper. At 30 seconds, some were still in the electrodeposited range, but most had broken through to laminate copper. At 40 and 50 seconds, all had broken through to laminate copper. For Alloy 1, none of the samples demonstrated break through to laminate copper at 20 second dwell times, but several samples (1 each) did show break through at 30, 40 and 50 seconds. For alloys 2 and 3, very little breakthrough to laminate copper was witnessed, even at 80 and 100 second dwell times. It is important to note that the purpose of Phase 1 of this study was to isolate the effects of the plated copper on erosion rates. In typical soldering processes, the overall erosion rates of both plated and laminate copper may be more pertinent. The ten samples tested demonstrated initial erosion rates (20-second for SAC305 and Alloy 1, and 40 second for alloys 2 and 3) ranging between um/sec to 1.04 um/sec, as shown in figure 13 (note that for SAC305, samples C and E broke through to laminate copper, and sample K was on the borderline of break through at the 20 second data point). Alloy 3 showed the slowest erosion rates, ranging from to um/sec. Alloy 2 was the next slowest, with erosion rates ranging from to um/sec. Alloy 1 showed erosion rates ranging from um/sec to um/sec. These are all far lower than SAC305 s erosion rates 259

6 of to um/sec. As a benchmark for comparison, Sample G was processed in eutectic tin-lead solder. The erosion rate for this sample in tin-lead was um/sec, less than half that of SAC 305 and roughly the same as alloys 2 and 3. Differences in the erosion rates of the electrodeposited copper varied by a factor of 1.4 to 1.7, depending on the alloy used in the tests. REFERENCES [1] A Study of Copper Dissolution in Pb-Free Solder Fountain Systems, Byle, F., et al, Proceedings of SMTA International, 2006 [2] A Study of Copper Dissolution in During PTH Rework Using a Thermally Massive Test Vehicle, Hamilton, C., et al, Proceedings of SMTA International, 2006 [3] Have High Copper Dissolution Rates of SAC305/405 Alloys Forced a Change in the Lead-Free Alloy Used During PTH Processes? Hamilton, C., et al, Proceedings of the Pan Pacific Microelectronics Symposium, 2007 [4] Examination of JEITA Second-generation Lead-free Solder Alloy Standardization for Flow Soldering, JEITA Second-generation Flow Solder Alloy Standardization Project Group, IPC-SPVC, APEX February [5] IPC-6012B, Qualification and Performance Specification for Rigid Printed Boards, IPC, Bannockburn, IL, 2007 Figure 13. Erosion rates of electrodeposited copper samples with different alloys. The findings of the grain structure, tensile elongation at break, and SAC305 copper loss behavior tests are collated and shown in Appendix A. The notations on erosion rates, 20-second copper loss, and erosion rank are based on behavior in the SAC305 alloy only. Samples C, E and K, whose 20-second copper loss met or exceeded the thickness of the electrodeposited copper in the SAC305 trials, were not ranked. [6] IPC-TM-650, Test Methods, IPC, Bannockburn, IL, 2007 [7] The Influence of the PWB Fabrication Electrodeposition Process on Copper Erosion During Wave Soldering, Shea, et al, Proceedings of SMTA International, 2007 CONTINUING WORK Considerable differences in the grain structures, appearances, mechanical properties, and erodibility of the ten electrodeposited copper samples were observed. While differences in the erosion rates of copper from different suppliers varied by up to a factor of 1.7, no strong correlations were identified between erosion rate, grain structure, or tensile properties. The expectation remains, however, that the erodability is affected by the grain structure of the copper and work will continue until the correlation factor is identified. Additional metallographic analysis in ongoing. The cross section samples that were etched and examined under SEM will be ground past the etched areas, and polished to a degree such that the grain boundaries should be observable under polarized light, without the darkening effect of the etchant. Additional information on the subject will be published as it becomes available. 260

7 Appendix A Results for all Samples 261

8 Sample A Erosion rate: 0.63 u/sec Total copper loss (20 sec): 13 u UTS: 289 MPa Elongation: 19.3 % Erosion Rank: 1 (lowest) ~ 26 u 262

9 Sample B SAC305 Erosion rate: 0.75u/sec Total copper loss (20 sec): 15 u UTS: 317 MPa Elong: % Erosion Rank: 2 (low) ~25u 263

10 Sample C SAC305 Erosion rate: 0.85 u/sec Total copper loss (20 sec): 17u Broke thru to laminate Cu Not Ranked ~ 10 u No foil submitted 264

11 Sample D SAC305 Erosion rate: 0.88 u/sec Total copper loss (20 sec): 18 u UTS: 215 MPa Elongation: 14.5% Erosion Rank: 3 (medium) ~24 u 265

12 Sample E SAC305 Erosion rate: 1.04 u/sec Total copper loss (20 sec): 21 u Bordering break thru to laminate Cu Not Ranked ~20 u 266 No foil submitted

13 Sample F SAC305 Erosion rate: 0.98 u/sec Total copper loss (20 sec): 20 u UTS: 293 MPa Elongation: % Erosion Ranking: 4 (high) ~28 u 267

14 Sample G SAC305 Erosion rate: 0.75 u/sec Total copper loss (20 sec): 15 u UTS: 245 MPa Elongation: 16.2% Erosion Rank: 2 (low) ~20 u 268

15 Sample H SAC305 Erosion rate: 0.76 u/sec Total copper loss (20 sec): 15 u UTS: 241 MPa Elongation: 22.6% Erosion Rank: 2 (low) ~23 u 269

16 Sample I (J) SAC305 Erosion rate: 1.1 u/sec Total copper loss (20 sec): 22 u UTS: 295 MPa Elongation: 6.14% Erosion Rank: 5 (highest) ~23u 270

17 Sample K SAC305 Erosion rate: 0.96 u/sec Total copper loss (20 sec): 19 u UTS: 299 MPa Elongation: 18.7% Break through to laminate Copper Not Ranked ~14 u 271

18 272 Sample B Surface SAC305 Erosion rate: 0.75u/sec Total copper loss (20 sec): 15 u UTS: 317 MPa Elong: % Erosion Rank: 2 (low)

19 273 Sample D Surface SAC305 Erosion rate: 0.88 u/sec Total copper loss (20 sec): 18 u UTS: 215 MPa Elongation: 14.5% Erosion Rank: 3 (middle)

20 274 Sample G Surface SAC305 Erosion rate: 0.75 u/sec Total copper loss (20 sec): 15 u UTS: 245 MPa Elongation: 16.2% Erosion Rank: 2 (low)

21 275 Sample H Surface SAC305 Erosion rate: 0.76 u/sec Total copper loss (20 sec): 15 u UTS: 241 MPa Elongation: 22.6% Erosion Rank: 2 (low)