Printed Circuit Design & Fab - October 2008 - (Page 36)

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 four 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 flowing solder were measured. The measurement locations are are shown in FiGurE 2. Total copper loss at each contact time was measured on both sides of the test coupons and averaged. To isolate the effect of the electrodeposited copper from the laminate copper, the total loss was compared to estimates of the copper thicknesses previously recorded from cross sectional analysis. Isolation of the electrodeposited layer is important in this study because the comparison focuses on the effects of different electroplating baths. For SAC305 at a 20-second dwell time, most of the Copper Loss Test Methodology samples demonstrated erosion in an amount approaching An Air-Vac PCBRM15 Selective Soldering & Rework systhe electrodeposited copper thickness range; several had tem was used to expose the samples to flowing solder. To broken through to the laminate copper. At 30 seconds, some limit sample-to-sample variation, a fixture was built for the were still in the electrodeposited thickness range, but most copper substrates to insure repeatable immersion depths had broken through to laminate copper. At 40 seconds and and positions relative to the solder flow well. The fixture is 50 seconds, all had broken through to laminate copper. For shown in FiGurE 1, both before and after installation on the Alloy 1, none of the samples demonstrated break through Air-Vac system. to laminate copper at a 20-second dwell time, but several Four solder alloys were tested. The first was the standard samples (one each) did show break through at 30, 40 and SAC alloy. Also tested were Alloy 1, a low silver SAC, Alloy 50 seconds. For alloys 2 and 3, very little breakthrough to 2 a SnCu with Ni solder and Alloy 3, a low silver SAC with laminate copper was witnessed, even at 80- and 100-second Ni. The solder pot temperature for the SAC305 alloy was dwell times. maintained at 265°C. The solder pot temperature for the The 10 samples tested demonstrated initial erosion rates alternative alloys was maintained at 270°C. All samples were (20 second for SAC305 and Alloy 1, and 40 second for alloys processed with a solder flow setting of 6.5 on the PCBRM15 2 and 3) ranging between 0.175 μm/sec to 1.04 μm/sec, (note unit. that for SAC305, samples C and E broke through to laminate Samples were dipped in a strong, organic acid flux immecopper, and sample K was on the borderline of break through diately before exposure to the solder. No preheat was used. at the 20 second data point). The details of the erosion rates Contact times for each alloy were determined in prescreening can experiments.Solder exposure test setup with test sample. be seen in FiGurE 3. SAC305 and Alloy 1 used dwell times of 20, Figure 1. Solder exposure test setup with test sample. Figure 1. Alloy 3 showed the slowest erosion rates, ranging from 0.175 μm/sec to 0.263 μm/sec. Alloy 2 was the next slowest, with erosion rates ranging from 0.263 μm/sec to 0.363 μm/sec, and Alloy 1 showed erosion rates ranging from 0.300 μm/sec to 0.538 μm/sec. These are all far lower than To perform the tensile tests, copper foil samples were conditioned at 125°C for one 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%. A total of 10 copper foils were evaluated. Four tensile specimens were prepared and tested from each supplier and the results were averaged. The tensile tests revealed a wide range of elongation results, ranging from 6.14% to 22.60%. The results for all samples are shown in tablE 1. It should be noted that suppliers C and E did not supply foils for tensile testing. Tensile Test Methodology FiGurE Solder exposure sample. Figure1.2. Copper losstest setup with testlocations Figure 2. Copper loss measurement locations measurement FiGurE 2. Table 1. Copper loss measurement locations. Table 1. 36 Figure 3. Erosion rates of electrodeposited copper sampl OCTOBER 2008 FiGurE 3. Erosion rates of electrodeposited copper samples with different alloys. printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - October 2008

Printed Circuit Design & Fab - October 2008 Contents Our Line Market Watch Around the World Happenings Test and Inspection ROI Tip Jar Interconnect Strategies Final Finish Forum The New Wave in High-Speed Modeling The PCB Design Library Mixed Signal Design Considerations Modeling Conductor Surface Roughness Copper Erosion: The Influence of Metallurgy on Copper Dissolution The Wave of the Future Ad Index Building a Profitable Niche Marketplace Off the Shelf BGA Bulletin

Printed Circuit Design & Fab - October 2008

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