Wave. Early Testing An evaluation was conducted by Hewlett-Packard using. Variable Oxygen Level Tests

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1 Development Wave By Cheryl A. Lechok, Martin 0. McDonald and Santiago Rodriguez-Sallaberry H ewlett-packard's Puerto Rico Manufacturing Operation (PRMO) in Aguadilla, PR, has been practicing no-clean wave soldering in an air atmosphere since The elimination of postsolder cleaning was a step toward achieving HP's corporate goal of total CFC elimination by Although inert atmosphere wave soldering has been proven to reduce manufacturing costs and improve board quality, Hewlett-Packard engineers at PRMO had to demonstrate that the cost savings resulting from reduced dross, maintenance and solder consumption would justify the addition of nitrogen.'s2 A key environmental benefit would be the reduction of lead-bearing solder dross in light of debate about its classification as a hazardous waste.' Both the cost of equipment and nitrogen operating costs were considered in the decision to inert the wave soldering process. n HP's case, the use of cryogenic nitrogen did not generate anticipated cost savings, so a cost-benefit analysis for an alternative nitrogen supply system was conducted. Nitrogen can be manufactured in noncryogenic membrane systems by passing compressed air through modules of semipermeable hollow fibers and purifying the resulting gas stream (Figure 1). FGURE 1: Noncryogenic membrane system. Early Testing An evaluation was conducted by Hewlett-Packard using two types of nitrogen-inerted wave solder machines-a long-hood retrofit inerting system, manufactured by Electrovert Ltd. (Grand Prairie, TX), and SEHOs (Kreuzwertheim, Germany) Nitrogenius inert wave solder machine. The retrofit system enclosed the entire wave solder machine from preheat section to solder pot. No significant difference in soldering quality was observed between the two inert wave soldering systems. However, installation, downtime and equipment costs proved obstacles to adoption of either-alternative. HP elected to test a "short-hood" wave solder retrofit system, citing its lower capital cost and ease of installation (Figure 2). The short-hood system enclosed only the wave area above the solder pot. Both the long- and shorthood inerting retrofits could maintain less than 10-ppm oxygen in nitrogen atmosphere. Variable Oxygen Level Tests Tests were conducted with the wave-only (short-hood) inerting system, which was installed on an Electrovert Econopak 1 machine. Oxygen levels in the inert gas supply system were controlled to process boards in O-, 100- and 1,000-ppm oxygen in nitrogen. Thus, HP could correlate the effects of increasing oxygen levels on solder defects, board residue and dross to establish the highest p " \ acceptable quality level while reducing operating costs. [ 2 A second factor to be examined was the effect of varying f flux quantity when soldering in a controlled atmosphere. Both solder-plated and SMOBC circuit boards were processed in air and nitrogen. The assemblies used in 42 CRCUTS ASSEMBLY APRL 1992

2 of No-Clean Soldering testing were manufactured at HP s Puerto Rico and Boise, D, facilities for installation in printers and plotters. The first test board, a MACE- 30, was a high-density, mixed-/ technology PCA with 994 solder pads and 796 plated through holes (PTHs). The second test board contained about 750 PTHs. Dynachem DM dry-film solder mask was used. Both through-hole and SMT components were soldered in the wave soldering process. A Multicore X33-04 low-solids flux was deposited by ultrasonic spray fluxer. A total of 60 PCAs were evaluated at various oxygen and flux levels. The experimental design for the tests was a full 3 X 2 factorial. Oxygen concentration and amount of flux were varied, while machine type, board type, preheat profile, solder mask and flux were held constant. The boards were examined for bridges, excess so der, skips, unfilled PTHs, solder balls and visual residue. - Oxygen concentration and the amount of flux were varied, while machine type, board type; preheat projile, solder mask and flux were held constant. A controlled amount of medical grade air was premixed into the nitrogen supply to simulate membrane nitrogen at various purities. Nitrogen flow rate was maintained at 1,800 SCFH. An oxygen probe installed in the inerting hood over the pumped solder wave measured the oxygen level in the inert atmosphere. Repeatability of oxygen level was verified with a strip chart recorder. FGURE 2 Nitrogen wave soldering hood. Results For all levels of oxygen tested, no unfilled via holes or visible residues were observed. Analysis of variances (ANOVA) was performed on each type of solder defect (Table 1). Effects on Bridges, Shorts and Skips Soldering at 10-ppm oxygen produced bright, shiny solder joints with no bridges or skips. At 100 ppm, noticeable oxides began to form on the solder wave, causing a slight increase in bridging between through-hole components. The solder joints remained shiny. -At!,000-ppm oxygen: excessive bridging occurred between through-hole components at a rate of 10 to 15 bridges per board. The formation of micro-dross on the solder wave was identified as the cause of this increase. The solder joints also appeared discolored, and there was a slight increase in skips and bridges on the bottom-side components. To minimize bridging, mechanical adjustments were made so that micro-dross would flow off the solder wave. The second series of boards showed no statistical difference in the number of bridges for through-hole or bottom-side component connections when soldered at levels of 10- and 1,000-ppm oxygen. The higher bridging rate and solder-joint discoloration APRL 1992 CRCUTS ASSEMBLY 43

E 8 P - Grand average.-w Error bounds for E u) FGURE 3 SMT solder skips V. ppm oxygen level. seen in the first series at 1,000 ppm was not present in the second series.

3 E 8 P - Grand average.-w Error bounds for E u) FGURE 3 SMT solder skips V. ppm oxygen level. seen in the first series at 1,000 ppm was not present in the second series. The total number of skips and bridges on the bottom-side components was re- TABLE 1: Summary of analysis of variances data duced by a factor (ANOVA). of five (Figure 3). The data also indicated that greater amounts of flux resulted in fewer solder skips at bottom-side component pads. Flux deposition was the main factor affecting skips. Effects on Excess Solder Solder volume at bottom-side component joints increased with the amount of flux deposited. Atmosphere oxygen content had only a SOLDERNG TECHNOLOGY NTERNATONAL The Source For All Your Training Supplies WORKMANSHP POSTERS TRANNG KTS (Through Hole & SMT) right ditection. NDUM CORPORATONOFAMERCA " " Vorkmanship Posters are 20' x 28" nd meet the reauirments of WS- 536E paragraph ' and Dodltd B paragraph 5 4. DUMMY COMPONENTS Call For nformation 1 Acosteffectivewaytoprovideeachstudeni with the m e lype of PWB's components and terminals for solder training. SST Kb contain two PWB's, and all the components to fill the boards plus 15 each of 5 dilferenl!ypes of terminals. SMT Kits contain one board and a wide variety of pat%. Both kits M static shielding bags to ~X'%aiareness. ALSO AVALABLE 0 Terminal Holding Blocks o Video Presentations Slide Presentations Lesson Plans 0 workshops SOLDERNG TECHNOLOGY NTERNATONAL, NC. P. 0. Box W. Allen Ave. Ste. 10 San Dimas, CA (714) FAX (71 4) CRCLE 27 ' 44 CRCUTS ASSEMBLY APRL 1992

minor effect on the number of joints with excess solder (Figures 4 and 5). Effects on Solder Balls The solder ball quantity, diameter and frequency of occurrence on through-hole vs.

4 minor effect on the number of joints with excess solder (Figures 4 and 5). Effects on Solder Balls The solder ball quantity, diameter and frequency of occurrence on through-hole vs. surface-mount technology component joints was measured in a 1-in.-sq. area on the printed circuit assembly, and ANOVA was performed. For both types of components, the oxygen level affected the total number of solder balls but only had a minor effect on solder ball density. After a thin layer of dross formed on the solder surface, it had a dampening effect on solder splashing from the pumped waves. This prevented backsplash of solder onto the circuit boards and reduced solder ball occurrence. Solder ball density at 1,000-ppm oxygen levels was 20% less than at 10-ppm oxygen levels. The results showed that the controlled oxygen atmosphere process is relatively insensitive to the amount of flux used. Because this decrease is slight, controlling atmosphere is obviously not the answer to controlling solder ball density. Oxygen level only marginally affected solder ball diameter. The average diameter was estimated to be 7.4 mils with a standard deviation of 2 mils. The quantity of flux was determined to be a statistically insignificant factor in the formation of solder balls. These findings confirmed HP s hypothesis that solder ball formation is more directly related to the type of solder mask used. t should be noted that the results are valid only within the range of oxygen levels and flux volumes tested. e 1 FGURE 4 Through-hole excess solder vs. ppm oxygen level. Economics vs. Nitrogen Purity After maintaining the 1,000-ppm oxygen level in the solder pot for four hours, a slight amount of dross accumulated. However, overall dross was still 85 to 88% less than the amount of dross formed in air (20.9% oxygen) (Figure 6). An economic analysis showed that Hewlett-Packard s cost savings with membrane nitrogen (1,000-ppm residual oxygen) were greater than with either air or cryogenic liquid nitrogen. Annual cost savings of over $22,000 (based on two-shift days and six-day weeks) were realized when dross reduction, makeup solder, maintenance and nitrogen cost were considered. Summary Hewlett-Packard s engineers found that soldering results obtained usin nitrogen with 1,000-ppm oxygen wer comparable in solder defect level an board quality to results using 1C ppm oxygen and superior to board processed in air. The results wer valid for each type of circuit boar tested. The results showed that the cor trolled oxygen atmosphere process 1 relatively insensitive to the amour of flux used. Thus, the assembler ca use less flux, and the printed circu assemblies will have less visible resl due (a cleaner look) than air-pro cessed printed circuit assemblies Hewlett-Packard anticipates that ai inert soldering process will also re duce maintenance on test fixture due to less visible flux residue at th test pads. - - Based on this series of tests, th FGURE 5 Surface-mount excess solder vs. ppm oxygen level. APRL 1992 CRCUTS ASSEMBLY 45

R. Crothers. Optimizing a No- Clean Circuit Board Process Using Nitrogen nerting of a Conventional Wave Solder Machine, NEP- CON West Conference Proceedings, February, 199 1. 2. Nowotarski, M. S., W.

5 R. Crothers. Optimizing a No- Clean Circuit Board Process Using Nitrogen nerting of a Conventional Wave Solder Machine, NEP- CON West Conference Proceedings, February, Nowotarski, M. S., W. Down and S. Davoudi. The Effect of a Nitrogen Atmosphere on the Wave Tinning of Component Leads, NEPCON West Conference Proceedings, February, Dammrich, T. J. Solder Dross Ruling Overturned, PC Regulatory Alert, December, FGURE 6: Dross formation rate with membrane nitrogen. Puerto Rico Manufacturing Operation will convert three wave soldering machines to nitrogen with waveonly inerting hoods. Economic comparisons determine that membrane nitrogen with a standard purity of 1,000-ppm oxygen can be more costeffective in the long term for the Aguadilla operation. Although this form of nitrogen supply produces 12% more solder dross than low-oxygen (3 to 5 ppm) cryogenic nitrogen, its lower cost relative to the high-purity liquid justifies its use. CB References 1. Hanaway, E., L. J. Hagerty and Cheryl A. Lechok is marketing manager of printed wiring board applications and Martin G. McDonald is a development engineer at the Linde Division of Union Carbide ndustrial Gases nc., Tarrytown, NY Santiago Rodriguez-Sallaberry is z manufacturing engineer at the Puerto Rico Manufacturing Operztion of Hewlett-Packard, Aguadilla, PR. olders. nlike tin-lead solders, they won t scaven e or lea& metallizations of precious metals or form troublesome intermetallics. Their stren th is derived from outstandine ductilitv whicf also Microscan s new AT100 kmes axid c6mponents per hour. That s fast! Want to see more? Like the photodetector that eliminates gap? Call Rick M& at 206/ for an even faster response NDUM CORPORATONOFAMERCA r S76 Lincoln Ave., Utiq NY w502 * (ns) or NDUM ndium corpontbn of Euro 7 Grldde Court, Wobum Road ndustrial Estate, Keopton, &oh MK42 7EE England (0234) Ma Of6ces-Wwdwide representation via: 1 Sassoon Metals ndiunt an Chemik taly (on)-7 and New York (p2) 7586% CRCLE 29 microscan P.O. Box 1227 Freeland. WA (206) CRCLE 30 APRL 1992 CRCUTS ASSEMBLY 47

Measuring the Benefits of Nitrogen for Reflow U.S. Robotics, Morton Grove, IL

Measuring the Benefits of Nitrogen for Reflow U.S. Robotics, Morton Grove, IL Measuring the Benefits of for Reflow U.S. Robotics, Morton Grove, IL Chad Boeding, Mark Nowotarski Praxair, 777 Old Saw Mill River Road, Tarrytown, NY 10591-6799 Julie Bradbury, Denis Jean U.S. Robotics,