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SMT surveyed soldering equipment manufacturers and solder suppliers to learn more about the latest practices and requirements. In Part I, we examine nitrogen and other reliability enhancers, as well as solder alloy R&D.
Gail Flower, Editor-at-Large, SMT
Is nitrogen essential to high-quality soldering? What other factors contribute to quality?
Solder equipment suppliers did not all agree on the answer to this question. Heller's Marc Peo notes that high-quality products can be soldering without nitrogen, but that certain applications (consider no-clean or bare copper) stipulate that nitrogen be used. "Nitrogen literally replaces the oxygen in the oven chamber and allows the flux to focus on reducing all the oxides on the pad and paste without being distracted by the oxygen in the chamber. So nitrogen can be helpful and certainly expands the process window but the benefit needs to be weighed against the cost."
There are other ways to improve reliability. "Nitrogen is used to avoid oxidation of the pad before the soldering process," says Carlo Perucca, product manager at Seica SpA. "The Seica Firefly system is controlling the temperature profile for each solder joint and the combination of the software and hardware avoid oxidation before soldering. The flux contained in the wire alloy is also de-oxidating any pre-existing oxidation on the pad." Nitrogen is not used on any of Seica's installed base, Perucca says.
Reader Survey. SMT asked readers about their soldering processes and production. Hundreds of electronics assemblers completed the survey.
Mick Austin, director, Americas, Vitronics Soltec, agrees that for most mass soldering applications, nitrogen gas is not essential: "This is primarily because materials have evolved considerably in recent years, whereas during the initial introduction of lead-free soldering, most everyone believed that nitrogen was indispensible." "Nitrogen is not essential to high-quality soldering. This has been proven many times; witness the fact that very few of the large EMS companies use this method and have no issues on quality of throughput," said Graham Norman, owner of EVS International. Nitrogen opens the process window, he adds, but is an expensive way to handle the problem. "With modern thermal profiling and more accurate control of the wave soldering inputs, it is not necessary." The real problem is dross removal and maintaining a clean wave solder pot, according to Norman. "Mechanical removal and recovery offer the best of both worlds –reduction of dross and a cleaner wave. The EVS solder recovery system is designed to handle both dross removal and metal recovery." Because of advances in flux technology and lead-free alloys composition, the process window has been widened and using nitrogen is now optional, agrees Austin, who says that most soldering processes benefit from nitrogen blanketing, but the cost of nitrogen must be balanced against the value that the manufacturer gains. If using nitrogen only improves appearance and not the robustness of the joint, then nitrogen isn't the answer. Products with lower margins generally aren't seeing payoff from the increased manufacturing costs involved in adding nitrogen.
Henry Mann, CEO of Manncorp Inc. concurs. "Today's soldering equipment buyers want lower-cost, compact, high-quality, lead-free convection reflow with independently controlled multi-zones and on-board profiling. Energy consumption is another important consideration. Reflow systems with these features are more suitable than costlier ovens with nitrogen capability," he says. Manncorp has sold several eight-zone nitrogen ovens to international assemblers, but in the U.S., computer-controlled benchtop reflow models with standby nitrogen capability are popular, he adds. Joachim Biegel, product manager reflow soldering systems at Essemtec, agrees that highly sensitive sectors use nitrogen, industries such as aerospace, medical, and military, where a reduced-oxygen atmosphere is required. "However, for standard SMT soldering processes, we cannot see big differences in solder quality with or without nitrogen. In fact, we see a much bigger influence of the soldering profile to the presence of voids in the solder joints," he said. Accurate temperature profile control is more important than an inert gas atmosphere in his assessment. Bob Wettermann of BEST Inc. adds that the rework market largely rejects nitrogen as a process aid.
Marc Dalderup, COO, Rehm Thermal Systems, supplier of condensation and convection soldering systems to the high-end electronics industry, points out that 90% of Rehm's equipment is sold as nitrogen versions, and of that 90% sold with nitrogen capability, 70–80% actually run with nitrogen regularly. Nitrogen isn't the only factor contributing to quality soldering. "A quality soldering process comes from items like board materials, solderability of the boards and components, solder paste quality, paste printing process, and in the reflow process, the thermal reproducibility of the profile used," said Rehm's Dalderup.
Christian Ott, senior sales and process manager at SEHO Systems GmbH, says that soldering in nitrogen atmosphere improves both quality and long-term solder joint reliability. Although it would seem that nitrogen would increase production costs, he suggests that the opposite can be true. Soldering in a full nitrogen atmosphere reduces typical failures, such as solder bridges, and results in lower dross formation and lowered consumption of solder alloy, Ott points out. "If production demands are high enough to run a wave soldering machine more than one shift, the return of investment (ROI) for a new full tunnel N2 machine can be as many as two years." SEHO offers both local inert systems and a tunnel nitrogen system.
Reader Survey. Respondents were engaged in diverse end-markets as well.
"Nitrogen is important to prevent build-up of oxides and improve the flow of liquefied solder paste during the reflow soldering process," said Manfred Maehl, president of SMT North America Inc. SMTNA's soldering equipment includes an intelligent nitrogen control system with integrated calibration for ensuring traceability throughout the entire reflow process cycle. The nitrogen system measures and controls the flow of nitrogen to develop consistent residual oxygen levels, which reduces nitrogen use to the minimum necessary, controlling cost.
John Wol, president of Pillarhouse USA Inc., agrees that nitrogen is an enhancement to selective soldering, providing an inert atmosphere and preventing oxidation from forming on the component and circuit board surfaces. Al Cable, president, ACE Production Technologies, states that nitrogen is a required consumable with all selective soldering. He explains that, without nitrogen, solder bridges and forms icicles. "Nitrogen reduces the surface tension of the solder, allowing the molten solder to readily break away from the solder site. In open atmosphere, dross formation will overwhelm the solder pump and nozzle causing excessive cleaning and pump maintenance," Cable points out. "Since nitrogen improves the flow characteristics of the molten solder, it enables soldering of tight spacing between adjacent components on a PCB," Wol adds. Some of Pillarhouse's selective soldering systems are equipped with a nitrogen curtain, which acts as a localized pre-heat for the solder joint while reducing thermal shock to nearby components. Companies like Pillarhouse and Juki offer nitrogen generators for producing nitrogen directly at the machine on demand, eliminating tank shipment, storage, and management.
A major issue in electronics manufacturing is cost of ownership (COO) for process equipment. This includes the original purchase price and ongoing operational costs. The cheapest reflow oven is not always the least expensive, warns Fred Dimock, manager, process technology, BTU International.
What are the latest alloy developments?
"The alloys themselves have no bearing on cleanability," clarify Steve Dowds and Mark Currie, Henkel. Rather, alloy composition dictates peak processing temperature; the hotter the temperature, the harder it is to clean residues. For instance, changing from a SAC387 to a SAC387 + Ni doesn't affect cleanability. "While alloys can impact rheology and metal loading for performance, as well as solderability, they really have no impact on open time," they add.
Alloys have the most impact on long-term reliability. Higher service temperatures (which Henkel addresses through their Innolot alloy) and improved drop resistance (addressed by doping a SAC alloy with minor metals) are driving trends in SMT.
Reader Survey. Lead-free has become more common in recent years.
Researchers are always developing new alloy formulations, asserts Karl Seelig, VP at AIM. "The original SAC alloys were found to have reliability and cost issues in certain applications and product types. The trend is to move away from silver and to turn to low- or no-silver tin/copper alloys." "Early in the lead-free conversion, many assemblers purposely avoided bismuth due to the potential complications that could result if it inadvertently mixed with lead on component leads. Many systems are now fully lead-free, and assemblers are realizing the benefits of using SnBiAg and SnBi alloys, such as lower alloy, laminate, and energy costs," notes Erik Bergum, Heraeus Materials Technology's VP, Americas.
In one study AIM chemists conducted, tin whiskers were less predominant in silver-free or low-silver alloys when compared to SAC alloys. Other studies indicate that after soldering, silver-free alloys are less prone to cracking. Therefore, AIM focuses on alloys with uniform grain structures and wide process windows. EMS providers prefer stable products with a broad process window and great wetting without the use of hazardous chemicals, Seelig adds. Indium Corporation develops alloys with shock resistance as well as good thermal cycling performance, says Tim Jensen, product manager for PCB assembly products. "Lead- (Pb-) free solder joints are not as reliable as their tin/lead (Sn/Pb) predecessors." Jensen explains that SAC305, formerly the most common Pb-free alloy, exhibits poor drop shock reliability because of the 3% silver (Ag) content (the brittleness that Seelig noted). To counter this, users look for lower-Ag alloys, such as SAC105, Jensen concurs. Balver Zinn has been introducing alloys with a focus on lower cost while maintaining joint integrity, Stan Renals, managing director and CEO, Cobar division, added. "Due to cost pressures, we supply an alternative to the common SAC alloys available." Balver Zinn has seen success with the SN100C alloy, for which Nihon Superior Co. Ltd. holds the original patent. "Nihon Superior continues to invest in internal and collaborative studies to gain a deeper understanding of the fundamentals of lead-free solder," said Tetsuro Nishimura, president, Nihon Superior. The company has yet to discover changes that would improve the original SN100C formulation. The proliferation of alloy compositions and related additives, or dopants, has escalated over the past few years, notes Derek Daily, GM/Advanced Technology, Senju Comtek Corp. (Americas). He credits alloy development with dramatic improvements in drop and shock performance. Qualitek's Sn100e is a lead-free alternative for those now using SAC alloys. The alloy comprises tin/copper/cobalt but no silver or nickel. Furthermore, it has good wetting characteristics compared to Sn63, the company claims. Cookson Electronics chemists have introduced the SACX Plus alloy product line with lower Ag content, using other additives to meet cost control and product performance reliability requirements. "Cookson continues to engineer alloys for applications such dual-alloy SMT processing or low-temperature processing," says Paul Cote, global marketing communications manager, Alpha. The lower Ag percentage allows cost reductions, while keeping silver content maintains the wetting standards we have come to expect from SAC processes, Daily concludes.
Jaspir Bath of Christopher Associates says Koki Solder has made changes in their paste formulations to deal with issues of warpage of the component or board, ball co-planarity issues for BGA/CSP components, and non-wetting or excessive oxidation. "Koki has developed both tin/lead and lead-free solder pastes with new flux formulations in the solder paste having higher heat resistance, which did not lose their flux activation properties as quickly and quicker wetting, which helps to reduce the head-in-pillow defect," said Bath.
Equipment suppliers vary widely in their views on whether nitrogen is essential as an inerting gas to prevent oxidation during soldering. Some say it is generally not necessary, while others point out that it is essential for critical applications, such as military, medical, and aerospace. Some equipment suppliers are seeing most new equipment requests including nitrogen processability.
Solder alloy suppliers agree that legacy SAC alloys did not meet the industry's reliability demands, particularly due to silver causing brittle joints. As a result of problems with SAC alloys containing silver, most suppliers developed other alloys to bridge the gap. John Vivari, applications engineering supervisor, Nordson EFD, states it best in saying that the performance of lead-free alloys must keep up, improving, as the industry creates more demanding lead-free soldering applications. Usability, cleanability, reliability, etc., cannot be sacrificed when adapting to materials restrictions.
Just as materials scientists have eased the transition to lead-free, their current task is making halogen-free a workable, palatable goal, predicts Brian Smith, business manager, EMS, Kester, an Illinois Tool Works Company. In Soldering Equipment and Materials Updates, Part II, solder material suppliers answer questions about the latest restrictions on hazardous materials. Also, solder equipment makers discuss new equipment or upgrades to existing equipment. Read Part II on smtonline.com. SMT
Gail Flower, editor-at-large, SMT, may be contacted at email@example.com.