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PROTECTING WAVE MACHINES FROM THE CORROSIVE EFFECTS OF PB-FREE ProtectingProtecting wavewave soldersolder machinesmachines fromfrom thethe

Lost in the conversion to (Pb)-free wave soldering is the corrosivecorrosive effectseffects ofof actual process of assuring that a wave solder machine is compatible with lead-free solder alloys. Determining Pb-freePb-free solderssolders compatibility can be a time consuming and expensive process. Several methods have evolved for protecting Table 1. Characteristics of lead-free solder alloys. wave solder machines from the corrosive effects of high Solder Solder Pot Temperature Tin Content Density content lead-free solder alloys. SnCu 227ºC 270-280ºC 99.3% 7.29 While some of these methods SnAg 221ºC 265-275ºC 96.5% 7.44 address certain unique requirements of lead-free wave SnAgCu 217ºC 260-270ºC 96.5% 7.38 soldering, this has created the SnAgCuSb 217ºC 260ºC 90.5% 7.24 need for an alternative SnPb 183ºC 250ºC 63.0% 8.80 solution that is total compatible with prolonged use Introduction The majority of lead-free (SnO2), forms at a high rate of highly corrosive lead-free An increasing number of solder alloys exhibit decreased due to higher tin content and solder alloys. One recurring discussion manufacturers are wetting characteristics and higher processing temperatures with lead-free wave soldering implementing lead-free wave slower wetting times in and results in more oxidation is the use of surface coatings soldering processes in comparison to tin-lead (SnPb) and dross. In a standard SnPb versus alternative base compliance with pending solders and flow characteristics wave pot, when impurities for machine elements in regulations. As has been of lead-free alloys are generally such as build up they contact with molten lead-free discovered, lead-free wave more viscous. Lead-free alloys form with the solder. This article analyzes the difference between these two soldering involves more than have melting points between tin. Reducing the temperature material alternatives and the simple replacement of one 30-40ºC higher than SnPb of the solder pot, allowing the compares they’re performance, solder alloy with another and solders requiring higher pot to sit idle for a few hours, longevity and cost. The cost requires a complete processing temperatures, and skimming the top surface and performance benefits are examination of the process putting increased demand on can easily remove these explored and methods of capabilities of the entire wave flux performance. The use of a impurities. This method works determining which material alternative offers greater solder process. For lead-free higher activity flux may be well for eutectic solder since service life, requires less wave soldering to be considered, however, the the density of the copper-tin recurring maintenance, and is successful in a production volume of flux applied to a (CuSn) is 8.28, therefore less expensive to environment, necessary circuit board and the and the density of the SnPb operate. It also addresses the changes to the entire process uniformity of deposition is solder is 8.80, allowing the benefits of field retrofitting this must be considered. essential. Increased preheat is copper-tin intermetallic to alternative base solution Lead-free processing generally required because of float to the top surface.With within existing wave solder machines, eliminating the requires a somewhat tighter the higher melting point of the use of lead-free solder replacement of wear parts process window than lead-free alloys. Solder pot alloys, the situation changes and minimizing equipment lead-bearing processing, temperature and solder since the density of lead-free downtime for lead-free making robust process control contact time must be alloys is less than that of the wave soldering. more critical when working increased but at the same time CuSn intermetallic (Table 1). with lead-free. Yet the basic controlled to avoid thermal As a result, instead of floating Tony Gyemant elements of forming good shocking, or excessive thermal on the surface where they can Technical Director quality solder joints remain differential, upon entering the be easily removed, the CuSn Specnor Tecnic International the same: flux, heat and solder. solder bath. intermetallics sink in lead-free St-Hubert, Quebec, Canada Therefore, critical variables in Compared to SnPb solder, alloys and are dispersed the wave soldering process lead-free alloys oxidize at a throughout the solder pot. In Keywords: such as flux deposition, more rapid rate when the addition, some lead-free alloys lead-free, wave preheat and solder solder is in liquidous state due dissolve copper at a faster rate soldering, solder alloys, temperature, and solder to their increased tin content. than SnPb solder. This surface coating, dwell time must be Tin oxide, consisting of effect can result in a higher base metals. precisely controlled. tin-oxygen (SnO) and rate of copper build-up and

10 Global SMT & Packaging October 2004 www.globalsmt.net PROTECTING WAVE SOLDER MACHINES FROM THE CORROSIVE EFFECTS OF PB-FREE SOLDERS contamination of the solder Table 2. Summary of base metals. *Longevity under sustained production conditions in direct contact with molten lead-free solder. pot. During long periods of operation, lead-free alloys can Material Advantages Disadvantages Longevity* begin to exhibit sluggish 304 stainless Inexpensive No resistance 1 month behavior caused by the 316 Inexpensive No corrosion resistance 3-6 months buildup of CuSn intermetallic Cast iron Inexpensive No corrosion resistance 1-3 years that forms at the bottom of Surface coated Good resistance to Will degrade when 6-12 months the solder pot. This problem stainless steel corrosive effects of tin coating is scratched generally doesn’t exist with Surface coated Good resistance to Will degrade when 3-5 years standard SnPb solders because cast iron corrosive effects of tin coating is scratched the CuSn intermetallic floats Titanium Excellent resistance to Can be expensive to fabricate 10 years and is easily removed. corrosive effects of tin

Optimized solder flow contents than their stability of the wave solder damaged after only a few The wetting of solderable lead-bearing counterparts. process. Wave solder pot months of operation with surfaces does not occur as Practice has shown many components such as flow lead-free alloys (Table 2). The readily with lead-free alloys as lead-free alloys cause corrosion ducts, solder nozzles and solder use of high-grade 316 stainless with eutectic solder. Longer of the base metals used for pump impellers should be steel does retard this effect dwell times and higher solder solder pot components due to replaced with a material that somewhat since 316 stainless pot temperatures are required, the aggressive nature of tin at prevents corrosion and steel exhibits a reduced rate of and the use of a nitrogen high temperatures. The surface ultimate failure. iron dissolution since it environment is recommended of many base metals such as An approach to alleviate contains less Fe due to its to assist the wetting process. stainless steel or cast iron this problem has been the higher 10-14% (Ni), It has become accepted generally show signs of pitting application of a surface 10-18% chromium (Cr), and practice to change the and start to dissolve during coating on solder pot 2-3% molybdenum (Mo) characteristics of the laminar prolonged contact with components. However, surface content. Unprotected cast iron solder nozzle since longer molten lead-free alloys. This coatings provide minimal solder pots are also prone to contact time is required due to leaching process releases iron corrosion protection and are high rates of iron dissolution the lower wetting properties of (Fe) particles into the molten susceptible to scratching due to the inherent high Fe lead-free alloys. The distance solder and can result in because of the higher content of cast iron. between the chip wave and contamination of the maintenance levels needed to The application of a laminar wave is also reduced solder alloy. remove the CuSn surface coating on stainless to increase the time above Unprotected solder pump intermetallics that sink to steel or cast iron solder pot liquidous, as well as to impellers, flow ducts and the bottom of the solder pot. components does provide minimize the temperature drop solder nozzles are subjected to some degree of corrosion between contact points, and to erosion caused by the Surface coated versus resistance, however allow the solder joint to corrosive effects of high tin base metals complications arise from their recover more quickly. content lead-free alloys. Over Experience has indicated that use. Prior to application of Increasing the length of the time, this erosion can degrade critical solder pot components the coating, all base metal chip wave also improves the performance of the wave such as pump impellers components must be subjected wetting, while increasing the solder machine that will result fabricated of unprotected 304 to a passivation process that preheat output produces a in an adverse effect upon the stainless steel can become consists of dipping into a similar effect. Reducing the fall height of the wave decreases the distance of overflowing and reduces the amount of dross generation with lead-free alloys. Nitrogen inerting of the solder pot is recommended since it minimizes exposure of the liquidous solder to oxygen resulting in decreased dross. Reducing the rate of oxidation and the resulting dross build-up significantly improves the performance of the wave soldering process.

Corrosive effects of lead-free solders Figure 1. Lead-free retrofit kit with titanium solder pump impellers, flow ducts, solder Most lead-free alloys have nozzles and solder pot liner. significantly higher tin

www.globalsmt.net October 2004 Global SMT & Packaging 11 PROTECTING WAVE SOLDER MACHINES FROM THE CORROSIVE EFFECTS OF PB-FREE SOLDERS nitric acid, phosphoric acid or eliminating the periodic Europe, Asia and North when all operational factors other similar acid solution to replacement of surface coated America. Their use represents are taken into consideration. clean the metal surface and parts (Figure 1). a cost effective solution to rapidly form a chromium oxide prevent erosion of critical Conclusions on the surface. This necessity End user benefits machine components and The use of titanium as an complicates the ability to field With a wide variety of wave assure consistent alternative base metal for retrofit components of an solder systems in use performance when using solder pot components has existing wave solder machine. throughout the global lead-free solder alloys. demonstrated high resistance A variety of surface coatings electronic assembly industry, When analyzing the cost to the corrosive effects of can be applied including upgrading existing and new considerations of surface lead-free solder alloys. It is Melonite, Teflon or other wave soldering machines for coated and alternative capable of consistent proprietary coatings. lead-free operation has titanium solder pot performance while Regardless of the type of become a complex task. components, cost elements eliminating the periodic surface coating, their A titanium solder pot liner is encompassing procurement, replacement of solder pot application thickness is available that can be placed installation, operation and components traditionally limited and they are inside of an existing cast iron sustaining engineering must be prone to wear, such as solder particularly susceptible to or stainless steel solder pot to taken into consideration. In pump impellers, flow ducts or scratching because of the convert an existing wave general, surface coated solder nozzles. This necessity to remove CuSn soldering machine for lead-free components may have a movement to titanium intermetallics that sink to the use without having to remove slightly lower purchase cost, solder pot components has bottom of the solder pot. the existing solder pot from but installation and shipping brought about manufacturing Over time repeated scratching the machine (Figure 2). of a heavier, surface coated productivity gains, increased breaks down the surface A significant advantage is that cast iron solder pot is machine performance, and coating, and the resulting the titanium solder pot liner, significantly more than most importantly, lower erosion damages the solder pot together with the replacement titanium. The additional operational costs critical components and contaminates titanium solder pump production downtime to in today’s competitive the solder alloy. impellers, flow ducts and physically exchange the surface environment. An alternative base metal solder nozzles, can be field coated cast iron solder pot is that is highly resistant to the retrofitted to most major another disadvantage. Tony Gyemant is Technical aggressive nature of brands of wave soldering When considering the Director for Specnor Tecnic continuous tin scavenging is machines with minimal lightweight of titanium, International in St-Hubert, titanium. Titanium is one downtime. They are equally installation costs are Quebec, Canada. He designs material that is ideally suited effective for upgrading existing substantially less than surface and develops wave soldering for solder pump impellers, flow wave solder machines or coated cast iron components. machines, reflow ovens and ducts and solder nozzles in equipping newly acquired The greater service life of mini-wave selective soldering contact with lead-free solder machines for lead-free operation. titanium means that recurring equipment for through-hole and alloys since it is impervious to Currently in use, titanium maintenance for periodic surface mount printed circuit the effects of tin scavenging. solder pot liners and retrofit replacement of wear parts such board assemblies. For 22 years, Traditionally titanium has kits have successfully met the as solder pump impellers is Gyemant has improved soldering been considered expensive to lead-free requirements of virtually eliminated. Therefore applications throughout the fabricate, but new major contract manufacturers titanium solder pot electronics assembly industry. developments in titanium and original equipment components are less expensive He may be contacted at fabrication techniques have manufacturers throughout than surface coated components [email protected] made its use cost competitive with other base metals, and have improved integrity, and quality when forming and welding complex shapes. The frequency with which solder pump impellers, flow ducts and solder nozzles made of traditional base metals wear out, and have to be replaced in lead-free wave soldering applications, has caused a rethinking of the use of titanium. Retrofit kits are now available that replace the solder pump impeller, flow ducts and solder nozzles with parts made of titanium that can easily withstand prolonged Figure 2. Titanium solder pot liner prevents iron dissolution and prolongs the life of the solder pot. service in lead-free applications

12 Global SMT & Packaging October 2004 www.globalsmt.net