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BRAZING & SOLDERING TODAY Challenges in Attaining -Free

The switch to cadmium-free alloys has been a little easier than finding a lead-free that performs similar to the one standard from the past

BY PHILIP BASKIN

ver the last 15 years, there have is similar to what occurred in the brazing nature was learned, it was widely agreed been many changes in the solder- industry as the conversion took place to that it was appropriate to remove it for the OOing and brazing industries as a re- make Cd-free BAg alloys. long-term health and safety of brazers. sult of chemical and metallurgical regula- One of the commonalities between the ead is being removed from con- tions that have changed the industrial Cd-free BAg alloys and the Pb-free soft sumer electronics soldering be- standards for making products. solders has been the heightened melt- LLcause when it leaches into the soil point of the filler alloys. The eutec- from a landfill, lead is deposited into water Impact of Removing tic of 63Sn/37Pb melts at 183°C (361°F). and the soil, which is detrimental to the By replacing lead with silver, , environment. Lead oxide is created during Cadmium and Lead and/or other additions, and raising soldering, and the objective of removing the percentage of to 91–99.3% of over- lead is to create a safer environment. The The brazing industry in the early 1990s all composition, the of cur- actual amount of lead used by the elec- removed cadmium (Cd) from the BAg rent conventional Pb-free solders is in the tronics industry totals 0.3–0.5%1 of the (boron-silver) alloys. Cadmium, a toxin, range of 217°–227°C (423°–440°F). An- overall lead use. The battery and ammu- was added to lower the melting point for other factor affected by the removal of nition industries represent more than 60% silver brazing filler . It promoted ex- lead is the density of the alloys having been of total lead use. cellent flow of the filler metal, and left a converted from a heavier 63Sn/37Pb to clean, shiny postbraze joint. With the re- lighter tin-based alloy. Common Alloys and moval of cadmium, the melting point/flow ue to the removal of major com- phase increased 40°–70°F, BAg filler metal position elements (Cd and Pb) Some Differences flow was slowed down, and the postbraze from filler metals in brazing and finish did not have the same shiny, bright DD While brazing and soldering employ electronics soldering applications, a more finish as the Cd-bearing alloy, which had restricted flow of the respective filler met- different alloys, there are certain com- set the visual standard. als occurred. Processes required a com- monalities to the filler metal metallurgy in any companies experiencing the plete reengineering to determine changes Pb-free solders and BAg alloys. Silver, lead (Pb)-free conversions to made to the overall physical characteris- copper, tin, , and are all com- MMmeet WEEE and RoHS stan- tics, efficiency, and durability of the newly mon ingredients in solders and BAg alloys. dards are experiencing similarities with manufactured products, as well as how to However, the alloy percentages and the SMT and PTH assembly applications as address the engineering of all the parts, formulation are very different to meet the those brazing companies that went Cd- the process, and long-term quality. temperature, strength, and heating re- free in the 1990s. While the term “drop- quirements for different applications. in” is used as a standard to explain the en- hen looking at the melting gineering required to be Pb-free, the Improving Health points (Table 1) of tin and lead, overall manufacturing process, metallur- and the Environment WWthere is less than a 100°C differ- gical evaluations, and strength/stress tests ence between the two metals. The combi- for the ‘old’ parts that have been produced Today, the brazing world speaks of cad- nation of these metals in a 63Sn:37Pb ratio for years have encountered more obsta- mium as an element whose properties cles than could have been imagined. This were much appreciated, but when its toxic 1. Source, Alpha/Fry Metals.

PHILIP BASKIN ([email protected]) is with Superior Flux & Mfg. Co., Cleveland, Ohio.

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lenges meet today’s PCBs as those that Table 1 — Melting Points of Various Alloys Table 2 — Different Alloy Compositions in challenged PCBs more than 25 years ago: Use • component and board sensitivity to Tin (Sn) Melting Point 232°C being overheated Lead (Pb) Melting Point 327.4°C 96.5Sn/3.5Ag 221°C E • consistent solder paste printing/ Silver (Ag) Melting Point 960.5°C 96.5Sn/3Ag/0.5Cu 218–220°C screening (a) Copper (Cu) Melting Point 1083°C 96.2Sn/2.5Ag/0.8Cu/0.5Sb 218°C E • and components re- Nickel (Ni) Melting Point 1455°C 95Sn/5Sb 232–240°C quiring a metallurgical consistency with (In) Melting Point 155°C 91Sn/9Zn 199°C E the assembly solder Bismuth (Bi) Melting Point 271°C SnBalance/0.7Cu/0.3Ni(b) 227°C E Antimony (Sb) Melting Point 630°C • consistent flow on pads and ® • noncracking, solid joints. (a) AIM Castin . Today’s boards, incorporating ball grid (b) Nihon Superior SN100C. array (BGA) components, flip chips, mini- quad flat pack (QFP) and much tighter yields a eutectic at 183°C — 49°C lower spaces between circuit leads, pads and than the melting point of tin. Silver-based mount technology (SMT) reflow of solder components, make for a more challenging BAg and Cu-based BCuP alloys are stan- paste was at its infancy, the double-sided solder paste application process and qual- dards for metal joining filler metals in the printed circuit board (PCB) was a new ity control of the soldering to the PCB, but brazing industry. Today, one of the most method of building PCBs to place more still incorporate the same disciplined tem- used solders, the SAC alloys, are combi- than twice the number of chips on both peratures to the process as were required nations of tin/silver/copper — all of which sides of the PCB. This improved the over- more than 25 years ago for the double- are components in BAg 7, 18, 21, 28, 34, all value of PCBs by using space far more sided boards. 36, and 37. Naturally, the difference lies in efficiently than the wave soldered plated- In addition to the SMT reflow using the ratios. Again, the BAg alloys are silver- through holes (PTH) and taking up less 96.5Sn/3.5Ag, there were wave solder PTH based, whereas Pb-free solder alloys are placement of space where the boards were applications where a higher-temperature all Sn-based. However, this does not housed. solder was required to meet the atmos- change the melting point of the silver and o achieve this objective, dual sol- phere and environmental heat that would copper. While the melting point of the dering processes were required. In have melted the 63Sn/37Pb and caused SAC alloys is higher than that of the Sn-Pb step one, 96.5Sn/3.5Ag solder board failure in sensitive operations areas. TT Again, while not common, wave soldering solder alloy, the addition of these very- paste was applied to the first side of the high melting point alloys to tin still brings board. With a eutectic at 221°C, this tem- with these Pb-free alloys was used for spe- the solder below the 232°C melting point perature was 38°C above the eutectic cialty applications on aerospace, automo- of tin. The true eutectic composition of a melting point of the Sn63/Pb37 alloy. tive, and other control boards requiring a SAC alloy is 93.6Sn/4.7Ag/1.7Cu at 217°C. Given this temperature difference, step higher-temperature solder. Even though However, the alloy most commonly used is two involved soldering the second side of today it is known as a Pb-free solder, that the SAC305, 96.5Sn/3Ag/0.5Cu. This alloy the board with the lower melting point tin- may not have been the objective for using has a eutectic at 218°C, however, there is lead solder and operating at a reflow tem- this specific solder. This Sn96.5/Ag3.5 bi- no true eutectic for this alloy, rather a perature of 210°–215°C max. This pre- nary alloy met the process requirements plastic phase at 218°–220°C. vented the silver solder side from meeting and helped to establish a base for what has liquidus and made dual-sided reflow become Pb-free solder. nother aspect to be considered Still Searching for the processes possible. To accomplish this, one of the chemical formulations incorpo- from the original days of using the Single Replacement Sn/Ag reflow alloy is that the stan- rated for the flux medium/binder of the AA spherical solders was a single dard flux formulations were rosin mildly With all of the Pb-free studies that have medium/binder that could maintain the activated (RMA) and rosin fully activated been conducted or are planned for the fu- consistency of printing, solder flow, and (RA) fluxes. Soldering was done using a ture, there is still no single alloy composi- temperature differences the distinctly dif- vapor-phase process capable of incorpo- tion that has taken the role of replacing ferent solders required. With rating a closed temperature profile for the 63Sn/37Pb. The Sn-Pb solder is a binary 96.5Sn/3.5Ag as the high-temperature sol- heating and activation of the flux solder alloy; the SAC, a ternary alloy. Ac- der used to ensure that the 63Sn/37Pb medium/binder to hold the spherical alloy tually, there are many different solder al- could be used for the lower-temperature in place on the board. This kept the com- loys in use that range from binary to quin- reflow application, Sn-Ag eutectic solder ponent leads in place on the pads as the tuple alloy compositions — Table 2. powder marks the initial work in the Pb- vapor heating process covered the board dditional alloys contain tin/sil- free solder process before the term Pb- surface evenly and created consistent ver/copper and indium, bismuth, free was applied. process parameters from board to board. AAand other balance metals. With all Post-reflow cleaning was done in of these choices, what makes one solder trichloroethane 1:1:1 and other CFC sol- advantageous over the others? The Present Application vent systems that removed all rosin One aspect that has been forgotten Process residues and left a very clean, shiny post- about the current conversion to Pb-free solder PCB. With the Montreal Protocol, solders is that today is not the first time Today, this history of soldering is for- which went into effect on January 1, 1996, that Pb-free soldering has taken place. If gotten or unknown to engineers working all chlorofluorocarbons (CFC) were re- we go back to the 1970–80s, when surface on the Pb-free conversion. The same chal- moved from the assembly process, making

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the overall challenge that much greater. removal of Cd from the brazing BAg al- soldering with the changes of the Mon- New heating processes for SMT reflow led loys in terms of the heightened tempera- treal Protocol for wave soldering. to convection ovens that today are accu- ture of the filler metals for all PCB assem- rate to ± 0.5°C. bly and the HASL/plating and component Solder Bath Differences tinning/plating operations. However, the The Lead-Free Challenge brazing process never required new fluxes While it is commonly stated that new to be formulated to meet the higher tem- Pb-free solders are a “drop-in,” the What makes Pb-free conversion so dif- perature of the Cd-free filler alloys. All process factors described here are only ficult today is that we do not have the same AWS A5.31, AMS 3410, and 3411 specifi- part of the challenge. While Pb is consid- tools available to use that we had 25 years cations covered fluxes to temperatures of ered an improper solder metal, the high ago. What makes it easier is that the 25- 1600°–1800°F respectively, and met the Sn-content products are also very aggres- plus years of SMT reflow experience have temperature requirements for the Cd-free sive alloys that can eat-up stainless . led to the development of accurate ovens, filler metals that had long been in use. In the solder baths of the Sn-Pb wave sol- profile tools to ensure that all temperature der machines, cast iron or steel were com- paths are consistent and equal, solder The Flux Question monly used for a solder bath that ranged paste squeegee/stencils that are consistent from 200–2000 lb of solder. However, the in laying out the paste onto very small mil Prior to the Montreal Protocol, the switch to the Pb-free alloys is commonly boards, adhesives that make double side Type RMA and RA fluxes were the stan- being dealt with by adding titanium (Ti) reflow with a single alloy a standard as- dard fluxes used for a vast majority of sol- form fittings into solder baths. Titanium is sembly process, new x-ray inspection units dering applications. In areas where the flux a neutral metal that develops its own for post reflow quality control of BGA and could be left on the PCBs, the post-solder oxide, which does not react to tin when it other closed components, and many more residues were inactive and actually could is heated to soldering temperature. In re- lessons learned by the hard-knocks of the be used as a protectant against moisture flow applications, this factor will not play early days. that could potentially cause electrical a role since no solder bath is being used. n additional difference between shorts, since water-white gum rosin is not nother factor that is widely differ- the Cd-free BAg alloy and the Pb- water soluble. The removal of vapor- ent in solder bath applications is the AAfree conversion are the number of phase soldering and CFC cleaning led to a AAdevelopment of dross. In the old factors that need to be brought together in replacement and/or reformulation of days when Sn-Pb solder was used, one could electronics assembly compared to silver rosin-based fluxes and research into what watch the top of the solder bath to see what brazing. In silver brazing applications, has become today’s standard for no-clean color the surface reflected. If the surface base metals, heat sources, and fluxes re- and water-soluble solder pastes and fluxes. maintained a silvery finish, the solder was mained the same. The removal of Cd in The source for determining what is the clean. However, if there was a copper/ BAg filler metal was the only difference in definition of liquid no-clean flux was insti- finish, the copper content could be growing the process changes that needed to be ad- tuted by Bell Corp. Research, the Bellcore too high in the bath. To remove the copper, dressed. In electronics assembly, SMT and 000078. This standard established chemi- one would set the temperature to bring the PTH soldering are the final steps of bring- cal tests to ensure that no halide was pres- copper to the surface as the heavy solder ing a printed circuit board and compo- ent in the flux formula, that copper mirror would sink and the light dross and metallic nents together. All of these elements are would not occur over a 24-h pe- free-floats would rise to be removed. This independently soldered or plated in Pb- riod in an environmentally controlled at- cannot be done in the Pb-free solders. The free alloys and brought together in final mosphere, and that surface insulation re- lesser density of the Pb-free solders and the assembly. However, the same question is sistance (SIR) values would exceed 108 at incorporation of copper into a number of asked at the final assembly process, 100 ohms for 168 to 500 h. These stan- the alloys used for wave soldering require all namely, what solder will be good for all ap- dards were adopted and incorporated into operations to monitor the copper levels and plications? That has to be answered for the IPC ANSI-J-STD Standard 004 and compensate for the buildup of copper. Ex- the PCBs, HASL, silver immersion, and 005 flux and solder paste definitions. cess copper can lead to tin whiskers, and other preparation activities needed to pre- hen comparing liquid RMA other defects such as joint cracking, lack of pare the PCB for final assembly process- fluxes (solids content: 15–50%) connection yielding peelable components, ing. The Pb-free solder or plating prepa- WWto no-clean fluxes (solids con- cosmetic shortcomings of cloudy, pitted ration of all semiconductors, flip chips, tent: 2–5%) the activity level was not only joints, and other quality control issues es- BGAs, passive components, and connec- much less than the RMAs’, the small op- tablished by a manufacturer. tors that will be attached to the PCB must erational envelope available with no-clean In terms of the solder pastes definition take place at completely different loca- fluxes forced all engineering to 1) strictly of no-clean, there is a great difference be- tions. For manufacturers to bring together ensure all PCBs met prestuffing cleanli- tween the no-clean liquid fluxes with a sol- these many different items, the Pb-free ness and coating specifications, 2) apply vent base of 95–98% vs. a solder paste that compliance of all of these items must be the flux consistently for each different has a spherical powder content of reviewed for the final assembly of the PCB, 3) incorporate a disciplined wave 85–91%. Since the solder pastes must en- PCB. Given the number of different sup- soldering preheat profile for top-side tem- dure a preheating profile that does not de- pliers for PCBs and components, the qual- perature for each different PCB, and 4) gas the solvents in the flux medium/binder ity control issues in the Pb-free conversion produce consistent solder joint finish and that hold the solder and lead in place over eclipse the changes that took place in the quality to meet the consistency require- and on the pad, a rosin/resin is used to Cd-free conversion. ments for mass production of PCBs. achieve the reflow requirements of the The Pb-free conversion is similar to the These are just four main factors to wave solder paste. This results in a postsolder

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residue of 2–8% that meets the no-clean alloy, upon the end-users specification, the other solders being used, except its standards of the IPC ANSI-J-STD 005. and/or upon cost. volume of Ag is 0.5% higher than SAC305, Additionally, this residue must be pin- hen breaking down the solders and the cost of Ag has increased. testable — meaning that all individual by application, we find that ne alloy that has taken no hold in joints can be electrically tested to ensure WWthere are certain Pb-free sol- North America, but is being they meet electric/electronics operations ders that work well in one application, but OOlooked at overseas, is 91Sn/9Zn. and specifications. Today, SMT boards use are not necessarily the choice for a second The Sn-Zn alloy has a eutectic at 199°C, much more densely packed, smaller com- application. With wave soldering and which is only 16°C higher than the eutectic ponents that have experienced an expo- SMT reflow as the main assembly of 63Sn/37Pb. However, Sn-Zn solders are nential growth over the PCBs of 25 years processes incorporated in board building, known as an aluminum soldering alloy with ago. However, the vapor phase solder a Sn-Cu alloy, the Nihon Superior no connection to PCB assembly. Cosmet- process of 25 years ago that worked so well SN100C, is an excellent choice for wave ics and flow are major issues for this alloy, with the Sn-Ag alloy is no longer available, soldering in terms of flow, joint strength, however, the potential for different chem- and the complete process analysis for the and a bright, shiny finish. With a eutectic ical reactions with its 9% Zn is the primary Pb-free conversion continues. of 227°C, this allows the solder to be op- reason this solder is not being given serious erated at a temperature of 245°–260°C, consideration as a Pb-free solder. No Equivalent to which is a similar temperature range used for 63Sn/37Pb solder. However, this tem- The Search Will Continue 63Sn/37Pb perature may not be preferred for SMT reflow due to the higher melting point of In conclusion, there is no hard and fast To date, a Pb-free equivalent to this alloy vs. other Pb-free solders. But this drop-in solution for the Pb-free conver- 63Sn/37Pb has yet to be announced. does not eliminate this solder, by any sion. While we have met and passed the Long-term life tests are being conducted, means, as a possible all-around solder. date when this change should have been and more trace metal additions in or , SAC 305 alloy incorporated globally as an easy conver- parts/million to a Sn-based multialloy sol- is used for the majority of SMT re- sion to make, the wide variety of different der are being evaluated to address the F flow applications due to the melting solders that have been introduced and joint cracking, tin whisker, ductility, and F point of this solder being in the new alloys being evaluated indicate that other common factors Pb-free alloys con- 218°–220°C range, the stress tests that this conversion is very difficult and still in tinue to encounter. have been conducted on this alloy in a a state of flux. road descriptions in this article de- wide array of solder tests, and due to many n the brazing industry, the Cd-free scribe the assembly processes of companies specifying this alloy to contract conversion was very tedious from the BBSMT reflow and PTH wave solder- manufacturers or subcontractors provid- assembly angle, but was capable of ing, but only briefly touch upon the major II ing boards for the overall chain on con- being achieved using and refining the same factors that must be analyzed in the Pb- trols on an assembly. Given this solder’s tools, heating methods, and process steps free conversion, such as 1) the finish of the ability to be used as a wave soldering alloy, that had been in place for many years. PCBs where all components are attached, the SAC305, to date, is the closest choice However, in electronics assembly, many of and 2) the and component to being the new 63/37 solder. However, the assembly and cleaning tools that were leads and what is the solder or plating that recent increases in the price of silver have available prior to 1996 are no longer in is used on the surface. In both cases, the made the cost of this solder for wave sol- place. While technology is certainly avail- elements that are provided to the final der applications reach a point where man- able for this conversion, there are still a PCB assembly are critical to the conver- ufacturers are looking for an alternative great many time consuming, equipment in- sion of Pb-free since the small amount of that is less expensive and has compatibil- vestment, and initial steps that need to be Pb in both the PCBs and components will ity with the SAC305 reflow alloy. implemented and understood for the Pb- cause a board that was thought to be Pb- Additional research has Sn-Ag-Cu ra- free conversion to take place. free to fail the RoHS requirements. This is tios other than the SAC305 being used for In the coming years, more metallurgi- leading to the manufacture of all compo- both wave soldering and reflow soldering. cal assessments of Pb-free solders will be nents with a Pb-free finish, and a similar Furthermore, there are quad and quintu- made to ensure strength, ductility, unifor- movement in PC fabrication. ple alloy combinations that are being sold mity, and other requirements for long- and proposed that use (or will use) bis- term stability. In this regard, the Pb-free muth, indium, and other trace element and Cd-free conversions share a great Which Alloys Are stability alloys to lower the temperature. many characteristics that require time, Available? The overall study time frame for these al- testing, evaluation, and many aspirin.◆ loys is still too early to provide sufficient There are many Pb-free alloys avail- information for them to become the new able, but there is no single alloy that has 63/37 solder. Change of Address? become an equivalent to the 63Sn/37Pb. final alloy that is being used is bi- Moving? Additionally, at this date, the IPC has not nary 96.5Sn/3.5Ag solder. It is the yet released test parameters for the ANSI- AAonly Pb-free alloy with military Make sure delivery of your Welding J-STD standards of the Pb-free solder(s) specifications, which lends proof to its Journal is not interrupted. Contact the and fluxes, which will be the guideline for overall reliability of as a Pb-free alloy. The Membership Department with your industry standards. As a result, the choice simplicity of this binary alloy, with a eu- new address information — (800) 443- of alloy(s) must be made by manufactur- tectic melting point of 221°C, does not 9353, ext. 217; [email protected]. ers upon their own research into a specific make this solder radically different from

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