Soldering Families and How They Work

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SOLDER FAMILIES AND HOW THEY WORK Low melting-temperature alloys are vital to successful electronics assembly. Eric Bastow Indium Corp. of America, Utica, New York

older is a critical material that physically indicates that the composition produces an alloy holds electronic assemblies together while with a distinct melting point, versus a melting allowing the various components to ex- range. pand and contract, to dissipate heat, and • 60Sn/40Pb: a variation from the eutectic, with Sto transmit electrical signals. Without solder, it a melting range of 183 to 188°C (361 to 370°F) would be impossible to produce the countless elec- • 62Sn/36Pb/2Ag: a composition that is often tronic devices that define the 21st century. chosen for silver metallizations, with a melting point Solder is available in numerous shapes and al- of 179°C (354°F). loys. Each has its particular properties, providing These alloys have reasonable melting points, ad- a solder for nearly every application. Many times, equate wettability and strength, and low cost. solder is an afterthought in the design and engi- Therefore, they account for perhaps 80 to 90% of neering process. However, by considering the sol- all solders in electronics assembly. The perform- dering step early in the design process, problems ance of these alloys is so well understood and doc- can be minimized. In fact, with the proper infor- umented that the electronics assembly industry has mation, the characteristics of a solder can be part designed and engineered products based on their of an optimal design. properties. Solders for assembly of electronic devices melt Increasing the lead content and reducing the tin at temperatures below 350ºC (660°F), and typically content results in solders with substantially higher bond two or more metallic surfaces. The elements melting points. Common versions are: commonly alloyed in solders include tin (Sn), lead • 90Pb/10Sn: has a melting range of 275 to 302°C (Pb), antimony (Sb), bismuth (Bi), indium (In), gold (527 to 575°F). (Au), silver (Ag), zinc (Zn), and copper (Cu). • 95Pb/5Sn: has a melting range of 308 to 312°C 5Another material commonly used in soldering (586 to 593°F). is flux. The primary function of flux is to remove These alloys solder the terminations within elec- existing oxides on the solder itself and on the tronic components. High melting-point solders pre- metallic surfaces being bonded, and to protect these vent the solder joint within the component from re- metals from further oxidation while at the high temperatures of the soldering operation. Fluxes typically contain rosin and/or resin, and organic acids and/or halides, which are combined to produce the appropriate fluxing strength for a particular metallization.. Electronic solders can be grouped into the fol- lowing five families: tin/lead, lead-free, indium/ lead, low-temperature, and high-temperature. This article discusses these five alloy families, and several members of each family. It also describes the wide variety of solder forms.

Tin/lead solder alloys Tin/lead alloys are the fundamental solders, with a history dating back to the early days of radio. This alloy family consists of three basic compositions that have melting points in the 180°C (355°F) region: • 63Sn/37Pb: the eutectic composition with a Solder preforms are available in a wide range of shapes and melting point of 183°C (361°F). The term “eutectic” sizes, primarily for surface mount technology. solder.qxd 11/13/03 2:30 PM Page 3

melting when the component is subsequently soldered to the printed circuit board (PCB), a step that typically involves the lower melting-point 63Sn/37Pb solder. High lead-containing solders, in general, have better fatigue performance, higher tensile strengths, and slightly reduced wettability when compared to the lower melting-point tin-lead compositions. Reducing-gas atmospheres, such as forming gas or pure hydrogen, are effective fluxing agents at these high soldering temperatures, and often substitute for chemical fluxes that may char at high soldering temperatures. In spite of all the beneficial attributes and famil- iarity associated with these alloys, the presence of lead, and its potential environmental impact when products are discarded to landfills, has caused the industry to seek lead-free alternatives.

Lead-free solder alloys These are solder balls on a ball grid array (BGA). Legislation in Europe will ban lead-containing solders, with a few exceptions, effective 01 July 2006. means more pollution. Thus, the environmental As a result, manufacturers, regardless of location, benefit of lead-free alloys is somewhat mitigated. will have to comply if they plan to sell electronic • Multiple soldering steps: The other main issue products into Europe after the deadline. revolves around the high-lead alloys (>85% Pb) that Lead-free alloy development (for replacing are often needed in assemblies requiring multiple Sn/Pb alloys) has largely focused on a group of al- soldering steps. These high-lead compositions melt loys that have become known by the acronym in the 245 to 327ºC (473 to 620°F) range. To date, “SAC” for its Sn/Ag/Cu (tin-silver-copper) com- the only lead-free alloy that can exist at these higher position. SAC alloys have compositions that range temperatures is 80Au/20Sn (eutectic at 280ºC, from 3.0% to 4.0% silver, and from 0.5% to 0.8% 536°F). The gold cost associated with this alloy, and copper, with the balance tin. They are generally re- the fact that no lower-cost alternative lead-free com- garded as eutectic, or nearly eutectic, at ~217ºC (422°F). It has been suggested that the properties of tin- older forms bismuth-silver alloys are better than those of the Solder is typically provided in these common forms: SAC alloys, because they exhibit improved wetta- • Bar/Ingot: Typically cast and used in solder pot or wave sol- bility and fatigue resistance. However, tin-bismuth- Sdering applications. silver solders do have some drawbacks. When com- • Shot: Small tear-drop shaped pieces of alloy. The relatively small bined with a lead-containing solder metallization, size offers flexibility in applications in which the alloy has to be weighed on the PCB or the component terminations, a small to a particular amount, such as filling crucibles for vapor deposition. amount of tin-lead-bismuth eutectic alloy will form. • Spheres: Also called precision solder balls, spheres are supplied with This resultant alloy has a melting temperature of diameters from 0.012 to 0.032 in. They are deposited as bumps on elec- only 96ºC (204°F)! Because many temperature- tronic packages such as BGAs (ball grid arrays). cycling regimens do cycle up to 125ºC (257ºF), this • Ribbon and foil: Typically thin (0.002 to 0.010 in.+ thick) pieces of solder, foil often has a square or rectangular geometry. Ribbon, on the presents an obvious problem. As a result, tin- other hand, is more of a long, narrow strip wrapped on a spool. Both bismuth-silver has been abandoned until the elec- can be hand cut to form simple preforms or to make shims and thermal tronics industry is certain that all lead has been interfaces. “purged” from electronics manufacturing. This is • Wire: Often applied in rework or cut to lengths and formed into expected to take at least five or ten years. rings or other simple shapes, wire diameters typically range from 0.010 Lead-free alloys, with all of their “environmen- to 0.030 in. However, smaller and larger diameters are available, de- tally friendly” hype, come with a few “issues” of pending on the alloy. Solder wire can be produced with a flux core. their own: • Preforms: Typically punched, these thin pieces of solder are manu- • Higher melting temperature: The ~35ºC (63°F) factured as squares, rectangles, frames, disks, washers, and custom geome- higher melting temperature (vs. eutectic tin-lead) tries. Solder preforms can be applied in surface mount technology (SMT), which is common to the manufacture of most consumer electronics such as has to be considered in component and assembly cellular phones and computers. Preforms separately attach a component to design. Standard solder processing temperatures a pad, or they augment the solder volume of the solder paste. Washers of 240 to 260ºC (464 to 500°F), associated with SAC serve as pin connectors or other through-hole components. alloys, can damage “standard” electronic compo- • Paste: A mixture of prealloyed spherical solder powder with a nents that are rated up to only 235ºC (455°F) be- flux/vehicle to form a pasty material. Paste is dispensed or stencil-printed cause they were designed for eutectic tin-lead. This onto the metallization pads of a printed circuit board, and components are higher processing temperature also results in higher automatically placed onto the solder paste. The tacky nature of the solder manufacturing cost due to the extra energy needed paste temporarily holds the components in place. The printed circuit to operate equipment at these higher temperatures. board is then reflow soldered, attaching the components to the pads. • Greater fuel consumption: More energy Solder pastes are available with RMA, no-clean, and water-soluble flux vehicle formulations. means higher fuel consumption, which in turn solder.qxd 11/13/03 2:30 PM Page 4

Selected lead-free solder alloys Indalloy No. Composition Solidus, °C Liquidus, °C Patents Comments 1E 52In/48Sn 118 (Eutectic) — Lowest melting-point practical solder. 281 58Bi/42Sn 138 (Eutectic) — Good thermal fatigue performance; established history. 227 77.2Sn/20.0In/2.8Ag 175 187 5,256,3702 and Not for use over 100°C due to Sn/ 5,580,5202 In eutectic at 118°C. 254 86.9Sn/10.0In/3.1Ag 204 205 5,256,3702 and No Sn/In eutectic problem; potential 5,580,5202 use for flip chip assembly. 241 95.5Sn/3.8Ag/0.7Cu 217-218 (Eutectic) — Common lead-free alloys. 246 95.5Sn/4.0 Ag/0.5Cu 217-218 (Eutectic) — Petzow (German) prior art reference makes this alloy patent-free. 2521 95.5Sn/3.9Ag/0.6Cu 217-218 (Eutectic) — NEMI promoted alloy (average composition of Indalloy #241 and #246). 249 91.8Sn/3.4Ag/4.8Bi 211 213 5,439,6393 Board and component metallizations must be lead-free. 121 96.5Sn/3.5Ag 221 (Eutectic) — Binary solder has history of use, marginal wetting. 244 99.3Sn/0.7Cu 227 (Eutectic) — Inexpensive, possible use in wave soldering. 133 95Sn/5Sb 235 240 — — 209 65Sn/25Ag/10Sb 233 (Melting point) — Die attach solder, very brittle. 1. Alloy of choice for general SMT assembly; 2. ICA patent; 3. ICA licensed Sandia patent.

positions exist, has forced the industry to recon- • 58Bi/42Sn: a eutectic alloy with a melting point sider a total ban on lead. As a result, the European of 138°C (281°F). lead-free legislation exempts lead-bearing alloys • 80In/15Pb/5Ag: melting range of 142 to 149°C that contain 85% or more lead. Certain defense, (287 to 300°F). telecommunications, and space applications are also exempt from lead restrictions. High-temperature solder alloys Other lower melting-point lead-free alloys that In addition to the 90Pb/10Sn and 95Pb/5Sn sol- are of some interest include 58Bi/42Sn (138ºC, ders discussed earlier, other high-temperature sol- 281ºF); Bi/Sn/Ag (~140ºC,~284ºF); and In/Sn ders have melting points in the 300°C range. For (118ºC, 244ºF). They offer alternatives for appli- example, 80Au/20Sn is a eutectic composition cations with temperature-sensitive components having a melting point of 280°C (536°F). This high and materials. They also serve well in step-sol- tensile-strength, precious metal solder is often se- dering applications in which the first level of as- lected for the “gold to gold” sealing of large pack- sembly may have been constructed with a SAC ages. When processed in an inert gas environment alloy. such as nitrogen, this solder has the advantage of requiring no flux when soldering to two gold met- Low-temperature alloys allizations. When added to various solder alloys, both in- The alloy 92.5Pb/5.0In/2.5Ag has a melting dium and bismuth reduce the melting point. Ad- range of 300 to 310°C (572 to 590°F). This solder has ditionally, high indium-containing, low melting- excellent thermal fatigue properties and is fre- point solders have good ductility that often can quently chosen for applications in which the elec- compensate for mismatches in the coefficient of tronic assembly is subjected to large thermal ex- thermal expansion (CTE) between component and cursions. board materials. Low temperature solders are useful in the sol- Indium-lead for thick gold metallizations dering of temperature-sensitive components or sub- Anyone who spends time perusing the various strates, as well as in step soldering. Step soldering is solder compositions will quickly realize that tin is the process in which an initial soldering step is one of the main constituents in most solders. How- made with a relatively high-melting point alloy, ever, tin has an affinity for alloying with precious followed by a soldering step with a lower-melting metals such as gold. Studies indicate that point alloy that can be applied without re-melting 63Sn/37Pb at 200ºC (392°F) will dissolve one mi- the previously soldered joints. cron (~40 micro-inches) of gold/second/unit area. Examples of low-melting point solders are: As tin reacts with gold, a brittle Au/Sn intermetallic • 52In/48Sn: a eutectic alloy with a melting point forms. When the concentration is high enough, of 118°C (244°F). these intermetallics have a deleterious effect on the solder.qxd 11/13/03 2:30 PM Page 5

thermal fatigue characteristics of the joint, and make it susceptible to fracture during thermal cycling. For tin-bearing solders in applications with gold- plated materials, it is advisable to keep the gold layer thin, < 0.38µ (15 micro-inches), thereby reducing the concentration of Au/Sn intermetallic that can form. However, many applications such as optoelectronics packages and defense/space electronics call for thicker gold metallizations. In such scenarios, in which the need for reliability is high, tin-bearing solders are not appropriate. Unlike tin, indium has a much lower affinity for precious metals and dissolves gold at a rate 13 to 14 times slower than tin. Also, in devices with op- erational temperatures below 125ºC (257°F), the intermetallic that forms between indium and gold is of a much more compliant and ductile nature, and is not susceptible to embrittlement. Therefore, the family of In/Pb solders is beneficial when soldering against thick gold film metallizations. The In/Pb alloys are a solid solution system in which the liquidus and solidus temperatures are close for all compositions (near-eutectic at Paste is dispensed or stencil-printed onto the metallization pads of a printed circuit all compositions). The indium-lead system offers board, and components are automatically placed onto the solder paste. The tacky na- alloys of varying melting points, with indium-rich ture of the solder paste temporarily holds the components in place. compositions having a lower melting range, and For more information: Eric Bastow is a Technical Sup- the lead-rich compositions having a higher melting port Engineer at Indium Corp. of America, 1676 Lincoln range. For examples: 70In/30Pb has a melting range Ave., Utica, NY 13503; tel: 315/853-4900; fax: 315/853- of 165 to 175°C (329 to 347°F), and 81Pb/19In has 1000; e-mail: [email protected]; Web site: www. a melting range of 260 to 275°C (500 to 527°F). ■ indium.com.