Source: Indium Solder Co.
Solder is critical material that physically holds electronic assemblies together while allowing the various components to expand and contract, to dissipate heat and to transmit electrical signals. Without solder, it would be impossible to produce the countless electronic devices that define the 21st century.
Solder is available in numerous shapes and alloys. Each has its particular properties, providing a solder for nearly every application. Many times, solder is an afterthought in the design and engineering process. However, by considering the soldering step early in the design process, problems can be minimized. In fact, with the proper information, the characteristics of a solder can be part of an optimal design.
Solders for assembly of electronic devices melt at temperatures below 350ºC (660 F), and typically bond two or more metallic surfaces. The elements commonly alloyed in solders include tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), indium (In), gold (Au), silver (Ag), zinc (Zn), and copper (Cu).
Another material commonly used in soldering is flux. The primary function of flux is to remove existing oxides on the solder itself and on the metallic surfaces being bonded, and to protect these metals from further oxidation while at the high tem-peratures of the soldering operation. Fluxes typi-cally 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 melting point of 183 C (361 F). The term “eutectic”
indicates that the composition produces an alloy with a distinct melting point, versus a melting range.
• 60Sn/40Pb: a variation from the eutectic, with a melting range of 183 to 188 C (361 to 370 F)
• 62Sn/36Pb/2Ag: a composition that is often chosen for silver metallizations, with a melting point of 179 C (354 F).
These alloys have reasonable melting points, ad-equate wettability and strength, and low cost. Therefore, they account for perhaps 80 to 90% of all solders in electronics assembly. The perform-ance of these alloys is so well understood and doc-umented that the electronics assembly industry has designed and engineered products based on their properties.
Increasing the lead content and reducing the tin content results in solders with substantially higher melting points. Common versions are:
• 90Pb/10Sn: has a melting range of 275 to 302 C (527 to 575 F).
• 95Pb/5Sn: has a melting range of 308 to 312 C (586 to 593 F).
Solder preforms are available in a wide range of shapes and sizes, primarily for surface mount technology.
These alloys solder the terminations within elec-tronic components. High melting-point solders prevent the solder joint within the component from re-melting when the component is subsequently sol-dered 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 familiarity 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
Legislation in Europe will ban lead-containing solders, with a few exceptions, effective 01 July 2006. As a result, manufacturers, regardless of location, will have to comply if they plan to sell electronic products into Europe after the deadline.
Lead-free alloy development (for replacing Sn/Pb alloys) has largely focused on a group of al-loys that have become known by the acronym “SAC” for its Sn/Ag/Cu (tin-silver-copper) com-position. SAC alloys have compositions that range from 3.0% to 4.0% silver, and from 0.5% to 0.8% copper, with the balance tin. They are generally regarded as eutectic, or nearly eutectic, at ~217ºC (422 F).
It has been suggested that the properties of tin-bismuth-silver alloys are better than those of the SAC alloys, because they exhibit improved wetta-bility and fatigue resistance. However, tin-bismuth-silver solders do have some drawbacks. When combined with a lead-containing solder metallization, on the PCB or the component terminations, a small amount of tin-lead-bismuth eutectic alloy will form. This resultant alloy has a melting temperature of only 96ºC (204 F)! Because many temperature-cycling regimens do cycle up to 125ºC (257ºF), this presents an obvious problem. As a result, tin-bismuth-silver has been abandoned until the electronics industry is certain that all lead has been “purged” from electronics manufacturing. This is expected to take at least five or ten years.
Lead-free alloys, with all of their “environmentally friendly” hype, come with a few “issues” of their own:
•Higher melting temperature: The ~35ºC (63 F) higher melting temperature (vs. eutectic tin-lead) has to be considered in component and assembly design. Standard solder processing temperatures of 240 to 260ºC (464 to 500 F), associated with SAC alloys, can damage “standard” electronic compo-nents that are rated up to only 235ºC (455 F) be-cause they were designed for eutectic tin-lead. This higher processing temperature also results in higher manufacturing cost due to the extra energy needed to operate equipment at these higher temperatures.
• Greater fuel consumption: More energy means higher fuel consumption, which in turn means more pollution. Thus, the environmental benefit of lead-free alloys is somewhat mitigated.
These are solder balls on a ball grid array (bga)
• Multiple soldering steps: The other main issue revolves around the high-lead alloys (>85% Pb) that are often needed in assemblies requiring multiple soldering steps. These high-lead compositions melt in the 245 to 327ºC (473 to 620 F) range. To date, the only lead-free alloy that can exist at these higher temperatures is 80Au/20Sn (eutectic at 280ºC, 536 F). The gold cost associated with this alloy, and the fact that no lower-cost alternative lead-free compositions exist, has forced the industry to recon-sider a total ban on lead. As a result, the European lead-free legislation exempts lead-bearing alloys that contain 85% or more lead. Certain defense, telecommunications, and space applications are also exempt from lead restrictions.
Other lower melting-point lead-free alloys that are of some interest include 58Bi/42Sn (138ºC, 281ºF); Bi/Sn/Ag (~140ºC,~284ºF); and In/Sn (118ºC, 244ºF). They offer alternatives for appli-cations with temperature-sensitive components and materials. They also serve well in step-sol-dering applications in which the first level of as-sembly may have been constructed with a SAC alloy.
Low-temperature alloys
When added to various solder alloys, both in-dium and bismuth reduce the melting point. Ad-ditionally, high indium-containing, low melting-point solders have good ductility that often can compensate for mismatches in the coefficient of thermal expansion (CTE) between component and board materials.
Low temperature solders are useful in the sol-dering of temperature-sensitive components or sub-strates, as well as in step soldering. Step soldering is the process in which an initial soldering step is made with a relatively high-melting point alloy, followed by a soldering step with a lower-melting point alloy that can be applied without re-melting the previously soldered joints.
Examples of low-melting point solders are:
•52In/48Sn: a eutectic alloy with a melting point of 118 C (244 F).
• 58Bi/42Sn: a eutectic alloy with a melting point of 138 C (281 F).
• 80In/15Pb/5Ag: melting range of 142 to 149 C (287 to 300 F).
High-temperature solder alloys
In addition to the 90Pb/10Sn and 95Pb/5Sn sol-ders discussed earlier, other high-temperature sol-ders have melting points in the 300 C range. For example, 80Au/20Sn is a eutectic composition having a melting point of 280 C (536 F). This high tensile-strength, precious metal solder is often se-lected for the “gold to gold” sealing of large pack-ages. When processed in an inert gas environment such as nitrogen, this solder has the advantage of requiring no flux when soldering to two gold metallizations.
The alloy 92.5Pb/5.0In/2.5Ag has a melting range of 300 to 310 C (572 to 590 F). This solder has excellent thermal fatigue properties and is fre-quently chosen for applications in which the elec-tronic assembly is subjected to large thermal ex-cursions.
Indium-lead for thick gold metallizations
Anyone who spends time perusing the various solder compositions will quickly realize that tin is one of the main constituents in most solders. How-ever, tin has an affinity for alloying with precious metals such as gold. Studies indicate that 63Sn/37Pb at 200ºC (392 F) will dissolve one mi-cron (~40 micro-inches) of gold/second/unit area. As tin reacts with gold, a brittle Au/Sn intermetallic forms. When the concentration is high enough, these intermetallics have a deleterious effect on the 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, < 0.38 (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 operational 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 all compositions). The indium-lead system offers alloys of varying melting points, with indium-rich compositions having a lower melting range, and the lead-rich compositions having a higher melting range. For examples: 70In/30Pb has a melting range of 165 to 175 C (329 to 347 F), and 81Pb/19In has a melting range of 260 to 275 C (500 to 527 F).
Solder is typically provided in these common forms:
• Bar/Ingot: Typically cast and used in solder pot or wave sol-dering applications.
• Shot: Small tear-drop shaped pieces of alloy. The relatively small size offers flexibility in applications in which the alloy has to be weighed to a particular amount, such as filling crucibles for vapor deposition.
• Spheres: Also called precision solder balls, spheres are supplied with diameters from 0.012 to 0.032 in. They are deposited as bumps on elec-tronic packages such as BGAs (ball grid arrays).
• 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 other hand, is more of a long, narrow strip wrapped on a spool. Both can be hand cut to form simple preforms or to make shims and thermal interfaces.
• Wire: Often applied in rework or cut to lengths and formed into rings or other simple shapes, wire diameters typically range from 0.010 to 0.030 in. However, smaller and larger diameters are available, de-pending on the alloy. Solder wire can be produced with a flux core.
• Preforms: Typically punched, these thin pieces of solder are manu-factured as squares, rectangles, frames, disks, washers, and custom geome-tries. Solder preforms can be applied in surface mount technology (SMT), which is common to the manufacture of most consumer electronics such as cellular phones and computers. Preforms separately attach a component to a pad, or they augment the solder volume of the solder paste. Washers serve as pin connectors or other through-hole components.
• Paste: A mixture of prealloyed spherical solder powder with a flux/vehicle to form a pasty material. Paste is dispensed or stencil-printed onto the metallization pads of a printed circuit board, and components are automatically placed onto the solder paste. The tacky nature of the solder paste temporarily holds the components in place. The printed circuit board is then reflow soldered, attaching the components to the pads. Solder pastes are available with RMA, no-clean, and water-soluble flux vehicle formulations.
Selected lead-free solder alloys
1E 52In/48Sn (118°C) (Eutectic) — Lowest melting-point practical solder.
281 58Bi/42Sn (138°C) (Eutectic) — Good thermal fatigue performance;
established history.
227 77.2Sn/20.0In/2.8Ag (175°C) 187 — Not for use over 100 C due to Sn/In eutectic at 118° C.
254 86.9Sn/10.0In/3.1Ag (204°C) 205 No Sn/In eutectic problem; potential use for flip chip assembly.
241 95.5Sn/3.8Ag/0.7Cu (217-218°C) (Eutectic) — Common lead-free alloys.
246 95.5Sn/4.0 Ag/0.5Cu (217-218°C) (Eutectic) — Petzow (German) prior art reference makes this alloy patent-free.
2521 95.5Sn/3.9Ag/0.6Cu 217-218°C) (Eutectic) — NEMI promoted alloy (average composition of Indalloy #241 and #246).
249 91.8Sn/3.4Ag/4.8Bi (211°C) 213 –board and component metallizations must be lead-free.
121 96.5Sn/3.5Ag (221°C) (Eutectic) — Binary solder has history of use, marginal wetting.
244 99.3Sn/0.7Cu (227°C) (Eutectic) — Inexpensive, possible use in wave soldering.
133 95Sn/5Sb (235°C) 240 — —
209 65Sn/25Ag/10Sb (233°C) (Melting point) — Die attach solder, very brittle.
Note: Alloy of choice for general SMT assembly; 2. ICA patent; 3. ICA licensed Sandia patent.
Source: Eric Bastow is a Technical Support Engineer at Indium Corp. of America, Web site: www. indium.com.