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5SOLDER FAMILIES

AND HOW THEYWORK

Low melting-temperature alloys are vital to successful electronics assembly.

Eric BastowIndium Corp. of America, Utica, New York

S

older is a critical material that physicallyholds electronic assemblies together whileallowing the various components to ex-

pand and contract, to dissipate heat, andto transmit electrical signals. Without solder, itwould be impossible to produce the countless elec-tronic devices that define the 21st century.

Solder is available in numerous shapes and al-loys. Each has its particular properties, providinga solder for nearly every application. Many times,solder is an afterthought in the design and engi-neering process. However, by considering the sol-dering step early in the design process, problemscan be minimized. In fact, with the proper infor-mation, the characteristics of a solder can be partof an optimal design.

Solders for assembly of electronic devices meltat temperatures below 350C (660F), and typically

bond two or more metallic surfaces. The elementscommonly 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 solderingis flux. The primary function of flux is to removeexisting oxides on the solder itself and on themetallic surfaces being bonded, and to protect thesemetals from further oxidation while at the high tem-peratures of the soldering operation. Fluxes typi-cally contain rosin and/or resin, and organic acidsand/or halides, which are combined to produce

the appropriate fluxing strength for a particularmetallization..

Electronic solders can be grouped into the fol-lowing five families: tin/lead, lead-free, indium/lead, low-temperature, and high-temperature. Thisarticle discusses these five alloy families, and severalmembers of each family. It also describes the widevariety of solder forms.

Tin/lead solder alloysTin/lead alloys are the fundamental solders, with

a history dating back to the early days of radio. Thisalloy family consists of three basic compositions thathave melting points in the 180C (355F) region:

63Sn/37Pb: the eutectic composition with amelting point of 183C (361F). The term eutectic

indicates that the composition produces an alloywith a distinct melting point, versus a meltingrange.

60Sn/40Pb: a variation from the eutectic, witha melting range of 183 to 188C (361 to 370F) 62Sn/36Pb/2Ag: a composition that is often

chosen for silver metallizations, with a melting pointof 179C (354F).

These alloys have reasonable melting points, ad-equate wettability and strength, and low cost.Therefore, they account for perhaps 80 to 90% ofall solders in electronics assembly. The perform-ance of these alloys is so well understood and doc-umented that the electronics assembly industry hasdesigned and engineered products based on theirproperties.

Increasing the lead content and reducing the tin

content results in solders with substantially highermelting points. Common versions are:

90Pb/10Sn: has a melting range of 275 to 302C(527 to 575F).

95Pb/5Sn: has a melting range of 308 to 312C(586 to 593F).

These alloys solder the terminations within elec-tronic components. High melting-point solders pre-vent the solder joint within the component from re-

Solder preforms are available in a wide range of shapes andsizes, primarily for surface mount technology.

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melting when the component is subsequently sol-dered to the printed circuit board (PCB), a step thattypically involves the lower melting-point63Sn/37Pb solder. High lead-containing solders,in general, have better fatigue performance, highertensile strengths, and slightly reduced wettabilitywhen compared to the lower melting-point tin-leadcompositions. Reducing-gas atmospheres, such asforming gas or pure hydrogen, are effective fluxing

agents at these high soldering temperatures, andoften substitute for chemical fluxes that may charat high soldering temperatures.

In spite of all the beneficial attributes and famil-iarity associated with these alloys, the presence oflead, and its potential environmental impact whenproducts are discarded to landfills, has caused theindustry to seek lead-free alternatives.

Lead-free solder alloysLegislation 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 electronicproducts 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 acronymSAC for its Sn/Ag/Cu (tin-silver-copper) com-position. SAC alloys have compositions that rangefrom 3.0% to 4.0% silver, and from 0.5% to 0.8%copper, with the balance tin. They are generally re-garded as eutectic, or nearly eutectic, at ~217C(422F).

It has been suggested that the properties of tin-bismuth-silver alloys are better than those of theSAC alloys, because they exhibit improved wetta-

bility and fatigue resistance. However, tin-bismuth-silver solders do have some drawbacks. When com-

bined with a lead-containing solder metallization,on the PCB or the component terminations, a smallamount of tin-lead-bismuth eutectic alloy will form.This resultant alloy has a melting temperature ofonly 96C (204F)! Because many temperature-cycling regimens do cycle up to 125C (257F), thispresents an obvious problem. As a result, tin-

bismuth-silver has been abandoned until the elec-tronics industry is certain that all lead has beenpurged from electronics manufacturing. This isexpected to take at least five or ten years.

Lead-free alloys, with all of their environmen-tally friendly hype, come with a few issues oftheir own:

Higher melting temperature: The ~35C (63F)higher melting temperature (vs. eutectic tin-lead)has to be considered in component and assemblydesign. Standard solder processing temperaturesof 240 to 260C (464 to 500F), associated with SACalloys, can damage standard electronic compo-nents that are rated up to only 235C (455F) be-cause they were designed for eutectic tin-lead. Thishigher processing temperature also results in highermanufacturing cost due to the extra energy neededto operate equipment at these higher temperatures.

Greater fuel consumption: More energymeans higher fuel consumption, which in turn

means more pollution. Thus, the environmentalbenefit of lead-free alloys is somewhat mitigated.

Multiple soldering steps: The other main issuerevolves around the high-lead alloys (>85% Pb) thatare often needed in assemblies requiring multiplesoldering steps. These high-lead compositions meltin the 245 to 327C (473 to 620F) range. To date,the only lead-free alloy that can exist at these highertemperatures is 80Au/20Sn (eutectic at 280C,536F). The gold cost associated with this alloy, andthe fact that no lower-cost alternative lead-free com-

older formsSolder 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 smallsize offers flexibility in applications in which the alloy has to be weighedto a particular amount, such as filling crucibles for vapor deposition.

Spheres: Also called precision solder balls, spheres are supplied withdiameters 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 ofsolder, foil often has a square or rectangular geometry. Ribbon, on theother hand, is more of a long, narrow strip wrapped on a spool. Bothcan be hand cut to form simple preforms or to make shims and thermalinterfaces.

Wire: Often applied in rework or cut to lengths and formed intorings 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 ascellular phones and computers. Preforms separately attach a component toa pad, or they augment the solder volume of the solder paste. Washersserve as pin connectors or other through-hole components.

Paste: A mixture of prealloyed spherical solder powder with aflux/vehicle to form a pasty material. Paste is dispensed or stencil-printedonto the metallization pads of a printed circuit board, and components areautomatically placed onto the solder paste. The tacky nature of the solderpaste temporarily holds the components in place. The printed circuitboard is then reflow soldered, attaching the components to the pads.

Solder pastes are available with RMA, no-clean, and water-soluble fluxvehicle formulations.

These are solder balls on a ball grid array (BGA).

S

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positions exist, has forced the industry to recon-sider a total ban on lead. As a result, the Europeanlead-free legislation exempts lead-bearing alloysthat contain 85% or more lead. Certain defense,telecommunications, and space applications arealso exempt from lead restrictions.

Other lower melting-point lead-free alloys thatare of some interest include 58Bi/42Sn (138C,281F); Bi/Sn/Ag (~140C,~284F); and In/Sn(118C, 244F). They offer alternatives for appli-cations with temperature-sensitive componentsand materials. They also serve well in step-sol-dering applications in which the first level of as-sembly may have been constructed with a SACalloy.

Low-temperature alloysWhen 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 cancompensate for mismatches in the coefficient ofthermal 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 isthe process in which an initial soldering step ismade with a relatively high-melting point alloy,followed by a soldering step with a lower-meltingpoint alloy that can be applied without re-meltingthe previously soldered joints.

Examples of low-melting point solders are:

52In/48Sn: a eutectic alloy with a melting pointof 118C (244F).

58Bi/42Sn: a eutectic alloy with a melting pointof 138C (281F).

80In/15Pb/5Ag: melting range of 142 to 149C(287 to 300F).

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 300C range. Forexample, 80Au/20Sn is a eutectic compositionhaving a melting point of 280C (536F). This hightensile-strength, precious metal solder is often se-lected for the gold to gold sealing of large pack-ages. When processed in an inert gas environmentsuch as nitrogen, this solder has the advantage ofrequiring no flux when soldering to two gold met-allizations.

The alloy 92.5Pb/5.0In/2.5Ag has a meltingrange of 300 to 310C (572 to 590F). 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 metallizationsAnyone who spends time perusing the various

solder compositions will quickly realize that tin isone of the main constituents in most solders. How-ever, tin has an affinity for alloying with preciousmetals such as gold. Studies indicate that63Sn/37Pb at 200C (392F) 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

Selected lead-free solder alloysIndalloy 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 100C due to Sn/5,580,5202 In eutectic at 118C.

254 86.9Sn/10.0In/3.1Ag 204 205 5,256,3702 and No Sn/In eutectic problem; potential5,580,5202 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 referencemakes this alloy patent-free.

2521 95.5Sn/3.9Ag/0.6Cu 217-218 (Eutectic) NEMI promoted alloy (averagecomposition of Indalloy #241and #246).

249 91.8Sn/3.4Ag/4.8Bi 211 213 5,439,6393 Board and component metallizationsmust 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 wavesoldering.

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.

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thermal fatigue characteristics of the joint, and makeit susceptible to fracture during thermal cycling.

For tin-bearing solders in applications with gold-plated materials, it is advisable to keep the goldlayer thin, < 0.38 (15 micro-inches), thereby re-ducing the concentration of Au/Sn intermetallicthat can form. However, many applications suchas optoelectronics packages and defense/space elec-tronics 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 forprecious metals and dissolves gold at a rate 13 to14 times slower than tin. Also, in devices with op-erational temperatures below 125C (257F), the in-termetallic that forms between indium and gold isof a much more compliant and ductile nature, andis not susceptible to embrittlement.

Therefore, the family of In/Pb solders is benefi-cial when soldering against thick gold film metal-lizations. The In/Pb alloys are a solid solutionsystem in which the liquidus and solidus temper-

atures are close for all compositions (near-eutectic atall compositions). The indium-lead system offersalloys of varying melting points, with indium-richcompositions having a lower melting range, andthe lead-rich compositions having a higher meltingrange. For examples: 70In/30Pb has a melting rangeof 165 to 175C (329 to 347F), and 81Pb/19In hasa melting range of 260 to 275C (500 to 527F).

For more information: Eric Bastow is a Technical Sup-port Engineer at Indium Corp. of America, 1676 LincolnAve., Utica, NY 13503; tel: 315/853-4900; fax: 315/853-1000; e-mail: ebastow@indium.com; Web site: www.indium.com.

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 na-ture of the solder paste temporarily holds the components in place.

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