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Indium Solders Operate at a Very High Homologous Failure of Joints Made of Indium-Base Giles Humpston Temperature

Indium Solders Operate at a Very High Homologous Failure of Joints Made of Indium-Base Giles Humpston Temperature

Indium.qxp 3/24/2005 10:27 AM Page 1

TECH SPOTLIGHT

Fig. 1 — Pure indium melts at a temperature of 156.7°C (314°F). Image courtesy Indium Corp. of America. Indium operate at a very high homologous failure of joints made of indium-base Giles Humpston temperature. That is, 25°C (77°F, solders tends to be caused by stress Tessera Inc. 298K) is close to the overload or unidirectional creep. San Jose, California when expressed in Kelvins. At high The ready creep of indium solders homologous temperatures, the rate implies that joints are unlikely to fail David M. Jacobson of -state diffusion in with on thermal cycling unless the load- Buckingham Chilterns University simple crystal structures is suffi- displacement curve is asymmetric. College ciently fast that microstructural It follows that indium solders are High Wycombe, England changes can occur in time scales that well suited for making joints be- are comparable to changes in the tween dissimilar materials that will service environment of joints in be subject to thermal cycling. In fact, ndium-base solders share the components. creep in joints made with indium sol- common characteristics of low This is exemplified by the stress- ders can usually take place suffi- melting point (Fig. 1) and ex- strain curve of a thick indium sol- ciently fast to ensure that the stress treme softness and ductility. dered joint and the continuum be- is always close to zero, with roughly Moreover, it remains ductile tween stress-strain and creep data 80% stress relaxation occurring I even at cryogenic temperatures. given in Fig. 2 and 3, respectively. within seconds of a step change in The mechanical properties are This means that recovery and recrys- strain. In that case, the will not mostly a reflection of the fact that at tallization occur as fast as work hard- work harden. If joints are suitably room temperature, indium solders ening is induced, and mechanical designed to take advantage of these ADVANCED MATERIALS & PROCESSES/APRIL 2005 45 Indium.qxp 3/17/2005 11:18 AM Page 2

12 Composition and melting ranges of indium-base solders

8 Composition, % Melting range, °C Melting range, °F In-50Pb 178-210 352-410 4

Stress. MPa In-30Pb 165-172 329-342

0.5 1 1.5 In-3Ag 141 286 Strain In-5Ag-15Pb 142-149 288-300 Fig. 2 — Shear stress-strain curve for In-40Sn-20Pb 121-130 250-266 a 500 µm thick indium (In-48Sn) joint held at 40 °C (104 °F) ambient and strained In-49Sn 120 248 x –4 at a rate of 5 10 /s. Adapted from Freer In-67Bi 110 230 Goldstein and Morris Jr. [1994] In-34Bi 72 162 -1 In-32.5Bi-16.5Sn 60 140

-3 65oC beneficial thermomechanical char- available because this is chem- 25oC -5 acteristics, indium solders can pro- ically extracted from residues as

log 1/s vide superior life (Fig. 4). a minor byproduct. Provided the in- -7 dium is of purity better than -9 Failure modes 99.99995%, it will wet and spread Steady-state strain rate, 0 1 2 3 4 5 6 Tensile stress, MPa When indium soldered joints do over unmetallized ceramics fail, the mechanism is predominantly and , in air, without flux. The re- Fig. 3 — Continuum between stress- by classical creep rupture, which has sulting joints do not have the same strain and creep data for indium-tin eutectic its origins in the nucleation and co- strength (5 to 10 MPa, or 725 to 1500 solder (In-48Sn) at room and elevated tem- perature. Adapted from Darveaux and Murty alescence of cavities that arise from psi) and fracture toughness as con- [1993] the material redistribution associated ventional soldered joints, but are with stress relaxation. In contrast, tin- nevertheless hermetic and usable in 16 base solders, when subject to condi- a limited range of applications. In-48Sn tions of low-cycle (low-strain rate) The low melting point of indium 12 Pb-62Sn fatigue, fail because the plastic de- solders is caused by the low melting 8 formation in the solder to mi- point of indium itself, which is 157°C crostructural changes in the region (315°F). It is interesting to note that the 4

Resistance, ohms of maximum strain. Internal cavities cited melting point of indium has in- 50 100 150 200 250 then develop, which first coalesce to creased by nearly 5°C (9°F) over the Number of cycles, N form voids and then grow into last 40 years with the development of Fig. 4 — Resistance of a flip-chip daisy cracks. improved refining methods, as the chain between and alumina, versus Indium-base solders for opto- melting point is very sensitive to low the number of cycles (N) of a thermal shock electronic and photonics applications levels of metallic impurities. Albeit test +25 to –196 °C, 30 sec dwell for the sol- can also fail by a process known as largely as a point of metallurgical cu- ders indicated. Adapted from Shimizu et al. segregation. This develops riosity, the melting point of indium is [1995] when an extreme and unidirectional also unusually sensitive to pressure; electrical and/or thermal gradient is the application of 4000 MPa (580 ksi) 450 sustained across a joint. The result is will cause the melting point to roughly

C migration of indium toward one joint double to 300°C (570°F), Fig. 5. o 350 interface, and the accumulation of voids at the other, until the joint fails. Alloying additions 250 This mechanism is generally ob- Alloying additions made to form served only in applications such as lower-melting-point eutectics confer Melting point, die-attach of microwave power am- a number of practical benefits. The 150 2 4 6 8 10 plifiers and laser diodes. Although table lists some of the more common Pressure, MPa x 1000 the absolute power levels are modest indium solders and their melting Fig. 5 — Melting point of indium as a in these devices, the small physical points. The merits of alloying are function of pressure. Adapted from Kennedy size of the parts results in very high principally the generation of multi- and Newton. [1963] energy flux. phase microstructures, which im- prove the mechanical properties of Pure indium the solder, and the formation of Pure indium is not often used as a mixed-composition , which solder because the and are generally easier to chemically re- spreading characteristics are move than pure indium oxide. mediocre, as are the mechanical Indium oxide forms readily as a properties of the joints. One excep- sticky and tenacious film that re- tion stems from exploitation of the quires specially formulated fluxes to complex oxide that forms on indium. effect its removal. Even so, as can be Very-high-purity indium is readily seen from Fig. 6, indium solders are 46 ADVANCED MATERIALS & PROCESSES/APRIL 2005 Indium.qxp 3/24/2005 10:27 AM Page 3

Excess temperature, oF Excess temperature, oF 50 100 150 200 250 300 50 100 150 200 250 300 15 15 Indium-based solders Sn-Pb 13 13 In-Pb Sn-Ag Sn-In 11 11 In-Sn 9 9 Sn-Bi

7 7 Spread ratio Spread ratio

5 In-Ag 5

3 In-Bi 3 Tin-based solders

1 1 25 50 75 100 125 150 25 50 75 100 125 150 Excess temperature, oC Excess temperature, oC

Fig. 6 — Spread characteristics of indium and tin binary alloys on an ideal substrate, as a function of excess temperature above the melting point. Indium solders are appreciably less fluid than their tin-base counterparts. The substrate is a flat microscope slide, sputter- metallized with 0.1 µm of chromium, overlaid with 0.1 µm of .

appreciably less fluid than their tin- Atomic percent Indium 10 20 30 40 50 60 70 80 90 100 base counterparts and require greater 350 superheats to achieve comparable flow. 327.5 300 Indium- solders L Indium-lead alloys are a useful 250 class of solders with readily ad-

justable melting ranges. Considera- C o tion of the phase diagram for this 200 system, which is given in Fig. 171.6 158.9 89.8 7, reveals that indium and lead form 156.8 an essentially miscible mixture that 150 Temperature, Temperature, extends between the melting points Pb of the two parent materials, that is, 100 157 to 328°C (315 to 622°F). Thus, by β In appropriate selection of the compo- sition, it is possible to obtain a solder, 50 albeit one with a melting range and mediocre mechanical properties, that 10 20 30 40 50 60 70 80 90 100 has any selected solidus temperature Pb Weight Percent Indium In between the two limits. Fig. 7 — Indium-lead phase diagram. Indium and lead form an essentially miscible mix- Although indium-lead alloys ture that extends between the two melting points. solidify by peritectic reaction, the solidification rate of most soldered reaction between these metals. solder, its price differential is a factor joints is usually sufficiently slow to In the presence of lead, the inter- of about 80, which restricts its use to prevent problems associated with facial layer takes the form of sepa- high-added-value and more spe- microstructural coring, but this must rate grains of the AuIn2 compound cialist applications. be considered if a fast heating embedded entirely in primary lead. method such as laser soldering is The lead provides an easy diffusion For more information: Giles Humpston, needed. path between the solder and the gold Tessera Inc., 3099 Orchard Drive, San Indium-containing solders are coating, and so permits the reaction Jose, CA95134; tel: 408/894-0700; fax: 408/ often recommended for joining to to continue, although at a modest 894-0768; e-mail: ghumpston@Tessera. gold-coated components, because rate over extended timescales. com. gold is less soluble in these alloys Intermediate melting temperature than in lead-tin solders. The re- indium-base solders, such as the In- This article is based on parts of Chapter stricted solubility of gold in a joint 15Pb-5Ag composition, are signifi- 2 in Principles of Soldering, by Giles containing indium is largely associ- Humpston and David M. Jacobson. Re- cantly more ductile than lead-tin eu- cently published by ASM International, ated with the formation of a contin- tectic, and can yield joints with the book is available online at www. uous layer of the com- correspondingly superior thermal asminternational.org, where you can pound AuIn2 at the solder/gold fatigue performance. Although the view chapter titles. Or contact customer interface. melting range of this alloy is close to service at 800/336-5152. Price is $149 for This effectively suppresses further the eutectic temperature of lead-tin ASM members, $182 for non-members. ADVANCED MATERIALS & PROCESSES/APRIL 2005 47