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Indium Corporation Tech Paper

Indium- (InPb) Alloys for Reliable Interconnects in Assembly Processes

Authored by: Maria Durham. Introduction MARIA DURHAM Gold (Au) interconnections are used in thicknesses that will generate significant many assembly processes levels of gold- . Often, because the surface of gold does the quoted desirable upper limit is 4% not change over time, unlike most by weight, but levels as low as 0.3% may other : even is subject to reduce the lifetime of the joint [1]. oxidation and tarnishing. Gold as a 2. The layer underlying the gold is solderable surface (metallization) is not solderable, and therefore may not often used in the form of a thin film of be wet by the solder after the gold is immersion gold on electrolessly-plated dissolved: one example is aluminum. nickel (AuNi), also called “ENIG”, Maria Durham, is a Product Specialist which is often used as a metallization to 3. Multiple reflow cycles (perhaps 5 or for Indium Corporation’s be soldered by standard Pb-free or tin- more) may be used after the initial and Advanced Assembly Materials . silver-copper (SnAgCu, “SAC”) solder soldering to the gold surface. This may Maria is responsible for interfacing with equipment partners and serving as a alloys. During reflow, the thin gold be in conjunction with either 1 or 2. In either situation, the brittle tin-nickel technical liaison between customers and layer rapidly dissolves into molten tin internal departments, including sales, intermetallic is formed, and may be (Sn)–based , allowing the solder technical support, R&D, and operations to wet onto the underlying pristine found to grow unacceptably thick. to perform experiments, gather data, and nickel surface and form a nickel-tin Indium-lead (InPb) alloys can be used to author technical articles. intermetallic. solder to surfaces, including email: [email protected] The gold dissolves into the tin to form gold. Unlike a Pb-free or SAC , gold Full biography: www.indium.com/ an AuSn intermetallic. However, when is much less soluble in lead, and lead also biographies the gold concentration becomes too dissolves gold at a much slower rate than high, the intermetallic appears as large tin, making it resistant to embrittlement. A QR (quick response) code contains encoded data. crystals in the bulk solder and can have For example, it has been noted that even at When scanned with a smart phone’s a significant, deleterious, effect on the 250°C, 50In/50Pb dissolves gold at a rate 13 times slower than 63Sn/37Pb [2]. camera (via a QR reader application), thermal fatigue characteristics of the it will take you to a specific URL or interconnection. Common Indium-Lead text message. Some applications call for gold • Download Alloys article metallizations where the need for Based on the perceived environmental reliability is high but Pb-free or SAC • Share with risks associated with lead toxicity, the a friend alloys are not acceptable. These may be use of lead in solders is restricted in some applications where: instances, therefore, compliance with 1. Gold is required to be used at well-known mandates such as RoHS or indium.us/E032 Form No. 98852 R0 From One Engineer To Another® 1 Indium Corporation Tech Paper

RoHS2 is necessary [3]. However, there are currently many wire form. The high helps the solder withstand high-reliability applications, such as those mentioned in the operational temperatures associated with engine the European ELV legislation [4], where lead is allowed in compartment electronics [2], such as in applications where automotive applications, such as where alloys with greater Indalloy® 151 (92.5Pb/5Sn/2.5Ag) is commonly used. than 85%Pb admit no substitute. Another advantage of using indium-lead alloys is their Table 1 shows the most common InPb alloys and their malleability. They exhibit high resistance to fatigue-cracking properties. Table 1 is sorted by liquidus temperature (the when thermal cycling from -55°C to 125°C. When compared temperature at which the solder is completely liquid); note that to tin-lead solder joints, 50In/50Pb solder joint fatigue life is eutectic materials have a single melting point. Three common about 100 times greater [2]. Indium-lead solder is the best comparison materials are highlighted in . to use on very rigid structures such as ceramic-to-metal or In addition to the wide selection of melting points, indium- ceramic-to-ceramic. containing alloys have comparable thermal and electrical For step-soldering type applications, an alloy such as conductivity to standard solder materials. 81Pb/19In (Indalloy® 150) may be used, as its melting point The most commonly used indium-lead alloy is Indalloy® range places it between standard high-melting high-Pb ® 205 (60In/40Pb), as it has the closest liquidus temperature solder such as Indalloy 151 or 163 (95.5Pb/2Sn/2.5Ag) and to the tin-lead eutectic (183°C), Indalloy® 106. Therefore, it lower melting solders such as the tin-lead eutectic. To fine- is a near drop-in for this tin-lead alloy and can be reflowed tune the properties required by the customer [3], the desired using a standard tin-lead eutectic profile. Indium-lead also temperature range of the step-soldering alloy can be adjusted has a similar mechanical strength and coefficient of thermal as needed by changing the indium-to-lead ratios. Indium expansion (CTE) to that of the tin-lead eutectic. The next Corporation can work with you on this to make solder to fit a commonly used alloys are Indalloy® 7, 204, and 206. specific application. Advantage of Indium-Lead Alloys Conclusion The much higher-melting Indalloy® 164 (92.5Pb/5In/2.5Ag) The contribution of InPb is understated but has the lowest coefficient of of all significant in modern electronics, especially in applications the indium-lead alloys. It is able to withstand the higher where the need for high reliability exists, but where Pb-free or temperature excursions that are seen in power electronics, SAC alloys are not acceptable. where solder is often used as a die-attach in solder paste or

Thermal Electrical CTE @ Tensile Shear Shear Conductivity @ Elongation Conductivity 20°C Strength Strength Modulus 85°C Indalloy® Liquidus Solidus Elemental Composition (% by Mass) % of IACS W/cm-°C PPM/°C PSI PSI PSI x 106 % # °C °C 2 154 149 80.0 In 15.0 Pb 5.0 Ag 13 0.43 28 2550 2150 58 4 157 157 100.0 In 24 0.86 29 273 890 1.57 22 to 41 204 175 165 70.0 In 30.0 Pb 8.8 0.38 28 3450 205 181 173 60.0 In 40.0 Pb 7 0.29 27 4150 106 183 183 63.0 Sn 37.0 Pb 11.5 0.5 25 7500 6200 4.35 37 7 210 184 50.0 In 50.0 Pb 6 0.22 27 4670 2680 55 206 231 197 60.0 Pb 40.0 In 5.2 0.19 26 5000 10 267 235 75.0 Pb 25.0 In 4.6 0.18 26 5450 3520 47.5 150 275 260 81.0 Pb 19.0 In 4.5 0.17 27 5550 151 296 287 92.5 Pb 5.0 Sn 2.5 Ag 11.02 0.26 29 4210 2240 2 164 310 300 92.5 Pb 5.0 In 2.5 Ag 5.5 0.25 25 4560 2830 11 313 300 95.0 Pb 5.0 In 5.1 0.21 29 4330 3220 52 Table 1. Indium-lead (InPb) alloy combinations and properties.

From One Engineer To Another® 2 Indium Corporation Tech Paper

Other Common Indium Alloys References: Commonly used Pb-free indium alloys include pure indium, 1. http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnum indium-tin, and indium-silver. Pure indium metal (available ber=4550381&url=http%3A%2F%2Fieeexplore.ieee. from Indium Corporation in purities up to 99.9999%) is org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4550381 frequently used for its unique ability to bond to ceramic 2. http://www.indium.com/techlibrary/whitepapers/the-value- materials and [5] in air at room temperature. Indium of-inpb-solders creates a hermetic seal by conforming very closely to and 3. http://en.wikipedia.org/wiki/Restriction_of_Hazardous_ Substances_Directive weakly bonding to a wide variety of substrates. Pure indium wire or a circular compressible seal may be used. Indium metal 4. http://en.wikipedia.org/wiki/End_of_Life_Vehicles_Directive remains malleable, even at cryogenic temperatures (less than 5. http://www.asminternational.org/static/Static%20Files/IP/ 100Kelvin, or < -173°C), and therefore works well in devices Magazine/AMP/V163/I04/amp16304p045.pdf?authtoken=c03 32933e47050258c71ae1f0980aec6c94c2c40 that require a compression seal in ultralow temperature 6. http://blogs.indium.com/blog/carol-gowans/indium-wire- applications. In addition, several vacuum applications use and-cryogenic-sealing pure indium because of its low vapor pressure and hermetic- sealing abilities [6]. Pure indium is also used in thermal interface materials (TIMs) for high-end device cooling because of its high , compressibility, and ease of application and reapplication. Indium’s unique indium-metal-based HeatSpring® technology enables users to eliminate hotspots, thus maintaining semiconductor die at an even temperature. Indium-tin and -tin-silver are two of the most commonly used lower melting point solders for applications such as LED. Indium-silver is typically used when an application needs a harder material—silver makes the soft indium less malleable. Like pure indium, indium-silver is often also used in vacuum applications. Flux Compatibility The fluxes most compatible with lower melting point (<200°C) indium-containing solders are NC-SMQ®80 (solder paste) and TacFlux® 012 (wire, preforms, and spheres). These are no-clean fluxes specifically formulated for lower temperature reflow. In low temperature reflow, these mildly-activated rosin fluxes leave behind benign residues that do not need to be cleaned off after the reflow step.

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