The Value of InPb Solders

Transcription

1 The Value of InPb Solders Authored by: Eric Bastow. Abstract The indium-lead (InPb) binary system offers numerous alloy combinations. These materials possess mechanical and physical properties which make them useful in many demanding applications. With small additions of silver (Ag), additional alloys are created, rounding out this set of materials and providing us with a full set of performance capabilities. This is especially valuable in the electronics assembly soldering arena. With melting temperatures ranging from C, this alloy family offers numerous choices for accommodating temperature-sensitive applications, as well as step-soldering operations. InPb alloys also possess metallurgical properties that make them particularly suitable for soldering to thick Au metallizations. Assemblers should not be dissuaded from using this versatile alloy family just because the joints that they form may not have the same visual characteristics that SnPb and Pb-free alloys have. ERIC BASTOW Eric Bastow, Senior Technical Support Engineer for Indium Corporation, supports the company's full range of solder products for the electronics assembly, semiconductor packaging, and thermal management markets. Eric is an SMTA-Certified Process Engineer (CSMTPE) and has earned his Six Sigma Green Belt. He is also a certified IPC-A-600 and 610D Specialist. Soldering to Gold The tin-lead (SnPb) alloy family has historically produced the mostused solder compositions. Standard SnPb compositions are: Sn63 (63Sn/37Pb eutectic composition), Sn62 (62Sn/36Pb/2Ag), and Sn60 (60Sn/40Pb). These alloys have proven to be excellent for soldering to tin, nickel (Ni), and other standard metallization schemes. In fact, due to their pervasiveness, metallization schemes were created just to suit these solders. Non-standard applications, such as high-reliability, military, satcom, etc., require different material sets, some of which do not respond well to Sn-based solders. One highly relied upon metallization is gold (Au). Sn63 will dissolve approximately 1 micron (40 micro-inches) of Au per second per unit area at 200 C [1]. Furthermore, studies have shown that when the Au content of the solder joint reaches or exceeds 4% by Figure 1. In60/Pb40 used to seal a sapphire lens with gold-plated ring to a gold-plated package. Image courtesy of SST International. ebastow@indium.com Full biography: biographies/eric-bastow A QR (quick response) code contains encoded data. When scanned with a smart phone s camera (via a QR reader application), it will take you to a specific URL or text message. Download article Share with a friend indium.us/d1115 Form No R0 From One Engineer To Another 1

2 Indalloy Composition (% by Weight) Liquidus C ( F) Solidus C ( F) 2 80In/15Pb/5Ag 154 (309) 149 (300) 4 100In (314) MP (314) MP In/30Pb 175 (347) 165 (329) In/40Pb 181 (358) 173 (343) 7 50In/50Pb 210 (410) 184 (363) Pb/40In 231 (448) 197 (387) 10 75Pb/25In 260 (500) 240 (464) Pb/19In 275 (527) 260 (500) Pb/5In/2.5Ag 310 (590) 300 (572) 11 95Pb/5In 313 (595) 300 (572) Pb 327 (621) MP 327 (621) MP Table 1. Indium-lead (InPb) alloy combinations. weight, the resultant solder joint becomes unacceptably brittle [2, 3], whereas Pb-free or SAC (SnAgCu) alloys start to exhibit brittleness at ~10% Au by weight [4]. As a result, SnPb alloys are only suitable for use with thin immersion-plated Au metallizations. This gold flash is typically <0.38 microns (15 micro-inches) and is used as a sacrificial layer to preserve the solderability of the underlying Ni layer. Thick Au metallizations are quite prevalent in high reliability/ critical applications. This thicker Au layer protects the device from corrosion, even in harsh environments, ensuring reliability. It is quite common for military specifications to call out Au platings on the order of 100 micro-inches (2.54 microns) [5]. Obviously, SnPb alloys cannot be used in such scenarios due to the large volumes of available Au which would readily produce brittle solder joints. Figure 2. Indium-based alloy hermetic seal ring on gold-plated base. Image courtesy of SST International. The compatibility of InPb alloys with Au metallizations has been extensively demonstrated in two papers by F.G. Yost of Sandia National Laboratories. He documented observations that strongly support the use of InPb against thick Au [6, 7]. Key findings include: 1. At 250 C, eutectic SnPb is capable of dissolving Au at a rate 13 times greater than 50In/50Pb. 2. In order to study the intermetallic layer that forms between 50In/50Pb and Au, a specimen was aged at 150 C for 40 hours. The reaction layer was quite ductile, with a Knoop hardness of 84.6 (compared to Au at 61.8, and soft Ni at 108.3). Yost explains this phenomenon by saying that the AuIn 2 intermetallics that grow during aging are embedded in a lead-rich phase. Another source [8] offers, The results of this testing suggest that the alloys (AuIn 2, Au 4 In) which form in the bond region are stable, non-brittle in thin film form... Yost did note that there are situations where InPb may not be the best option. These two situations are directly related to the thickness of the Au metallization: 1. To avoid excessive scavenging of the Au, InPb alloys are not recommended against metallizations thinner than 1 micron (40 micro-inches). (It is the opinion of the author that this is not necessarily a problem, assuming that there is a solderable metallization, such as Ni, beneath the Au for the solder to bond to.) 2. Because of void problems, it is recommended that PbIn solder should not be used on Au films thicker than 10 microns. Yost found that Au layers with thickness of 10 microns<x<15 microns lead to the depletion of In at the reaction interface. This causes the formation of Au-rich intermetallics, such as Au 9 In 4. In a sample aged at 150 C for 40 hours, Yost noticed voids forming at the Au-Au 9 In 4 interface. These voids widened and eventually covered considerable portions of the Au interface. In certain situations, InPb alloys should be used cautiously against Au in applications with high operational temperatures. Honeywell research [9] offers the following: The microstructure developed at 125 C is suggestive of recrystallization. The coarse microstructure developed at 125 C is detrimental to the mechanical and electrical integrity of the bond. Couples aged at 125 C had zones which were non-uniform, had large columnar grains and large voids. The alloy used in their experiments was 50In/50Pb and the joints were examined after 320, 640, 1280, 2000, and hours. From One Engineer To Another 2

3 CTE Mismatch Some electronics applications require that materials with markedly different coefficients of thermal expansion (CTE) be joined. Other situations subject the final assembly to repeated extreme temperature excursions. Due to excellent ductility characteristics, the InPb alloy system offers exceptional thermal fatigue resistance. It has been reported [10] that when exposed to temperature cycling (-55 C 125 C), the fatigue life of lead-indium (50/50) solder joints averages about 100 times greater than tin-lead joint fatigue life. InPb alloys absorb the stresses created by bonded materials possessing different coefficients of thermal expansion. Some higher temperature InPb alloys, such as 95Pb/5In and 92.5Pb/5In/2.5Ag have been selected for service in automotive under-the-hood applications, which offer CTE differences and extreme temperature excursions. Also, their high, 300 C+ melting temperatures help them to withstand the operational temperatures that can be associated with engine compartment electronics. Thermal and Electrical Conductivity Indium has respectable thermal and electrical conductivity when compared to traditional electronics assembly materials, including solders. As points of reference, the thermal conductivity of Kovar is 17W/mK and the electrical conductivity of Sn63 (standard SnPb) solder is 11.5% IACS. Additional Mechanical and Physical Properties of InPb Alloys The mechanical strength of InPb alloys is notably greater than their parent metals. They are generally strong enough to be used as direct replacements for the SnPb family. Some of the high Pb-containing InPb alloys offer more strength than their SnPb counterparts. A technical paper issued by Texas Instruments [11] wrote that the electrical and structural performance (within the range of HARM requirements) of Indalloy 7 (50In/50Pb) has been determined to be equivalent to that of more conventional Sn63 solder. Figure 5. InPb tensile/shear strength. Alloy (% by Weight) Tensile (PSI) Shear (PSI) 95Pb/5Sn Pb/5In Table 2. Comparison of tensile/shear strength of SnPb and InPb solders. Figure 3. Thermal conductivity of InPb alloys at 85 C. Figure 4. Electrical conductivity of InPb alloys. Alloy (% by Weight) Tensile (PSI) Shear (PSI) 95Pb/5In Pb/5In/2.5Ag Table 3. Comparison of tensile/shear strength of InPb and InPbAg solders. The addition of Ag to certain InPb alloys can also impact alloy strength. Two examples of Ag-containing alloys are 80In/15Pb/5Ag and 92.5Pb/5In/2.5Ag. The thermal coefficient of expansion of InPb alloys is similar to that of the SnPb alloy family (25 30 ppm/ C) [12].This is another example where InPb alloys could be used in applications where SnPb alloys may have been designed in. From One Engineer To Another 3

4 Figure 6. Thermal coefficient of expansion of InPb alloys at 20 C. Corrosion Concerns InPb alloys exhibit poor corrosion resistance when exposed to high humidity (85%RH) and temperature (85 C). In a paper from the IBM System Products Division [13], it was observed that corrosion products containing In(OH) 3, formed under the conditions previously stated, in which PbSn has been found to be stable. They go on to say that the presence of chlorides has been found to stimulate the reaction. It would be prudent to consider an appropriate conformal coating, or other protection, for InPb joints that are exposed to humidity and/or halides. Wetting Properties and Solder Joint Workmanship Neither IPC J-STD-001D or IPC-A-610A devote much attention to solder joint workmanship for non-standard or specialty alloys. This can impose a significant challenge to assemblers required to meet IPC criteria. There is some verbiage in both IPC documents, but neither offers significant direction to assemblers when assessing non-standard solder joints, such as those made with an InPb alloy. Below is an excerpt that captures the extent to which current IPC documents cover these scenarios. IPC J-STD-001D 4.14 There are solder alloy compositions, component lead and terminal finishes, or printed board platings and special soldering processes (e.g., slow cooling with large mass PCBs) that may produce dull, matte, satin, gray, or grainy appearing solders that are normal for the material or process involved. These solder connections are acceptable. Wetting cannot always be judged by surface appearance. The wide range of solder alloys in use may exhibit from low or near zero contact angles to nearly 90-degree contact angles as typical. The acceptable solder connection shall indicate evidence of wetting or adherence where the solder blends to the soldered surface. In an attempt to address the lack of workmanship criteria for non-standard joints, a simple experiment was performed by the author to give some visual guidance as to what can be expected of InPb-based alloys. The experiment was performed on clean copper coupons for the sake of availability and low cost, as compared to thick Au plated metallizations. (It should be noted that when soldering to a thick gold-plated surface, the solder joints may actually appear grainier depending on how much gold goes into solution in the solder joint.) Select alloys were chosen to represent the spectrum of InPb alloys. Solder preforms and paste of each alloy were used. The dimensions of the preform were selected to roughly equal the respective solder paste deposits. Sn63 and SAC305 alloys were also reflowed as controls. All preforms were reflowed using the same generic ROL1 tacky flux. The amount of flux applied was not controlled. This means that the amount of flux residue will vary, but is in no way a function of the alloy. Fluxes optimized to specific alloys may produce better wetting. All reflow was performed in air with an appropriate profile. This simple experiment demonstrates that InPb solder joints are noticeably more grainy in appearance and may exhibit less wetting than their SnPb and Pb-free counterparts. The solder joint appearance should not alarm assemblers when attempting to build to IPC workmanship requirements. Figure 7. Sn63 (left image - preform; right image - paste). Figure 8. SAC305 (left image - preform; right image - paste). From One Engineer To Another 4

5 as the range of InPb alloys itself. Many applications can and do benefit from the properties and performance that these alloys offer. Assemblers should not be deterred from using these alloys simply because they may not look the same as common SnPb and Pb-free alloy solder joints. Note: All charts in this work are best-fit curves based upon available data. Special thanks to Nick Hodgman of ITT Industries of Fort Wayne, Indiana for the plating specifications, and Paul W. Barnes of SST International for the indium-lead solder joint photos. Figure 9. Indalloy 2 (80In/15Pb/5Ag) (left image - preform; right image - paste) Figure 10. Indalloy 205 (60In/40Pb) (left image - preform; right image - paste). Figure 11. Indalloy 164 (92.5Pb/5In/2.5Ag) (left image - preform; right image - paste). Pb-Free Legislation Many see the current RoHS legislation in Europe as an obstacle to using InPb alloys due to the Pb content [14]. This legislation will definitely have an impact on some applications, particularly for commercial products. It is strongly recommended that concerned parties consult the RoHS Directive and pay special attention to item #7 in the annex. Undeniably Valuable The value and potential contribution of the InPb solder alloy system is very meaningful in the modern electronics world. InPb alloys are virtually necessary when thick Au metallizations are required. Their uses are as wide and varied First published in OnBoard Technology, December References 1. "Surface Mount Technology, Principles and Practices" by Ray Prasad, p. 358, Table 10.1, Copyright 1989 by Van Nostrand Reinhold 2. "Effect Of Au On The Reliability Of Fine Pitch Surface Mount Solder Joints" by Judith Glazer of Hewlett Packard Company, Palo Alto, California and Pamela A. Kramer and J.W. Morris, Jr. of Department of Materials Science and Mineral Engineering, University of California and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, California 3. "Soldering in Electronics" by R.J. Klein Wassink, p , Copyright 1984 by Electrochemical Publications Ltd. 4. "Solder Selection For Photonic Packaging" by Brian O'Neill of AIM Specialty Materials, Cranston, Rhode Island, USA 5. MIL-G-45204C, Section 1.2.3, Classes 2 6 ASTM B Aspects Of Lead-Indium Solder Technology by F.G. Yost, Sandia National Laboratories, Albuquerque, New Mexico 7. Soldering to Gold Films (subtitled The Importance Of Lead-Indium Alloys ) by F.G. Yost, Sandia National Laboratories, Albuquerque, New Mexico 8. Indium Gold Alloy Bonding by Morabito and Hartwig of Bell Telephone Laboratories, Allentown, Pennsylvania 9. Thermal Aging Characteristics of In-Pb Solder Bonds to Gold by Maciolek, Pisacich and Speerschneider of Honeywell, Inc., Bloomington, MN and Phoenix, AZ 10. Lead-Indium Solder Alloy Won t Embrittle in Gold by C.R. Jackson of IBM, Owego, NY 11. Use of Indium Alloy Solder For Microwave PWB Component Attachment by Holt, Kennedy, Bergman and Gowen of Texas Instruments, Lewisville, Texas 12. Indium Corporation data 13. Lead-Indium for Controlled-Collapse Chip Joining by Goldman, Herdzik, Koopman and Marcotte of IBM System Products Division, East Fishkill, Hopewell Junction, NY Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment, Article 4.1, Official Journal of the European Journal, L37/21 From One Engineer To Another 5