Journal of Surface Mount Technology - SMTA

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SMTA Journal Volume 26 Issue 2, 2013BiographiesRobert Darveaux, Ph.D. is Vice President atSkyworks Solutions, Inc. Robert has over 20years of experience in the IC packaging field atthe Microelectronics Center of North Carolina,Motorola, and Amkor. Robert has a B.S. inNuclear Engineering from Iowa State Universityand a Ph.D. in Materials Science and Engineeringfrom North Carolina State University. His areas of expertise in ICpackaging include thermal and mechanical simulation, materialscharacterization, failure analysis, and fatigue life prediction for solderjoints. Mr. Darveaux has won several major awards including bestpresentation at 1995 Surface Mount International Conference, bestpaper at 2003 SMTA Flip Chip and BGA Packaging TechnologiesWorkshop, best paper at SMTAI 2006, and best paper at ECTC2006. Robert has published 77 technical papers and has 24patents.Corey Reichman is an R&D product managerat Amkor Technology. He’s been helping to bringadvanced IC package technologies to productionsince 2005. He is currently working on Amkor’sembedded die fan-out technology for fine pitch andultra-thin IC package applications. He has severalyears’ experience in material characterization andadvanced testing methods. Corey has an M.S. Degree in Technologyfrom Arizona State University.20Darveaux and Reichman
SMTA Journal Volume 26 Issue 2, 2013A Nano Silver Replacement for High Lead Soldersin Semiconductor JunctionsKeith Sweatman 1 , Tetsuro Nishimura 1 , and Teruo Komatsu 21Nihon Superior Co., Ltd. and 2 Applied Nanoparticle Laboratory Co., Ltd.Osaka, JapanAbstractWhile it is now widely accepted that most electronic assembly canbe reliably effected with lead-free solders, a practicable alternativeto the high-lead high-melting-point solders has not been available.That reality has been acknowledged by the interim exemption fromthe requirements of the EU RoHS Directive granted for solderswith 85% or more of lead. With no direct replacement yet foundby conventional alloying of elements permitted by the RoHSDirective the search for a replacement for these high-lead soldershas extended to alternative joining materials. One approach hasbeen to take advantage of the reactivity of nano particles of silverto make a product that while ultimately having a melting point ator near the silver melting point of 961.8°C can combine to formreliable connections at temperatures much lower than that. Thechallenge in this approach is that the very reactivity that makesthe formation of a joint possible at a relatively low temperaturemeans that the nano silver tends to be unstable. In this paper theauthors report the development of a unique nano silver materialthat is manufactured and stabilized in an alcohol environment toproduce a material that can be used to make reliable joints betweena wide range of the substrates commonly used in electronics inprocess conditions similar to those used with high-lead solders.This material can be used to make joints to ferrous materials (e.g.stainless steel) as well as non-ferrous materials such as copper andnickel. And most importantly for component manufacture thisnew material bonds strongly to semiconductor materials such assilicon. Where even longer life in thermal cycling is required thesilver structure can be reinforced by the addition of other materialsin the form of particles of the appropriate size. The paper willinclude details of mechanical and reliability testing of joints madewith these materials under a range of temperature, pressure andatmosphere conditions.Key words: Nano Silver, die attachINTRODUCTIONThe main application for high- Pb solders that resulted in thegranting of an exemption from the requirement of the EU RoHSDirective has been in semiconductor packaging. The relativelyhigh melting point of these solders (around 300°C) means thatjoints made with them in earlier stages of component assembly arenot disturbed by subsequent stages of assembly with lower meltingsolders (step soldering) or by the high peak temperature sometimesrequired in final reflow soldering.For power semiconductors the die-attach material has to beable to maintain a reliable joint at the relatively high operatingtemperature of these devices while at the same time providing ahigh thermal conductivity path for dissipation of the heat generatedduring their operation.In this application the trend to wide bandgap semiconductordevices such as SiC power diodes and transistors means that thereis an advantage if the die- attach material can sustain a higheroperating temperature than can the high-Pb solders. With SiCdevices a substantial increase in power density can be achievedbecause of its higher thermal conductivity, higher breakdownvoltage and higher saturated carrier velocity. The larger bandgap ofSiC can allow higher junction temperatures without compromisingperformance. [1]The conventional metallurgical alternatives to the high-lead solderssuch as the gold-tin eutectic have a processing temperature that ishigher than the semiconductor can tolerate without degradation,And another alternative, silver-filled epoxy cannot always providethe thermal conductivity that is required to keep the die at atemperature at which it can operate at maximum efficiency [1] [2].The relatively high surface activity of nano particles means that,for example a 2.4nm Silver particle would be expected to have amelting point of 350°C [3], much less than the 961.8°C meltingpoint of bulk silver (Figure 1). The outer layers of such a particlewould have a mobility similar to that of the molten state at evenlower temperatures so that they will bond to each other or to othercompatible materials by wetting and interdiffusion at temperatureswell below those required for conventional sintering of conventionalSilver powder.Although the application of external pressure during the sinteringprocess does increase the area of contact of the particles it is notessential in sintering nano silver particles. Even at temperaturesless than those used in reflowing the high-lead solders the capillaryforces generated by the mobile atoms at the surface of the silver aresufficient to ensure the wetting of adjacent particles with which theyare in contact.Sweatman, et al.21
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