<strong>SMTA</strong> <strong>Journal</strong><strong>Journal</strong> <strong>of</strong> <strong>Surface</strong> <strong>Mount</strong> <strong>Technology</strong>April - June 2013Volume 26, Issue 2 (Quarterly)ISSN: 1093-73585200 Willson Road, Suite 215Edina, MN 55424-1316Phone: 952-920-7682Fax: 952-926-1819smta@smta.orgwww.smta.orgEditor/<strong>Journal</strong> Committee ChairSrinivas Chada, Ph.D.SchlumbergerGood summer to all the members. Now that schools are out<strong>of</strong> session and many <strong>of</strong> you are ready for summer vacationor already on it, we bring you this second quarter issue ata leisurely pace; not that summer has anything to do with it, but itsounded like a good excuse. In this issue we present 3 interestingpapers reviewed and revised from last year’s <strong>SMTA</strong> International.Please enjoy reading at the beach, by a swimming pool or whichevervacation spot you are lounging at.Do remember <strong>SMTA</strong> International 2013 is coming up fast. If youhaven’t registered yet, please do so. As usual we have a slate full <strong>of</strong>great presentations and presenters in one single location.To submit your original papers to the <strong>Journal</strong> <strong>of</strong> SMT, pleasecontact <strong>SMTA</strong> at 952-920-7682, or send me an e-mail atdrschada@hotmail.com.— Srini Chada, Ph.D.The <strong>Journal</strong> <strong>of</strong> SMT Editor/<strong>Journal</strong> Committee ChairPublisherMichelle OgiharaSeika Machinery, Inc.ProductionRyan Flaherty<strong>SMTA</strong>JOURNAL REVIEW COMMITTEEKola Akinade, Ph.D.Cisco Systems, Inc.Raiyomand Aspandiar, Ph.D.Intel CorporationTom BorkesThe Jefferson ProjectJean-Paul Clech, Ph.D.EPSI, Inc.Rob ErichMedtronicGary FreedmanColab Engineering, LLCReza Gaffarian, Ph.D.Jet Propulsion LaboratoryCarol Handwerker, Ph.D.Purdue UniversityDavid HillmanRockwell CollinsPradeep Lall, Ph.D.Auburn UniversityAndrew MawerFreescale SemiconductorGary PangelinaEnvironmental Soldering<strong>Technology</strong>Viswam Puligandla, Ph.D.Nokia (Retired)Rob RowlandAxiom ElectronicsPolina Snugovsky, Ph.D.Celestica, Inc.Laura Turbini, Ph.D.International ReliabilityConsultantPaul Vianco, Ph.D.Sandia NationalLaboratories10
<strong>SMTA</strong> <strong>Journal</strong> Volume 26 Issue 2, 2013SOLDER ALLOY CREEP CONSTANTSFOR USE IN THERMAL STRESS ANALYSISRobert Darveaux, Ph.D.Skyworks Solutions, Inc.Irvine, CA, USACorey ReichmanAmkor <strong>Technology</strong>, Inc.Chandler, AZ, USAABSTRACTCreep constants were determined by conducting mechanicaltests on solder joint array specimens. Either double lap shearor tensile loading was employed. Thirty combinations <strong>of</strong> alloy,pad metallization, and joint size were characterized. The testtemperature range was from -55C to 134C. The strain rate rangewas from 10 -8 /sec to 10/sec. The stress range was from 0.1MPa to100Mpa. All <strong>of</strong> the data sets were fit to the same basic constitutiverelation.INTRODUCTIONMechanical testing <strong>of</strong> solder is used for a two primary purposes:1) alloy development and selection, 2) determining properties foruse in predictive simulation. There are several mechanical testmethods which put the material under different types <strong>of</strong> loading,e.g., tension, shear, torsion, bending, etc. For any particular test,the specimen can be fabricated purely <strong>of</strong> the bulk alloy, or it can bean actual solder joint (or an array <strong>of</strong> joints). A particularly goodcomparison <strong>of</strong> steady state creep data from several different sourcesis given by Clech [1].Solder joints have some features that are unique from bulkspecimens. Joints have intermetallic compounds at the interfacebetween the bulk material and the substrate. They can also haveintermetallic compounds dispersed throughout the joint. Joints aremechanically constrained at the interface between the substrate andthe solder because the substrate is deforming elastically as the solderdeforms inelastically. In most cases, a joint size is much smaller thana bulk specimen. Hence, the alloy microstructure <strong>of</strong> a joint is <strong>of</strong>tendifficult to reproduce in bulk. The advantage <strong>of</strong> testing actual jointsis that one can be confident the microstructural effects <strong>of</strong> actualproduct have been accounted for.All <strong>of</strong> these factors can result in a different mechanical responsewhen a solder joint is subjected to loading as compared to a bulkalloy specimen. Several previous studies on testing actual solderjoints are given in Refs [2-12] A comparison <strong>of</strong> room temperaturecreep data on near eutectic tin-lead alloy revealed a 5 orders <strong>of</strong>magnitude range in creep rate at a given stress level [6]. At agiven strain rate, there was a 10X range in strength data for jointsversus bulk specimens [6]. Solder joint data typically shows higherstrength than bulk alloy specimen data. This trend was also evidentin data on lead-free alloys compared in Refs. [8,10].It was shown in Refs [6,12] that joint size and pad metallizationalso affect the creep behavior <strong>of</strong> solder alloys. The general trendswere that larger joints and NiAu pad metallizaitons produce morecreep resistant solder. The influence <strong>of</strong> these factors was found tobe greater at lower test temperatures.The present paper is a compilation <strong>of</strong> all the author’s work todate on testing solder joint specimens. For each sample type (alloy,joint size, pad metallizaiton), creep constants are given which willenable accurate mechanical simulation using finite element analysisor other analytical methods.CONSTITUTIVE RELATIONSSince solder is above half <strong>of</strong> its melting point at room temperature,creep processes are expected to dominate the deformation kinetics.Steady state creep <strong>of</strong> solder can be expressed by a relationship <strong>of</strong> theform [13-15]where dε s/dt is the steady state strain rate, k is Boltzmann’sconstant, T is the absolute temperature, σ is the applied stress, Qa isthe apparent activation energy, n is the stress exponent, α prescribesthe stress level at which the power law dependence breaks down,and C is a constant.Both shear and tensile loading modes were utilized in the presentpaper. It has been shown in previous work [6,10] that Von MisesDarveaux and Reichman11
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