Thermally induced deformation of solder joints in real packages: Measurement and analysis

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<ul><li><p>atiem</p><p>n S</p><p>n Un</p><p>Received 27 July 2005; received in revised form 2 October 2005Available online 18 November 2005</p><p>condition, etc. In the point of view of solid mechanics, accelerated test routines and failure statistic based dataanalysis. On the other hand, the interest in the physics-based, predictive methodologies is notably growingdriven mainly by the time and cost reduction for newpackage development, design and prototyping. The</p><p>0026-2714/$ - see front matter 2005 Elsevier Ltd. All rights reserved.</p><p>* Corresponding author.E-mail address: (H. Lu).</p><p>Microelectronics Reliability 46 (1. Introduction</p><p>The global transition to lead-free solders has given anew thrust to the packaging reliability research e.g., [16]. The solder joint reliability performance as commonlyacknowledged is a complex issue due to its sensitivity tovarious factors such as the package structure and geo-metric scale, the manufacturing process and the service</p><p>the stress and strain dictate the solder joint failure. Thedecades long studies have led to the establishment of aframework for quantitatively assessing the solder jointreliability. Based on that, some bunch mark structuresimulation and material modeling have provided neededguidance to the understanding of failure mechanismsunder dierent circumstances. Yet to date the packagethermal reliability assessment is heavily relied on theAbstract</p><p>The paper presents a hybrid experimental and analytical approach to track the deformation of solder joints in anelectronic package subject to a thermal process. The solder joint strain is directly measured using a computer visiontechnique. The strain measurement is analyzed following an approach that is devised based on an established solderconstitutive relation. The analysis leads to the determination of the solder joint stress and in turn, to the separationof the elastic, plastic and creep strain from the measured total strain. The creep strain rate and stressstrain hysteresisloop are also obtained. Two case studies are presented to illustrate the applications and to show the viability of theapproach. Each case involves a resistor package with SAC (Sn95.5Ag3.8Cu0.7) solder joints, which is subjected to atemperature variation between ambient and 120 C. The results conrm that shear is a dominant strain componentin such solder joints. The shear strain varies nearly in phase with the temperature whereas the shear stress exhibits adierent trend of variation due to stress relaxation. The peak shear stress of around 10 MPa to 15 MPa are found,which occur at near 70 C in both cases, when the temperature ramps up at approximately 3 C/min. The creep shearstrain goes up to 0.02 and accounts for over 80% of the total shear strain. The creep strain rate is in the order of mag-nitude of 105 s1. Responding to the temperature cycling with such moderate rate, the creep strain shows modest rat-cheting while the stressstrain hysteresis stabilizes in two cycles. 2005 Elsevier Ltd. All rights reserved.Thermally induced deformpackages: Measur</p><p>Hua Lu *, Hele</p><p>Department of Mechanical and Industrial Engineering, Ryersodoi:10.1016/j.microrel.2005.10.002on of solder joints in realent and analysis</p><p>hi, Ming Zhou</p><p>iversity, 350 Victoria Street, Toronto, Canada ON M5B 2K3</p><p>2006) 11481159</p><p></p></li><li><p>other, the solder joint creep fatigue can be demonstrated</p><p>tor is dened and related to the motion of the points in asubset of the deformed image with respect to the corre-sponding points in the un-deformed counterpart. Themotion is assumed to obey the continuum mechanicsbased deformation kinematics. An iterative algorithmis adopted to assess the factor initially calculated withgross estimates of the deformation parameters, and sub-sequently to improve the parameters. The iteration pro-</p><p>Fig. 1. Experimental setup used for strain measurement: (a) asketch of the system layout and (b) a picture of the system.</p><p>H. Lu et al. / Microelectronics Reliability 46 (2006) 11481159 1149by the stressstrain hysteresis, and the per-cycle damagecan be quantied by the area enclosed in a stabilized hys-teresis loop. For a practical assessment, it is desirable toconduct such an evaluation on real assembly packagessubjected to a non-accelerated, operating thermal pro-cess. In a pioneering work, Hall et al. [17] used opticaland strain gauge techniques to observe the thermal defor-mation of LCCC (leadless ceramic chip carrier) assem-blies. Pan and Pao [18] studied the solder jointdeformation using a beam specimen. In both studiesthe solder joint hysteresis loops are examined. The pres-ent study, though similarly motivated, uses the methodof digital speckle correlation (DSC) to measure a solderjoint of a package. Specically, the strain variation withthe temperature in a stress concentrated local area isobtained. The analysis uses a scheme that yields the solu-tion of solder joint stress based on the strain measure-ment. This in turn enables the separation of the elastic,plastic and creep strain components from the measuredtotal strain, and the determination of the creep rateand the strain energy density.</p><p>2. Solder joint strain measurement</p><p>2.1. DSC technique</p><p>The key of the study is to obtain direct strain mea-surement in the solder joints, which relies mainly onthe application of a technique called digital speckle cor-relation (DSC). With its high and adjustable resolution,existing solder joint failure theories are diverse as over-viewed and categorized by Lee et al. [7] in 2000, whichare recently enriched by the additions devoted to lead-free solders e.g., [812]. A class of the models [1316] thathave gained wide acceptance attributes the thermal creepfatigue to the solders microstructure damage that accu-mulates as the temperature cycles. Given that the damageand the strain energy dissipation are correlated to each</p><p>Abbreviations</p><p>SAC Sn95.5Ag3.8Cu0.7 alloyDSC digital speckle correlationDSC enables a gross search for the sites of strain concen-tration in a package, if necessary. When such sitesbecome apparent, the system can be zoomed in to obtaindetailed measurement. In Fig. 1 a sketch and a photo aregiven to help illustrate the experimental setup used. Themethodology is based on evaluating the correlationbetween a pair of the images that are recorded at dier-ent deformation states of a surface. A de-correlation fac-LCCC leadless ceramic chip carrierPCB printed circuit boardceeds to turn the deformed image back to its originalshape, evidenced by the progressively improved imagecorrelation. A good correlation reached at the end ofthe process signies that the deformation parametersat the subset center have been converged to the respec-tive true values. The deformation of an area is deter-mined in a point-by-point manner repeating the aboveprocess. The computer vision technique is quick in</p></li><li><p>generating large quantities of measurements upon thetest completion. And its features of non-contact andnon-coherent make it suitable for thermal applications.Detailed descriptions of the methodology and the mea-surement procedures can be found in Refs. [1921].</p><p>2.2. Test vehicles and sample preparation</p><p>The test vehicles used in the study are the type ofceramic resistor packages chosen to use in a Pb-free reli-ability research by National Electronics ManufacturesInitiative (NEMI). Fig. 2 gives a three-dimensionalsketch and another of a cross-section detailing the pack-age structure. The packages feature dimensions arelisted in Table 1. The sample preparation includescross-sectioning, polishing, photographing and specklecoating, etc. Fig. 3 gives photos of a partial cross-sectionshowing the solder llet and the surrounding area beforeand after a speckle pattern is applied to. The articialspeckle pattern provides a desired random variation oflight reectivity across the surface, which is criticallyrequired by the image correlation calculation. Should anaked cross-section be measured, the grey-value varia-tion across the image would be contributed mainly bythe surface texture or by variable reectivity of individ-ual solder grains and cell boundaries in case a very large</p><p>optical magnication is used. Yet under heat and stress,the solder grains will grow to cause the solder micro-structure coarsening, which could aect the macroscopicappearance of the surface texture. Given that the meth-odology attributes the image de-correlation only to thesurface deformation, the microstructure coarseninginduced surface texture change could confuse the corre-lation calculation and thus cause measurement error.</p><p>2.3. Image recording and processing</p><p>In this study, each test records a series of images ofthe area centered at the solder joint llet, as typically</p><p>Ceramic thickness(mm)</p><p>PCB thickness(mm)</p><p>Data area(mm2)</p><p>0.508 1.15 0.032 0.0440.508 1.15 0.050 0.044</p><p>Fig. 3. Pictures showing a solder joint and its vicinity: (a) aphoto of a joint llet and surrounding area with no specklecoverage, (b) a photo of same area covered by speckles, and (c)a sketch giving geometric dimensions of the joint.</p><p>1150 H. Lu et al. / Microelectronics Reliability 46 (2006) 11481159Fig. 2. A 3D sketch and a sketch of a XY cross-section of atest package.</p><p>Table 1Geometric dimensions of the package cross-section</p><p>Test Solder-joint thickness(mm)</p><p>Copper trace thickness(mm)</p><p>Case A 0.044 0.068Case B 0.044 0.063</p></li><li><p>shown by Fig. 3(b). The image processing involves a pairat a time, namely a reference image taken at the ambientand another at an elevated temperature. The processingrestricts to a portion of the image that covers a part ofthe solder joint. To exactly outline the solder joint underthe speckle coverage is assisted by feature lines andpoints that are commonly identiable in both images.Upon the determination of the rectangular area andthe measurement resolution, a processing grid is laiddown in the reference image. The area is indicated bythe solid-line box in both schematics shown inFig. 3(a) and (c), as well as by the processing left traceson the image as seen in Fig. 3(b). Each processingobtains a set of the measurements at a grid point, includ-ing two displacement components and four displace-ment partial derivatives. It is noted that the DSCtechnique yields the six parameters simultaneously andindependently. Raw measurements in the solder layerobtained at a particular temperature are typically shownin Fig. 4 in the form of area contours. The displacementcomponents u and v are the direct output. The partialderivatives ou/ox and ov/oy are also direct output andrepresent approximately the respective normal strains.And the shear strain is obtained via ou/oy + ov/ox.Within the processed area, a small portion near the sol-der joint llet is named the data area. This chosen</p><p>directly attached to the test package. The test data andthe results from a nite-element thermal modeling both</p><p>H. Lu et al. / Microelectronics Reliability 46 (2006) 11481159 1151sub-area, as indicated by a dash-line box in bothFig. 3(a) and (c), has linear dimensions of a few dozenmicrons. Based on the original data, the average shear</p><p>Fig. 4. Typical contour maps of strain and displacement</p><p>components directly obtained from image processing.conrm a fairly uniform temperature distribution acrossthe sample surface and in the body. The modelingproves that the temperature in-uniformity throughoutthe package is limited and ranges from a fraction of adegree to a couple of degree Celsius. In actual testing,additional monitoring of the sample temperature is pro-vided by thermal couples attached to the sample stand atthe samples very vicinity. The system vibration and theair circulation, enhanced by the use of large optical mag-nication, are the main sources of disturbance to aectthe image stability and thus to cause scattered measure-ments. The chamber temperature control is automatedby adjusting the air ow, which functions well in keepinga limited temperature uctuation during dwelling. Thenature of the control, however, appears to have contrib-uted to the data scatter. The troubling instable air owbecomes apparently severer in the dwell periods duringwhich the feedback controls the electric fan to be alter-nately on and o. The resulted turbulence causes theoptical refractive index of the air to vary, and in turn,the image to distort. An eort made to counter that isto manually turn the fan o for a few seconds beforeeach image recording. This however results in that thesample temperature quickly drops from the preset testprole by as much as 10 degree Celsius.</p><p>3. Strain analysis leading to solutions of stress and</p><p>separation of strain components</p><p>3.1. Solder constitutive relations</p><p>Numerous constitutive equations have been pro-posed for dierent solder alloys. This study gives theattention to those that are based on testing actual assem-blies e.g., [13,22,23]. For surface mount packages, it is awell documented e.g., [2] phenomenon that the sheardominates the thermally induced solder joint deforma-tion. This analysis focuses on the shear strain despitethe availability of normal strain measurements. Adoptedstrain in the sub-area is calculated for o-setting the ran-dom errors that exist in the single-point measurements.In the analysis to follow, this average shear strain isreferred to as the measured total shear strain.</p><p>2.4. Temperature control</p><p>A forced convective thermal chamber as seen inFig. 1(b) is used. The chamber temperature can be setto vary between 70 C and 250 C with chosen ratesand at the accuracy of 3.5 C. Since the sample tem-perature always lags behind the chamber reading, a sep-arate calibration test is conducted using thermal couplesas follows is a commonly accepted, additive constitutive</p></li><li><p>3.2. Solder deformation basics</p><p>Some basic aspects of the solder deformation arenoted in formulating the stress solution as presented inthe next section. First, the measured strain generallyconsists of mechanical and thermal components. Themechanical part is stress related and usually has morethan one contributor as indicated by Eq. (1); the thermalpart represents the materials thermally induced dimen-sional change while the material is under no force andconstraints. The thermal strain is usually non-stress</p><p>Table 2Solder constitutive parameters [13]</p><p>G0 (GPa/Mpsi) 19.3/2.8</p><p>G1 (MPa/K/Kpsi/K) 69/10C6 2.04 1011</p><p>m 4.39C (K/s/Pa/K/s/psi) 0.454 106/3.13 103</p><p>a 1500n 5.5Q (eV) 0.5</p><p>Fig. 5. Test temperature vs. time curve: (a) for test package A</p><p>1152 H. Lu et al. / Microelectronics Reliability 46 (2006) 11481159relation that regards the shear strain as a linear superpo-sition of separate components, each representing contri-bution of a dierent deformation mechanism. The totalshear strain ctl is expressed as</p><p>cel cpl ccr ctl 1where cel, cpl and ccr are, respectively, the elastic, plasticand creep strain components. The specic model andparameters adopted are proposed by Darveaux et al.[13]. Darveauxs model extends to several solder alloys(60Sn40Pb, 96.5Sn3.5Ag, 97.5Pb2.5Sn and 100In, etc.),covers a wide strain range and inc...</p></li></ul>