Showing papers in "Journal of Electronic Packaging in 2002"

Effect of Simulation Methodology on Solder Joint Crack Growth Correlation and Fatigue Life Prediction

Abstract: A generalized solder joint fatigue life model for surface mount packages was previously published in Refs [1,2]. The model is based on correlation to measured crack growth data on BGA joints during thermal cycling. It was subsequently discovered by Anderson et.al. that the ANSYS 5.2 finite element code used in the model had an error in its method for calculating plastic work [3]. It was shown that significant error in life prediction could result by using a recent version of the code where the bug has been fixed. The error comes about since the original crack growth constants were derived based on plastic work calculations that had the bug. In this paper, crack initiation and growth constants are recalculated using ANSYS 5.6. In addition, several other model related issues are explored with respect to the crack growth correlations. For example, 3D slice models were compared to quarter symmetry models. Anand’s constitutive model was compared with Darveaux’s constitutive model. It was shown that the crack growth rate dependence on strain energy density always had an exponent of 1.10 +/0.15. This is in the range of the original correlation, so the accuracy of relative predictions should still be within +/25%. However, the accuracy of absolute predictions could be off by a factor of 7 in the worst case, if the analyst uses a modeling procedure that is not consistent with that used for the crack growth correlation. The key to good accuracy is to maintain consistency in the modeling procedure. Introduction Analytical models in engineering have several practical uses: 1) rapid design optimization during the development phase of a product, 2) predicting field use limits, and 3) failure analysis of product returned from the field or failed in a qualification test. The solder joint fatigue model presented here was first published in Ref [1]. An outline of the procedure to predict fatigue life is shown schematically in Figure 1. The model utilizes finite element analysis to calculate the inelastic strain energy density accumulated per cycle during thermal or power cycling. The strain energy density is then used with crack growth data to calculate the number of cycles to initiate a cracks, and the number of cycles to propagate cracks through a joint. In reference [2], more work was presented regarding sensitivity of the life prediction to the FEA procedure. As a result, the procedure was modified slightly to include volume averaging of the strain energy values near the joint interface. The model has been successfully applied to TSOP, CQFP, CBGA, PBGA, and power hybrid packages [1,2,4-9]. Calculate Strain Energy Density Accumulated per Cycle Calculate Number of Cycles to Crack Initiation Calculate Crack Growth Rate Calculate Fatigue Life Based on Joint Length Figure 1. Solder joint fatigue life prediction method. It was subsequently discovered by Anderson et.al. that ANSYS 5.5.2 and earlier versions of the finite element code had an error in their method for calculating plastic work [3]. Even though there had generally been good correlation to measured results, it was shown that significant error in life prediction could result by using a recent version of the code where the bug has been fixed (e.g. ANSYS 5.5.3 and later versions). The error comes about since the original crack growth constants were derived based on plastic work calculations that had the bug. In this paper, crack initiation and growth constants are recalculated using ANSYS 5.6. In addition, several other model related issues are explored with respect to the crack growth correlations. Several recommendations are made so the analyst can get accurate results more efficiently. Constitutive Relations Since solder is above half of its melting point at room temperature, creep processes are expected to dominate the deformation kinetics. Steady state creep of solder can be expressed by a relationship of the form [10-12] dεs dt = Css[sinh(ασ)] n exp( -Qa kT ) (1) where dεs/dt is the steady state strain rate, k is Boltzmann's constant, T is the absolute temperature, σ is the applied stress, Qa is the apparent activation energy, n is the stress exponent, α prescribes the stress level at which the power law dependence breaks down, and Css is a constant. Steady state creep data for 62Sn36Pb2Ag solder joints is shown in Figure 2.

A New Creep Constitutive Model for Eutectic Solder Alloy

Abstract: In this study, a large number of creep tests were carried out to study the effect of stress level and testing temperature on the creep behavior of 63 Sn/37Pb solder in a systematic manner. Based on the dislocation controlled creep mechanism and Gibbs’ free-energy theory, a new creep constitutive model was proposed. The model was found to describe accurately the creep flow of the solder and to be capable of explaining the issues of stress and temperature dependent stress exponent and activation energy in the Arrhenius powerlaw creep model. Furthermore, the model was employed to predict accurately the longterm reliability of solder joints in a PBGA assembly. @DOI: 10.1115/1.1462624#

Accelerated Life Testing (ALT) in Microelectronics and Photonics: Its Role, Attributes, Challenges, Pitfalls, and Interaction With Qualification Tests

Abstract: Accelerated life tests (ALTs) are aimed at the revealing and understanding the physics of the expected or occurred failures, i.e. are able to detect the possible failure modes and mechanisms. Another objective of the ALTs is to accumulate representative failure statistics. Adequately designed, carefully conducted, and properly interpreted ALTs provide a consistent basis for obtaining the ultimate information of the reliability of a product - the predicted probability of failure after the given time of service. Such tests can dramatically facilitate the solution to the cost effectiveness and time-to-market problems. ALTs should play an important role in the evaluation, prediction and assurance of the reliability of microelectronics and optoelectronics devices and systems. In the majority of cases, ALTs should be conducted in addition to the qualification tests, which are required by the existing standards. There might be also situations, when ALTs can be (and, probably, should be) used as an effective substitution for such standards, or, at least, as the basis for the improvement of the existing qualification specifications. We describe different types (categories) of accelerated tests, with an emphasis on the role that ALTs should play in the development, design, qualification and manufacturing of microelectronics and photonics products. We discuss the challenges associated with the implementation and use of the ALTs, potential pitfalls (primarily those associated with possible shifts in the mechanisms and modes of failure), and the interaction of the ALTs with other types of accelerated tests. The role of the nondestructive evaluations is also briefly outlined. The case of a laser welded optoelectronic package assembly is used to illustrate the concepts addressed.

Thermomechanical Analysis of Solder Joints Under Thermal and Vibrational Loading

Abstract: Due to the coefficient of thermal expansion (CTE) mismatch between the bonded layers,the solder joint experiences cycling shear strain, which leads to short cycle fatigue. Whensemiconductor devices are used in a vibrating environment, additional strains shorten thefatigue life of a solder joint. Reliability of these joints in new packages is determined bylaboratory tests. In order to use the FEM to replace these expensive reliability tests aunified constitutive model for Pb40/Sn60 solder joints has been developed and imple-mented in a thermo-viscoplastic-dynamic finite element procedure. The model incorpo-rates thermal-elastic-viscoplastic and damage capabilities in a unified manner. The con-stitutive model has been verified extensively against laboratory test data. The finiteelement procedure was used for coupled thermo-viscoplastic-dynamic analyses for fatiguelife predictions. The results indicate that using Miner’s rule to calculate accumulativedamage by means of two separate analyses, namely dynamic and thermo-mechanical,significantly underestimates the accumulative total damage. It is also shown that a simul-taneous application of thermal and dynamic loads significantly shortens the fatigue life ofthe solder joint. In the microelectronic packaging industry it is common practice to ignorethe contribution of vibrations to short cycle fatigue life predictions. The results of thisstudy indicate that damage induced in the solder joints by vibrations have to be includedin fatigue life predictions to accurately estimate their reliability.@DOI: 10.1115/1.1400752#

Lithium Doped Polyethylene-Glycol-Based Thermal Interface Pastes for High Thermal Contact Conductance

Abstract: Polyethylene-glycol-based thermal interface paste containing trifluoroacetic acid lithium salt (1.5 wt. percent optimum) and boron nitride particles (;18.0 vol. percent optimum), as well as water and N, N-dimethylformamide for helping the dissociation of the salt to release Li ions, gives thermal contact conductance that is almost as high as that given by Sn-Pb solder, similar to that given by boron nitride particle filled sodium silicate, and much higher than that given by boron nitride particle filled silicone. @DOI: 10.1115/1.1477191#

Could Shock Tests Adequately Mimic Drop Test Conditions

Abstract: Drop tests are often substituted in qualification or life testing of microelectronic and optoelectronic products by shock tests. The existing (e.g., Telcordia) qualification specifications require that a short term load of the given magnitude and duration (say, an "external" acceleration with the maximum value of 500 g, acting for 0.001 s) is applied to the support structure of the product under tests. The natural frequencies of vibration are not taken into account. The objective of our study is to develop simple analytical ("mathematical") predictive models for the evaluation of the dynamic response of a structural element in a microelectronic or an optoelectronic product/package to an impact load occurring as a result of drop or shock tests. We use the developed models to find out if a shock tester could be "tuned" in such a way that the shock tests would adequately mimic drop test conditions. We suggest that the maximum induced curvature and the maximum induced acceleration be used as suitable characteristics of the dynamic response of a structural element to an impact load. The results of the analysis can be helpful in physical design and qualification testing of microelectronic and photonic products, experiencing dynamic loads of short duration.

Micro-Mechanics of Creep-Fatigue Damage in PB-SN Solder Due to Thermal Cycling—Part I: Formulation

Abstract: This paper presents a micro-mechanistic approach for modeling fatigue damage initi due to cyclic creep in eutectic Pb-Sn solder. Damage mechanics due to cyclic cre modeled with void nucleation, void growth, and void coalescence model based on m structural stress fields. Micro-structural stress states are estimated under viscop phenomena like grain boundary sliding, its blocking at second-phase particles, and fusional creep relaxation. In Part II of this paper, the developed creep-fatigue dam model is quantified and parametric studies are provided to better illustrate the utilit the developed model. @DOI: 10.1115/1.1493202 #

Parametric Design and Reliability Analysis of Wire Interconnect Technology Wafer Level Packaging

Abstract: The demands for electronic packages with lower profile, lighter weight, and higher input/ output (I/O) density have led to rapid expansion in flip chip, chip scale package (CSP) and wafer level packaging (WLP) technologies. The urgent demand high I/O density and good reliability characteristics have led to the evolution of ultra high-density non-solder interconnection, such as wire interconnect technology (WIT). New technology, which uses copper posts to replace the solder bumps as interconnections, has improved reliability. Moreover, this type of wafer level package produces higher I/O density, as well as ultra fine pitch. This research focuses on the reliability analysis, material selection and structural design of WIT packaging. This research employs finite element method (FEM) to analyze the physical behavior of packaging structures under thermal cycling conditions to compare the reliability characteristics of conventional wafer level and WIT packages. Parametric studies of specific parameters will be performed, and the plastic and temperature-dependent material properties will be applied to all models. @DOI: 10.1115/1.1481368#