This Is AuburnElectronic Theses and Dissertations

Ridge Regression based Development of Acceleration Factors and closed form Life prediction models for Lead-free Packaging




Arunachalam, Dinesh Kumar

Type of Degree



Mechanical Engineering


The thermo-mechanical mismatch caused by the difference in the coefficient of thermal expansion between the electronic part and the printed circuit board results in shear strains in the solder interconnects during thermal excursions. Widely used life prediction models include the Manson [1966] and Coffin Model [1954, 1963] which correlates plastic strain amplitude, Goldmann Model [1969] which correlates the geometry and material parameters with cyclic life, Δεp, with fatigue life, Norris-Landzberg`s Model [1969] which correlates the thermo-mechanical material and geometry parameters with cyclic life. Norris-Landzberg acceleration factors for lead-free solders have been developed based on ridge regression models (RR) and on PCR for reliability prediction and part selection of area-array packaging architectures under thermo-mechanical loads. The principal component transformation has been used to rank the new orthogonal principal components in the order of their importance. Scree plots, Eigen values and proportion of total variance explained by each principal component are then used to eliminate the least important principal components. Multiple linear regressions have been performed with the original response variable and reduced set of principal components. Ridge regression adds a small positive bias to the diagonal of the covariance matrix to prevent high sensitivity to variables that are correlated. The proposed procedure proves to be a better tool for prediction than multiple-linear regression models. Models have been developed in conjunction with Stepwise Regression Methods for identification of the main effects. Package architectures studied include, BGA packages mounted on copper-core and no-core printed circuit assemblies in harsh environments. The models have been developed based on thermo-mechanical reliability data acquired on copper-core and no-core assemblies in four different thermal cycling conditions. Packages with Sn3Ag0.5Cu solder alloy interconnects have been examined. The models have been developed based on perturbation of accelerated test thermo-mechanical failure data. Data has been gathered on nine different thermal cycle conditions with SAC305 alloys. The thermal cycle conditions differ in temperature range, dwell times, maximum temperature and minimum temperature to enable development of constants needed for the life prediction and assessment of acceleration factors. Norris-Landzberg acceleration factors have been benchmarked against previously published values. In addition, model predictions have been validated against validation data-sets which have not been used for model development. Convergence of statistical models with experimental data has been demonstrated using a single factor design of experiment study for individual factors including temperature cycle magnitude, relative coefficient of thermal expansion, and diagonal length of the chip. Life prediction models have been developed over the years trying to assess the influence of the different geometrical, material and thermo-cycling parameters on the life of an electronic package. In this study the influence of silver content on packages based on SAC alloys have been investigated. Along with silver content, the solder ball configuration parameters such as ball pitch, ball count, ball height, ball diameter and cycle conditions such as dwell time and delta T have been considered .An assortment of packages such as CBGAs, PBGAs, flips chips based on a variety of SAC alloys with a set of different silver contents were considered for the analysis