Alexandrine Guédon-Gracia

Microstructure evolution observation for SAC solder joint: Comparison between thermal cycling and thermal storage

Abstract This paper describes a reliable and reproducible method to reveal the microstructure of lead-free solder joints. This method is then used to follow the aging of lead free assembled chip resistors and BGA, with either ENIG or immersion Sn finishes. Two types of aging were applied: accelerated thermal cycles (ATC), and iso-thermal storage at three different temperatures. The associated failure mechanisms are compared and discussed. During ATC, microstructural changes under thermo-mechanical fatigue are evidenced, whereas thermal storage only induces intermetallic evolution, depending on the finish type.

Electrically driven matter transport effects in PoP interconnections

The migration issue of Package-on-Package (PoP) is investigated in this article. The migration phenomenon can be amplified as the density of solder bumps increases, while the paths are very finer as well in PoP.

Reliability of Lead-Free BGA Assembly: Correlation Between Accelerated Ageing Tests and FE Simulations

In 2006, lead was banished in electrical and electronic equipment in Europe, but the reliability of lead-free technology is not yet well known. This paper describes a methodology using experiments and FE simulations that allow us to assess acceleration coefficients (ACs) and to predict assembly lifetimes. Two accelerated ageing tests have been conducted on lead-free ball grid array (BGA) assemblies. During the ageing test, cycles-to-failure have been accurately determined by an event detector from AnaTech. The 3-D nonlinear finite-element model of the BGA assembly has been designed and built using ANSYS software. Thermomechanical simulations have been carried out on this model to compute the strain energy density (SED) that dissipated in the solder joints during the thermal cycles. Then, the correlation between the experimentations and the simulations has allowed us to assess the ACs of two different kinds of thermal cycles and, thus, the cycles-to-failure for another test.

Correlation between experimental results and FE simulations to evaluate lead-free BGA assembly reliability

Lead-free solder is taken into use in the electronics industry and there is a lack of data about the reliability of the lead-free assemblies. In this paper, a methodology on the acceleration factor (AF) assessment is described. The lead-free BGA assemblies are submitted to various accelerated ageing test (thermal cycles). An event detector from AnaTech is used in order to determine the cycles-to-failure. The three-dimensional nonlinear finite element model of BGA assembly is designed and built using ANSYS software. The thermo-mechanical simulations are carried out on this model in order to compute the strain energy density (SED) dissipated in the solder joints during the thermal cycles. Then the correlation between the experiments and the simulations allows to assess a SED critical value in order to predict the cycles-to-failure for another test.

Virtual prototyping in a Design-for-Reliability approach

Abstract The main purpose of the paper is to present a methodological approach combining experiments and simulations for virtual prototyping in a Design-for-Reliability approach. Two examples illustrate the methodology and the importance of a wise choice of the adequate failure criterion for numerical modelling. In the first example, the stress induced in silicon is axial whereas it is radial in the second case. Strength data recorded from three-point-bend tests have been regarded in the first case, whereas in the second case, ball-on-ring test results have been considered.

Moisture induced effects in PoP

This paper presents a combined numerical and experimental methodology for predicting moisture-induced effects in Package-on-Package (PoP) assemblies. The hygroscopic warpage of the assembly “PoP” is measured. Its behavior is compared with the individual components “top” and “bottom”. The assembly “PoP” absorbs more moisture than the sum of the two individual components, while deforms less than either of the two individual components, which holds much more constrain inside the assembly. In addition, the hygroscopic warpage (20µm) adds to the thermal warpage (30µm) for the assembly “PoP”, which might lead to more serious reliability problem. Furthermore, a new experimental methodology for prediction of the moisture-induced warpage is validated, and simulation models match the experimental results.

Analytical model for thermally-induced warpage of POP

The thermally-induced deformations of package-on- package (POP) are investigated theoretically, numerically and experimentally in this paper. An analytical model of POP is newly proposed for prediction of the warpage of POP, which can be achieved by solving a few linear multiple polynomial equations rather than building a complex model in ANSYS, and leads to satisfying results. All the parameters needed for the analytical model are temperature-dependant material properties, and geometrical parameters. Measurements of the the warpage of POP under various temperature loads are made with a Profilometer Altisurf 500. Moreover, a 3-D linear finite element model was built. Both measurements and simulation validate our analytical model. The results from the experiments are presented and compared in detail with those from the 3-D linear finite element and theoretical analysis. The good agreements in the experimental and modelling results reveal the validation of 3-D linear finite element model in mechanics sense and the effectiveness of the numerical and theoretical models in physics predictions. The interest lies in a very easy use of this theoretical model to quickly obtain the warpage information for deciding whether the design is suitable and/or comparing the different POP circuits design.

Assessment of Constitutive Properties of Solder Materials Used in Surface-Mount Devices for Harsh Environment Applications

To perform a thermomechanical simulation of surface-mount devices (SMD), relevant solder constitutive properties are needed. This paper reviews some promising Pb and Pb-free solders that are candidates to harsh environment applications. It presents the discrepancy in their common mechanical properties reported in the literature. The mechanical test systems, solder microstructure under test, and the test conditions (temperature and strain rate) are among the major causes for the discrepancy. The use of such variations in the solder thermomechanical properties as input data for simulation studies may cause reliability prediction less meaningful. A novel and reliable test methodology to extract relevant solder properties for surface mount applications is presented in this paper. To fulfill the methodology requirements, lap shear tests with accurate control of the reflow profile and dimensionally accurate solder joint thickness need to be conducted to reproduce the SMD solder joint microstructures for determining solder joint properties for elastic, plastic, and creep model simulations. Two case studies with high-Pb and Pb-free solders are presented. Time-independent yield strength and ultimate tensile strength data obtained from our studies were found to be higher than the reported literature data. This methodology can be extended for other solder material systems with any surface finish plating materials.

Influence of PCB design and materials on chip solder joint reliability

This paper describes chip solder joint reliability on three substrate types: one board SMI (aluminium substrate) and two different boards in FR4. Several chip sizes and types (resistor, capacitor) were assembled on these boards. Accelerated Thermal cycles (ATC) -55/+125°C were applied to evaluate the lifetime of chip solder joints. The different results obtained showed an important dispersion in Time To Failure (TTF) according to the substrate type. Literature data confirm this dispersion. To understand this discrepancy Finite Element Modelling (FEM) analyses were used to evaluate the influence of PCB design and materials on solder chip component reliability. The simulations permit to identify which parameters are the most influent.

Creep measurement and choice of creep laws for BGA assemblies' reliability simulation

Abstract Since the replacement of hazardous materials by the RoHS directive, the intensive use of lead free solder materials generates countless studies to find new acceleration factors. In that scope, identifying the failure mechanisms involved in accelerated thermal tests has become of great concern for reliability design. Numerical tools are commonly developed for predictive analysis but their accuracy and capacity to be transposed to multiple technologies rely in part on the right choice of material behaviour. This study proposes to fit material coefficients of 4 different available creep laws to a same set of experiments. A unique couple of Finite Element models is defined using global/local calculations. Global calculation gives the assembly behaviour under thermal loads that is used to feed a part of the assembly with a more refined mesh to calculate locally a BGA ball that behaves following the Garofalos, Anands, Darveauxs laws of creep. The discrepancies that this comparison highlights, show how much care needs to be taken at that step of modeling for reliability prediction.