H. Preu

Modeling and characterization of molding compound properties during cure

Abstract During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. In this paper we focus on three commercial molding compounds and analyze in detail the increase in elastic modulus and the change in viscoelastic behaviour during cure. This was done with a special shear tool which allows to measure mechanical properties with sufficient accuracy in the liquid as well as in the solid state. The cure dependent viscoelastic material behaviour was modeled using a cure dependent shift factor and rubber modulus. The viscoelastic behaviour of the molding compounds is also shown not to be stable. During postcure the materials slowly continue to crosslink thereby systematically changing their viscoelastic behaviour. The material models presented here therefore only account for the initial curing stage and do not include postcure.

Characterization and modeling of molding compound properties during cure

During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. In this paper we focus on three commercial molding compounds and and analyze in detail the increase in elastic modulus and the change in viscoelastic behaviour during cure. This was done with a special shear tool which allows to measure mechanical properties with sufficient accuracy in the liquid as well as in the solid state. The cure dependent viscoelastic material behaviour was modeled using a cure dependent shift factor and rubber modulus. The viscoelastic behaviour of the molding compounds is also shown not to be stable. During postcure the materials slowly continue to crosslink thereby systematically changing their viscoelastic behaviour. The material models presented here therefore only account for the initial curing stage and do not include postcure.

Kinetic Characterisation of Molding Compounds

During the packaging of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. For a reliable simulation of the stresses generated in a package during cure and subsequent cooling it is essential to have accurate data for the thermal and mechanical properties of the molding compound, die, solder and substrate materials. Of these materials the molding compound is by far the most difficult one to model since these properties vary widely and are time, conversion and temperature dependent. The present paper consists of an extensive study on several commercial molding compounds of which the kinetic parameters we obtained by analysing both isothermal and non-isothermal differential scanning calorimetry (DSC) data. For the modelling of the kinetics data it turned out to be necessary to take the diffusion limitation effect (incomplete cure at lower temperatures) into account. The glass transition versus conversion could be modelled satisfactory using the well known DiBenedetto equation.

Characterization and Modelling of Cure Kinetics of A Molding Compound

Packaging failures has always been the bottleneck of the development on the new techniques of IC packaging using molding compounds. Numerous simulations are developed to minimize the maximum stress during or after the packaging process. Within those simulations, a reliable kinetic model is indispensable since the viscoelastic properties are related strongly with conversion during the cure process. In tins paper, the cure kinetics of a molding compound is described by the Kamal-Sourour equation, including the effect of diffusion limitation, and the corresponding parameters are determined. Furthermore, a preliminary dynamic mechanical experiment during cure is presented to show an application of our cure kinetic model.

Characterization of adhesives for microelectronic industry in DMA and relaxation experiments for interfacial fracture toughness characterization — Difficulties and solution

Electrically conductive adhesives are widely used in semiconductor technology. The focus of this work is set on Isotropic Conductive Adhesives (ICA) with a high amount of electrically conductive filler particles. The aim of this work is the material characterization of highly filled epoxy based die attaches materials by dynamic mechanical analysis (DMA) and relaxation experiments in order to derive elastic and viscoelastic material models in a wide temperature range. The measurement of the epoxy based highly filled die attach material is a challenging topic. We show how to overcome the difficulties in measuring these materials. Critical interface fracture data, which include the Critical (Strain) Energy Release Rate Gc(Ψ) as a function of temperature, humidity or aging, are crucially needed in microelectronic industry for failure modeling, lifetime prediction and design evaluation associated with reliability [1], but they are rarely given in literature. Therefore fast measurement methods are needed [2, 3]. This work shows a measurement method of the critical fracture mechanic properties with the micro Mixed Mode Tester (μMMT) [2] on samples cut from real products and their numerical evaluation using linear elastic fracture mechanics and cohesive zone modeling.

A study on electrochemical effects in external capacitor packages

Abstract Exposing semiconductor devices with external capacitors to harsh environmental conditions may lead to electrical failures with the formation of conductive paths. This paper presents examples of the analysis of modules with the purpose to understand the respective failure modes. Appropriate sample preparation, sensitive analytical methods like micro-X-ray fluorescence spectroscopy (μXRF), ToF-SIMS, SEM/EDX, X-ray-microscopy as well as micro computed X-ray-tomography (μCT) have been applied to identify the root causes of the electrical failures. As a main conclusion of these investigations, we found that electrolytes can easily penetrate thermoplastic overmold materials which are typically used by module manufacturers. This can lead to either reversible electrical failures which can be eliminated by drying or irreversible electrical failures because of material migration. The effective failure mode depends on mechanical and climate conditions inside the module which could not be simulated up to now under laboratory but only under application conditions.