Numerical analysis of thermal stresses induced during VFM encapsulant curing

Encapsulant curing using a Variable Frequency Microwave (VFM) system is analysed numerically. Thermosetting polymer encapsulant materials require an input of heat energy to initiate the cure process. In this article, the heating is considered to be performed by a novel microwave system, able to perform the curing process more rapidly than conventional techniques. Thermal stresses are induced when packages containing materials with differing coefficients of thermal expansion are heated, and cure stresses are induced as thermosetting polymer materials shrink during the cure process. These stresses are developed during processing and remain as residual stresses within the component after the manufacturing process is complete. As residual stresses will directly affect the reliability of the device, it is necessary to assess their magnitude and the effect on package reliability. A coupled multiphysics model has been developed to numercially analyse the microwave curing process. In order to obtain a usefully accurate model of this process, a holistic approach has been taken, in which the process is not considered to be a sequence of discrete steps, but as a complex coupled system. An overview of the implemented numerical model is presented, with particular focus paid to analysis of induced thermal stresses. Results showing distribution of stresses within an idealised microelectronics package are presented and discussed.

Comparison of encapsulant curing with convention and microwave systems

Comparison of the performance of a conventional convection oven system with a dual-section microwave system for curing thermosetting polymer encapsulant materials has been performed numerically. A numerical model capable of analysing both the convection and microwave cure processes has been developed and is breifly outliines. The model is used to analyse the curing of a commercially available encapsulant material using both systems. Results obtained from numerical solutions are presented, confirming that the VFM system enables the cure process to be carried out far more rapidly than with the convection oven system. This capability stems from the fundamental heating processes involved, namely that microwave processing enables the heating rate to be varied independently of the material temperature. Variations in cure times, curing rates, maximum temperatures and residual stresses between the processes are fully discussed.

Impact of assembly process technologies on electronic packaging materials

Assembly processes used to bond components to printed circuit boards can have a significant impact on these boards and the final packaged component. Traditional approaches to bonding components to printed circuit boards results in heat being applied across the whole board assembly. This can lead to board warpage and possibly high residual stresses. Another approach discussed in this paper is to use Variable Frequency Microwave (VFM) heating to cure adhesives and underfills and bond components to printed circuit boards. In terms of energy considerations the use of VFM technology is much more cost effective compared to convection/radiation heating. This paper will discuss the impact of traditional reflow based processes on flexible substrates and it will demonstrate the possible advantages of using localised variable frequency microwave heating to cure materials in an electronic package.

Does an incremental filling technique reduce polymerization shrinkage stresses

It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to the hard tissue result in stress transfer and inward deformation of the cavity walls of the restored tooth. Deformation of the walls decreases the size of the cavity during the filling process. This fact has a profound influence on the assumption-raised and discussed in this paper-that an incremental filling technique reduces the stress effect of composite shrinkage on the tooth. Developing stress fields for different incremental filling techniques are simulated in a numerical analysis. The analysis shows that, in a restoration with a well-established bond to the tooth-as is generally desired-incremental filling techniques increase the deformation of the restored tooth. The increase is caused by the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. This leads to a higher-stressed tooth-composite structure. The study also shows that the assessment of intercuspal distance measurements as well as simplifications based on generalization of the shrinkage stress state cannot be sufficient to characterize the effect of polymerization shrinkage in a tooth-restoration complex. Incremental filling methods may need to be retained for reasons such as densification, adaptation, thoroughness of cure, and bond formation. However, it is very difficult to prove that incrementalization needs to be retained because of the abatement of shrinkage effects.

Macro-micro modelling of moisture induced stresses in an ACF flip chip assembly

Purpose – This paper discusses the use of modelling techniques to predict the reliability of an anisotropic conductive film (ACF) flip chip in a humid environment. The purpose of this modelling work is to understand the role that moisture plays in the failure of ACF flip chips. Design/methodology/approach – A 3D macro-micro finite element modelling technique was used to determine the moisture diffusion and moisture-induced stresses inside the ACF flip chip. Findings – The results show that the ACF layer in the flip chip can be expected to be fully saturated with moisture after 3?h at 121°C, 100%RH, 2?atm test conditions. The swelling effect of the adhesive due to this moisture absorption causes predominately tensile stress at the interface between the adhesive and the metallization, which could cause a decrease in the contact area, and therefore an increase in the contact resistance. Originality/value – This paper introduces a macro-micro modelling technique which enables more detailed 3D modelling analysis of an ACF flip chip than previously.

Singular stresses at the tip of a crack terminating at frictional interface of fibre reinforced composites

The stress singularities at the tip of a crack that terminates at a frictional interface between two layers in anisotropic composites are investigated. The order of stress singularities is determined by solving the characteristic equations obtained from the boundary conditions and the frictional interface conditions for the cases concerned. The interface is assumed to be governed by Coulomb's law of friction. Numerical results are presented for the cases with a crack terminating at a frictional interface of a fibre reinforced composite, and it is shown that there is a big difference of stress singularities between cases with and without considering friction along the interface.