Effects of solder reflow on the reliability of flip-chip on flex interconnections using anisotropic conductive adhesives

This work describes the work of an investigation of the effects of solder reflow process on the reliability of anisotropic conductive film (ACF) interconnection for flip-chip on flex (FCOF) applications. Experiments as well as computer modeling methods have been used. The results show that the contact resistance of ACF interconnections increases after the reflow and the magnitude of the increase is strongly correlated to the peak reflow temperature. In fact, nearly 40 percent of the joints are open when the peak reflow temperature is 260°C, while there is no opening when the peak temperature is 210°C. It is believed that the coefficient of thermal expansion (CTE) mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. To understand this phenomenon better, a three-dimensional (3-D) finite element (FE) model of an ACF joint has been analyzed in order to predict the stress distribution in the conductive particles, adhesive matrix and metal pads during the reflow process. The stress level at the interface between the particle and its surrounding materials is significant and it is the highest at the interface between the particle and the adhesive matrix.

Computational modelling for reliable flip-chip packaging at sub-100 micron pitch using isotropic conductive adhesives

This paper presents the assembly process using next generation electroformed stencils and Isotropic Conductive Adhesives (ICAs) as interconnection material. The utilisation of ICAs in flip-chip assembly process is investigated as an alternative to the lead and lead-free solder alloys and aims to ensure a low temperature (T < 100 °C) assembly process. The paper emphasizes and discusses in details the assembly of a flip-chip package based on copper columns bumped die and substrate with stencil printed ICA deposits at sub-100 μm pitch. A computational modelling approach is undertaken to provide comprehensive results on reliability trends of ICA joints subject to thermal cycling of the flip-chip assembly based on easy to use damage criteria and damage evaluation. Important design parameters in the package are selected and investigated using numerical modelling techniques to provide knowledge and understanding of their impact on the thermo-mechanical behaviour of the flip-chip ICA joints. Sensitivity analysis of the damage in the adhesive material is also carried out. Optimal design rules for enhanced performance and improved thermo-mechanical reliability of ICA assembled flip-chip packages are finally formulated.

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.

Thermo-mechanical analysis of the packaging process for micro-electronic displays

Micro-electronic displays are sensitive devices and its performance is easily affected by external environmental factors. To enable the display to perform in extreme conditions, the device must be structurally strengthened, the effects of this packaging process was investigated. A thermo-mechanical finite element analysis was used to discover potential problems in the packaging process and to improve the overall design of the device. The main concern from the analysis predicted that displacement of the borosilicate glass and the Y stress of the adhesive are important. Using this information a design which reduced the variation of displacement and kept the stress to a minimum was suggested

Analysis of the thermo mechanical effects on packaging process of performance enhanced AMLCD's and the optical performance of the display

The performance enhancement of AMLCD's has been hindered with problems encountered during the curing process, such as window framing and de-lamination of the glass and adhesive. A thermo-mechanical analysis using FEA was conducted to help optimise the design of the rugged display and enhance the optical performance.

Thermal-mechanical analysis of flexible substrates during lead-free solder reflow

This paper presents modeling results about the performance of flexible substrates when subjected to higher lead-free reflow temperatures. Both adhesiveless and adhesive types of polyimide substrates were studied. Finite element (FE) models of flex substrates were built, two copper tracks located in the centre of the substrate was considered. The thermal induced shear stress in the flex substrate during the lead-free reflow process was studied and the effect of the design changes including the track thickness, flex thickness, and copper width were studied. For both types of flexes, the one of most important variables for minimizing damage to the substrate is the height of the copper tracks. The height of flex and the width of copper track show less impact. Beside of the geometry effects, the increase in reflow peak temperature can also result in a significant increase in the interfacial stress between the copper track and flex. Higher stresses were identified within the adhesive flex due to the big CTE mismatch between the copper and adhesive/dielectric

Application of modelling for predicting the behaviour of polymers and adhesives in microelectronic and photonic devices

With the growth in computing power, and advances in numerical methods for the solution of partial differential equations, modeling technologies based around computational fluid dynamics, finite element analysis and optimisation are now being widely used by researchers and industry. Polymer and adhesive materials are now being widely used in electronic and photonic devices. This paper will illustrate the use of modeling tools to predict the behaviour of these materials from product assembly to its performance and reliability.

Thermal analysis of LEDS for liquid crystal display's backlighting

This paper investigated the thermal design of the light emitting diode (LED)onto the board and its packaging. The LED was a 6-lead MultiLED with three chips designed for LCD backlighting and other lighting purposes. A 3D finite element model of this LED was built up and thermal analysis was carried out using the multi physics software package PHYSICA. The modeling results were presented as temperature distributions in each LED, and the predicted junction temperature was used for thermal resistance calculation. The results for the board structure indicated that (1) removing the foil attach decreased the thermal resistance, (2) Increasing the copper foil thickness reduced the thermal resistance. package design indicated that the SMT designed LED with integrated slug gave lower thermal resistance. Pb-free solder material gave lower thermal resistance and junction temperature when compared with conductive adhesive

Microwave curing for high density package

Summary form only given. Currently the vast majority of adhesive materials in electronic products are bonded using convection heating or infra-red as well as UV-curing. These thermal processing steps can take several hours to perform, slowing throughput and contributing a significant portion of the cost of manufacturing. With the demand for lighter, faster, and smaller electronic devices, there is a need for innovative material processing techniques and control methodologies. The increasing demand for smaller and cheaper devices pose engineering challenges in designing a curing systems that minimize the time required between the curing of devices in a production line, allowing access to the components during curing for alignment and testing. Microwave radiation exhibits several favorable characteristics and over the past few years has attracted increased academic and industrial attention as an alternative solution to curing of flip-chip underfills, bumps, glob top and potting cure, structural bonding, die attach, wafer processing, opto-electronics assembly as well as RF-ID tag bonding. Microwave energy fundamentally accelerates the cure kinetics of polymer adhesives. It provides a route to focus heat into the polymer materials penetrating the substrates that typically remain transparent. Therefore microwave energy can be used to minimise the temperature increase in the surrounding materials. The short path between the energy source and the cured material ensures a rapid heating rate and an overall low thermal budget. In this keynote talk, we will review the principles of microwave curing of materials for high density packing. Emphasis will be placed on recent advances within ongoing research in the UK on the realization of "open-oven" cavities, tailored to address existing challenges. Open-ovens do not require positioning of the device into the cavity through a movable door, hence being more suitable for fully automated processing. Further potential advantages of op- - en-oven curing include the possibility for simultaneous fine placement and curing of the device into a larger assembly. These capabilities promise productivity gains by combining assembly, placement and bonding into a single processing step. Moreover, the proposed design allows for selective heating within a large substrate, which can be useful particularly when the latter includes parts sensitive to increased temperatures.

Failure mechanisms of ACF joints under isothermal ageing

The possible failure mechanisms of anisotropic conductive film (ACF) joints under isothermal ageing conditions have been identified through experiments. It has been found that ACF joints formed at higher bonding temperatures can prevent increases in the contact resistance for any ageing temperature. The higher the ageing temperature the higher the electrical failure rate is. The formation of conduction gaps between the conductive particles and the pads and damages to the metal coatings of the particle have been identified as the reasons behind the electrical failures during ageing. In order to understand the mechanism for the formation of the conduction gap and damages in metal coatings during the isothermal ageing, computer modelling has been carried out and the results are discussed extensively. The computer analysis shows that stresses concentrate at the edges of the particle–pad interface, where the adhesive matrix meets the particle. This could lead to subsequent damages and reductions in the adhesion strength in that region and it is possible for the conductive particle to be detached from the pad and the adhesive matrix. It is believed that because of this a conduction gap appears. Furthermore, under thermal loading the thermal expansion of the adhesive matrix squeezes the conductive particle and damages the metal coatings. Experimental evidences support this computational finding. It is, therefore, postulated that if an ACF-based electronic component operates in a high temperature aging condition, its electrical and mechanical functionalities will be at risk.

Rheological characterisation of solder pastes and isotropic conductive adhesives used for flip-chip assembly

Solder pastes and isotropic conductive adhesives (ICAs) are widely used as a principal bonding medium in the electronic industry. This study investigates the rheological behaviour of the pastes (solder paste and isotropic conductive adhesives) used for flip-chip assembly. Oscillatory stress sweep test are performed to evaluate solid characteristic and cohesiveness of the lead-free solder pastes and isotropic conductive adhesive paste materials. The results show that the G' (storage modulus) is higher than G '' (loss modulus) for the pastes material indicating a solid like behaviour. It result shows that the linear visco-elastic region for the pastes lies in a very small stress range, below 10 Pa. in addition, the stress at which the value of storage modulus is equal to that of loss modulus can be used as an indicator of the paste cohesiveness. The measured cross-over stress at G'=G '' shows that the solder paste has higher stress at G'=G '' compared to conductive adhesives. Creep-recovery test method is used to study the slump behaviour in the paste materials. The conductive adhesive paste shows a good recovery when compared to the solder pastes. (C) 2008 Elsevier B.V. All rights reserved.

Controlled release from directly compressible theophylline buccal tablets

The aim of the current study was the development of theophylline buccal adhesive tablets using direct compression. Buccal adhesive formulations were developed using a water soluble resin with various combinations of mucoadhesive polymers. The prepared theophylline tablets were evaluated for tensile strength, swelling capacity and ex vivo mucoadhesion performance. Ex vivo mucoadhesion was assessed using porcine gingival tissue and the peak detachment forces were found to be suitable for a buccal adhesive tablet with a maximum of 1.5N approximately. The effect of formulation composition on the release pattern was also investigated. Most formulations showed theophylline controlled release profiles depended on the grade and polymer ratio. The release mechanisms were found to fit Peppas' kinetic model over a period of 5h. In general the majority of the developed formulations presented suitable adhesion and controlled drug release. Copyright © 2010 Elsevier B.V. All rights reserved.