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.

Effects of thermal cycling profiles on the performance of chip-on-flex assembly using anisotropic conductive films

Anisotropic conductive films (ACFs) are widely used in the electronic packaging industries because of their fine pitch potential and the assembly process is simpler compared to the soldering process. However, there are still unsolved issues in the volume productions using ACFs. The main reason is that the effects of many factors on the interconnects are not well understood. This work focuses on the performance of ACF-bonded chip-on-flex assemblies subjected to a range of thermal cycling test conditions. Both experimental and three-dimensional finite element computer modelling methods are used. It has been revealed that greater temperature ranges and longer dwell-times give rise to higher stresses in the ACF interconnects. Higher stresses are concentrated along the edges of the chip-ACF interfaces. In the experiments, the results show that higher temperature ranges and prolonged dwell times increase contact resistance values. Close examination of the microstructures along the bond-line through the scanning electron microscope (SEM) indicates that cyclic thermal loads disjoint the conductive particles from the bump of the chip and/or pad of the substrate and this is thought to be related to the increase of the contact resistance value and the failure of the ACF joints.

Investigation of wall-slip behaviour in lead-free solder pastes and isotropic conductive adhesives

Slippage due to wall depletion effect is well-known in rheological investigation. The aim of this study was to investigate the influence of the paste microstructure on slip formation for the paste materials (lead-free solder paste and isotropic conductive adhesives). The effect of different flowgeometries, gap heights and surface roughness on the paste viscosity was investigated. The utilisation of different measuring geometries has not clearly showed the presence of wall-slip in the paste samples. The existence of wall-slip was found to be pronounced when gap heights were varied using the parallel plate geometry. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall-slip as expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates and to a certain extent inducing structural breakdown in the paste. Most importantly, the study also demonstrated on how the wall-slip formation in the paste material could be utilised for understanding of the paste microstructure and its flow behaviour.

Thixotropic studies of lead-based solder, lead-free solder and conductive adhesive pastes

The stencil printing process is an important process in the assembly of Surface Mount Technology (SMT)devices. There is a wide agreement in the industry that the paste printing process accounts for the majority of assembly defects. Experience with this process has shown that typically over 60% of all soldering defects are due to problems associated with the flow properties of solder pastes. Therefore, the rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. This paper presents results on the thixotropic behaviour of three pastes; lead-based solder paste, lead-free solder paste and isotropic conductive adhesive (ICA). These materials are widely used as interconnect medium in the electronics industry. Solder paste are metal alloys suspended in a flux medium while the ICAs consist of silver flakes dispersed in an epoxy resin. The thixotropy behaviour was investigated through two rheological test; (i) hysteresis loop test and (ii) steady shear rate test. In the hysteresis loop test, the shear rate were increased from 0.001 to 100s-1 and then decreased from 100 to 0.001s-1. Meanwhile, in the steady shear rate test, the materials were subjected to a constant shear rate of 0.100, 100 and 0.001s-1 for a period of 240 seconds. All the pastes showed a high degree of shear thinning behaviour with time. This might be due to the agglomeration of particles in the flux or epoxy resin that prohibits pastes flow under low shear rate. The action of high shear rate would break the agglomerates into smaller pieces which facilitates the flow of pastes, thus viscosity is reduced at high shear rate. The solder pastes exhibited a higher degree of structural breakdown compared to the ICAs. The area between the up curve and down curve in the hysteresis curve is an indication of the thixotropic behavior of the pastes. Among the three pastes, lead-free solder paste showed the largest area between the down curve and up curve, which indicating a larger structural breakdown in the pastes, followed by lead-based solder paste and ICA. In a steady shear rate test, viscosity of ICA showed the best recovery with the steeper curve to its original viscosity after the removal of shear, which indicating that the dispersion quality in ICA is good because the high shear has little effect on the microstructure of ICA. In contrast, lead-based paste showed the poorest recovery which means this paste undergo larger structural breakdown and dispersion quality in this paste is poor because the microstructure of the paste is easily disrupted by high shear. The structural breakdown during the application of shear and the recovery after removal of shear is an important characteristic in the paste printing process. If the paste’s viscosity can drop low enough, it may contribute to the aperture filling and quick recovery may prevent slumping.

Investigation of wall-slip effect on lead-free solder paste and isotropic conductive adhesives

Wall-slip plays an important role in characterising the flow behaviour of solder paste materials. The wall slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry.These interactions could lead to the presence of a thin solvent layer adjacent to the wall, which gives rise to slippage. The wall slip effect can play an important role in ensure successfulpaste release after the printing process. Wall-slip plays animportant role in characterising the flow behaviour of solderpastes and isotropic conductive adhesives. The study investigates the wall-slip formation in solder paste andisotropic conductive adhesives using flow visualisation technique. The slip distance was measured for parallel plate with different surface roughness in order to quantify the wallslip formations in these paste materials. An ink marker line was drawn between the parallel plate and the free surface of the sample. The parallel was rotated slowly at a constant shear rate of 0.05 sec-1 and the displacement of the ink marker was observed using a video microscope and image capturing software was utilised to capture the displacement of ink marker. From this study, it was found that the wall-slip effect was evident in all the paste materials. In addition, the different surface roughness of the parallel plates did not prevent the formation of wall-slip. This study has revealed that the wallslip effect could used to understand the flow behaviour of the paste in the stencil printing process.

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.

Viscoelastic properties of solder paste and isotropic conductive adhesives used for flip-chip assembly

The printing of pastes (solder pastes and isotropic conductive adhesives) through very small stencil apertures required for flip-chip pitch sizes is expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit board pads. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance.

Self-lubricating silicone elastomer biomaterials

Silicone has a relatively high coefficient of friction and silicone medical devices therefore lack inherent lubricity, leading to pain on device insertion and potential tissue trauma. In this study, higher molecular weight tetra(alkoxy) silanes, particularly tetra(oleyloxy) silane, have been used as crosslinkers in the condensation cure of a hydroxy end-functionalised linear poly(dimethylsiloxane). The resulting elastomers displayed a persistent lubricous surface of oleyl alcohol, and coefficients of friction (static and dynamic) approaching zero. Chemical structures of the synthesised silanes and surface alcohol exudate were confirmed by nuclear magnetic resonance spectroscopy. Mechanical properties of the elastomers, which were chemically identical to conventionally cured systems, suggested that an 80/20 mixture of tetra(oleyloxy) silane and tetra(propoxysilane) gave the best compromise between desirable mechanical and frictional properties.

A dynamic mechanical method for determining the silicone elastomer solubility of drugs and pharmaceutical excipients in silicone intravaginal drug delivery rings

The silicone elastomer solubilities of a range of drugs and pharmaceutical excipients employed in the development of silicone intravaginal drug delivery rings (polyethylene glycols, norethisterone acetate, estradiol, triclosan, oleyl alcohol, oxybutynin) have been determined using dynamic mechanical analysis. The method involves measuring the concentration-dependent decrease in the storage modulus associated with the melting of the incorporated drug/excipient, and extrapolation to zero change in storage modulus. The study also demonstrates the effect of drug/excipient concentrations on the mechanical stiffness of the silicone devices at 37°C.

Influence of silicone elastomer solubility and diffusivity on the in vitro release of drugs from intravaginal rings

The in vitro release characteristics of eight low-molecular-weight drugs (clindamycin, 17beta-estradiol, 17beta-estradiol-3-acetate, 17beta-estradiol diacetate, metronidazole, norethisterone, norethisterone acetate and oxybutynin) from silicone matrixtype intravaginal rings of various drug loadings have been evaluated under sink conditions. Through modelling of the release data using the Higuchi equation, and determination of the silicone solubility of the drugs, the apparent silicone elastomer diffusion coefficients of the drugs have been calculated. Furthermore, in an attempt to develop a quantitative model for predicting release rates of new drug substances from these vaginal ring devices, it has been observed that linear relationships exist between the log of the silicone solubility of the drug (mg ml(-1)) and the reciprocal of its melting point (K-1) (y = 3.558x - 9.620, R = 0.77), and also between the log of the diffusion coefficient (cm(2) s(-1)) and the molecular weight of the drug molecule (g mol(-1)) (y = - 0.0068x - 4.0738, R = 0.95). Given that the silicone solubility and silicone diffusion coefficient are the major parameters influencing the permeation of drugs through silicone elastomers, it is now possible to predict through use of the appropriate mathematical equations both matrix-type and reservoir-type intravaginal ring release rates simply from a knowledge of drug melting temperature and molecular weight. (C) 2003 Elsevier Science B.V. All rights reserved.

Characterisation of Silicone Elastomer Vaginal Rings Containing HIV Microbicide TMC120 by Raman Spectroscopy

Silicone elastomer vaginal rings are currently being pursued as a controlled-release strategy for delivering microbicidal substances for the prevention of heterosexual transmission of HIV. Although it is well established that the distribution of drugs in delivery systems influences the release characteristics, in practice the distribution is often difficult to quantify in-situ. Therefore, the aim of this work was to determine whether Raman spectroscopy might provide a rapid, non-contact means of measuring the concentrations of the lead candidate HIV microbicide TMC120 in a silicone elastomer reservoir-type vaginal ring. Vaginal rings loaded with TMC120 were manufactured and sectioned before either Raman mapping an entire ring cross-section (100 µm resolution) or running line scans at appropriate time intervals up to 30 h after manufacture. The results demonstrated that detectable amounts of TMC120, above the silicone elastomer saturation concentration, could be detected up to 1 mm into the sheath, presumably as a consequence of permeation and subsequent reprecipitation. The extent of permeation was found to be similar in rings manufactured at 25 and 80°C.