The effect of co-planarity variation on anisotropic conductive adhesive assemblies

Anisotropic Conductive Adhesives (ACAs) consist of a polymer adhesive matrix containing fine conductive particles dispersed either randomly, or more rarely in an ordered way. The primary objective of this experimental research was to understand the effects of a non-uniform bond thickness due to non co-planarity of the component or substrate terminations in ACA assemblies. This has been achieved through measurements of the conductivity variations of ACA joints in a number of ACA assemblies, where the component bump plane and substrate plane were deliberately held in different degrees of relative rotation from parallel during adhesive cure. Measurements of the joint resistances versus rotational angle, for a constant bonding force, were made for 10 levels of rotation of the chips relative to the substrates. The results showed that the resistances of the joints in the assemblies exhibited three distinct types of behaviour: stable joint resistances; gradually increasing resistances and unstable resistances. In conclusion, it is shown that ACA joints are very sensitive to the uniformity of the bond thickness, as the larger the rotations were, the lower and less uniform the joint conductivities were, however, the joints were uniform if the rotation angles were controlled within certain limits

Thermal stability of high temperature Pb-free solder interconnect characterised by in-situ electron microscopy

The present investigation aimed to use in-situ heating experiment in a transmission electron microscope (TEM) to live characterize the thermal stability of a Cu/Ni-W-P interlayer/ZnAl solder interconnect. It demonstrated the TEM was able to detect live intermetallic compounds (IMCs) growth during heating. In addition, stress building up was evidenced by the progressive evolving of the dislocations at the interface between NiW-P interlayer and the ZnAl Solder. However, due to the μm to nm scale of specimens' dimensions required for electron microscopy, the sample preparation and data interpretation remains a challenge

The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing

This paper reports the penetration limit of via holes through dissolving dielectric polymer thin films by inkjet printing. It was found that both the outer diameter of via holes and the polymer thickness affect the penetration depth from the experimental results. Based on this finding, a more accurate relationship between the inner diameter of via holes and the diameter of in-flight droplets for different polymer thicknesses is obtained

Numerical analysis of thermo-mechanical behavior of indium micro-joint at cryogenic temperatures

Microelectronic packaging plays an important role in cryogenic engineering; in particular, a solder joint as interconnection, which offers a mechanical, thermal and electrical support, undergoes much larger and harsher thermal changes during its service compared with conventional customer electronic products. The impact of thermo-mechanical properties of such solder joints under cryogenic service conditions becomes even more significant due to the continuing miniaturization of solder joints. Indium, a solder material with a low melting point and excellent cryogenic properties, has been favorable in various low temperature applications, in particular, to form solder joints for electronics interconnections. In order to understand the fundamental aspects of reliability of indium joints, this paper reports a constitutive model accounting for the effect of temperature change on thermo-mechanical behavior of indium joints. Especially, the model is used and subsequently implemented in a FE analysis to simulate a hybrid pixel detector system, in which indium micro-joints are manufactured to serve at cryogenic conditions. The response of indium joints to low-temperature cycling (300-76 K) was analyzed based on the proposed model, which not only offers a tool to understand the performance and experimental testing of solder joints under cryogenic temperatures, but can also be used for design optimization of the microelectronic package

Effect of thermal aging on interfacial behaviour of copper ball bonds

Thermosonic copper ball bonding is an interconnection technology that serves as a viable and cost-saving alternative to gold ball bonding. However, the reliability of copper bonds remains to be ascertained. Intermetallic compounds (IMCs) and possible voids and cracks may grow and propagate at the interface of bonds during their service. The proper IMCs formation is beneficial to bonding strength but an excessive growth of IMCs, voids and cracks can induce a mechanical failure and increase a contact resistance. In this study, a 99.99% copper wire with diameter 50.4 mum was bonded to a Al-1%Si-0.5%Cu metallisation pad by thermosonic bonding. Scanning electron microscopy, energy dispersive X-ray spectrometry, dual focused ion bean and transmission electron microscopy (TEM) were used to investigate the interfacial evolution of such formed joints during the thermal ageing, and kinetics of Cu-Al IMCs growth was established. The results showed no IMCs at the initial bonded Cu/Al interface. To study the Cu-Al IMCs growth, the samples were thermally aged for different times at a temperature from 200 degC to 300 degC to accelerate interfacial evolution. The growth of Cu-Al IMCs followed the parabolic law as a function of aging time at a certain aging temperature, and it is more sensitive to temperature compared to time. The activation energy of Cu-Al IMC growth was obtained from the Arrhenius plot. Voids and cracks, which are commonly present in gold ball bonds due to thermal aging, were not observed in copper ball bonds even after aging at 200 degC for 2900 hours. Finally, the structure of Cu-Al IMCs was confirmed to be Cu9Al4 by selected area electron diffraction with TEM

The impact of substrate temperature on the size and aspect ratio of inkjet-dissolved via holes in thin poly(4-vinyl phenol) dielectric layers

The authors demonstrate the effect of substrate temperature on the relationship between the inkjet-etched via hole size and the number of drops of etchant dispensed. A mechanism for the different via hole size evolution versus the number of drops is proposed. An explanation for the interrelationship between the solvent evaporation rate and polymer re-deposition is presented. The aspect ratio of via holes produced is found to increase with the substrate temperature. Therefore, higher temperatures can be used to reduce the size and increase the aspect ratio of via holes fabricated by inkjet etching

Mechanical and electrical characterisation of individual ACA conductor particles

Anisotropic conductive adhesives (ACAs) consist of a polymer adhesive matrix containing fine conductive particles. The primary objective of this experimental research is to establish a clearer understanding of the effects of the bonding force on the deformation of individual ACA particles and their resulting conductivity when in contact with an appropriate metallic surface. This has been achieved through simultaneous measurements of the deformation and electrical resistance whilst applying force using a specially configured nano-indenter machine, where the "indenter", instead of being pointed, had a flat tip about 20-30 mum in diameter. The merit of using this machine is that very small forces, of the order of 100 mN, can be accurately applied to the particles to a resolution of 100 nN and the resulting deformations, of less than 6 mum, can then be recorded to a resolution of 0.1 nm. The results showed that the ACA particle deformation was nonlinear and that the force/deformation at which particle crushing occurs was affected by the load rate. The resistance was observed to decrease as the deformation increased up to the crush point at which stage it increased slightly. The voltage versus current behaviour of a deformed ACA particle was also found to be linear

A mechanism of the penetration limit for producing holes in poly(4-vinyl phenol) films by inkjet etching

A penetration limit has been experimentally demonstrated for inkjet etching of holes in thin polymer layers. A mechanism combining the competing coffee ring flow, polymer dissolution and diffusion into the solvent drop, and the interaction between the contact line during evaporation and the softened deformable polymer, is proposed to explain the existence of such a penetration limit. The height-averaged velocity of the coffee ring flow within the evaporating sessile drop is calculated during the initial stage of this etching process when the spherical cap geometry assumption is valid. This is compared with the diffusion velocity of the disentangled polymer into the solvent. The two competing flows are used to elucidate why a hole could be formed initially. The complex wetting dynamics of the receding contact line is included to explain the via hole profile evolution in the later stage of the etching process and the existence of a penetration limit. These two stages are differentiated by the drop volume with respect to the volume of the via hole produced by the preceding drop. The competition between the coffee ring flow transferring polymer away from the central region and the polymer diffusion within the solvent drop is postulated to contribute to either via hole formation or a penetration limit, depending on which one of the two processes is dominant within the solvent evaporation time scale

The thermal stability of alkanethiol self-assembled monolayers on copper for fluxless soldering applications

The ability of alkanethiol monolayers deposited on copper to prevent surface oxidation has suggested their application as preservatives for fluxless soldering. However, the utility of such coatings for this purpose will critically depend on their ability to continue to preserve the substrate during exposure to elevated temperatures throughout the electronics manufacturing process. Consequently, the aim of this paper is to systematically determine the effect of storage temperature and duration on the ability of alkanethiol coated copper samples to undergo fluxless soldering. Similarly, the effect of pre-heating copper immediately prior to soldering is also investigated. The effect of reducing atmospheric oxygen concentration during storage and soldering is also considered as a potential route to improve the thermal resilience of the coatings. Parallel to ascertaining these industrially relevant performance parameters, a quantitative correlation between surface chemistry and solder wetting is established, and the temperature dependence of the kinetics of surface oxidation through an alkanethiol barrier layer is discussed

Mechanical response of indium micro-joints to low temperature cycling

In this study, mechanical properties of indium micro-joints exposed to low temperature cycling were investigated. The metallization structure of Ni/Cu was specially used as a substrate to form indium joints for mechanical tests. This paper focuses mainly on the tensile test of indium joints at thermal cycling from 300 K to 77 K. The Young's modulus, the ultimate strength and yield strength of the material were obtained. The failure mode after different loading histories was analyzed. The results indicate the decline of the Young's modulus with the increase in the number of thermal cycles; however, the ultimate/yield strength of indium joints did not show an obvious trend with the number of thermal cycling. It was confirmed that indium joints still maintain a high ductility even after 20 thermal cycles. Finally, the associated modeling provides an insight predicting that an interface between indium and copper could be the potential site for failure in thermal cycling