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Polyester connector strips were joined to polyimide substrates with anisotropic electrically conductive adhesives. Copper conductors as well as Au/Ni‐coated copper conductors were used on flexible circuits. The adhesives were composite materials consisting of heat curing, one‐component epoxy resin and powdered ternary solder alloys: tin‐bismuth‐zinc, tin‐indium‐zinc and tin‐zinc‐aluminium. An adhesive filled with eutectic tin‐bismuth alloy powder was used as reference. The effect of bonding parameters (e.g., temperature, dwell time and pressure) on contact resistance values was evaluated. The contact resistance values were measured for evaluating the reliability of adhesive joints during a 60°C/95%RH test. Furthermore, the joint microstructures were examined with optical and scanning electron microscopy. The results showed that with the copper conductors the initial contact resistance values were lower than with the Au/Ni‐coated copper conductors. The most reliable joints were produced with low melting filler alloys (with respect to bonding temperature) on bare copper metallisation. The most likely reason for failure of the Au/Ni‐coated circuits was strong oxidation of locally exposed nickel in the presence of moisture.

The dissolution processes and subsequent intermetallic reactions between high tin solder bump alloys and Cu‐ or Ni‐based UBM‐metallisations were investigated both theoretically and experimentally. The results showed that when the Cu UBM layer is used together with eutectic or higher Sn‐based solder alloys the dissolution of Cu and the rate of the Cu₆Sn₅ formation is too high for reliable interconnections. On the contrary, Ni provides feasible solution for UBM/high tin solder applications. Although there is strong chemical interaction between nickel and high Sn solder bump alloys, the dissolution and subsequent Ni₃Sn₄ layer growth rates are very low. Thus, a thin Ni layer can sustain interactions with high Sn liquid as well as solid solders during high temperature use. On the basis of the results obtained flip chip bonding with Ni‐based UBM structures provides a viable interconnection solution for reliable fine‐pitch applications.

Tape automated bonding (TAB) circuits were joined by hot compression bonding to copper or nickel conductors on glass with two anisotropic electrically conductive adhesives. One of the adhesives had a thermoplastic polystyrene‐polyester matrix which contained easily deforming metal‐coated polymer particles, while the other was a thermosetting bisphenol (A) based epoxy resin filled with nickel particles. The resistance values and the mechanical strengths of the joints were measured before and after the ageing treatments. The thermoplastic adhesive had the lowest resistance values with copper conductors and the joints produced with this adhesive showed increasing strength values during the ageing tests. The joints between the Ni conductors had smaller values of electrical conductivity irrespective of the adhesive used. The SEM/EPMA technique revealed that particles of the thermoplastic adhesive tended to agglomerate. This may cause problems when components with very fine lead pitch are joined, either by short circuiting or leaving some contacts without particles.

The dissolution rate of a solid metal such as Cu, in contact with molten solder can be calculated with the use of the Nernst‐Brenner equation. We describe how this equation should be correctly used in cases when the solder is in contact with both the base metal and any intermetallic compounds that have formed. We also show that the concentration of solute in the solder will generally lie between the metastable solubility limit and the equilibrium solubility limit, illustrating these ideas with reference to a system comprising Nb as the base metal and eutectic In‐Sn as the solder, where the concentration levels can be directly correlated to the crystal growth rate.

The purpose of this paper was to develop a physics-based mathematical model to estimate the amount of substrate metal lost during the wet soldering process.

A mathematically rigorous model depicting the actual physics of the substrate/solder interaction and dissolution has been proposed to simulate the dissolution of the substrate metal in the liquid lead-free solder. The basic mass diffusion equation with the implementation of interface reaction kinetics was solved numerically using the finite volume approach. The moving interface was tracked by utilizing the coordinate transformation technique.

It was observed that the process of metal dissolution in the liquid solder was governed by two important parameters, viz., interface kinetics and long-range diffusion in the liquid solder. Non-equilibrium behavior was observed in the early stage of the process. The early stage of the dissolution process was seen as governed by interface kinetics, while diffusion became the rate-controlling mechanism at the later phase of soldering.

Substrate dissolution can be accurately estimated for a particular substrate–solder combination and for the given process conditions. This early estimation will help in ensuring the reliability and health of the solder joint.

A model based on actual physics is proposed, and interface reaction kinetics has been introduced to capture the actual behavior of the process. The model will serve as the basis for two- and three-dimensional analysis, including the formation of an intermetallic compound in the solder joint.

To investigate the influences of environmental stresses on board‐embedded polymeric waveguides.

Optical multimode waveguides were embedded on printed circuit boards using commercial polymers. The optical‐PCBs varying in board structure and in optical build‐up materials were exposed to heat, moisture and ionic‐contaminants in accelerated reliability tests. The influence of stress factors on the structural integrity and functional parameters, namely the refractive index and optical transmissivity, was investigated at the key communication wavelengths.

Isothermal annealing reduced the refractive index to the greatest extent. The optical‐PCB structure with an optical surface build‐up layer was observed to be more vulnerable under temperature shock when compared with the optical‐PCB with optical inner layer. The buffer layer beneath the optical build‐up was found to improve the stability of the optical waveguides significantly. The results indicated of wavelength dependence to the aging factor with a failure mechanism. The factors affecting the performance and reliability of polymer‐based optical waveguides on PCBs were discussed.

More experimental data and investigations of failure mechanisms are required to ultimately obtain sufficient reliability statistics for accurate life‐time prediction models.

Optical interconnects are seen as a promising solution to overcome performance limitations encountered with high‐frequency electrical interconnections. As an emerging technology, only a limited amount of reliability data on optical/electrical packages is available. The paper investigates the influences of environmental stresses on board‐embedded polymeric waveguides.

As a literature review article, the purpose of this paper is to highlight the intricate interaction and correlation between the interconnection microstructure and the failure mechanism. It is therefore critical to summarize all the challenges in understanding solder solidification of interconnections.

Literature review.

Solidification of solder interconnections is therefore critical because it is the process during which the solder interconnection is formed. The as‐solidified microstructure serves as the starting point for all failure modes. Because of the miniaturization of electronics, the interconnection size decreases continuously, already to such a range that solder solidification takes place remarkably differently from the bulk ingot, on which solidification studies have been focused for decades. There are many challenges in understanding the solidification of tiny solder interconnections, including the complex metallurgical system, dynamic solder composition, supercooling and actual solidification temperature, localized temperature field, diverse interfacial IMC formation, and so on, warranting further research investment on solder solidification.

This paper provides a critical overview of the concerns in solidification study for lead‐free solder interconnection. It is probably an article initiating more attention towards solidification topics.

Personal and portable electronic devices are becoming an important part of the everyday lives. Electronics miniaturization has been and continues to be the most important driver in this development. The current level of miniaturization has made the use of solder joints challenging and new methods, such as adhesive attachments, have been developed. The applicability of these methods depends crucially on their long‐term reliability. Typical failures mechanisms in adhesive connection include cracking, open joints and delamination. Moisture is the principal cause of failures in adhesive attachments. The purpose of this paper is to examine the long‐term effects of moisture and extended temperature on non‐conductive adhesive (NCA) attachments.

Moisture and extended temperature on NCA attachments are examined by strength tests and by finite element models.

The increase in temperature and moisture induces stresses on the interface of the adhesive and the chip. In the experiments, it is found that the adhesion strength of the adhesive decreased as a function of the time for which the samples are in a humid environment. Failures due to delamination are seen to be the result of these two mechanisms.

Reduction of pitch causes manufacturing problems in direct solder attachments. This paper examines a promising technique to overcome this problem by using adhesive attachment. Instead of solder joints in flip chip attachment, the chips can be attached to the substrate, or components can be attached to a printed wiring board, with adhesives.

A new under bump metallurgy (UBM) solution consisting of the TiW‐, Au‐ and Ni‐layers for solder flip chip applications has been developed. The metallurgy, being based on the well‐known TAB metallisation procedure, was modified by producing the galvanic nickel layer on the top of the Au‐TiW metallisation. Nickel is needed between high Sn liquid solder bumps and the Au layer to prevent fast and extensive dissolution of thin Cu or Au layers and consequently excessive intermetallic formation. The flip chip joints were manufactured, employing both the 60Sn40Pb‐ and pure Sn‐bumped chips together with the new UBM, which is relatively easy to implement into volume production. The different ageing tests demonstrated that reliable joints can be produced by using the UBM. On the basis of the results obtained the new UBM was found to be a significant improvement in the production of reliable flip chip joints.

The purpose of this paper is to visualise the activities of three solders; Sn‐37Pb, Sn‐9Zn and Sn‐3.5Ag on Cu substrates during reflow near their melting points and to relate them with reflow reactions between solder and substrate.

Melting activities of three solders near their melting points on copper substrates are visualised in an infrared reflow furnace.

Solder balls demonstrate different ways of melting and reflowing behaviours in dissimilar times and temperature intervals. Melting of Sn‐9Zn solder balls is initiated simultaneously at the surface and joint between solder balls. This is followed by the melting at the joint between solder balls and the Cu substrate. During melting, solder balls are first merged into each other and then reflow on the substrate from top to bottom. Opposite to Sn‐9Zn, Sn‐3.5Ag solder balls start to melt at the surface and the joint between the solder and substrate, simultaneously. Balls are first reflowed from top to bottom and, in the process, liquid solder is merged. Unlike Sn‐9Zn and Sn‐3.5 Ag, melting of Sn‐37Pb solder balls is initially commenced at the surface only. This is followed by simultaneous melting at both joints. Variation in melting activities of these solders is found to be closely related to the coalescence mechanism of solder balls and the reflow reactions between the solders and the Cu substrate.

The elementary melting activities of different solders on Cu substrates is related with their reflow behaviours. This provides better understanding of solder behaviour and selection of good lead‐free solder for applications in the electronic industry.