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A model SMD test‐piece has been developed which permits mechanical stressing of a solder joint in a similar mode to that occurring in the process of thermal fatigue. The failure mechanisms in the 60Sn40Pb alloy studied have been in agreement with those frequently observed in thermal fatigue. Further, the fatigue life of model joints was found to increase with increasing solder volume whilst, upon ageing at room temperature, fatigue resistance decreased. These effects were attributed to microstructural changes occurring within the solder.

The electronics industry seems to be under slightly less pressure, at present, to move to lead‐free solders. Nevertheless, many companies are interested in switching to comply with initiatives aimed at reducing the use of toxic materials such as lead. At the same time, these and other companies are hoping to use the improved properties of many lead‐free solders to increase solder joint reliability. While lead‐free solders can indeed give improved properties in the proper system, using them with lead‐containing materials can actually give poorer properties and uncertain reliability. It is important to understand these materials' interactions as thoroughly as possible before introducing new solder compositions. To illustrate this point, some case histories will be presented, along with a general discussion of how solders and substrates can and do interact.

The growing concern over the possibility of lead elimination from electronics solders has prompted the initiation of several studies by various groups. The same basic approach is a feature of most of these efforts — a programme of alloy development aimed at finding a new alloy which may substitute for most, if not all, applications for tin‐lead. Many factors must be considered in such an approach and these are outlined in the present paper, along with some consideration of the alloys available. An alternative approach of process adaptation is also presented.

Eutectic Sn‐Ag solder is being considered as a potential replacement for Sn‐Pb solders. A potential drawback to using the eutectic Sn‐Ag solder is its higher melting point, 221°C, compared with the eutectic Pb‐Sn solder. Owing to its higher melting temperature, the eutectic Sn‐Ag solder is also being considered for automotive under‐the‐hood applications, which experience high temperature environments. Electronic components and/or circuit boards are often coated with Pb‐bearing solder to facilitate soldering operations. Soldering Pb‐bearing solder coated components and/or boards with eutectic Sn‐Ag solder will result in joints contaminated with Pb. In this study, the effects of Pb contamination on eutectic Sn‐Ag solder joints were investigated using three ternary alloys made by incorporating some Pb into eutectic Sn‐Ag solder. These ternary alloys all showed a peak at 178°C in heating curves obtained using Differential Scanning Calorimetry (DSC), which resulted from the ternary eutectic composition in the Sn‐Ag‐Pb system. The Pb phases in the ternary alloys were found to be dispersed throughout the microstructure. A practical implication of Pb contamination in eutectic Sn‐Ag solder joints is that the service temperature of such joints would be limited by the lower melting temperature of the ternary eutectic phase.

The increased interest in the electronics industry to search for alternatives to lead‐containing solders is evidenced by the number of recently published articles on lead‐free solders in this journal and other journals. At the latest Surface Mount International conference, several papers were presented on lead‐free solder alloys, conductive adhesives and organic preservatives, all in search of replacements for lead‐containing finishes. The efforts to find a replacement for tin/lead are in response to possible legislation banning lead or possible taxation on the use of lead. In an attempt to reduce the use of lead in this company's assembly operation, five lead‐free solder pastes and four corresponding flux vehicles (for A, B, C and E pastes) were evaluated. All of the flux vehicles passed the standard industry tests except for two flux vehicles (pastes B and C) that failed the copper mirror test. An assembly trial of the lead‐free pastes was carried out by building liquid crystal display panels with minimal process modification. Printability, process compatibility and solder joint reliability were assessed. This paper describes the preliminary results of the authors' investigation.

Studies have been made of ageing effects, both at room temperature and at 125°C, on the microstructure of 60:40 tin‐lead solders. A comparison was made of the effect of ageing on slow and rapidly cooled matrices. Precipitation of tin within lead dendrites was observed to occur very rapidly after solidification of the alloy. Subsequently the precipitates coarsened markedly over a period of a few weeks. The matrix of the alloy also coarsened at room temperature over this period. At elevated temperatures a similar sequence of events occurred, but substantially faster. The microstructural origins of the known loss in mechanical strength of solders with ageing are discussed.

Establishes a quantitative evaluation method on solder bridging in microsoldering using a printed wiring board with comb pattern conductors. Bridge tests were conducted by immersing the comb pattern board into a molten solder bath. The total length of solder bridge between conductors against the total length between conductors was measured as a measure of the occurrence of solder bridging. The occurrence of bridging depended on the number of immersions, flux activity including solid content, conductor spacing, solder bath temperature and solder composition. The increase in number of immersion enhanced bridging. The rosin flux without activators showed higher bridging than the activated flux. Sn‐37Pb solder showed lower bridging than Sn‐3.5Ag‐5Bi solder. Solder bridging was found to be closely correlated with wettability, therefore, the improvement of wettability could be effective to suppress solder bridging. The proposed method is believed to be suitable for the quantitative evaluation of solder bridging.

The lifetime of flip chip solder joints can be greatly increased by applying underfill encapsulation. This paper presents a case study where the thermal cycle lifetime was increased by more than 20 times, that is from ∼100 cycles to more than 2,000 cycles for a chip size of 5.6mm × 6.3mm. By combining electrical, acoustic and metallographic investigation, the degradation of the solder joints was monitored, some important factors relevant to solder joint reliability were analysed and discussed. Delamination was found not to be the dominant failure mode.

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.

In terms of science, technologies and end‐use applications, ‘lead‐free solders’ are a very broad subject. This paper is intended to highlight the key issues of this broad subject for its applications in electronic packaging and assembly. The areas covered include the main thrusts behind the research and development effort, the scientific approaches, economic and practical considerations, and regulatory and legislative perspectives of lead. In addition, this paper will present the new findings as a result of research and development effort on lead‐free solders. Technologically, in view of the current and future needs in interconnecting materials, the first objective of this work is to demonstrate the ability to provide lead‐free solders in lieu of conventional lead‐bearing solders without mandating significant changes in manufacturing processes and/or equipment which have been established and proven in the industry. The second objective is to design the lead‐free solders which are able to offer superior performance to their Sn/Pb counterparts, thus providing solder interconnections with higher reliability and endurance during service life. The fulfilment of these two objectives will concurrently address the two critical technological areas in the field of solders, namely the inherent vulnerability of conventional solders to temperature and stress, and the concerns of toxicity and health hazard of lead. Based on materials principles, ternary and quaternary lead‐free systems are at present being designed. The work conducted thus far demonstrates that two of the designed solder compositions show great promise. In comparison with 63Sn/37Pb eutectic solder, the new lead‐free compositions exhibit superior performance in shear strength, creep resistance and thermomechanical fatigue resistance. This paper will also cover the data in mechanical testings, the summary of microstructure evaluation (with detailed elucidation omitted), and low‐cycle fatigue fractoqraphy examination of the designed lead‐free compositions.