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An electronic assembly may consist of a printed circuit and various types of electrical components. Soldering to make the electrical/mechanical connection is a critical process. Both printed circuit and component leads must promote acceptable solder wetting if high reliability is to be obtained. Bulk purchasing of these items can lead to long periods of storage often in poor conditions. This paper describes some of the work which simulates storage conditions by accelerated ageing so that a prediction can be made as to whether solderability will be affected. Due acknowledgement is hereby made to the EIPC for their permission to publish this paper which was presented at a recent EIPC seminar.

Thermoanalytical spectra of the components of a typical rosin based flux and the interactions of the components with each other, the substrate and the solder are presented. The study includes heating rates up to those experienced in typical soldering operations.

In the production of printed circuit assemblies, the demand for higher reliability levels has increased over the years. In order to achieve a high level of soldering quality, it is essential that solderability is built into the system at all stages and various factors must be taken into account. In the first section of this paper some of these factors are discussed. The various solderable coalings that are available are reviewed, some of the problems that can be encountered are illustrated and the effects of impurities in solders discussed. In the second part of the paper, the use of circuit boards having fused tin/lead coatings is discussed from the solderability point of view.

Amalgams, which are mechanically alloyed mixes of a liquid metal with a powder, offer advantages in situations where large devices are to be bonded to materials with significant coefficient of expansion differences or where extremely temperature‐sensitive devices are to be bonded. This is because these materials will set or harden at or near room temperature to yield hard metallic bonds with melting points from 280°C up to ∼600°C depending upon the systems used. In this paper the results of a survey study of three binary systems of gallium with copper, nickel and silver are described. Wetting characteristics, bond strengths with and without metallisation, bulk properties including electrical and thermal properties and thermal cycle performance of joints are described. The feasibility of using these materials for bonding metallised and unmetallised surfaces of a variety of ceramics and semiconductors is clearly demonstrated.

Tin and solder coatings interact with substrates commonly used in the electronics industry to produce layers of intermetallic compounds at temperatures above and below the melting point of the coatings. Observations on the rates of compound growth at room temperature for durations of up to 12 years are reported and related to the published results for shorter times at higher temperatures. Recent results concerning the effect of intermatallic compound growth on the solderability of coatings and on the strength of soldered joints are presented. In both cases it is apparent that retarding the rate of compound growth could be useful and the use of barrier layers for this purpose is considered.

Rinsing treatments on copper after etching are investigated by using the adverse effect they have on solderability. The wetting time, as measured by the GEC Meniscograph, is a sensitive indicator of the effect of varying contact times with water (the most obvious way of removing etching residues). Several variables of the water were examined such as dissolved impurities (as indicated by a comparison of demineralised, boiled demineralised and tap‐water) rinsing time in tap water and water temperature. Wetting time is found to rise with all of these and demineralised water is superior to tap water, as might be expected. Very small amounts of acid added to the rinsing solution appear to retard almost completely the contamination effects of the water. All the findings underline how important surface cleanliness is to a successful soldering operation. By corollary it appears that solderability tests (especially the GEC Meniscograph) are very good indicators of extremely low levels of surface contamination.

Reaction rate curves for a complete range of rosin based fluxes, calculated from observed solderability wetting time curves on progressively more heavily oxidised copper, are presented. These are compared with the form expected on the basis of a concentration‐dependent fluxing mechanism. Either a chain reaction type or an autocatalytic type mechanism is suggested from the observations presented.

The introduction of high density surface mount technology may lead to a number of metallurgical problems. This paper considers two aspects which are currently being actively discussed. Firstly, incompatibility in expansion properties of the materials used and severe thermal cycling may induce creep and fatigue stresses on soldered joints, and the merits of different alloys are considered. Secondly, the necessary thermal treatments such as burn‐in and elevated service temperature can lead to intermetallic compound layer growth between the solder and the metallised layer on components which may be considered a potential source of joint strength reduction. Mention is also made of the different visual appearance of joints to chip components compared with conventional soldered joints.

A feasibility study into alternative methods of producing interconnection between a PCB and flip‐chip has been undertaken. A number of initial ideas were investigated, the least promising being discarded at an early stage, while the ideas showing the greatest chance of success were subject to a more rigorous examination. Of the initial ideas the most promising were amalgam materials and magnetic alignment of ferromagnetic particles. These two ideas were combined to produce a new type of anisotropic conducting adhesive (ACA), which may have the potential to overcome problems owing to co‐planarity issues and have the ability to form fine pitch metallurgical bonds. In order to promote bonding, amalgam compositions that enhance surface wetting, while retaining good mechanical properties have been investigated. The possibility of incorporating liquid/semi‐solid metallic interconnects, within the ACA, which will retain contact during the thermal expansion of the polymeric materials was also explored. During the course of the study, various techniques such as DSC and SEM have been used to characterise thermal stability of Ga‐based alloys and discrepancies with current phase diagrams have been found.

Current classification of fluxes is typically on the basis of a qualitative process with parameters like residue activity in a water extract or the ability to dissolve copper in a thin mirror film. The capability of quantitatively ranking fluxes by their total reaction capacity and reaction rate with copper oxide would provide valuable data for process design. Results are reported of experiments to establish a standard procedure to measure a flux activity number using a controlled reaction of a measured amount of flux reacted with an excess of a standard metal oxide.