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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.

Quality Control and/or Assurance and similar phrases are in common use but can have different meanings, depending on the type of product involved and on economic considerations. It is proposed that, from the soldering point of view, component design and solderability must be as good as possible and must be regularly checked. There must be adequate control over every aspect of the soldering process, and inspection of the completed assembly must be meaningful with realistic accept/reject levels decided upon at the outset. Having established a complete production scheme incorporating quality checking, it is proposed that no alterations should be made without full consultation and collaboration with all the personnel concerned.

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.

Solder is a major use for tin and in his function as Chief Metallurgist of the Tin Research Institute, C. J. Thwaites has become internationally recognised as an authority on soft soldering, particularly for the electronics industry. It is with pleasure that the Institute announces that he has recently been awarded the degree of Doctor of Science (Eng.) by London University, for his published work in the field of soldering and related topics.

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.

The paper presents the results of extensive studies on circuit board solderability comparing wetting balance and IPC test methods through performance in vapour phase and wave soldering operations. The effects on solderability of key parameters are examined and compared with storage times of one year, and accelerated ageing using damp heat, dry heat and steam oxygen. An evaluation is made of tin‐lead alloys from 40/60 to 70/30 in solder coating thicknesses from 0·1 to 1·0 mil.

Joints made to the Surface Mount configuration for a chip capacitor using either 60Sn40Pb or 62Sn36Pb2Ag solders were found to weaken substantially during storage at room temperature. This loss in joint strength is attributed to microstructural changes in the solder, particularly precipitation of the β phase and recrystallisation of the matrix, i.e., the two solid solutions (α + β), and the subsequent coarsening of the microstructure.

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.

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.

An empirical study was conducted to determine the thermal fatigue behaviour of 1.27 mm pitch, J‐bend and gullwing surface mount solder joints, manufactured with four low‐temperature solders. Selected solder alloys were: 58Bi‐42Sn (wt %), 43Sn‐43Pb‐14Bi, 52ln‐48Sn and 40ln‐40Sn‐20Pb. Accelerated thermal cycling was used in conjunction with metallographic analysis and mechanical (pull) strength measurement to test their behaviour. The relative merit of each solder composition was determined by comparing it with 63Sn‐37Pb solder, subjected to identical testing conditions. The strength decreased linearly with increased number of thermal cycles for gullwing solder joints of all four solder alloys. The fatigue lifetime was relatively longer for 58Bi‐42Sn and 40ln‐40Sn‐20Pb than for other alloys, but significantly lower than that obtained with 63Sn‐37Pb solder. No discernible degradation of strength was observed with the J‐bend solder joints of any solder alloy, even after the completion of 6000 thermal cycles. Thermal fatigue resistance of the latter joints was attributed to a more favourable coefficient of thermal expansion (CTE) mismatch. Solder joint cracking occurred only in gullwing components soldered with 52ln‐48Sn, 40ln‐40Sn‐20Pb and 43Sn‐43Pb‐14Bi alloys, after 1000 or 2000 thermal cycles. The crack initiated on the outside surface of the solder fillet, and appeared to propagate through both phases of the microstructure. The stress‐induced heterogeneous coarsening of the microstructure was evident only with 43Sn‐43Pb‐14Bi solder, although not as prevalent as that usually observed with eutectic Sn‐Pb solder. Fatigue cracks were absent from solder joints of 58Bi‐42Sn and 63Sn‐37Pb alloys.