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The wetting balance is used for the measurement of solderability of electronic components. The wetting force is measured dynamically and the technique gives information about both the degree and the speed of wetting. For practical quality assessment of electronic components, a simple‐to‐use index is required that incorporates the data on both degree and speed of wetting. The index must also have a uniform discrimination between different wetting properties, across the full range encountered in practical soldering. This paper reviews critically the numerous indices suggested in the literature, and supports with quantitative data the choices previously made subjectively in some soldering standards.

An experimental method for measurement system improvement is presented and applied to development of a protocol for solderability measurement with a wetting balance. Protocol development is central to development of reliable monitoring systems for manufacturing. This paper illustrates the method with an experiment in which sets of nearly identical test leads, each with a different solderability, are obtained by steam ageing of hot‐solder‐dipped copper and then measured according to alternative protocols. The protocols entail different flux types and solder bath temperatures. This method can be used wherever solderability measurements are made and thus satisfies the need for in‐house refinement of wetting balance protocols.With the experimental method, one can both compare alternative measurement protocols and estimate the relative solderability of sets of test leads. The results of both depend on what feature of the wetting force curve one selects to portray solderability. The comparison of measurement protocols is based on what is variously called precision, sensitivity, or signal‐to‐noise ratio. The solderability estimates show that different physical properties of the test leads affect different parts of the wetting force curve, and that changes in the steam ageing procedure affect solderability in a generally predictable way.

The availability of a wetting balance which can be easily interfaced to a microcomputer has made possible a practical receiving inspection solderability test for component leads that avoids the subjectivity of the present dip‐and‐look test. The wetting balance, in effect, detects the size and shape of the solder meniscus on the lead. Since it is the solder meniscus more than the degree of coverage that is evaluated by inspectors of the completed solder joint, the wetting balance provides a more realistic test of how well the components will perform on the PWA. The software that has been developed for the wetting balance is designed to make it easy for inspection workers to perform the test with a minimum of training. It asks for identification of the part, manufacturer, date code, purchase order number, etc., so that the final results are adequately documented. Use of a computer to present the results means that the wetting force as a function of time can be plotted as a normalised curve (automatically accounting for differences in number and size of leads), and also that the results can be accumulated in a factory computer for statistical quality control. For a given lot of components, there is usually little spread in the observed results. This indicates that a sample size as small as three is sufficient to characterise the lot. With further data accumulation, it should be possible to devise a skip‐lot sampling plan for those manufacturers showing consistently good solderability. Also, the accumulated results of lots from problem manufacturers, coupled with microscopic studies of the causes of poor solderability, can be used as a basis for negotiations. The results are reproducible from one plant to another because they do not require visual interpretation. Judicious application of this method of solderability testing by a component user should allow removal of problem lots (for return or solderability enhancement), and therefore lead to a virtual elimination of solderability‐related defects observed after PWA soldering. Widespread application by component manufacturers (after burn‐in) should lead to a virtual elimination of cases of shipping unsolderable components.

Visual inspection remains the dominant method of assessing component lead solderability and finished board solder joint quality. In recent years the wetting balance has received much attention as an attractive alternative to the inherently subjective visual inspection method of assessing component termination solderability. Whether direct visual inspection or wetting balance methods are used, the method can be shown to be effective only if the results are in agreement with board‐level soldering performance. This paper addresses the issue of the agreement of visual board‐level solder joint quality with both visual ‘dip and look’ solderability assessment and wetting balance measurement of the components prior to board assembly. A description of visual ‘dip and look’ solderability test assessment and of wetting balance methodology for components is presented, and a compendium of wetting balance tests and indices are documented in the Appendix. The experimental strategy employed is outlined, and details of the experimental technique (including the equipment, materials and component sample preparations) are provided. The experimental results present a comparison of both ‘dip and look’ visual solderability assessment and wetting balance measures with regard to actual board‐level soldering performance. The ability of the various assessment methods to predict board level defects is also explored.

With the increased use of surface‐mounted devices, both the component density and the board complexity are greatly increased on many circuit pack assemblies. Good solderability for the components as well as for the printed circuit boards has become one of the most important elements in achieving the ultra‐high efficiency and quality required of an assembly soldering process in today's competitive environment. Solderability evaluation generally uses the ‘dip and look’ method that relies entirely on the individual inspectors' often inconsistent interpretations resulting from the examination of the specimen dip‐tested in a molten solder bath. This method is subjective, so the results can vary from person to person and from day to day. In addition, the sensitivity of such a method is inadequate in discerning the differences in solder wetting characteristics of very small device leads and terminations. Consequently, components with marginal or even bad solderability may pass through inspection and move onto the production line to cause many easily avoidable defects and their subsequent repairs. A sensitive and quantitative wetting balance method has been studied with the purpose of developing a better alternative to the ‘dip and look’ procedure. Special sample holders and test conditions have been developed for testing various types of components and printed circuit board coupons. Examples of solderability testings are provided to illustrate the capability of the instrument when proper testing procedures are followed. More effort is under way to simplify the test procedure, and to establish a practical solderability test standard for the wetting balance method.

In the last decade through‐hole mounting to printed wiring boards has matured and people now have the tools to diagnose and correct any solderability problems which might arise. Such is not the case with surface mount soldering technology. In surface mount the connections are smaller and are often hidden from view. Therefore when a solderability problem does occur it may never be known until the assembly fails. The solution to the situation is to understand the nature of the problems and provide assurance that they will not occur during assembly soldering. This paper is structured in two parts. The first details the types of solderability problems unique to surface mounting. Examples of these problems will be shown and discussed with reference to solder joint life. The second part of the paper discusses the solderability testing of surface mount devices and printed wiring boards intended for surface mounting. This discussion will concentrate on the new quantitative solderability test methods being developed in this company's laboratory for leadless devices and printed wiring boards. As part of this development, new solderability criteria have been defined which reflect the unique problems associated with surface mounting.

Solderability testing, according to Military Standard 883, has evolved through four sets of different test conditions during the past ten years. These relate to the duration of artificial steam ageing and the utilisation of either mildly activated or pure rosin flux. The European Space Agency (ESA) has experienced few solderability problems with leadless ceramic chip carriers (LCCCs) supporting tin‐lead finished castellations. However, similar packages having gold on nickel plated finishes will only produce satisfactory solderability results when activated flux is applied to samples exposed to steam ageing for eight hours. A reason for poor solderability is given, based on an evaluation of test samples and metallography. It is concluded that tin‐lead finished devices should be subjected to eight hours of steam ageing followed by solderability testing with pure rosin flux. An ESA prerequisite for soldering is that all gold finishes must be removed, possibly with the aid of RA type fluxes. For this reason it is recommended that RA flux is permitted (when necessary) during the standard solderability test.

To establish a measurement scale for the solderability of electronic components using the wetting balance, there is a requirement for a set of standard reference surfaces of defined solderabilities to calibrate the instrument and the measurement chain. Ideally, a single material will be used, whose surface can be tuned to a given solderability by a well characterised procedure. This paper describes the development of ranges of solderable surfaces suitable for calibrating wetting balances and other solderability test and measurement methods.

An overview has been presented on the topic of alternative surface finishes for package I/Os and circuit board features. Aspects of processability and solder joint reliability were described for the following coatings: baseline hot‐dipped, plated, and plated‐and‐fused 100Sn and Sn‐Pb coatings; Ni/Au; Pd, Ni/Pd, and Ni/Pd/Au finishes; and the recently marketed immersion Ag coatings. The Ni/Au coatings appear to provide the all‐around best options in terms of solderability protection and wire bondability. Nickel/Pd finishes offer a slightly reduced level of performance in these areas which is most likely due to variable Pd surface conditions. It is necessary to minimize dissolved Au or Pd contents in the solder material to prevent solder joint embrittlement. Ancillary aspects that include thickness measurement techniques; the importance of finish compatibility with conformal coatings and conductive adhesives; and the need for alternative finishes for the processing of non‐Pb bearing solders are discussed.

To study the influence of storage time and environment on the solderability of electroless nickel plated samples with Sn‐3.8Ag‐0.7Cu and Sn‐3.5Ag lead‐free solders and to provide criteria for the use of an electroless nickel (Ni‐P) under bump metallization (UBM) without immersion gold protection.

Electroless nickel coatings were deposited onto pure aluminium foil through a procedure developed for the UBM of wafers prior to flip chip bumping. Their solderability with lead‐free solders was studied using the wetting balance technique. Samples stored in different environments for different periods of time were tested to study the dependence of the solderability of Ni‐P coatings on the storage time and temperature. The degree of oxidation of the Ni‐P coatings was examined by means of X‐ray photoelectron spectroscopy and the surface microstructure and roughness of the coatings were analyzed by scanning electron microscopy and atomic force microscopy.

It was found that the Ni‐P coatings were unacceptable for direct soldering without the assistance of a flux, due to poor wettability, even when using a freshly prepared Ni‐P coating. Therefore, a suitable flux with nitrogen inerting had to be applied to assist the soldering process. The results also show that the solderability of Ni‐P coatings was affected by the phosphorus content, and the Ni‐P coating with high phosphorus content had a good solderability. The storage time and temperature did not influence the wettability significantly with the assistance of strong flux.

The stability of the plating solution and the consistence of the phosphorus content in the coating are not easily controlled. This has resulted in implications for surface analysis and wetting testing. Ni‐P coatings with different levels of phosphorus content are being investigated in detail.

The value of the paper lies in its study on the solderability of lead‐free solders to Ni‐P coating after storage in different environments and for different periods, which can provide some criteria for the use of Ni‐P UBM without immersion gold protection.