INTRODUCTION

What does good solderability mean?

Solderability has been defined to varying levels of detail depending on the observer's immediate perspective. The fundamental definition from which they develop, however, s that good solderability means the ability of a base mental to be evenly wetted with an adherent coat of solder [1] [2]. Due to a desire for a cosmetically "shiny" joint, the definition has been pragmatically expanded by some to also include additional characteristics of solder fillet surface luster or smoothness. [3]

How is poor solderability identified? How is it measured?

Poor printed circuit (PWD) solderability may be identified in any one of several ways both prior and subsequent to the soldering process. A number of industry standard tests[4] and equipment exist for evaluating both the extent of solder wettability of a surface and the resulting mechanical strength of the bonds accomplished. Frequently, statistically validated, testing has become standard practice at most of the world's major PWB suppliers to provide manufacturing process control and screen outgoing lots.

Why is it important? Results of poor solderability.

Poor product solderability at the least involves added inspection and touchup costs. At worst it may precipitate a catastrophic circuit failure.

Unfortunately, often the most common point of solderability problem identification is after components have already been assembled to the circuit and it has passed through the soldering process. The purpose of soldering components to a circuit is to provide both a low resistance electrical path and a primary (surface mount) or secondary means of mechanical retention in/on the circuit.

Inadequate solder at the component/circuit interface leads increases the probability of increased electrical resistance, circuit "hot spots", intermittent performance, and potential circuit failure. In a consumer product, this may be of relatively small consequence. In a system control or life support application, it may have catastrophic results.

One of the most frequent symptoms of poor solderability is a dewetted PWB surface[5]. A second may involve plated through holes which fill partly or not at all. The lack of fill may not decrease the reliability of any specific hole, but in the case of an MLB, can be a "process indicator" alerting the assembly operation of other latent problems with overall interconnection quality and reliability.

What are the primary design issues influencing soldering capability?

Good product soldering performance occurs as a result of good design practices, well specified and sourced materials, strong manufacturing process controls, and attention to detail. The first phase of ensuring product solderability is rooted in the fundamentals of the product design process. The product should be viewed as a system of interrelated parts, assembled to accomplish a customer defined function (who else would you build it for?)

Given a failure of any one of those composite parts will result in some limitation of product performance to specification. The must take into consideration issues relating to materials, layout, process capability/limitations.