"You are what you eat"! Perhaps you have seen this as a book title, but it’s a concept that has attracted increasing focus in recent years. Just look at the pressure to influence the diet of school-children in order to improve their prospects for long-term good health, and even their behaviour in the classroom.
What you put in affects what you get out, and this applies just as much to electronics as to people. Some of the materials form part of the eventual product, others are consumed during the processing, and still others merely influence the outcome.
As with our diet, we can choose the materials that we use for our electronic products. Our choices need to be appropriate for the application, so that the products give the required reliability in the given environment, and our choices have to be considered as a whole, because the material chosen can affect the other materials in the assembly. After all, not every material is compatible with every other, or is compatible with every process, particularly when we are considering joining and coating techniques.
Whether we are making components, packaging semiconductors, fabricating printed circuit boards, or making an assembly, materials are involved. A few of these are gases or liquids, most are solids. But changes of state are also important, most crucially the change from liquid to solid that happens during soldering.
Electronics uses many different types of materials. Concentrating on the solid materials, we use metals, polymers, ceramics and glasses; some are conductors, some are insulators, and a key group are semiconductors. They exist in different forms, ranging from large single crystals to materials that have only a microcrystalline structure, or no structure at all.
And they come in different physical formats – films and foils, wires and fibres, solid materials and foams. All of which have to be correctly chosen for the application.
Although some are relative simple single materials, many are more complex, designed to give improved performance and complementary qualities. For example, combining fibres that give tensile strength with a matrix of a material such as a resin that binds the fibres together and makes the resulting material more rigid.
As well as such composites, we use a number of multi-layer structures, where the properties depend on striking the right balance between the properties of the individual materials, and having some mechanism for keeping the layers together.
Although it is tempting to include a lot of basic physics and chemistry, particularly when we consider materials, we have tried to keep this to a minimum, focusing on questions such as Where are the materials used? Why are they used? What different materials are used? And what properties are important?
But we still need to know something of the basic properties of the material itself, and the aspects of processing and life that introduce stresses and defects, and may eventually lead to failure.
Most of our study in this unit will consider each of the different types of material in turn, and ask the questions suggested by Rudyard Kipling in the Just So stories, in the short poem that follows the story of the Elephant’s Child and which you might remember from your youth.
“I keep 6 honest serving men (they taught me all I knew),
their names are What, and Why and When
and How and Where and Who.”
Certainly the first five of these are exactly the sort of questions that we need to ask when we look at the different kinds of materials used in electronics. For the last question, we should probably replace “Who?” by “How much?”, as this aspect is important in what has always been a cost-sensitive industry.
Of course, as a former production man, I would also want to ask questions like, “What yield?” and “How many problems?”, and I would encourage you, as you study the unit and consider the different types of material and their uses, to think of the other people in the supply chain.
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