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BOOKSHELF Surface Mount Technology Surface Mount Technology by Rudolph Strauss. Published in 1994 by ButterworthHeinemann, Oxford. Hard covers, 361 pages, 240 x 160mm, ISBN 0-7506-1862-0, $99.00 This book will provide informative reading for any person presently involved with wave soldering equipment for through-hole mounting components or anyone considering the assembly of PC boards with surface mounted devices (SMD's). In the first chapter the Author asks the same question we all ask. Why SMD's? What is wrong with existing components? But after a moment's thought we realise that components have always been getting smaller. From valves to transistors to integrated circuits, the package size has shrunk dramatically. Microprocessors have gone from 16 pin devices to 40 pins to 169 pins and so on. The trend to higher packaging density of IC's and to higher frequency operation, requires shorter IC interconnecting leads and components with very short (low inductance) leads. Thus SMD's have come to the rescue. In chapter 2 the Author discusses SMD shapes including MELF's (Metal Electrode Faced Components), chips, small outline devices (Transistors-SOT's, Integrated Circuits-SOIC's etc) and discusses their solderability and mechanical stability in wave soldering machines. Chapter 3 covers soldering methods, fluxes, solder composition, the effects of impurities in solder baths and the properties of soldered joints. Chapter 4 describes wave soldering in detail, including the relative simplicity before SMD's. Wave soldering is a technique where the PC board, with all components physically mounted or glued, is passed over a wave of molten solder. The solder is pumped through a vertical nozzle, usually the width of the machine and as it flows up and out the PC board is moved through this "wave". Other topics in this chapter include fluxes, board preheating, the solder wave, oxygen-free atmospheres and the role of adhesives. Chapter 5 covers reflow soldering. What is the difference? In wave soldering, as in hand soldering the flux comes first, the solder and heat come at the same time. With reflow, the heat comes last, the flux and solder or solder paste having already been placed on the PC board and sometimes on the component leads as well. The Author points out reflow soldering is not new. Plumbers use reflow when joining modern copper capillary fittings. These are supplied with an insert of flux and solder. The plumber only needs to clean the copper pipe he intends to use, push it into the fitting and apply heat. The flux and solder inside the fitting will now do their job and make a waterproof joint. Strauss then discusses one pass and two pass soldering, where the PC board is reflowed on one side then turned over and the second side soldered. You would think that the components on the bottom would fall off on the second pass and they will if they are too heavy. Believe it or not, the surface tension of the softened solder holds the smaller components in place. One layout requirement for two pass SMD's is to keep all the heavy components on one side of the board. The rest of the chapter covers various methods of reflow, including vapour phase, infrared, hot air or gas, laser and impulse. Chapter 6 details the requirements for PC boards for SMD's. This covers layout procedures, which should take into account the direction of the board through the solder machine as most components will be more mechanically stable in one orientation. Chapter 7 discusses the placing of components on the PC board, from manual methods to fully automatic placement. Chapter 8 discusses the methods used to clean the boards after soldering. Historically CFC's have been used but now they have been phased out due to their environmental unfriendliness; new cleaning compounds and methods have evolved. The final chapters cover quality control, inspection and rework. These are interrelated, for obviously, the better the quality control the less rework. Inspection before soldering and rectification of faults can be far easier and cheaper than repairing the same fault after the solder bath. To sum up, this is a very interesting book, written by an Author who knows his subject - a must for the production manager of any electronics manufacturing company not yet into SC SMD. (R.J.W.) August 1995  53