chapter12 manufacturing-processes

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  • 1. The University of New South WalesSchool of Electrical Engineering and TelecommunicationsELEC3017 ELECTRICAL ENGINEERING DESIGNCHAPTER 12: SOME ELECTRONIC MANUFACTURING TECHNIQUES Lecture Notes Prepared byMr D. Williams,1 Prof. W. H. Holmes, & Dr A. P. Bradley1Chief Executive Officer, Associative Measurement Pty Ltd.ELEC3017 ELECTRICAL ENGINEERING DESIGN 1 ELECTRONICS MANUFACTURING

2. ELECTRONIC ASSEMBLY TECHNOLOGIESNearly all modern electronic circuits are constructed by mounting most of the electroniccomponents on a basically two dimensional substrate with conducting tracks to connectthem. By far the commonest form for the board or substrate is a printed circuit board(PCB), though occasionally one of several hybrid technologies is used. PCBs inparticular are described in more detail in this chapter.These substrates have two important functions: They are the means for physically mounting electronic components; They are the means of interconnecting the electronic components.There are three basic electronic component assembly (or mounting) methods forcomponents on substrates:1. Through-Hole Technology The wire component leads are inserted into holes which have been drilled through the PCB (including the copper tracks), and then soldered to the copper tracks. Component insertion may be manual or automatic. Usually all soldering is carried out in a single operation using a wave solder machine. Through-hole technology is still the dominant approach, but surface mount technology (see below) is likely to play an ever increasing role, offering much higher component densities and being more suitable for automatic manufacturing methods.Figure 1. Through-hole, hybrid and surface mount technologiesELEC3017 ELECTRICAL ENGINEERING DESIGN 2ELECTRONICS MANUFACTURING 3. 2. Surface Mount Technology The component leads in surface mount technology (SMT) are soldered directly to the surface tracks without drilling. The components are held in place before soldering using a dab of glue. Special leadless components with pre-tinned attachment areas (not pigtails) are used, such as chip capacitors and chip resistors. High component densities are more easily achievable than with through-hole mounting. This process is especially suitable for automatic manufacture indeed, it almost requires automatic techniques to be viable. Some other acronyms often encountered in connection with surface mount technology (SMT) are SMA (assembly), SMC (component) and SMD (device). The application of SMT components is generally limited to the smaller discrete devices and larger leaded components that can provide some form of mechanical compliance in their means of attachment. The problem arises because of the disparity in thermal expansion characteristics between the current types of substrate materials (such as FR4) and the components themselves. If the effects of this are not properly accommodated then the overall reliability of the assembled board is reduced due to the propensity for component to substrate soldered joint to fail under heat induced mechanical stress. The increased circuit density with SMT carries with it an increased need to evacuate the heat produced by dissipative components and special heat sinks and/or improved board substrate materials have to be employed. Except for consumer applications where paper-based substrates are used, the predominant substrate used for double-sided PTH boards is glass- fibre epoxy material such as the ubiquitous FR4. There are improved versions of FR4 being developed and used for heat critical applications, while special applications require polyimide resins or, for RF applications, PTFE.3. Hybrid Technology A hybrid circuit uses a substrate (similar to surface mount assembly) on which there is a combination of film technologies and discrete component technologies. The film technology is used to realize most passive components, especially the component interconnections (conductors), resistors and capacitors. Either thick or thin films may be used. In thin film technology, the components are created by vapour deposition onto unmasked areas of the substrate, followed by selective etching or machining. In thick film technology, they are made by screen printing with a paste onto the substrate, which is then dried and fired, and finally machined (if required). The advantage of film technologies is that they can realize many passive components in a very small area with a single manufacturing step. Automatic precision trimming is also possible.ELEC3017 ELECTRICAL ENGINEERING DESIGN3 ELECTRONICS MANUFACTURING 4. The remaining components (active and passive) are then added to the substrate to complete the circuit. Although leaded components (as with through-hole PCBs) can be used, leadless components offer the most benefit, just as they do for surface mount technology.PRINTED CIRCUIT BOARDS (PCBs)The rest of this chapter is concerned with PCB design and technology, the mostimportant electronics assembly method. We will concentrate here mainly on through-hole technology.A printed circuit board consists of a sheet of insulating material with one or more layersof conducting copper tracks, either attached to the surface or buried within it.Occasionally other conductive metals or alloys are added for special purposes.TYPES OF PRINTED CIRCUIT BOARDSThe major classes of PCBs are:1. Single Layer Boards These are boards having only one conducting layer. While this is one of the oldest types of printed circuit board, it is still produced in volume since it offers the most economical solution for a large range of consumer electronics products. The components are usually mounted on the side of the board without the conducting tracks. This side is hence called the component side of the board, whereas the side with the conducting layer is called the solder side, since the component leads will be soldered on that side.2. Two Layer Boards, with or without Plated-Through Holes (PTHs) These are boards with two conductive layers, usually one on each side of the insulating substrate. Their main advantage over the single layer boards is that it is much easier to design them without crossing tracks (which require wire jumpers in practice.) Another advantage is that it is often possible to use areas of copper on the component side to provide either shielding or low-impedance earth or power supply connections. However, geometric constraints still often lead to long tracks, which are electrically undesirable (higher resistance and inductance, increased problems with cross coupling, etc.). In the simplest cases there will be no interconnections between conducting tracks on opposite sides of the board except those provided by the component leads (about 5% of consumer product boards are like this). However, the majority of two layer boards use plated-through-holes (PTHs) to connect conducting tracks on opposite sides of the board. PTH connections are usually made by depositing copper on the sides of the holes drilled to connect tracks on opposite sides of the board.ELEC3017 ELECTRICAL ENGINEERING DESIGN4ELECTRONICS MANUFACTURING 5. The double-sided board is readily adapted to surface mount techniques, sometimes having conventional components on one side of the board and surface mount devices (SMDs) on the other. However, as SMT components have become more readily available, more and more double-sided boards are being produced with SMT components on both sides of the board and with very little application of conventional through-leaded componentry.3. Multilayer Boards Multilayer boards can be used to further ease the geometric problems of interconnecting complex circuits, so that track lengths can be reduced and the density of electronic components can be much larger, especially with SMT components. Also, special layers are often dedicated to earths, power supply rails or for shielding purposes. Such layers can greatly improve the electrical performance of the circuit. For example, with multilayer boards it is very easy to provide separate low impedance earths and power supply rails for analogue and digital components, and to use guard tracks and/or shields for sensitive analogue or high frequency connections. Many computer and telecommunications products, as well as low volume special circuits, call for PCBs with layer counts from 4 to 18 or so, which are not always much more expensive than two-layer boards. Some manufacturers can produce boards with up to 60 layers. However, because of the increase in complexity and the increased requirements for capital equipment to cope with laminating, fine tolerance holes and inspection and testing, there are far fewer manufacturers of multilayer boards than of single and two-layer boards. There are some special problems that can arise in the use of multilayer PCBs, especially the fact that the thermal expansion of rigidly bonded SMT components may possibly be mismatched to that of the substrate, which can lead to stress between the conducting tracks and the substrate, with possibly catastrophic effects over time (layer separation). Multilayer boards are usually fabricated from a number of cores, each of which resembles a very thin double-sided board. Holes are not always drilled and through-plated at this stage unless the design calls for connections solely between the two layers of a core. The cores are assembled in a stack, separated from each other by one or more layers of partially cured substrate material called pre-preg, and with outer layers of copper foil. Heat and pressure are applied to fuse the whole into a rigid board. Holes are drilled to provide, when plated with copper, for through connections (called vias) between layers and the copper tracks etched onto the outside layers. In this way a variable (usually even) number of circuit planes is interconnected to form a complex whole. Choice of materials in multilayer PCBs is important because of the h