Also known as SMT, surface mount technology places electronic components and fixes directly onto the circuit boards. Advances in technology have meant that SMT now allows for smaller board level components and consequently, smaller end products. Think of the size difference in early mobile phones and today's tiny cell phones, for example. Surface mount components can be soldered using two basic techniques - using a soldering iron or using an oven. Though the process is fairly easy, it requires care and a stable hand specially when using a soldering iron. The process starts with applying a thin layer of solder paste, which is a sticky mixture of tiny solder particles and flux, to the SMT pads which are present on the PCB and are used to hold the SMT components. Then the component is placed on the pads and carefully aligned. Next, using a clean solder iron or the oven, the component needs to be tacked into place properly by soldering two opposite pins. Once in place, the alignment is rechecked and the soldering completed. The SMT method of mounting has a number of main advantages over older methods, allowing as it does for the placement of components in exact positions on both sides of a circuit board, reducing costs and reducing size at the same time. Though cost reductions have only come over time, the initially cost of creating the machines and designs needed to undertake surface mounting are higher than the old through-hole based mounting. Many people still suggest that the older methods of mounting components are easier to repair, but as SMT has developed enough, along with advances in the repair tools, the methods and costs of repairing surface mount boards has fallen. While it is still possible to undertake manual soldering repairs, there have been professional rework systems designed as well which have improved the method and cost of repairs. Surface mount components now range into three main categories, Passive SMDs (including resistors and capacitors), Transistors and Integrated Circuits. There are currently still some electronic components that require the old drilled through and lead wire methods, but the numbers are on a dramatic decline. Look for further advances in surface mount technology in the coming years. As the technology improves, the electronic components will get smaller and smaller. Learning how to use them now will keep you on top of the industry. PCB Assembly refers to the attaching of electronic components to a printed circuit board. PCBs are used to mechanically connect and support electronic components using conductive pathways, traces or tracks which are etched from copper sheets laminated onto a non-conductive substrate. Once the PCB is completed then electronic components can be attached to produce a printed circuit assembly. The leads of the components are inserted through holes in the PCB, and in surface mount construction, the electronic components are placed on lands or pads on the outer surfaces of the printed circuit board. The component leads in both types of this construction are then mechanically fixed to the PCB with soft metal molten solder. There are all kinds of various techniques for attaching components to printed circuit boards, and most very high volume production is generally produced by machine placement and bulk wave soldering or reflow ovens. Sometimes very skilled technicians are employed to solder very small parts by hand under a microscope. This is achieved by using tweezers and a very fine soldering tip. Some parts such as ball grid array packages are impossible to solder by hand. Very often, surface mount and through the hole construction must be combined on a single printed circuit board, as some of the required electronic components are only available in surface mount packages, while others are only available in through hole packages. One reason to use both of the above methods is that surface mount techniques take up less space and will go largely unstressed, while through the hole mounting can provide needed strength for any components which are likely to endure any physical stress. When the printed circuit board or PCB has been built or populated with the desired components, it can be tested in a number of different ways and these may include: o Power on; functional test, checking if the PCB is doing what it is designed for o Power on; in circuit test, physical measurements o Power off; visual inspection & automated optical inspection o Power off; analogue signature analysis, power off testing Some printed circuit boards may have a conformal coating applied by dipping or spraying once the components have been soldered. This coating provides a protective layer which helps prevent corrosion and leaking current due to condensation. Assembled PCBs are sensitive to static and are often placed in antistatic plastic bags for handling and transport purposes. SMD Schablonen
