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Just for Starters

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Just for Starters by Mark Balch Just For Starters Basics For Beginners Just For Starters What is a PCB? ircuit boards are familiar items ple layers of thin fiberglass. Some lay- mon weight is half ounce, but one and to electronics enthusiasts and ers are coated with copper foil and two ounce copper is common, as well. Clay people alike. Clear cell- others are bare fiberglass. The thick- Half-ounce copper is approximately phone bezels reveal small circuit ness of each layer ranges from several 0.006" thick. Thicker copper can carry boards jammed with tiny components. mils (a mil is 1/1000 of an inch) to more current, but poses etching prob- When you open a PC, the mother- tens of mils. Layer thickness is deter- lems as the patterns get smaller. board is visible — populated with a mined by the desired electrical proper- variety of integrated circuits and other ties and dimensions of the finished Vias components. PCB. The PCB is built up by stacking With the exception of certain pro- layers of fiberglass and copper until Wires from multiple layers are totype circuitry, the great majority of the specifications are met. Figure 1 connected after the stacking process. electronic circuits are constructed shows a cross-section of a multi-layer A small hole — called a via — is pre- using printed circuit boards, or PCBs. PCB. Note the thin copper foil layers cisely drilled through the fiberglass If you plan on working with a PCB, it sandwiched between thicker fiberglass stack and the surface of the hole is helps to understand some basic termi- layers. Electrical connections are metal plated. The plating makes elec- nology, as well as what a PCB actually formed by etching patterns into the trical connections to copper that is is. PCB design and construction is a copper layers. This is where the term penetrated in each layer. The vertical vast area of concentration that fills vol- "printed" comes from. A photographic metal surfaces in Figure 1 illustrate umes of text. This article provides a mask is created and each layer of cop- this concept. Each via appears as a brief overview of PCB basics. per foil is treated with a photo-resistive small hole from the surfaces of a PCB. chemical. Much like a normal photo- Figure 2 shows a finished PCB with Fiberglass and graph, wiring patterns are "printed" vias clearly visible. Some vias have Copper onto the copper by exposing it to light etched copper connections visible on through the mask. The exposed cop- the top layer. A PCB is constructed from multi- per is chemically etched and individ- Printed wires on each copper ual wires remain behind. layer are very narrow. Mature con- Figure 1. Cross-section of a multi-layer Copper foil is manufactured in struction technologies allow copper PCB with a plated hole. three common thicknesses, or weights "traces" as thin as 8 mils. More per square foot. A PCB fabricator, or advanced manufacturing techniques "fab-house," will specify the copper routinely produce traces down to 3 weights that it stocks. The most com- mils. Below 3 mils is possible, but Figure 2. Surface view of vias with wires expensive. on the top layer. Each fab-house has its own manufacturing capability. Some offer finer pitch PCBs than oth- ers. A fab-house should tell you how small they can reliably manufacture traces and the spaces between them. For exam- ple, a quote of "8-mil trace and 7-mil space" means that the fab-house can etch copper traces DECEMBER 2003 17 Just For Starters Figure 3. Wire with circular via pad. down to 8 mils wide with 7 mils seen in Figure 4, the between each trace. With such small result is that vias do not copper features, properly aligning each take up the entire layer layer and drilling vias is not trivial. Vias stack and two or more Figure 4. Blind and buried vias connect only a few are not simply drilled to hit wires such vias can be located layers of the PCB. because a wire's cross-section does not in the same place on dif- contain enough copper to make a reli- ferent layers. High density PCBs may These electrical properties become able electrical connection with the via require blind and buried vias. This tech- critical in fast and sensitive circuitry. plating. Instead, Figure 3 illustrates nology is more expensive than conven- Planes are continuous except for how via connections are designed with tional vias because additional manu- via clearance holes. Clearance holes circular pads at the end of each wire. facturing steps are required and layer are necessary to prevent vias shorting When drilled, the pad forms a ring that stacking becomes a more difficult their signals to the power planes. This surrounds the via. The via's plating has process. would normally happen because vias much more area to form a connection. are drilled through all PCB layers (or Planes some layers with blind and buried Blind and Buried Vias vias). The solution is to etch away a cir- Not all PCB layers are used for cle of copper in each plane where each Blind and buried vias are special conventional wiring. Most PCBs via hole is drilled. There can be no elec- types of vias used in leading-edge sys- reserve some layers for use as power trical connection between the via and tems. Conventional vias are drilled planes. As their name implies, planes the plane if there is no copper between through the entire PCB after all layers are nearly continuous sheets of cop- the via hole and plane. Most PCBs have have been stacked. Blind and buried per. A plane is used to carry power to several planes and assign each plane vias are drilled only through a few lay- many or all components on the PCB to carry a specific power rail. For exam- ers during the stacking process. As with low inductance and resistance. ple, a microprocessor board may have 5-volt and 3.3-volt power supplies plus ground return. The PCB may have four planes: 5, 3.3, and two grounds. In some cases, a product cannot accept the added cost of one plane for each power rail. It may be possible to split a plane into multiple areas for multiple power rails. Whether or not this split is possible depends upon the circuit's electrical constraints. A split plane is constructed by etching a copper void into an otherwise continuous plane. Moats Sometimes a plane is partially split for electrical reasons. Noisy circuits OLTS may cause problems in a sensitive sys- V tem. Some engineers attempt to mini- mize noise transmission by etching a void into part of a plane, but leaving a small electrical connection for power Everything For Electronics UTS & and signal routing purposes. This type N 18 DECEMBER 2003 Just For Starters of void is called a moat. Figure 5 shows several moats on a single PCB as the board is held up to a light. The fiber- glass is translucent, while the copper planes are opaque. Moats can cause as much trouble as they solve, so it is best to fully understand their dynamics before proceeding. Figure 5. Moats are often visible when the PCB is held up to the light.The moats allow Figure 6. Through-hole PLCC and LED land Land Patterns light to pass through the fiberglass. patterns with solder finish. We've discussed wiring up till now, component. The features are usually tors, and more. Land patterns some- but PCBs exist to connect components. made larger than the leads or pins so times include special visual markers Most components are soldered to that a sufficient quantity of solder can called fiducials. The figures also show PCBs. In order for soldering to be effec- adhere. Land patterns may be combi- several examples of fiducials. Fiducials tive, the leads or pins on a component nations of plated holes and surface are typically round patterns located in must be capable of reliable mechanical pads, depending on the type of compo- the corner of surface-mount land pat- and electrical connections to the nent being mounted. Figures 2, 5, 6, terns that robotic assembly machines etched copper wiring. and 7 show a variety of land patterns. key off of to accurately place compo- Each component is provided with There are patterns for through-hole nents onto the PCB. These machines a matching land pattern — a set of connectors, quad-flat packs (QFP), are often called "pick-and-place" etched copper features that directly small-outline integrated circuits machines. The fab-house finishes correspond to the leads or pins of the (SOIC), discrete surface-mount resis- exposed copper land patterns different- DECEMBER 2003 19 Just For Starters default color for solder-mask that most fab-houses use. Solder-mask is applied About the Author over the entire surface of a PCB, Mark Balch is the author of except for land patterns and other cop- Complete Digital Design (see per areas that must make electrical www.completedigitaldesign.com) connections. The solder-mask eases He is an electrical engineer in Silicon assembly by preventing solder from Valley, CA, who designs high-perform- adhering. Solder-mask is available in a ance computer-networking hardware. rainbow of colors including blue, red, His responsibilities have included PCB, and clear. Silkscreen patterns and let- FPGA, and ASIC design. Prior to working in telecommunications, Mark designed tering are applied to most PCBs to Figure 7. Through-hole headers, SOIC, and products in the fields of HDTV, plated mounting hole with gold finish.
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