A PCB board consists of multiple layers of copper separated by layers of insulating FR4 fire-resistant epoxy. A trace is a wire on a layer of the board. A via is a copper-plated hole that goes through the board to connect layers. The easiest way to build vias is to drill them through the entire PCB board, though for more money, it is possible to manufacture blind vias between selected layers. A typical four-layer board consists of a top signal layer, a ground plane layer, a power plane layer, and a bottom signal layer. The components are mounted on the top layer. SMT components are soldered directly to the top layer, while through-hole components are soldered to the bottom layer. The power plane carries the power supply voltage used by the important components (often 3.3 V). If a digital PCB board is operating at greater than 100 MHz or so and requires multiple power supplies, consider providing additional layers for each supply. If the board has a huge number of signals to route, you may need extra signal layers. Each signal layer should be stacked adjacent to a PCB power or ground plane for good signal integrity. For example, a 6-layer board stack might look like signal – power – signal – signal – ground – signal. Manufactured PCB boards include solder mask and silkscreen layers over the outer signal layers.
It is a layer of lacquer to protect the fine copper traces underneath. Solder mask helps make assembly easier by preventing the solder from bridging between unrelated traces. Finally, the manufacturer will use a silkscreen to lay down ink. This is used to denote where components will go, through the use of text and outlines. The silkscreen also typically specifies the name and version number of the board, the date of manufacturing, and the designers’ names. microcontroller, by Leo Altman ’11 and Christian Jolivet ’11. The white text and shapes are from the silkscreen layer; together with the solder pads, these make up the PCB footprint for each part. The IC labeled EPCS4 is a surface-mount chip (specifically, the small outline integrated circuit, or SOIC, package); the rest of the parts in the foreground are all through-hole.
The soldermask layer covers everything but the pads where components are placed. The lighter green lines are copper traces underneath the soldermask. The small green circles at the ends of traces are vias. As you can see, they are covered in soldermask because no components will be affixed to them.The board must satisfy design rules PCB related to the minimum size and spacing between components so that it can be cost-effectively manufactured. The width and spacing of traces is measured in mils, orthousandths of an inch. A good set of standard design rules is to use traces that are at least 6 mils wide and spaced at least 6 mils from adjacent traces.
Advanced Circuits charges a PCB premium for trace width/spacing less than 6 mils. Vias should have a drill hole 15 mils in diameter. The diameter of the copper pads PCB surrounding drilled holes should be at least 14 mils greater than the diameter of the drilled hole. Through-hole components typically have a drill hole diameter of 35 mils and a copper pad with a 60 mil diameter. Components on a 100 mil pitch thus have a PCB spacing of 40 mils between pads, which is sufficient to route two wires between the PCB pads using a width and spacing of 6-8 mils. If you have to route a power signal on a signal layer (because you have some minor extra power supplies or are building a 2-layer board), use a fatter trace to provide lower resistance where possible. 20 mils is a reasonable choice if you can accommodate it PCB.
