Circuit design rules
Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. If you want to develop good wiring habits, you should master the following layout rules.- Set the clearances of the printed board according to the structural drawing and manufacturing capabilities of the chosen fabricator. Set the clearances according to the special requirements of some components.
- Follow the layout principle of “big first, then small, difficult first, then easy “, that is, important unit circuits and core components should be laid out first.
- The layout should meet the following requirements as much as possible: the total connection is as short as possible, the key signal line is the shortest; high voltage, large current signal and low current, low voltage weak signal are completely separated; analog signal and digital signal are separated; high frequency signal and low frequency signal are separated; the spacing of high frequency components should be sufficient.
- The device layout grid setting. For general IC device layout, the grid should be 50-100 mil. For small surface mount devices, such as surface mount component layout, the grid setting should be no less than 25mil.
- Optimize the layout according to uniform distribution. In particular, the circuit parts of the same structure shall adopt the “symmetrical” standard layout as much as possible.
- The heating elements should generally be evenly distributed to facilitate heat dissipation. Temperature sensitive devices other than the temperature detection elements should be kept away from the components that generate large amounts of heat.
PCB design theory
Board Size and thickness
PCB manufacturer needs to set a maximum size for all the PCB boards. As well as their panel size limit. In order to save more space and reduce the costs in mass production, people always use more boards on a single panel. Board thickness also need to be specified. A standard thickness and type of board is FR4 .062″ and .010″, .020″, .031″, and .092″.
Width and Spacing
The parameters are always specified as “ x/y rules” where x is for the minimum of trace width and y is for the minimum trace spacing. As a instance, “8/10 rules” would indicate 8 mil minimum trace width and 10 mil for the minimum trace spacing. There is a requirement on the minimum spacing between traces and minimum width of trace when producing a PCB. There is some chance trace will open if a trace is made smaller than this minimum width when manufactured. There is also some chance they will short if two traces are closer together than the minimum spacing when manufactured.
In the modern process, the rules of x/y is 8/8, but the small values as 2/2 is also available. Those beginner developers can use 12/12 rules, and larger value can make work consistently. Nevertheless, please remember that the board must be soldered and a trace within 8 mils as 8/8 rules. A pad is easier to short , use one with greater spacing when hand soldered instead. Like 10/10 rules is much more easier to solder if you have such spacing in your designed board.
PCB Thickness
The 63 mil PCB thickness appears often as the thickness specification for PCBs. The question often asked is why is this thickness specified and is it an industry standard? This is one of those topics that warrants looking at the history of PCBs as they have evolved from simple single-sided boards to dozens of layers. After this review it will be seen that this is one of those specifications not unlike the often told story of how American railroad rails came to be 4’ 8.5” apart.
The variety of laminate thickness options offered to our customers are range from 0.008 inches to 0.240 inches and they include 0.2 mm (0.0079 inch), 0.4 mm (0.016 inch), 0.5 mm (0.020 inch), 0.6 mm (0.024 inch), 0.8 mm (0.032 inch), 1.0 mm (0.04 inch), 1.2 mm (0.047 inch), 1.5 mm (0.062 inch), 1.6 mm (0.063 inch), 2.0 mm (0.079 inch), 2.3 mm (0.091 inch), etc.
The manufacturers process four layer PCB boards with final thickness of 0.020″, 0.031″, 0.040″, 0.047″, 0.062″, 0.093″ and 0.125″.Six layer boards are produced in thickness of 0.031″, 0.040″, 0.047″, 0.062″, 0.093″ and 0.125″ with the same inner layer foil options. Both eight and ten layer boards are available in finished thickness of 0.062″, 0.093″ and 0.125″.