1. The shortest principle of the trace: How short the trail can be shorter, the simplest principles are also the most important principles. This is a decisive factor that your board function is stable.  
2. Component layout, connector must consider good location, all people's DB9 painting does not consider your serial line can insert in the board. If you add USB, you should also consider a good location. After the PCB layout is completed, you can print out and try it with the original.  
3. Component layout, components with electrical connections are as close as possible, such as the 4 capacitances of MAX232, need to be placed around the MAX232, so that you can go shorter, if your trace is on the board, a large circle, It is necessary to consider changing the layout of the element.  
4. The thickness of the signal line is generally designed to be 10 mil (about 0.2mm), the power cord is at least 25mil, preferably 30mil, because the current is, the wider, the width of the required line width, the current line current is relatively small, and the signal line current is relatively small.  
5. The hypertension is generally set to 12-24mil (inner diameter-outer diameter)  
6. The trace must not appear 90 degrees of the line, the reason is very simple, you will turn a 90 degree in the case of a hurry? The line should be turned, 45 exens.  
7. Crystal oscillator as the heart of the microcontroller, must try to close to the chip, the minimum system we designed, except that the single-chip microcratient is crystal, as well as the USB turn serial port has a crystal.  
8. TheUSB turn serial port must be available, you can solve the problem of power supply and download procedures  
9. Copper copper is an indispensable part of the PCB, must have, and the copper area should be as large, try to cover a complete copper.  
10. When the copper is copper, the overfo is fully connected with the copper you covered together, the pad is connected to the copper with 4 wires, because the pad is also completely connected to copper, the welding is too fast, and it is very easy to lead to virtual Welding, and the via is only used to connect, not welding.  
11. Your board is finally sent to the processing plant, whether it is in the company's PCB, or to do itself, it is necessary to consider cost, board area control at 10 * 10cm