PCB design, engineers how to avoid not inflow!

- Sep 01, 2018-

The basic design process of PCB is as follows:

Preliminary preparation - PCB structural design - PCB layout - wiring - wiring optimization and screen printing - network and DRC inspection and structural inspection - platemaking.

The preliminary preparation includes preparing the component library and schematic diagram

"If you want to do well in your work, you must make good use of your tools.Before PCB design, prepare the component library of schematic diagram SCH and PCB.The component library can use peotel's own library, but in general it is difficult to find the appropriate one. It is better to make the component library by yourself according to the standard size data of the selected device.In principle, first make the PCB component library, then make the SCH component library.

PCB has a higher requirement of component library, which directly affects the installation of board.SCH's component library needs to be relatively loose, as long as the pin attributes and corresponding relations with PCB components are well defined.PS: note the hidden pins in the standard library.After that, the design of the schematic diagram is ready to start the PCB design.

2. PCB structure design.In this step, according to the determined circuit board size and various mechanical positioning, the PCB board surface is drawn under the PCB design environment, and the required connectors, buttons/switches, screw holes, assembly holes and so on are placed according to the positioning requirements.And fully consider and determine the wiring area and the non-wiring area (such as the area around the screw hole belongs to the non-wiring area).

3. PCB layout

The layout is essentially putting devices on the board.At this point, if all the preparatory work mentioned above is done, you can generate the network table (design-> Create Netlist) on the schematic diagram and then import the network table (design-> Load Nets) on the PCB diagram.You can see that all the devices are going up, and there are flying wire between the pins.Then you can layout the device.The general layout is based on the following principles:

1 according to reasonable electrical performance, it is generally divided into: digital circuit area (i.e. fear of interference and interference), analog circuit area (fear of interference), power drive area (interference source);

2. The circuit with the same function should be placed as close as possible, and the components should be adjusted to make the connection as simple as possible.At the same time, adjust the relative position between the functional blocks so that the connection between the functional blocks is the most concise;

The installation position and installation strength should be considered for high quality components.The heating element should be placed separately from the temperature sensitive element.

4 I/O drive devices as close as possible to the edge of the printing plate, close to the lead connector;

5 clock generator (such as crystal oscillator or clock vibration) should be as close as possible to the device using the clock;

6. A decoupling capacitance shall be added between the power input pin and the ground of each integrated circuit (generally using a sololite capacitor with high frequency performance);When the circuit board space is relatively dense, a tantalum capacitor can be added around several integrated circuits.

7 discharge diode should be added at relay coil (1N4148 is enough);

8 to have a balanced layout requirements, density and orderly, not top-heavy or YiTouChen - need to pay special attention to, when placed components, components should be considered when the actual size (in the area and height) and the relative position between the components, to ensure the feasibility of the electrical properties and production of circuit boards installed and convenience at the same time, should be on the premise of guarantee the above principle to reflect, appropriate change device, make it tidy and beautiful, such as the same devices to be neat, in the same direction, can't put strewn at random have send.

This step is related to the overall image of the board and the next level of wiring is difficult, so it takes a lot of effort to consider.When layout, not quite sure place can make preliminary wiring first, consider adequately.

Four, wiring

Wiring is the most important process in PCB design.This will directly affect the performance of the PCB board.In the design process of PCB, the wiring is generally divided into three levels: first, the layout, then the most basic requirements of PCB design.If all the lines are not connected, you will find a flight line everywhere, which will be a disqualified board, so to speak, not yet.Next is the satisfaction of electrical performance.

This is the measure of a printed circuit board.This is after the layout, careful adjustment of the wiring, so that it can achieve the best electrical performance.And then aesthetics.If your wiring cloth is connected, also did not have what to affect the place of electrical performance, but see past desultorily, add colorful, colorful green, that calculate your electrical performance how good, still be rubbish in other people eye piece.This brings great inconvenience to testing and maintenance.The wiring should be uniform and not crisscrossed without any rules.All of these should be realized under the condition of ensuring electrical performance and meeting other individual requirements, otherwise, it is a waste of time.

Five main principles of wiring

In general, the power cord and ground wire shall be first wired to ensure the electrical properties of the circuit board.As far as possible, the width of power supply and ground wire should be extended as far as possible, preferably with the ground wire wider than the power cord. Their relationship is as follows: the ground wire > power cord > signal line, usually the signal wire width is 0.2-0.3mm, the minimum width can be 0.05-0.07 mm, and the power cord is generally 1.2-2.5mm.The PCB of digital circuit can be used to form a loop with a wide ground wire, namely a ground network. (the ground of analog circuit cannot be used in this way.)

In advance, the wire (such as high frequency line) with strict requirements shall be wired, and the side line of the input end and the output end shall be adjacent and parallel, so as to avoid reflection interference.When necessary, ground separation should be added, two adjacent layers of wiring should be perpendicular to each other, parallel easy to generate parasitic coupling.

The oscillator housing is grounded, and the clock line should be as short as possible and not be triggered everywhere.Under the clock oscillating circuit, the part of the special high-speed logic circuit should increase the area of the ground, but should not go to other signal lines to make the surrounding electric field close to zero;

As far as possible, 45o polyline should be adopted for wiring, and 90o polyline should not be used to reduce the radiation of high frequency signals.(the high-demand lines should be double-curved.)

Any signal line should not form a loop. If it is unavoidable, the loop should be as small as possible.The signal line through the holes should be as few as possible;

The key lines should be as short and thick as possible, with a protective cover on both sides.

When transmitting the sensitive signal and noise field signal through flat cable, the signal shall be introduced by means of ground wire - signal - ground wire.

Test points should be reserved for key signals to facilitate production and maintenance testing. After the wiring is completed, the wiring should be optimized.

At the same time, after the initial network inspection and DRC inspection, the unwired area was filled with ground wire, and a large area of copper layer was used as ground wire.Or make multi-layer board, power supply, ground line occupied one layer each.