Electronic Design - Explanation of PCB design process and production of PCB board

PCB design full analysis: from the preparation of information to the wiring check of the exhaustive process

Key information required for PCB design:

Schematic: a detailed electronic document that generates an accurate netlist.

Mechanical dimensions: Clearly labelled with specific orientation and direction identification of positioning devices, as well as specific height limit areas.

BOM List: Verifies and checks the packaging information of the device on the schematic.

Wiring Guidelines: Detail specific requirements for particular signals, as well as design criteria such as impedance and lamination.

Core steps of PCB design:

1.Pre-Preparation

Preparation of component libraries and schematics: As the saying goes, ‘To do a good job, you must first make good use of your tools’. In order to create a high-quality circuit board, in addition to the design principles, but also must be well drawn. Before embarking on PCB design, you must first prepare the schematic (SCH) component library and PCB component library (this is a critical step). Component library can choose Protel comes with the library, but it is often difficult to find exactly the right one, it is best to create their own component library according to the standard size data of the selected device. In principle, the first production of PCB component library, and then the production of SCH component library. PCB component library requirements are higher, it directly affects the installation of the circuit board; SCH component library requirements are relatively loose, as long as attention to the definition of the pin attributes and the PCB components can be defined with the corresponding relationship. At the same time, pay attention to the hidden pins in the standard library. After completing the schematic design, PCB design can begin.

When making libraries for schematic diagrams, make sure that the pins are correctly connected to the output/input PCB boards, and carefully verify the library files.

2.PCB Structure Design

According to the determined board dimensions and various mechanical positioning, draw the outline of the PCB in the PCB design environment and place the required connectors, buttons/switches, digital tubes, indicators, input/output ports, etc., and set up screw holes and mounting holes according to the positioning requirements. In this process, the wiring area and non-wiring area (e.g., around the screw holes is non-wiring area) should be fully considered and determined. Special attention should be paid to the actual size of the devices (occupied area and height), the relative position between the devices (space size), and the level at which the devices are placed to ensure the electrical performance of the circuit board, the feasibility and convenience of production and installation. Under the premise of ensuring that the above principles can be reflected, the device can be adjusted to make the layout more neat. If similar devices need to be placed neatly and in the same direction, not randomly put together.

3.PCB Layout

Ensure that the layout of the schematic diagram before the accuracy of this is essential! After the completion of the schematic diagram, you need to check the power grid, ground network, etc. is correct.

Layout needs to pay attention to the placement of the device level (especially plug-ins, etc.) and device placement (direct insertion horizontal or vertical placement) to ensure the feasibility and convenience of the installation.

Place the devices on the board for layout. At this point, if all the above preparations have been completed, the netlist can be generated on the schematic (Design->Create Netlist) and then imported onto the PCB (Design->Load Nets). At this point, you can see the complete stack of devices, and between the pins have flying wire tip connection, followed by device layout.

Layout should follow the following principles:

Determine the placement level of the device: usually, SMD devices should be placed on the same side, and plug-in devices need to be determined on a case-by-case basis. According to the reasonable division of electrical properties, generally divided into digital circuit areas (susceptible to interference, interference generation), analogue circuit areas (susceptible to interference), power-driven areas (sources of interference). Place circuits with the same function as close together as possible and adjust components to ensure the simplest connection; at the same time, adjust the relative position between functional blocks to make the most concise connection between functional blocks. For devices with high quality requirements, consider their mounting position and mounting strength; heat-generating components should be placed separately from temperature-sensitive components, and thermal convection measures should be considered if necessary. Clock generators (such as crystals or clocks) should be located as close as possible to the device using the clock.The layout should be balanced and sparse, avoiding top-heavy or one-sided.

4.Cabling

Cabling is the most critical part of PCB design,will directly affect the performance of the PCB.

In the PCB design process, wiring is usually divided into three levels.The first is connection, which is the basic requirement of PCB design. If the lines are not paved and there are flying wires everywhere, then this will be a substandard board, and it can be said that the design has not yet begun. The second is to meet the electrical performance, which is an important indicator of whether the PCB board is qualified or not. After the connection, the wiring needs to be carefully adjusted to achieve the best electrical performance, while taking into account the aesthetics. If the wiring, although connected correctly, is messy and colourful, then even if the electrical performance is good, in the eyes of others is a poor quality board. This brings great inconvenience to the test and maintenance.Wiring should be neat and even, not cluttered. These must be achieved while ensuring electrical performance and meeting other individual requirements.

The following principles should be followed for wiring:

Normally, power and earth wires should be routed first to ensure the electrical performance of the board. Within these conditions, try to widen the power and earth wire widths as much as possible. Ground lines are wider than power lines. Their relationship is: ground > power > signal lines. Usually the width of the signal line is 0.2~0.3mm, and the thinnest width can be 0.05~0.07mm; the power line is usually 1.2~2.5mm. For digital circuit PCBs, a wide ground line can be used to form a loop in order to form a grounding network (grounding for analogue circuits cannot be used in this way).

Pre-treatment of demanding lines (e.g. high-frequency lines), the edges of inputs and outputs should be avoided adjacent to each other in parallel to avoid reflective interference. If required, an earth wire can be added and two adjacent layers of wiring should be perpendicular to each other, as parallel wiring is prone to parasitic coupling. The oscillator case is grounded, and the clock wire should be as short as possible, and not led long at will. Below the clock oscillation circuit and special high-speed logic circuit part, should increase the grounding area, should not go to other signal lines, in order to make the surrounding electric field tends to zero. Try to use 45 ° folded lines, avoid the use of 90 ° folded lines to reduce the radiation of high-frequency signals (for demanding lines can be used double arc).

Avoid loops on any signal lines, if unavoidable, the loops should be as small as possible; the number of over-holes in signal lines should be as small as possible. Critical lines should be as short and thick as possible and protected on both sides. When transmitting sensitive signals and noisy field signals over flat cables, they should be extracted by means of ‘ground-signal-ground’. Reserve test points for critical signals for commissioning, production and maintenance testing. After completing the schematic wiring, the wiring should be optimised. At the same time, after the initial network check and DRC check are correct, carry out grounding of non-wiring areas and use a large copper layer as a ground to connect unused areas of the printed circuit board to the ground, or make a multilayer board so that the power supply and the ground each occupy one layer.

5.Adding Teardrops

Teardrops are drop-like connections between pads and wires or between wires and lead-through holes. The purpose of setting up a tear drop is to prevent the contact point between pads and wires or wires and lead-through holes from disconnecting when the board is subjected to a large external force. In addition, the setting of the tear drop can also make the PCB circuit board look more beautiful. In circuit board design, in order to make the pads more solid, to prevent breakage between the pads and conductors in the production of mechanical boards, between the pads and conductors are usually set up between the transition strip copper film, the shape of which is similar to a tear drop, and is therefore often referred to as a tear drop.

6.Inspection and verification

First check Keepout layer, top layer, bottom layer, top silkscreen and bottom silkscreen. Then the electrical rules are checked: checking for through holes (0 through holes are not allowed; border line width is 0.8), disconnected netlists, minimum spacing (10 mil) and short circuits (analysing each parameter one by one). Next check the power and ground lines for interference (filter capacitors should be close to the chip). After completing the PCB design, the netlist is reloaded to check that it has not been modified and to ensure that the board will work properly. Finally, check the wiring of the core devices to ensure accuracy.

PCB design is the core of electronic product innovation, continuous optimisation of the design process and the application of technology will greatly enhance the competitiveness of products, leading the electronics industry to a new height.