PCB Design and Assembly ServicesPosted on 2020-07-06
PCB design steps
The basic design process of general PCB circuit board design is as follows: pre-preparation -> PCB structure design -> PCB layout -> wiring -> wiring optimization and screen printing -> network and DRC inspection and structural inspection -> plate making.
1. Preliminary preparation
(1) Before the PCB design, we must first prepare the schematic SCH component library and PCB component library (this is the first step-very important). The component library can use Protel’s own library, but in general it is difficult to find a suitable one. It is best to make your own component library based on the standard size information of the selected device.
In principle, make the PCB component library first, and then make the SCH component library. PCB component library requirements are high, which directly affects the installation of the board; SCH component library requirements are relatively loose, as long as you pay attention to define the pin attributes and the corresponding relationship with PCB components.
Note: Pay attention to the hidden pins in the standard library. After that, it is the schematic design, and when you are done, you are ready to start PCB design.
(2) When making a library of schematics, pay attention to whether the pins are connected to/output the PCB board and check the library.
2. PCB structure design
In this step, according to the determined circuit board plane size and various mechanical positioning, draw the PCB board in the PCB design environment, and place the required connectors, buttons/switches, digital tubes, indicator lights, input, output according to the positioning requirements , Screw holes, assembly holes, etc. and fully consider and determine the wiring area and non-wiring area (such as how much range around the screw hole belongs to the non-wiring area).
Special attention is required, when placing components, the actual size of the components (occupied area and height), the relative position between the components-the space size, the surface on which the device is placed, to ensure the electrical performance of the circuit board At the same time as the feasibility and convenience of production and installation, the placement of devices should be appropriately modified to ensure that they are neat and beautiful on the premise that the above principles can be reflected. For example, the same devices should be placed neatly and in the same direction.
3. PCB layout
- Make sure the schematic diagram is correct before layout-this is very important! Check items after the schematic drawing is completed: power network, ground network, etc.
- Pay attention to the placement surface of the device (especially the plug-in, etc.) and the placement of the device (whether it is straight or vertical) to ensure the feasibility and convenience of installation.
- Layout is to put the device on the board. At this time, if the preparatory work mentioned above is completed, you can generate a network table (Design->CreateNetlist) on the schematic diagram, and then import the network table (Design->LoadNets) on the PCB diagram. You can see the full stack of devices and there are flying leads to remind the connection between the pins, and then you can lay out the device.
The general layout is as follows:
- The layout of the device should be determined on the layout: Generally speaking, the patch should be placed on the same side, and the plug-in depends on the specific situation.
- According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, fear of interference and interference), analog circuit area (fear of interference), power drive area (interference source).
- Circuits that accomplish the same function should be placed as close as possible, and the components should be adjusted to ensure the most concise connection; at the same time, the relative position between each functional block should be adjusted to make the connection between the functional blocks the most concise.
- For high-quality components, the installation location and installation strength should be considered; the heating element should be placed separately from the temperature-sensitive component, and if necessary, thermal convection measures should also be considered.
- The I/O drive device should be as close as possible to the side of the printed circuit board and close to the leading connector.
- The clock generator (such as: crystal oscillator or clock oscillator) should be as close as possible to the device using the clock.
- The layout requirements should be balanced, dense and orderly, not top-heavy or heavy.
The wiring is mainly carried out according to the following principles:
- Under normal circumstances, the power cord and ground wire should be wired first to ensure the electrical performance of the circuit board. Within the scope of the conditions, try to widen the width of the power supply and the ground line. It is better to make the ground line wider than the power line. Their relationship is: ground line>power line>signal line, usually the signal line width is: 0.2~0.3mm , The thinnest width can reach 0.05~0.07mm, the power line is generally 1.2~2.5mm. For the digital circuit PCB, a wide ground wire can be used to form a loop, that is, to form a ground network to use (analog circuit ground cannot be used in this way) ).
- First of all, it is necessary to route the more demanding wires first(such as high-frequency lines). The side lines of the input end and the output end should avoid adjacent parallel to avoid reflection interference. If necessary, ground lines should be added to isolate. The wiring of two adjacent layers should be perpendicular to each other, and parallel will easily cause parasitic coupling.
- The shell of the oscillator is grounded, the clock line should be as short as possible, and can not be led everywhere. The area of the special high-speed logic circuit under the clock oscillation circuit should increase the area of the ground, and should not take other signal lines to make the surrounding electric field approach zero.
- Use 45° polyline wiring as much as possible. Do not use 90° polyline to reduce the radiation of high-frequency signals (requires high-line to use double arc).
- Do not form a loop in any signal line. If it is inevitable, the loop should be as small as possible. The signal line should have as few vias as possible.
- The key line should be as short and thick as possible, and add protective grounds on both sides.
- When transmitting the sensitive signal and the noise field band signal through the flat cable, it should be led out by the method of “ground wire-signal-ground wire”.
- Test points should be reserved for key signals to facilitate debugging, production and maintenance testing.
- After the schematic wiring is completed, the wiring should be optimized; at the same time, after the preliminary network inspection and DRC inspection are correct, fill the unrouted area with a ground wire, use a large area copper layer for the ground wire, and print the circuit board. The unused places are connected to the ground.
5. Add teardrop
6. The first item to check, look at the Keepout layer, top layer, bottom layer topoverlay, bottomoverlay in turn.
7. Electrical rules inspection
Vias (0 vias-very incredible; 0.8 dividing line), whether there is a broken netlist, minimum spacing (10mil), short circuit (parameters must be analyzed one by one).
8. Check the power cord and ground wire-interference. (The filter capacitor should be close to the chip).
9. After the PCB is completed, reload the net logo to check whether the netlist has been modified-it works.
10. After the PCB is completed, check the core device lines to ensure accuracy.
The following are commonly used techniques for PCB assembly:
1. Surface Mount Components
Surface mount components (or surface mount technology, also called SMT method is a method of directly connecting electronic components standing on the PCB to the surface of the circuit board. The final circuit board is called surface mount device (SMD). This technology was originally called “flat installation”.
Most electronic boards currently produced are manufactured using SMT. Due to its increased circuit density and the ability to produce results on smaller boards, it has replaced the more traditional through-hole technology (more on the next point). Usually, SMT uses solder to connect the component to the circuit board, but in some cases, it is also possible to use an adhesive dot on the second side to keep the component in the reflow oven.
2. Board through hole assembly
Through-hole technology (also called “through-hole”) is a mounting technology in which electronic components are inserted through holes built into the circuit board and then soldered to the pads on the other hand. Welding can be done manually or using a plug-in mounting machine.
Through-hole technology replaces the traditional point-to-point structure. As double-sided and multilayer boards become more and more popular, through holes become popular. It is used to connect the components with the conductive layer to ensure the normal operation of the circuit board. These components are equipped with leads for connecting to the other side of the via plate.
Contrary to the SMT method, through-hole technology provides greater cohesion for circuit board components because the cells are placed through the circuit board itself.
3. Electronic mechanical assembly
Electro-Mechanical components use electrical and mechanical units to cover a wide range of functions, such as power generation, controlling switches and other mechanical tasks. Since electromechanical components are often used in applications with complex wiring and components, the components need to be put together manually.