First: preparation. This includes preparing component libraries and schematics. “To do a good job, one must first sharpen one’s tools”.To make a good board, in addition to good PCB design principles,drawing should also be well. Before PCB design,firstly component library of SCH schematic and PCB should be ready.Component library can use protel own libraries, but under normal circumstances it is difficult to find a suitable one.It is better to do the component library yourself according to the standard size data of the selected device. In principle, do first PCB component library, then do SCH component library. Requirements of PCB component library are higher,which directly affects the board installation.SCH component library requires relatively loose.As long as the attention to pin attributes and corresponding relationship with the PCB components. PS: Note hidden pins in the standard library. After that it is the schematic design.After finishing the schematic design,it is ready to do PCB design.
Second: PCB design. This step draws PCB board in the PCB design environment according to determined board size and the mechanical positioning,and according to the positioning requirements put the connector, button or switch, screw holes, mounting holes and so on. And fully consider and determine the wiring area and the non-wiring area .For example, how much area around the screw holes belongs to the non-wiring area?
Third: PCB layout. Layout means to put the device on the board. Then if the preparations mentioned above are ready, then you can generate a netlist (Design-> Create Netlist) on the schematic, and then import the netlist (Design-> Load Nets) on the PCB drawing.The components are all put up.There are fly line prompt connection among the pins.Then you can make layout for the device.
The following principles of the general layout :
①. According to the electrical performance,do the reasonable zoning,which generally can be divided into: digital circuit area where is fear of interference, and also makes interference, analog circuit area where is fear of interference,power driver region where is interference source;
②. When performing the same function circuit,they should be placed as close as possible, and we should adjust various components to ensure the connection is the most simple; At the same time, adjust the relative position of each block to make sure the connection among the function block is the most simple;
③. For large mass components, the installation location and mounting strength should be considered. The heating elements should be placed separately from the temperature-sensitive components.When necessary, thermal convection measures should be considered.
④. I / O driver device should be as close to the edge of the printing plate as possible,and near the lead connector;
⑤. A clock generator (eg: Crystal or Restructure) tries to close the device which may use this clock ;
⑥. Between power input pin and ground in each IC, you need to add a decoupling capacitor (generally monolithic capacitors with good high-frequency performance are used.) when board space is more dense, you can also add tantalum capacitors around a few integrated circuits.
⑦.The place of relay coil should be added the discharge diode (1N4148 is OK).
⑧. Layout requirements should be balanced and orderly. The layout can not be biased towards one end.
– Pay special attention:when placing components,the actual size (occupied area and height) of components and the relative position between the components must be considerred,in order to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time,on the premise of guarantee to reflect the above principles ,appropriate modification of device placement makes it neat and beautiful.For example, the same device should be placed neatly in the same direction,and can not be put too “patchwork.” This step is related to the overall image of the board and the degree of difficulty in the next wiring.So it takes great effort to consider.When making the layout,you can make the preliminary wiring for the place where you are not sure.And you should have full consideration.
Fourth: cabling. Cabling is the most important step in entire PCB design.This will directly affect the performance of PCB is good or bad .In the PCB design process, the wiring is generally divided three realms: first,the wiring is smooth,which is the basic requirement of PCB design. If the line is not smooth, and everywhere is the fly line,that would be defective board. Followed by the meet of the electrical performance,which is a standard that can be used to measure the board is qualified or not.After the wiring is smooth, carefully adjust the wiring so that it can achieve the best electrical performance. Next it is beautiful.If your wiring is smooth,and there is no place which can effect electrical properties,but all is in a mess,even if electrical performance is good, in others’ eyes,it is just a piece of garbage,which cause great inconvenience for the test and maintenance.Wiring should be uniform,not maintenance.These can be achieved under the condition than can ensure the electrical properties and meet other individual requirements, or you concentrate on the trifles and neglect the essentials.
When wiring mainly according to the following principles:
①. Under normal circumstances,firstly do the wiring of power and ground wiring to ensure the electrical performance of the circuit board.Within the range of allowing conditions ,try to widen the power and ground line.It is better that ground line is wider than power line.Their width relationship is :Ground lines > power lines > signal lines. Generally signal line width is 0.2 ~ 0.3mm , the smallest width is up to 0.05 ~ 0.07mm,and the power lines is usually 1.2 ~ 2.5mm.For the PCB of digital circuit, you can use wide ground wire to form a loop,and that become a ground network to use(analog circuits can not be usde like this)
②.Do the wiring in advance for lines with the more stringent requirements,such as high frequency line.The edges of input and output terminal should avoid adjacent parallel,in order to avoide reflection interference.If necessary, add ground line to isolation.The wiring in two adjacent layers should be vertical.If they are paralles, it is to cause parasitic coupling.
③. Chassis of oscillator is on ground.The clock line should be as short as possible,and can not be attracted everywhere.
④. Do the wiring with 45° polyline as far as possible.Do not use 90° polyline,which reduce radiation of high frequency signal
⑤. Any signal lines do not form a loop.If it is inevitable, the loop should be as small as possible; via of signal lines should be as little as possible;
⑥. The key line should be short and thick, and is surrounded with protective ground.
⑦. When transmitting sensitive signal and the noise field signal through the flat cable, use the way of “Ground – Signal – ground” to leads.
⑧. Test points should be set aside for the key signal, to be convenient for the production and maintenance .
⑨. After completion of schematics wiring, wiring should be optimized.
– PCB wiring process requirement
①Line
In general, width of the signal line is 0.3mm (12mil), and the power line width is 0.77mm (30mil) or 1.27mm (50mil); the distance between lines and the distance between the lines and the pad are not less than 0.33mm (13mil ).In the actual application, if conditions permit, increasing the distance should be considered.When wiring density is higher,using two lines between IC feet can be considered,but not recommended.Line width is 0.254mm(10mil),and line spacing is not less than 0.254mm (10mil). In special cases, when the device pin is dense, the line width and line spacing can be appropriately reduced.
②. Pad (PAD)
The basic requirements of PAD and VIA are: diameter of PAD is larger 0.6mm than the hole . For example,for universal pin-type resistors, capacitors and integrated circuits,if the diameter of PAD is 1.6mm(32mil) and the diameter of holes is 0.8mm(32mil),the size of sockets, pins and diode 1N4007, etc should be 1.8mm （71mil）or 1.0mm（39mil）.In practical application, the size of PAD should be based on the actual size of the element.Conditionally, it may be appropriate to increase the size of the pad;The diameter of element mounting holes in PCB should be larger 0.2 ~ 0.4mm than the actual size of the element tin.
③. Vias (VIA)
Typically it is 1.27mm (50mil ) or 0.7mm ( 28mil);
When witing density is high,the via size can be ruduced appropriately,but not too small.You can consider 1.0mm(40mil) or 0.6mm ( 24mil)
(4)Spacing requirements of pad, line, and hole
PAD and VIA ： ≥ 0.3mm（12mil）
PAD and PAD ： ≥ 0.3mm（12mil）
PAD and TRACK ： ≥ 0.3mm（12mil）
TRACK and TRACK ： ≥ 0.3mm（12mil）
When the density is higher:
PAD and VIA ： ≥ 0.254mm（10mil）
PAD and PAD ： ≥ 0.254mm（10mil）
PAD and TRACK ： ≥ 0.254mm（10mil）
TRACK and TRACK ： ≥ 0.254mm（10mil）
Fifth: wiring optimization and screen printing.
“There is no best, only better”! No matter how you rack their brains to design,after you finish the painting, and take a look, still feel a lot of places that you can modify. General design experience are: the time of wiring optimization is twice than the initial wiring time.If I feel no need to modify the place,you can shop the copper (Place-> polygon Plane).The shopped copper is generally ground wire(note that the analog ground shoulde separated from the digital ground).If multilayers. power may also be paved. For the silk screen,it can not be blocked by the device or removed by vias and pads . Meanwhile,when design,you should face the component layer,and the bottom layer should be mirrored, in order to avoid confusion level.
Sixth: The network and the DRC and structural inspection. First, under the circumstance of the right circuit schematic design,do the physical connection relationship check for the generated PCB network files and schematics network file.And timely correct the design according to the output file results, to ensure that the wiring connection relationship is right.After the network is checked and passed,check the pcb design by DRC.And timely correct the design according to the output file results, to ensure electrical performance.Finally, check and confirm the pcb mechanical mounting strcutre.
Seventh: making the board. Before that, it is better to have a review process.
PCB design is the work of a mind .People, whose mind is dense and experience is rich, design the board that is indeed good.So be extremely careful when design, and fully consider all aspects of the factors (for example, ease of maintenance and inspection that a lot of people do not consider).Be excelsior , we will be able to design a good board.