PCB wiring skills
1. The edges of the input end and the output end should be avoided adjacent and parallel to avoid reflection interference. If necessary, ground wire should be added for isolation. The wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling can easily occur in parallel.
2. Add decoupling capacitors between the power supply and ground. Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire> power wire> signal wire, usually the signal wire width is: 0.2~0.3mm, the smallest width can be reached 0.05~0.07mm, power cord is 1.2~2.5mm
3. Common ground processing of digital circuit and analog circuit. The frequency of digital circuit is high and the sensitivity of analog circuit is strong. For signal lines, high-frequency signal lines should be as far away as possible from sensitive analog circuit devices, and for ground lines , The entire PCB has only one node to the outside world, so the problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground are actually separated inside the board. They are not connected to each other. The interface of the external connection (such as plug, etc.). There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.
4. Shorten the wiring between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. The components that are susceptible to interference should not be too close to each other, and the input and output components should be as far away as possible. There may be a high potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuits caused by discharge. The components with high voltage should be arranged as far as possible in a place that is not easy to reach during debugging.
5. For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld. It is easy to mass produce.
6. The wires used for the input and output terminals should try to avoid adjacent parallel. It is best to add ground wires between wires to avoid feedback coupling.
7. The bends of printed conductors are generally arc-shaped, and right angles or angles will affect electrical performance in high-frequency circuits. If you have to take a right angle, two 135 degree angles are generally used instead of right angles.
8. Power cord design
According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.
9. Ground wire design
The principles of ground wire design are:
(1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the grid-like large-area ground foil should be used around the high-frequency component.
(2) The ground wire should be as thick as possible. If the ground wire uses a very thready line, the ground potential changes with the current change, which reduces the noise resistance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2~3mm.
(3) The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.
10. Decoupling capacitor configuration
One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed board.
The general configuration principles of decoupling capacitors are:
(1) Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.
(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the printed board gap is not enough, a 1-10pF capacitor can be arranged for every 4~8 chips.
(3) For devices with weak anti-noise ability and large power changes when shutting down, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and ground of the chip.
(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.
11. In addition, the following two points should be noted:
(1) When there are contactors, relays, buttons and other components in the printed board. Large spark discharges will be generated when operating them, and the RC circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47UF.
(2) The input impedance of CMOS is very high and it is susceptible to induction, so the unused terminal must be grounded or connected to a positive power supply during use
Welding principle and welding tools
Principle of Welding
At present, the soldering technology of electronic components is mainly used. The tin soldering technology uses tin-based tin alloy materials as solder. The solder melts at a certain temperature, and the metal solder and tin atoms attract, diffuse and combine with each other to form a wet bond layer. The appearance of the printed board copper platinum and the component leads are very smooth.In fact, their surfaces have many tiny bumps and gaps. The molten tin solder spreads along the surface of the solder by capillary suction to form solder and solder. The infiltration of the parts, the components and the printed board are firmly bonded together, and it has good electrical conductivity.
The conditions for tin welding are: the surface of the weldment should be clean, and grease and rust will affect the welding; only the metal that can be wetted by the tin solder is solderable, and the material that is easy to form an oxide film on the surface of brass, etc., can be used Flux, first tin-plating the surface of the soldering parts, and then soldering; it must have a proper heating temperature to make the solder have a certain fluidity, so that the purpose of soldering can be achieved, but the temperature should not be too high. When it is high, it is easy to form an oxide film and affect the welding quality.
Electric soldering iron
The main tool for manual soldering is an electric soldering iron. There are many types of electric soldering irons, including direct heating, induction, energy storage and temperature regulation. The electric power is 15W, 2OW, 35w…300W, which are mainly determined by the size of the weldment. Generally, 2OW internal heating electric soldering iron is suitable for welding components; energy storage electric soldering iron can be used when welding integrated circuits and vulnerable components; 150W~300W high-power external heating electric soldering iron can be used when welding large weldments. The tip temperature of low-power soldering iron is generally between 300 and 400 ℃.
The soldering iron tip is generally made of copper material. In order to protect the soldering iron head from being oxidized and rusted under the high temperature conditions of welding, the soldering iron tip is often electroplated, and some soldering iron tips are also made of alloy materials that are not easily oxidized. The new soldering iron tip should be tinned before the formal soldering. The method is to polish the soldering iron tip with fine gauze paper, then soak it in rosin perfume, dip the soldering tin on a hard object (such as a wooden board) and grind repeatedly, so that all surfaces of the soldering iron tip are tinned. If the use time is long and the soldering iron tip has been oxidized, use a small file to gently file off the surface oxide layer. After the bright copper is exposed, use the same method of tinning the new soldering iron tip. When only one electric soldering iron is used, the temperature of the soldering iron tip can be adjusted by inserting the tip of the soldering iron into the core of different depths. The longer the soldering iron tip is pulled out from the soldering iron core, the lower the temperature of the soldering iron tip, and vice versa, the higher the temperature. It is also possible to change the size and shape of the soldering iron tip to achieve the purpose of adjusting the temperature of the soldering iron tip. The thinner the soldering iron tip, the higher the temperature; the thicker the soldering iron tip, the lower the relative temperature.
According to the type of components to be soldered, the appropriate shape of the soldering iron tip can be selected. The tip shape of the soldering iron tip has a conical shape, an oblique ellipse shape, and a chisel shape. The small welding spot can be conical, and the larger welding spot can be chisel or cylindrical.
There is also a kind of soldering iron, which is formed by adding a soldering mechanism to the direct heating electric soldering iron. This electric soldering iron is used when desoldering components in the circuit
Solder and flux
Solder is the main material for soldering. The solder for soldering electronic components is actually a tin-lead alloy. The melting point temperature of solder with different tin-lead ratios is different, generally 180~230 ℃. The most suitable for manual welding is the tubular solder wire. The solder wire contains high-quality rosin and activator, which is very convenient to use. Tubular solder wire has various specifications such as 0.5, 0.8, 1.0, 1.5…, etc., which can be easily selected.
Flux, also known as flux, is a material that can clean and protect the surface of the welded metal after being heated. An oxide film is easily formed on the metal surface in the air. This oxide film can prevent the infiltration of solder on the solder metal. Proper use of flux can remove the oxide film, make the welding quality more reliable, and the surface of the solder joints is smoother and rounder.
There are three types of fluxes: inorganic series, organic series and rosin series. Among them, inorganic flux has the strongest activity, but it has a strong corrosive effect on metals. It is not allowed to be used in the welding of electronic components. Organic fluxes (such as diethyl hydrochloride) are second in activity and are also slightly corrosive. The most widely used is rosin flux. Melt rosin with alcohol (1:3) to form “pine perfume”. Dip a small amount of pine perfume on the solder joints during welding to achieve a good soldering effect. Too much amount or welding for many times, when the black film is formed, the rosin has lost the fluxing effect, and it needs to be cleaned up before welding. For metal components that are difficult to weld with rosin flux, you can add about 4% hydrochloric acid diethyl glue or triethanol glue (6%). As for the various fluxes sold on the market, we must understand their composition and corrosion effects on components before using them. Do not use it blindly, which may cause corrosion of components in the future, which will cause endless troubles.
Manual welding method
Manual welding is a traditional welding method. Although manual welding is rarely used in the production of mass electronic products, manual welding will inevitably be used in the maintenance and debugging of electronic products. The quality of welding also directly affects the maintenance effect. Manual welding is a very practical skill. After understanding the general method, you must practice more; more practice can have a better welding quality.
There are three ways to manually solder and hold the electric soldering iron. It is more convenient to hold a pen when welding components and repairing circuit boards.
Manual welding is generally carried out in four steps. ①Preparation for soldering: Clean the dust and oil stains on the components to be welded, and then break the components around the components to be welded, so that the soldering iron head can touch the soldering part of the components to be welded, so as to prevent the soldering iron head from extending Scald other components during welding. When soldering new components, the leads of the components should be tinned. ②Heat welding: Touch the soldering iron tip with a little solder and rosin to the components to be welded for about a few seconds. If you want to remove the components on the printed board, after the soldering iron tip is heated, gently pull the components with your hands or silver to see if they can be removed. ③Clean the soldering surface: If there is too much solder on the soldered part, you can shake off the solder on the soldering iron tip (be careful not to burn your skin or throw it on the printed circuit board!), and “dip” some solder with a light soldering tip . If the solder joint is too small and not smooth, you can use an electric soldering iron tip to “dip” some solder to repair the solder joint. ④Check the solder joints: check whether the solder joints are round, bright and firm, and whether they are connected to the surrounding components.
Reasons for poor welding quality
The requirements of manual welding for solder joints are: ①good electrical connection performance; ②has a certain mechanical strength; ③smooth and round.
The common reasons for low soldering quality are: ① Too much solder is used to form the accumulation of solder joints; too little solder is not enough to cover the solder joints. ②Cold welding. When soldering, the temperature of the soldering iron is too low or the heating time is insufficient, the solder is not completely melted, infiltrated, the surface of the solder is not shiny (not smooth), and there are small cracks (like tofu!). ③For soldering with rosin, a layer of rosin is mixed between the solder and the components or the printed board, causing poor electrical connection. If there is insufficiently heated rosin, there will be a yellow-brown rosin film under the solder joint; if the heating temperature is too high, there will be a black carbonized rosin film under the solder joint. In the case of insufficiently heated rosin film, a soldering iron can be used for repair welding. For the black film that has formed, it is necessary to “eat” the net solder, clean the surface of the soldered components or the printed board, and re-soldering. ④ Solder bridge. Refers to the excessive amount of solder, causing short circuits between the solder joints of the components. This should be particularly noticed when soldering ultra-small components and small printed circuit boards. ⑤ Excessive flux, there are many rosin residues around the solder joints. When a small amount of rosin remains, you can use an electric soldering iron to lightly heat it to let the rosin evaporate, or use a cotton ball dipped in absolute alcohol to wipe off the excess rosin or flux. ⑥The solder on the surface of the solder joint forms a sharp tip. This is mostly caused by insufficient heating temperature or too little flux, and improper angle when the soldering iron leaves the solder joint.
Welding of vulnerable components
Vulnerable components refer to components that are easily damaged when heated or contacted with an electric soldering iron during the installation and welding process, such as organic casting components, MOS integrated circuits, etc. Fragile components should be carefully prepared for surface cleaning, tinning, etc. before welding. Avoid repeated hot soldering for a long time during welding. The temperature of the soldering iron tip and soldering iron should be selected appropriately to ensure a successful soldering. In addition, use less flux to prevent the flux from invading the electrical contacts of components (such as relay contacts). It is best to use an energy storage electric soldering iron for welding MOS integrated circuits to prevent damage to the integrated circuit due to the weak leakage of the electric soldering iron. Because the lead spacing of integrated circuits is very small, it is necessary to choose a suitable soldering iron tip and temperature to prevent tin connection between the leads. It is best to weld the ground terminal, output terminal, power terminal first, and then the input terminal when welding the integrated circuit. For those components that are particularly sensitive to temperature, you can use tweezers to clamp a cotton ball dipped in ethanol (alcohol) to protect the roots of the components, so that the heat is transmitted to the components as little as possible.