Circuit board wiring, soldering skills and precautions, PCB wiring skills

PCB wiring skills

1. The edges of the input end and the output end should not be adjacent and parallel to avoid reflection interference. If necessary, ground isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is simply generated in parallel.
2, add a decoupling capacitor between the power supply and the ground wire. 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. The 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 equipment. 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 imitation ground inside the board are actually separated from each other. They are only between the PCB and the outside world. The connecting interface (such as plug, etc.). There is a short connection between the digital ground and the imitation 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 connection between high-frequency components as much as possible, and try to reduce their dispersion parameters and electromagnetic interference between them. Elements that are susceptible to interference should not be placed too close to each other, and input and output components should be kept as far away as possible. There may be a high potential difference between some components or wires, and the interval between them should be increased to avoid accidental short circuits caused by discharge. Components with high voltage should be placed as far as possible in places that are not easy to reach during debugging.
5. For circuits operating at high frequencies, the dispersion parameters between the components must be considered. Generally, the circuit should be placed in parallel as far as possible. In this way, it is not only beautiful, but also simple to install and weld. It is easy to produce in batches.
6. The wires used for the input and output terminals should try to avoid being parallel adjacent to each other. It is best to add ground wires between wires to avoid feedback coupling.
7. The corners of printed conductors are generally arc-shaped, and right angles or angles will affect electrical functions 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 and the direction of data transmission the same, which helps to enhance the anti-noise ability.
9. Ground wire design
The principle of ground wire design is:
(1) The digital ground is separated from the imitation 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 low frequency circuit should be connected in parallel with a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then connected 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 as much as possible.
(2) The ground wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the current change, which reduces the anti-noise function. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.
(3) The ground wire forms a closed loop. For printed boards composed only of digital circuits, the grounding circuits are mostly looped 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 principle of the decoupling capacitor is:
(1) The power input terminal is connected with a 10 ~ 100uf electrolytic capacitor. 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 gap of the printed board is not enough, a 1-10pF capacitor can be installed for every 4~8 chips.
(3) Regarding devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the chip's power line and the ground line.
(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.
11. In addition, you should pay attention to the following two points:
(1) When there are touch devices, 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 selected 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 it is not necessary to ground or connect to a positive power supply during use.
Welding principle and welding tools
One, welding principle
At present, the soldering technology is the first choice for the soldering of electronic components. The 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, disperse, and combine with each other to form a moisturizing bonding layer. It seems that the copper platinum of the printed board and the lead wires of the element equipment are very lubricated.In fact, they have many tiny bumps and gaps on the surface. The moisturizing of the parts, the element equipment and the printed board are firmly bonded together, and it has an outstanding conductive function.
The conditions for tin welding are: the appearance of the weldment should be clean, and grease and rust will affect the welding; only the metal that can be moistened by the tin solder is solderable, and the material that is easy to form an oxide film on the appearance of brass, etc., can be assisted Flux, first tin-plating the surface of the weldment and moisturizing, then soldering; it must have a proper heating temperature to make the solder have a certain fluidity to achieve the purpose of soldering, but the temperature should not be too high or too high At times, the oxide film is simply formed and the welding quality is affected.
Second, electric soldering iron
The most important thing for craft welding is the 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 is mainly determined by the size of the weldment. Generally, 2OW internal heating electric soldering iron is suitable for welding of general components; energy storage electric soldering irons can be used when welding integrated circuits and vulnerable components; 150W~300W high-power external heating electric soldering irons can be used when welding large weldments. The temperature of the tip of the low-power soldering iron is generally between 300 and 400°C.
The soldering tip is generally made of copper materials. In order to prevent oxidation and rust under the high temperature conditions of welding, the soldering iron tip is often electroplated, and some soldering iron tips are 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 clean 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 repeat the grinding, so that all surfaces of the soldering iron tip are tin-plated. If the application 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 as the tinning of 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 soldering iron tip into the soldering iron core with 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 is, and vice versa. It is also possible to use the size and shape of the replacement soldering iron tip to achieve the purpose of regulating 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.
The soldering iron tip with proper shape can be selected according to the types of components to be soldered. The tip shape of the soldering iron tip has a conical shape, an oblique ellipse shape, and a chisel shape. Welding small welding spot can choose conical shape, welding larger welding spot can choose chisel shape or cylindrical shape.
There is also a soldering iron, which is formed by adding a soldering mechanism to the direct heating electric soldering iron. This kind of electric soldering iron is used when unsoldering the components in the circuit
Three, solder and flux
Solder is the primary material for soldering. The solder for soldering electronic components is actually a tin-lead alloy, and the melting point temperature of different tin-lead solders 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 unusually convenient to use. Tubular solder wire has a variety of 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 maintain the surface of the welded metal after being heated. The surface of the metal in the air is very simple to form an oxide film, which can prevent the moisturizing effect of the solder on the weld metal. Appropriate use of flux can remove the oxide film, make the welding quality more reliable, and the appearance of the solder joints is more lubricated and rounded.
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 flux (such as diethyl hydrochloride) is second in activity, and it is 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 at the solder joints during welding to achieve an outstanding soldering effect. Too much use or repeated welding, when forming a black film, the rosin has lost the welding effect, and it needs to be cleaned before welding. Regarding metal components that are difficult to weld with rosin flux, it is possible to 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 the equipment before using them. Don't use it blindly, which will cause corrosion of the equipment in the future, which will cause endless troubles.
Craft welding
One, technical welding method
Technical welding is a traditional welding method. Although the production of batch electronic products has seldom used craft welding, it is inevitable that craft welding will be used in the repair and debugging of electronic products. The quality of welding also directly affects the repair effect. Manual welding is a very practical technology. After understanding the general methods, you must practice more; more practice can have a better welding quality.
There are three ways to hold an electric soldering iron for welding. It is more convenient to hold the pen when welding components and repairing circuit boards.
Craft welding is generally carried out in four processes. ①Preparation for welding: 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 electric soldering iron head can touch the soldering part of the components to be welded, so as not to extend the soldering iron tip. Scald other components during welding. When soldering new components, the leads of the components should be tin-plated. ②Heat soldering: Touch the soldering iron tip with a little solder and rosin to the device to be welded for about a few seconds. If you want to remove the meta equipment on the printed board, after the soldering iron tip is heated, pull the meta equipment quietly with your hands or silver to see if it can be removed. ③Clean the soldering surface: If there is too much solder on the soldering part, you can shake off the solder on the soldering iron tip (be careful not to burn the skin or throw it on the printed circuit board!), and "dip" some solder with a light soldering tip come out. If the solder joint is too small and not oily, 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 strong, and whether they are connected to the surrounding components and equipment.
2. Reasons for poor welding quality
The requirements for welding joints in technical welding are: ①Excellent electrical connection function; ②There is a certain mechanical strength; ③Lubrication and roundness.
The common reasons for the low welding quality are: ① Too much amount of solder, the accumulation of tin that constitutes the solder joint; too little solder, lack of wrapping the solder joint. 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, moisturized, the solder is not shiny (not lubricated), and there are small cracks (like tofu dregs!). ③When soldering with rosin, there is a layer of rosin mixed between the solder and the equipment or the printed board, resulting in poor electrical connection. If it is mixed with rosin that lacks heating, there will be a layer of yellow-brown rosin film under the solder joint; if the heating temperature is too high, there will be a black film of carbonized rosin under the solder joint. In the case of rosin film that lacks heating, it can be repaired with a soldering iron. Regarding the black film that has been formed, it is necessary to "eat" the net solder, clean the surface of the welded element equipment or the printed board, and perform the soldering again. 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 heat it quietly to let the rosin volatilize, or use a cotton ball dipped in absolute alcohol to wipe off the remaining rosin or flux. ⑥ The solder on the surface of the solder joint constitutes a sharp tip. This is mostly due to lack of heating temperature or too little flux, and improper viewpoint when the soldering iron is separated from the solder joint.
3. Welding of vulnerable equipment
Vulnerable components refer to components that simply cause damage when heated or touched by the soldering iron during the soldering process of the device, such as organic casting components, MOS integrated circuits, etc. Before welding, the fragile components should be carefully prepared for appearance cleaning, tinning, etc. Do not repeat the hot soldering for a long time when welding, and 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 touch points of human components (such as the contacts of relays). It is best to use an energy storage electric soldering iron for welding MOS integrated circuits to avoid 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 avoid connecting tin 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. Regarding those meta-devices that are particularly sensitive to temperature, you can use tweezers to clamp a cotton ball dipped in water and ethanol (alcohol) to maintain the roots of the meta-devices, so as to minimize heat transfer to the meta-devices.
The above is the wiring, soldering skills and precautions of the circuit board, and the introduction of PCB wiring skills. I hope it can help everyone and want to learn more about PCB wiring information. Please follow us.