PCB wiring skills

1. The side lines of the input end and the output end should avoid adjacent parallel lines to avoid reflection interference. If necessary, the ground wire should be isolated, and the two adjacent layers should be perpendicular to each other.

2. Add decoupling capacitors between power supply and ground wire. Try to widen the power supply, ground wire width, it is best to ground wide than the power cord, their relationship is: ground > power > signal lines, signal lines usually wide: 0.2 ~ 0.3 mm, the most by the fine width up to 0.05 ~ 0.07 mm, the power cord is 1.2 ~ 2.5 mm

3, digital circuit and analog circuit processing, high frequency digital circuits, analog circuit sensitivity is strong, for signal lines, high frequency of the signal lines as far as possible away from the sensitive analog devices, for the ground, the PCB to the outside world is only one node, so must be within the PCB processing, mold has problem, and inside the plate to digital and analog is actually are divided between them, just connected with the outside world in PCB interface (e.g., plugs, etc.). There is a short digital connection to analog, please note that there is only one connection point. There are also differences on PCB, which are determined by system design.

4. Shorten the connection between high frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Sensitive components should not be too close to each other. Input and output components should be kept as far away as possible. There may be a high potential difference between some components or conductors, so the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed as far as possible in areas not easily accessible by hand during debugging.

5. In the circuit working at high frequency, the distribution parameters between components should be considered. In general, the components should be arranged as parallel as possible. So, not only beautiful. And it's easy to weld. Easy to mass produce.

6. The wires used at the input and output terminals should avoid parallel connection. It is best to add ground wire between lines to avoid feedback coupling.

7. The curve of the printed conductor is usually rounded, while the right Angle or the Angle between them will affect the electrical performance in the high-frequency circuit. If a right Angle is required, two 135-degree angles are usually used instead of a right Angle.

8. Power line design

According to the current of printed circuit board, try to add the width of leased power line to reduce the circuit resistance. At the same time, make the power line and ground line move in the same direction as the data transmission, which will help enhance the anti-noise ability.

9. Ground wire design

Ground wire design principles are:

(1) digital and analog separation. If there are both logic circuits and linear circuits on the circuit board, they should be kept as separate as possible. The ground of low frequency circuit should adopt single point parallel grounding as far as possible. High frequency circuit should adopt multi-point series grounding, ground wire should be short and rent, high frequency components should be surrounded by large area of grid foil.

(2) the ground wire should be as thick as possible. If the ground wire is made of an adhesive line, the ground potential will change with the change of current, so as to reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printing plate. If possible, the ground wire should be above 2~3mm.

(3) the ground wire forms a closed loop circuit. A printed board consisting only of digital circuits, whose grounding circuits are arranged into a mass loop, can improve the anti-noise capability.

10. Delotus root capacitor configuration

One of the common ways of PCB design is to configure the appropriate de-lotus capacitors in key parts of the PCB.

The general configuration principle of the lotus root capacitor is:

(1) the power input end spans the electrolytic capacitor of 10 ~100uf. If possible, it is better to connect more than 100uF.

(2) in principle, each IC chip should be equipped with a 0.01pF ceramic chip capacitor. If there is insufficient space for the printing plate, a 1-10 pF capacitor can be arranged for every 4-8 chips.

(3) for devices with weak anti-noise capability, such as RAM and ROM storage devices with large power changes when power is turned off, direct access to the de-lotus capacitance should be made between the power line and ground wire of the chip.

(4) capacitance leads should not be too long, especially high frequency bypass capacitors should not have leads.

11. In addition, the following two points should be noted:

(1) when there are contactor, relay, button and other elements in the printing plate. When operating them, large spark discharge will be generated, and the RC circuit shown in the attached diagram must be adopted 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 easy to be induced. Therefore, it is necessary to ground or connect the power supply to the unused end when it is used

Welding principles and tools

a

Welding principle

At present, the welding technology of electronic components is mainly tin welding. Soldering technology adoption is given priority to with tin tin alloy solder materials, solder melt under a certain temperature, metal weldment and attract each other, between tin atom diffusion, combine, form the combination of infiltration layer. Appearance printing plate copper platinum and component lead is very smooth, in fact they surface, there are many small bump clearance flow pattern of molten tin solder by capillary suction along the spread on the surface of the weld, solder and the infiltration of the weldment, the components with the printing plate tightly together, and it has good electrical conductivity.

The conditions of tin welding are as follows: the surface of the welding parts should be clean. Can be tin solder wetting of metal is the weldability, that is easy to generate oxide film on the surface of a brass and other materials, can with the aid of flux, first after tinning infiltration on the surface of the butt welding, welding again; Must have the proper heating temperature, make the solder material has a certain liquid, can achieve the purpose of the weld, but the temperature is not too high, too high easy formation of oxide film and affect the welding quality.

The second

Soldering iron

The main tool for manual welding is electric soldering iron. There are many kinds of electric soldering iron, including direct heating type, induction type, energy storage type and temperature regulation type More than 300W, mainly depending on the size of the welding parts. The welding of general components is suitable for 2OW internal heating soldering iron. Energy storage electric soldering iron can be used in welding integrated circuit and vulnerable components. 150W~300W high-power external heat soldering iron can be used when welding large welds. Small power electric soldering iron head temperature is between 300 ~ 400 ℃.

The soldering iron head is generally made of copper. In order to protect the soldering iron from being oxidized and rusted under high temperature condition, the soldering iron head is often electroplated. The new soldering iron head should be tinned before formal welding. The method is to use fine gauze paper to polish the iron head clean, then dip into the rosin water, dip in the soldering tin on the hard objects (such as wood boards) repeated grinding, so that the iron head all face tin plating. If the soldering iron head has been oxidized for a long time, use a small file to file away the surface oxidation layer. After exposing the light of copper, use the same method of tin plating on the new soldering iron head. When only one electric soldering iron is used, the temperature of the soldering iron can be adjusted by inserting the soldering iron head into the human soldering iron core in different depth. The longer the soldering iron head is pulled from the soldering iron core, the lower the temperature of the soldering iron head is, and vice versa. The size and shape of the soldering iron head can also be changed to adjust the temperature of the soldering iron head. The thinner the soldering iron head, the higher the temperature; The thicker the soldering iron head, the lower the relative temperature.

According to the type of soldering element, the proper shape of soldering iron head can be selected. The top shape of the soldering iron head is conical, inclined oval and chisel. Small welding points can be conical, larger welding points can be gouged or cylindrical.

There is also a kind of tin - absorbing soldering iron, is in the direct - heating type of electric soldering iron on the addition of tin - absorbing mechanism. This kind of electric soldering iron is used when disassembling and welding components in the circuit

three

Soldering tin and flux

Soldering tin is the main material for welding. Welding electronic components solder is actually a tin lead alloy, different proportion of tin lead solder melting point temperature is different, generally for 180 ~ 230 ℃. In manual welding, the most suitable use is tubular solder wire, with high quality rosin and activator sandwiched between the solder wire, which is very convenient to use. The tubular solder wire has 0.5, 0.8, 1.0, 1.5... Such as a variety of specifications, can be conveniently chosen.

Flux, also known as flux, is a material that can clean and protect the metal surface after being heated. The surface of metal in the air is easy to produce oxide film, this oxide film can prevent the solder from infiltrating the weld metal. Proper use of flux can remove the oxide film, make the welding quality more reliable, the surface of the welding point more smooth, round.

There are three kinds of flux: inorganic series, organic series and rosin series, among which inorganic flux has the strongest activity, but it has a strong corrosion effect on metal, and it is not allowed to be used in the welding of electronic components. Organic soldering agents (such as diethyl hydrochloride) are less active and mildly corrosive. The most widely used is rosin flux. The rosin is melted into alcohol (1:3) to form "rosin water". When welding, dip a small amount of rosin water in the welding spot to achieve a good welding effect. Too much or too many welding, when forming black film, rosin has lost the role of welding aid, it needs to be cleaned before welding. For metal components that are difficult to solder with rosin solder, about 4% diethyl hydrochloride or triethanol-gel (6%) can be added. As for all kinds of flux on the market, it is important to understand its composition and corrosion effect on components before use. Do not use blindly, so that the corrosion of components will be caused in the future, its consequences are endless.

Manual welding

a

Manual welding method

Manual welding is one of the traditional welding methods, although the bulk electronic products are less likely to use manual welding, but for electronic product maintenance, debugging, inevitably will also be used in manual welding. The quality of welding also influences the maintenance effect directly. Manual welding is a very practical skill, after understanding the general methods, to practice more; More practice, can have better welding quality.

Manual welding of electric soldering iron, there are three types of positive grip, reverse grip and pen grip. It is more convenient to hold pen when welding components and repairing circuit board.

Manual welding is generally carried out in four steps. (1) to prepare welding: be welded components and clean the dust and oil, and will be welded components around the components or so bye bye, let the soldering iron head can touch to the welding components of the solder, lest the welding head to welding when burning the other components. When welding new components, the lead wire of components should be tinned. Furnace welding: contact the soldering iron head with a small amount of soldering tin and rosin for a few seconds. If you want to remove the components on the printing plate, after heating the soldering iron head, gently pull the components with your hand or silver to see if you can remove them. Troubleshoot soldering surfaces: if you have too much soldering tin in your soldering area, you can remove soldering tin from the soldering iron's head (be careful not to burn your skin, nor to throw it on the PCB!). Burn the tin head with light and "dip" some solder out. If the soldering point solder is too little, not smooth, you can "dip" some soldering tin with the electric soldering iron head to the soldering point. Checking soldering spots: see whether the soldering spots are round, bright and firm, and whether there is a phenomenon of connecting welding with surrounding components.

The second

The reason of low welding quality

The requirement of manual welding for solder joints is: Substances have certain mechanical strength; Press is smooth and round.

The common reason for low welding quality is: the density of soldering solder is too high, forming the soldering point's tin accumulation. Too little soldering tin is not enough to cover the soldering point. (2) cold welding. Soldering iron temperature is too low or the heating time is insufficient, soldering tin has not been fully melted, soaked, the surface of soldering tin is not bright (not smooth), there are small cracks (like bean curd residue!). . The solder is mixed with a layer of rosin between the solder and the component or printing plate, resulting in poor electrical connection. If the mixed rosin is not heated enough, there is a layer of tan rosin film under the welding spot. If the heating temperature is too high, there is a black film of carbonized rosin under the welding spot. In the case of under-heated rosin film, soldering iron can be used for rewelding. For those that have formed the black film, it is necessary to "eat" the clean solder, clean the surface of the welded component or printing plate, and re-weld. Welding solder bridge. Refers to the excessive amount of soldering tin, resulting in a short circuit between the soldering points of components. Special attention should be paid to the welding of ultra-small components and small printed circuit boards. Negative flux too much and a lot of rosin residue around the solder joint. When a small amount of rosin remains, can use electric soldering iron again gently heat, let rosin volatilize, also can dip in the cotton ball that has no water alcohol, wipe off redundant rosin or flux. The solder on the surface of the bead forms a sharp spike. This is mostly due to insufficient heating temperature or solder, and soldering iron out of the welding point when the Angle is improper.

three

Welding of vulnerable components

The vulnerable component refers to the component which is easy to be damaged when it is heated or contact with electric soldering iron during installation and welding, for example, organic casting plastic component, MOS integrated circuit, etc. Vulnerable components carefully prepared surface cleaning before welding, tin plating, such as preparation, welding of avoid by all means when long time repeatedly hot welding, welding head and solder iron temperature should choose appropriate, to ensure a successful welding. In addition, less flux should be used to prevent the flux from infecting the electrical contact points of the components (such as relay contacts). It is advisable to use energy storage soldering iron for welding MOS integrated circuit to prevent damage to the integrated circuit due to weak leakage of electric soldering iron. Because the lead spacing of the integrated circuit is small, the proper soldering iron head and temperature should be selected to prevent the lead wire from being connected with tin. It is better to weld the ground end, output end and power end before welding the input end. For those components that are particularly sensitive to temperature, a cotton ball dipped in methydrate ethanol (alcohol) can be used to protect the root of the component with tweezers to minimize heat transfer to the component.