PCB Technical - Precautions for copper sheet wiring of printed board

Line current density: Most electronic lines now consist of copper bound by an insulating plate. Common circuit boards have a copper skin thickness of 35 μ M, the current density value can be obtained according to the 1A/m m empirical value, and the specific calculation can be referred to the textbook. The line width should be greater than or equal to 0.3mm in order to ensure the mechanical strength of the wiring (other non-power circuit boards may have a smaller line width). Copper skin thickness is 70 μ M circuit boards are also common in switching power supply, so the current density can be higher.

Additionally, the current commonly used circuit board design tool software generally has design specifications, such as line width, line spacing, and drying tray hole size parameters can be set. When designing the circuit board, the design software can be automatically executed according to the specifications, which can save a lot of time, reduce part of the workload and reduce the error rate.

In general, double panel can be used for lines with high reliability or high wiring density. It is characterized by moderate cost, high reliability and can meet most applications.

There are also some products with multi-layer plates in the power supply line of the module, which are mainly convenient for integrating transformer inductance and other power devices, optimizing wiring, power tube cooling and so on. It has the advantages of good workmanship, good consistency and good heat dissipation of transformers, but its disadvantage is high cost, poor flexibility and is only suitable for large-scale industrial production.

Single-panel, market-circulating universal switching power supplies almost all use single-sided circuit boards, which have the advantage of low cost, and take some measures in the design and production process to ensure their performance.

Today we talk about some experience of single-sided printed circuit board design. Because of its low cost and easy manufacturing, single-sided printed circuit board is widely used in switched power lines. Because it only has one side bound to copper, the electrical connection of devices and mechanical fixing depend on that layer of copper, so care must be taken when handling it.

In order to ensure good structural performance of the welding machine, the single-panel bonding pad should be slightly larger to ensure the good binding force of the copper sheet and the base plate, without the copper sheet peeling off and breaking off when subjected to vibration. Generally, the weld ring width should be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the pin diameter of the device, but not too large. Make sure that the distance between the pin and the pad is short, the size of the pad hole should not obstruct the normal inspection. The pad hole diameter is generally larger than the pin diameter 0.1-0.2mm. Multi-pin devices can be larger to ensure smooth inspection.

Electrical connection should be as wide as possible, the principle width should be larger than the diameter of the pad. In special cases, the line must be widened (commonly called tear drop) when connecting to the pad to avoid breaking between certain conditional lines and the pad. The principle line width should be greater than 0.5mm.

Components on a single panel should be closely attached to the circuit board. For devices that require overhead heat dissipation, the addition of sleeves to the pins between the devices and the circuit board can play the dual roles of supporting the devices and increasing the insulation, minimizing or avoiding the impact of external force on the bond pad-to-pin connection, and enhancing the solidity of the welding. Heavy parts on the circuit board can increase the support connection points and enhance the connection strength with the circuit board, such as transformers, power device radiators.

The pin of single panel welded surface can be retained longer without affecting the distance between the outer shell. Its advantage is to increase the strength of the welded area, increase the welded area, and instantly detect the phenomenon of virtual welding. When the pin is long enough to cut the leg, the force on the welded part is less. In Taiwan and Japan, the process of bending the pin of the device on the welded surface at a 45 degree angle to the circuit board before welding is often used, which is the same reason. Today, I will talk about some issues in the design of double-panel. In some application environments with high requirements or high line density, the performance and various indicators of double-sided PCB are much better than that of single-panel.

Because the holes have been metallized to a higher strength, the ring can be smaller than that of the single panel, and the hole diameter of the double panel can be slightly larger than that of the pin, because during the welding process it is beneficial for the tin solution to penetrate through the holes into the top layer of the pad to increase the reliability of the welding. However, there is a disadvantage, if the hole is too large, some parts of the device may float under the impact of jet tin during peak soldering, which may cause some defects.

For the treatment of high current line, the line width can be treated according to the previous one. If the width is not enough, tin plating on the line can generally be used to increase the thickness. There are many ways to solve this problem.

1. Set the route as the bonding pad property so that it will not be covered with solder during the manufacturing of the circuit board and will be plated with tin during hot air conditioning.

2. Place the pad in the wiring and set the pad to the desired shape. Be careful to set the pad hole to zero.

3. This method is flexible when placing wires in the solder-resistant layer, but not all circuit board manufacturers will understand your intentions and need to write them down. No solder will be applied at the place where the wire is placed on the

solder layer

As mentioned above, it should be noted that if a wide line is completely tin-plated, a large amount of solder will be bonded after soldering, and the distribution is very uneven, which will affect the appearance. Generally, long and thin tin-plated strips can be used with a width of 1~1.5mm, and the length can be determined according to the line. Double-sided circuit boards with a distance of 0.5~1mm between the tin-plated parts provide a great selection for layout and wiring, which makes the wiring more reasonable. With regard to grounding, the power ground must be separated from the signal ground. The two ground can be joined at the filter capacitance to avoid unexpected factors that may cause instability when large pulsed current is connected through the signal connection. The signal control circuit uses a point grounding method as far as possible. There is a technique that tries to place the non-grounded lines in the same wiring layer and lay the ground wire on another layer. In general, the output line passes through the filter capacitance before reaching the load, and the input line must pass through the capacitance before reaching the transformer. The theoretical basis is that the ripple current passes through the filter capacitance.

Voltage feedback sampling, in order to avoid the impact of large current through the line, the sampling point of feedback voltage must be placed at the power output end to improve the load effect index of the whole unit.

Wiring from one wiring layer to another is usually connected through holes, which is not suitable through the pin pads of the device because it is possible to destroy this connection relationship when inserting the device, and there should be at least two holes per 1A current. The principle of hole size should be greater than 0.5mm, and generally 0.8mm to ensure the reliability of processing.

Device heat dissipation, in some small power sources, the wiring of the circuit board can also have the function of heat dissipation. Its feature is that the wiring is as wide as possible to increase the area of heat dissipation, and no solder is applied. Conditionally, holes can be evenly placed to enhance thermal conductivity.