How to control line width and current in PCB wiring?

qwqwqw2088

How to control line width and current in PCB wiring? [Copy link]

     When we draw PCB, we generally have a common sense, that is, use thick wires (such as 50mil or even more) for places with large current, and use thin wires (such as 10mil) for small current signals. For some electromechanical control systems, sometimes the instantaneous current flowing through the trace can reach more than 100A, so thinner wires will definitely cause problems. A basic empirical value is: 10A/square mm, that is, the current value that can safely pass through a trace with a cross-sectional area of 1 square mm is 10A. If the line width is too thin, the trace will burn when a large current passes through it. Of course, the current burning trace must also follow the energy formula: Q=I*I*t. For example, for a trace with a current of 10A, if a current glitch of 100A suddenly appears and lasts for us, then a 30mil wire can definitely withstand it.

At this time, another problem will arise: the stray inductance of the wire. This glitch will generate a strong reverse electromotive force under the action of this inductance, which may damage other devices. The thinner and longer the wire, the greater the stray inductance, so in practice, the length of the wire must be considered comprehensively. Three copper plating methods General PCB drawing software often has several options for plating copper on the via pads of device pins: right-angle spokes, 45-degree spokes, and straight plating . What is the difference between them? Newbies often don't care much and just pick one that looks good. In fact, it's not the case. There are two main considerations: one is that the heat should not be dissipated too quickly, and the other is that the overcurrent capability should be considered. The characteristic of using the straight plating method is that the pad has a strong overcurrent capability, and this method must be used for device pins on high-power circuits. At the same time, its thermal conductivity is also very strong. Although it is good for device heat dissipation, it is a problem for circuit board welders, because the pad heats up too quickly and it is not easy to tin. It is often necessary to use a larger wattage soldering iron and a higher welding temperature, which reduces production efficiency.

Using right-angle spokes and 45-degree spokes will reduce the contact area between the pin and the copper foil, slow down the heat dissipation, and make soldering much easier. Therefore, the connection method of copper plating for via-hole pads should be selected according to the application scenario, and the comprehensive overcurrent capacity and heat dissipation capacity should be considered together. Do not use straight plating for low-power signal lines, but for pads that pass large currents, straight plating must be used. As for right angles or 45-degree angles, it depends on the appearance. For example, I have been studying a motor driver for a while. The H-bridge device in this driver always burns out, and the reason has not been found for four or five years. After a lot of hard work, I finally found out that the pad of a device in the power circuit used the copper plating method of right-angle spokes when plating copper (and because the copper plating was not well drawn, only two spokes actually appeared). This greatly reduces the overcurrent capacity of the entire power circuit. Although there is no problem with the product during normal use, it works completely normally under the condition of 10A current. However, when the H-bridge is short-circuited, a current of about 100A will appear in the loop, and the two spokes will burn out instantly (uS level). Then, the power circuit becomes open-circuited, and the energy stored in the motor will be dissipated through all possible channels without a discharge channel. This energy will burn the current measuring resistor and related op amp devices, destroy the bridge control chip, and penetrate into the signal and power supply of the digital circuit part, causing serious damage to the entire device. The whole process is as thrilling as using a hair to detonate a large landmine. Then you may ask, why are only two spokes used on the pad in the power circuit? Why not let the copper foil go straight over? Because the production department staff said that the pin would be too difficult to solder in that case! The designer listened to the production staff, so... Alas, it took a lot of thought to find this problem, it's not as simple as it sounds! If the via hole is smaller than 0.3mm, there is no way to use mechanical drilling. Laser drilling is required, and the production and processing of the board is more difficult. So my personal opinion is that if it is not absolutely necessary, the minimum is 0.5mm outside/0.3mm inside. But for computer motherboards, memory sticks, dense BGA packages, etc., sometimes it may be as small as 14mil/8mil. My personal opinion is that the size of the hole inner diameter is generally 1.5 times the line width, of course, special thickened lines (such as power supplies, etc.) do not need this.

star_66666

study

yjguohua

Are there any disadvantages to using 40mil/20mil vias for 10mil or even thinner (such as 7mil) wires?

JLCS

Thanks for sharing!! ~~ Jiali Chuang proofing contact QQ800058652

shakencity

A basic empirical value is: 10A/square mm, that is, the current that can safely pass through a trace with a cross-sectional area of 1 square mm is 10A.