X mm
Your position: Home >> News
About the PCB's line width and vias
We usually have a common sense when drawing PCBs, that is, where heavy currents are used (for example, 50 mils or more), small current signals (eg, 10 mils) can be used.

For some electromechanical control systems, sometimes the instantaneous current flowing in the trace can reach more than 100A. In this case, the relatively thin line will definitely cause problems. A basic empirical value is: 10A/square mm, that is, a current with a cross-section of 1 square millimeter can safely pass a current of 10A. If the line width is too thin, the traces will burn when large currents pass. Of course, current burned traces must follow the energy formula: Q=I*I*t. For example, for a 10A current trace, a 100A current glitch suddenly occurs for the duration of us, and the 30mil wire is Certainly can withstand. (There will be another problem at this time: stray inductance of the wire, this glitch will produce a strong back electromotive force under the action of this inductor, which may damage other devices. The thinner the longer the wire spurious. The greater the inductance, the more practical it is to consider the length of the conductor.)

The general PCB drawing software often has several options for copper via pads on the device pins: right angle spokes, 45 degree spokes, and straight shop. What's the difference between them? Newcomers often don't care too much. Just pick one and they will look beautiful. actually not. There are two main considerations: First, consider that you cannot dissipate heat too quickly, and second, consider overcurrent capability.

The characteristic of using the straight-laid method is that the over-current capability of the pad is very strong, and this method must be used for the device pins on the high-power loop. At the same time, its thermal conductivity is also very strong, although it works to heat the device, but this is a problem for the board soldering staff, because the pad heat is too fast to hang tin, often need to use a larger wattage soldering iron and Higher welding temperatures reduce production efficiency. The use of right-angled spokes and 45-degree spokes will reduce the contact area between the pins and the copper foil, slow heat dissipation, and welding will be much easier. Therefore, the choice of the connection mode of the via pad copper plating should be based on the application, comprehensive over-current capacity and heat dissipation capabilities together, the small power signal line should not use direct shop, but for the pad through the large current must be straight shop. As for the right angle or 45 degree angle to see beautiful.

Why did you mention this? Because the previous one has been studying a motor driver, the components of the H bridge in this driver are always burned, and no reason can be found for four or five years. After some hard work, I finally discovered that it was originally the copper pad of a device in the power loop that used right-angled spokes for copper plating (and because of poor copper plating, only two spokes actually appeared. ). This greatly reduces the overcurrent capability of the entire power loop. Although the product did not have any problems during normal use, it worked perfectly at 10A. However, when a short circuit occurs in the H-bridge, a current of about 100 A appears on the circuit, and the two spokes are instantaneously blown (uS level). Then, the power circuit becomes an open circuit, and the energy stored in the motor is emitted through all possible channels without a drain channel. This energy will burn the current-carrying resistance and related op-amp devices and destroy the bridge control chip. It also broke into the signal and power supply of the digital circuit, causing serious damage to the entire device. The whole process is like using a hair wire to detonate a large land mine as thrilling. Then you may want to ask, why only use two spokes on the pad in the power circuit? Why not let copper foil go straight? Because, Oh, the production department's staff said that this pin is too hard to weld! The designer just listened to the production staff, so it was... Hey, I found this problem really cost me a lot of brains, which is as simple as saying! The bittersweet know it, bittersweet know it...

If the via hole is less than 0.3mm, there is no way to use mechanical drilling. To use laser drilling, the production and processing of the board is more difficult. So my personal thought is that if not very much needed, the minimum is 0.5mm outside/0.3mm inside. . However, such as computer motherboards, memory sticks, dense BGA packages, etc., may sometimes be as small as 14mil/8mil. . My personal idea is that the size of the hole's inner diameter is usually 1.5 times the width of the line. Of course, special bold lines (such as power supplies, etc.) do not need to be. . .