PCB Line Width And Cias

We generally have a common sense when drawing PCBs, that is, use thick wires (such as 50 mil or even more) where high current flows, and thin wires (such as 10 mil) for small current signals.

For some electromechanical control systems, sometimes the instantaneous current flowing in the traces can reach more than 100A, so thin 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 is passed. Of course, the current burnt trace must also follow the energy formula: Q=I*I*t. For example, for a trace with a 10A current, a 100A current burr suddenly appears, and the duration is us level, then the 30mil wire can definitely bear it. (At this time, there will be another problem? The stray inductance of the wire. This burr will generate a strong back electromotive force under the action of this inductance, which may damage other devices. The thinner and longer the wire is, the stray the larger the inductance, the actual length of the wire should be considered).

General PCB drawing software often has several options when laying copper on the via pads of device pins: right-angle spokes, 45-degree angle spokes, and straight lay. What’s the difference between them? Novices often don’t care much. There are two main considerations: one is to consider not to dissipate heat too fast, and the other is to consider the overcurrent capability.

The feature of using the direct laying method is that the overcurrent 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 is good for the heat dissipation of the device, it is a problem for the circuit board soldering personnel, because the heat dissipation of the pad is too fast and it is not easy to hang tin, so it is often necessary to use a higher wattage soldering iron and Higher soldering temperatures reduce productivity. The use of right-angle spokes and 45-angle spokes will reduce the contact area between the pins and the copper foil, the heat dissipation will be slow, and the soldering will be much easier. Therefore, the connection method of copper laying on the via pad should be considered according to the application, and the comprehensive overcurrent capability and heat dissipation capacity should be considered together. Do not use straight laying for low-power signal lines, but must be straight for the pads that pass through high current. shop. As for the right angle or 45 degree angle, it depends on the beauty.

Why bring this up? Because I have been working on a motor driver a while ago, the H-bridge devices in this driver are always burned, and the reason has not been found for four or five years. After a lot of hard work, I finally found out that it turned out that the pads of a device in the power loop used the copper-plating method of right-angle spokes when laying copper (and due to the poor copper-plating painting, only two spokes actually appeared. ). This greatly reduces the overcurrent capability of the entire power loop. Although the product does not have any problems during normal use, it works perfectly under the condition of 10A current. However, when the H-bridge is short-circuited, a current of about 100A will appear on the loop, and the two spokes will be blown out instantaneously (uS level). Then, the power loop becomes an open circuit, and the energy stored on the motor is dissipated through all possible channels without a discharge channel. This energy will burn the current measuring resistor and related op amp devices, and destroy the bridge control chip. And into the signal and power supply of the digital circuit part, causing serious damage to the entire equipment. The whole process is as thrilling as detonating a large landmine with a strand of hair. So you might be asking, why are only two spokes used on the pads in the power loop? Why not let the copper foil go straight over it? Because, hehe, the people in the production department said that this pin is too difficult to solder!

If the via hole is smaller than 0.3mm, there is no way to use mechanical drilling. If laser drilling is used, the production and processing of the board is more difficult. So my personal thought is that the minimum is 0.5mm outside / 0.3mm inside if it is not very necessary. But like computer motherboards, memory sticks, dense BGA packages, etc., sometimes it may be as small as 14mil/8mil. My personal idea is that the inner diameter of the hole is generally 1.5 times the line width, of course, special thick lines (such as power supplies, etc.) do not need this.