PCB Tech - PCB copy board accuracy and energy storage capacitor

1. What is the choice of the accuracy of PCB copy board?

The accuracy of copy board depends on two links, one is the accuracy of the software and the other is the accuracy of the original image. All kinds of copying companies have different technical capabilities in copying accuracy. Some PCB companies have poor circuit board copying technology and low copying accuracy, while some have strong copying technology and can achieve board copying accuracy. Below 1mil.

For PCB software accuracy, the 32-bit floating point representation can be said to have no accuracy limitations, so the most important thing depends on the image accuracy of the original scan. For example, if you use 1 million pixels to take photos, you can Wash a 5-inch photo, but if you want to wash it into a 20-inch photo, it’s not clear at all. The reason is the same, so for a circuit board with high precision, if you want to copy a very high-precision PCB picture, Choose a higher DPI when scanning.

The meaning of DPI is how many dots per inch. That is to say, the distance between every two points on the scanned image is 1000/DPI, the unit is mil.

If the DPI is 400, then the distance between two points on the image is 1000/400 = 2.5 mil, which means that the accuracy at this time is 2.5 mil.

This is the most scientific basis, so some people say that the accuracy can be less than 1mil, and that is a prerequisite. In fact, the accuracy of circuit board copying mainly depends on the original scanning accuracy.

In summary, when scanning the board, setting the DPI should be based on the accuracy required by the actual board. If the accuracy of the circuit line of the mobile phone board is required to be below 1mil, then the scanning DPI should be set above 1000DPI. . Scanners on the market can meet this condition.

The higher the DPI, the clearer the picture and the higher the accuracy, but the disadvantage is that the picture is too large and requires higher hardware, so it should be set according to the specific situation. For boards with general accuracy, 400DPI is generally good, and mobile phone boards can be set above 1000DPI.

Two, PCB energy storage capacitor

The PCB energy storage capacitor can ensure that the power supply voltage does not drop when the load changes rapidly. Energy storage capacitors can be divided into system-level energy storage capacitors, board-level energy storage capacitors, and device-level energy storage capacitors.

1: System-level energy storage capacitor The energy storage capacitor collects charge through the rectifier and transfers the stored energy to the output of the power supply through the lead of the converter. Aluminum electrolytic capacitors (such as B43504 or B43505 from EP-COS) with a voltage rating of 40 to 450V (DC) and a capacitance of 220 to 150,000 μF are more commonly used. According to different power requirements, devices sometimes adopt series, parallel or combination forms. For power supplies with a power level of more than 10 kW, larger tank-shaped screw terminal capacitors are usually used.

2: The board-level energy storage capacitor ensures that the power supply voltage on the board will not drop when the load changes quickly to the maximum. Place a certain number of large-capacity tantalum capacitors (such as 22μF, 33μF) on the high-frequency and high-speed single board to ensure that the voltage of the single board is consistent.

3: The device-level energy storage capacitor ensures that the voltage around the device will not drop when the load changes quickly to the maximum. For devices with higher operating frequencies and higher power consumption, it is recommended to place 1 to 4 large-capacity tantalum around them Capacitor to ensure the stability of its voltage. Principles for the placement of energy storage capacitors:

Near the input end of the power supply connector

Near the power terminals for connecting daughter boards, peripherals, and auxiliary circuits

Near high-power digital components

The farthest position from the power input terminal

A component-intensive area far away from the digital terminal of the power supply

Close to the clock generation circuit