What exactly are the "special pads" on PCB boards?
I. Plum Blossom Pad
1: Fixed holes need to be non-metallic. In wave soldering, if the fixed hole is a metallized hole, tin will block the hole during reflow soldering.
2. Fixed installation holes are usually used to make Plum Blossom Pad for GND network of installation holes, because PCB is usually copper-coated for GND network. Plum Blossom holes are equipped with PCB shell devices, which in fact makes GND connect with earth, and in some cases makes PCB shell play a shielding role. Of course, some do not need to connect the installation holes to the GND network.
3. Metal screw holes may be squeezed, resulting in zero-boundary state between grounding and non-grounding, resulting in strange abnormalities of the system. Plum blossom holes, no matter how the stress changes, can always keep the screw grounded.
II. CROSS PAD
Cross pad is also called hot pad, hot air pad and so on. Its function is to reduce the heat dissipation of the pad during welding, so as to prevent virtual welding or PCB peeling caused by excessive heat dissipation.
1. When your pad is ground wire. Cross can reduce the area of ground wire connection, slow down heat dissipation speed and facilitate welding.
2. When your PCB needs a machine patch and a reflow solder, the cross pad prevents the PCB from peeling (because more heat is needed to melt the solder paste).
Ⅲ. Tear droplet pad
Tear droplets are excessive connection between pad and wire or between wire and guide hole. The purpose of setting tear droplets is to avoid disconnection between wire and pad or contact point between wire and guide hole when circuit board is impacted by huge external force. In addition, setting tear droplets can make PCB board more beautiful.
Teardrop's function is to avoid the reflection caused by the sudden reduction of the signal line width. It can make the connection between the pad and the component pad transition smoothly, and solve the problem that the connection between the pad and the pad is easy to break.
1. Welding pad can be protected from falling off during multiple welding.
2. Strengthen the reliability of the connection (production can avoid uneven etching, cracks in the hole offset, etc.)
3. Smoothing the impedance and reducing the sharp jump of impedance
IV. Discharge teeth
Have you ever seen the sawtooth bare copper foil deliberately reserved under the common-mode inductance of someone else's switching power supply?
This is called discharge tooth, discharge gap or spark gap.
The sparkgaps are a pair of triangles pointing at the opposite sharp angles of each other. The maximum distance between fingertips is 10 mil and the smallest is 6 mil. One triangle is grounded and the other is connected to the signal line. This triangle is not a component, but is made by using copper foil in PCB routing. These triangles need to be set on the top of the PCB board (component side), and can not be covered by anti-welding paint.
When switching power supply surge test or ESD test, high voltage will be generated at both ends of common-mode inductance, and flying arc will appear. If the distance between the device and the surrounding device is close, it may damage the surrounding device. Therefore, a discharge tube or varistor can be connected on it in parallel to limit its voltage, thus playing an arc extinguishing role.
Lightning protection devices are effective but cost-effective.
Another method is to add discharge teeth at both ends of common mode inductance in PCB design, so that inductance can discharge through two discharge tips, avoiding discharging through other paths, thus minimizing the impact on peripheral and post-stage devices.
Discharge gap does not require additional cost, it can be drawn when drawing PCB board, but it should be noted that this form of discharging gap is in the form of air discharging gap, which can only be used in occasional ESD environment.
If it is used in the occasion where ESD often occurs, the carbon deposit between discharge gaps will occur at two triangles due to frequent discharge, and ultimately short circuit will occur on discharge gap, which will cause permanent short circuit between signal lines and ground, thus causing system failure.
