What do the “special pads” on the PCB do?
First, the plum pad
1, The fixing hole needs to be non-metallized.If the fixing hole is a metalized hole during the wave soldering, the tin will block the hole during the reflow process.
2, The fixed mounting hole to make the plum pad is generally to give the mounting hole GND network, because the general PCB copper is GND network copper, after the plum hole is installed PCB shell device, in fact, is to make GND and earth connected, in some occasions, the PCB case acts as a shield. Of course, some do not need to connect the mounting holes to the GND network.
3, the metal screw hole may be squeezed, resulting in grounded and ungrounded zero-bound state, causing the system to be strangely abnormal, plum hole, regardless of the any changes of stress, always keep the screw grounded
Second, the cross flower pad
The cross flower pad is also called a thermal pad, a hot air pad etc. Its role is to reduce the heat dissipation of the pad during soldering to prevent virtual welding or pcb peeling due to excessive heat dissipation.
1 When your pad is grounded. The cross flower can reduce the connected ground area, slow down the heat dissipation and facilitate welding.
2 When your PCB requires a machine patch and is a reflow soldering machine, the cross flower pad can prevent the PCB from peeling (because more heat is needed to melt the solder paste)
Third, the teardrop pad
The teardrop is the drip connection transition between the pad and the wire or between the wire and the guide hole. The purpose of setting the teardrop is to avoid the contact point disconnect between the wire and the pad or the wire and the guide hole when the circuit board is subjected to a large external force. In addition, to make the PCB board look more beautiful.
The function of the teardrop is to avoid the signal line width suddenly becoming smaller and causing reflection, which makes the connection between the layout and the component pad tend to be smoothly transitioned, and solves the problem that the connection between the pad and the circuit is easily broken.
1, On the solder, the pad can be protected, and multiple soldering is prevented from falling off the pad.
2, To strengthen the reliability of the connection (production can avoid uneven etching, cracks in the via hole deviation, etc.)
3, Smooth impedance, reduce the sharp jump of impedance
In the circuit board design, in order to make the pad stronger, to prevent the break between the pad and the wire when the mechanical board is made, a transition zone is often arranged between the pad and the wire with a copper film, and the shape is like a teardrop, so it is often called Teardrops
Fourth, the discharge tooth
Do you see the intentionally reserved serrated bare copper foil under the common mode inductance of the switching power supply by others? What is the specific effect?
This material is called a discharge tooth, a discharge gap or a spark gap.
The discharge gap (Sparkgaps) is a pair of triangles pointing at an acute angle opposite each other with a fingertip distance of up to 10 mils and a minimum of 6 mils. One triangle is grounded and the other is connected to the signal line. This triangle is not an element, but is made using a copper foil layer during PCB layout. These triangles need to be placed on the top side of the PCB and cannot be covered by solder mask.
In the switching power supply surge test or ESD test, high voltage is generated at both ends of the common mode inductor, and arcing occurs. If it is close to the surrounding devices, it may damage the surrounding devices. Therefore, a discharge tube or a varistor can be connected in parallel to limit its voltage, thereby functioning as an arc extinguishing.
The lightning protection device is effective but costly. Another method is to add a discharge tooth to both ends of the common mode inductor during the PCB design, so that the inductor discharges through the two discharge tips, avoiding discharge through other paths, thereby making the effects on the surrounding and downstream devices are minimized.
The discharge gap does not require additional cost, and it can be drawn when drawing the pcb board, but it is important to note that this type of discharge gap is a discharge gap in the form of air, which can only be used in an environment where ESD is generated. If it is used in the case where ESD is often generated, carbon deposits will occur at the two triangular points due to frequent discharge, and the short circuit will eventually be caused on the discharge gap, causing the signal line to be permanently shorted to the ground. This causes the system to malfunction.
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