PCB News - The role of the "special pad" on the PCB board

1. Plum blossom pad

1: The fixing hole needs to be non-metallized. During wave soldering, if the fixing hole is a metalized hole, the tin will block the hole during the reflow soldering process.

2. Fixing mounting holes as quincunx pads is generally used for mounting hole GND network, because generally PCB copper is used to lay copper for GND network. After the quincunx holes are installed with PCB shell components, in fact, GND is connected to the earth. On occasion, the PCB shell plays a shielding role. Of course, some do not need to connect the mounting hole to the GND network.

3. The metal screw hole may be squeezed, resulting in a zero-boundary state of grounding and ungrounding, resulting in a strange abnormality of the system. The plum blossom hole, no matter how the stress changes, can always keep the screw grounded.

Two, cross flower pad

Cross flower pads are also called thermal pads, hot air pads, and so on. Its function is to reduce the heat dissipation of the pad during soldering, so as to prevent false soldering or PCB peeling caused by excessive heat dissipation.

1 When your pad is ground. The cross pattern can reduce the area of the ground wire, slow down the heat dissipation speed, and facilitate welding.

2 When your PCB requires machine placement and a reflow soldering machine, the cross-pattern pad can prevent the PCB from peeling (because more heat is needed to melt the solder paste)

Third, the teardrop pad

Teardrops are excessive dripping connections between the pad and the wire or the wire and the via. The purpose of the teardrop is to avoid the contact point between the wire and the pad or the wire and the via when the circuit board is hit by a huge external force. Disconnect, in addition, the setting of teardrops can also make the PCB circuit board look more beautiful.

The function of teardrop is to avoid the sudden decrease of signal line width and cause reflections, which can make the connection between the trace and the component pad become a smooth transition, and solve the problem that the connection between the pad and the trace is easily broken.

1. When soldering, it can protect the pad and avoid the falling off of the pad due to multiple soldering.

2. Strengthen the reliability of the connection (production can avoid uneven etching, cracks caused by via deviation, etc.)

3. Smooth impedance and reduce the sharp jump of impedance

In the design of the circuit board, in order to make the pad stronger and prevent the pad and the wire from being disconnected during the mechanical manufacturing of the board, a copper film is often used to arrange a transition area between the pad and the wire, which is shaped like a teardrop, so it is often called Teardrops

Four, discharge gear

Have you seen other people’s switching power supplies deliberately reserved sawtooth bare copper foil under the common mode inductance? What is the specific effect?

This is called a discharge tooth, discharge gap or spark gap.

The spark gap is a pair of triangles with sharp angles pointing to each other. The maximum distance between the fingertips is 10 mils and the minimum is 6 mils. One delta is grounded, and the other is connected to the signal line. This triangle is not a component, but is made by using a copper foil layer in the PCB routing process. These triangles need to be set on the top layer of the PCB (componentside) and cannot be covered by the solder mask.

In the switching power supply surge test or ESD test, high voltage will be generated at both ends of the common mode inductance and arcing will occur. If it is close to the surrounding devices, the surrounding devices may be damaged. Therefore, a discharge tube or varistor can be connected in parallel to limit its voltage, thereby playing the role of arc extinguishing.

The effect of placing lightning protection devices is very good but the cost is relatively high. Another way is to add discharge teeth at both ends of the common mode inductor during PCB design, so that the inductor discharges through two discharge tips, avoiding discharge through other paths, so as to make the surrounding And the influence of later stage devices is minimized.

The discharge gap does not require additional cost. It can be drawn when drawing the PCB board, but it is important to note that this type of discharge gap is an air-type discharge gap, which can only be used in an environment where ESD is occasionally generated. If used in occasions where ESD occurs frequently, carbon deposits will occur between the two triangular points due to frequent discharges between the discharge gaps, which will eventually cause a short circuit in the discharge gap, and cause permanent short-circuit of the signal line to the ground. Resulting in system failure.