What Are The Principles of PCB Design?

I. Layout of components on PCB

1. When the PCB to the reflow soldering machine furnace conveyor belt, the long axis of the components should be perpendicular to the direction of transmission of the equipment, so as to prevent the components in the welding process on the board drift or the phenomenon of “vertical monument.

2. PCB components should be evenly distributed, especially the high-power devices should be dispersed, to avoid circuit work on the PCB localized overheating to produce stress, affecting the reliability of the solder joints.

3. Double-sided mounting components, the two sides of the larger devices to be staggered on the installation position, otherwise the welding process will be due to the increase in local heat capacity and affect the welding effect.

4. In the wave soldering machine surface can not be placed on the PLCC / QFP and other four sides of the device with pins.

5. Mounted on the wave soldering surface of the SMT large devices, its long axis and the direction of the flow of the solder wave should be parallel to the direction of flow, so as to reduce the electrode between the solder bridging.

6. Wave soldering surface of large and small SMT components can not be lined up in a straight line, to be staggered position, so as to prevent soldering due to the “shadow” effect of the solder wave caused by virtual welding and leakage of welding.

II. The pad on the PCB

1. Wave soldering surface of the SMT components, the larger components of the pad (such as transistors, sockets, etc.) to be appropriately increased, such as SOT23 pad can be extended by 0.8-1mm, so that you can avoid the “shadow effect” due to the components of the empty soldering.

2. The size of the pad should be determined according to the size of the component, the width of the pad is equal to or slightly larger than the width of the electrode of the component, the best welding results.

3. In the two interconnected components, to avoid using a single large pad, because the solder on the large pad will be connected to the middle of the two components, the correct approach is to separate the pads of the two components, in the middle of the two pads with a thinner wire connection, if you require the wire to pass a larger current can be connected in parallel to a few wires, wires covered with green oil.

4. SMT component pads or in its vicinity can not have through holes, otherwise in the REFLOW process, the pads on the melting of the solder will flow along the through holes, which will produce a virtual soldering, less tin, but also may flow to the other side of the board resulting in a short circuit.