SMT-PCB design principles

1. The layout of the components on the SMT-PCB When the circuit board is placed on the conveyor belt of the reflow soldering furnace, the long axis of the components should be perpendicular to the driving direction of the equipment, so that the components can be prevented from drifting on the board during welding. Or "tombstone" phenomenon.

The components on the PCB should be evenly distributed. In particular, the high-power devices should be dispersed to avoid local overheating on the PCB when the circuit is in operation, which may affect the reliability of solder joints.

Double-sided placement of components, on both sides of the larger device to stagger the installation location, otherwise in the welding process will affect the welding effect due to the increase of local heat capacity.

Pin-edge devices such as PLCC/QFP cannot be placed on the wave solder surface.

The large SMT devices mounted on the wave soldering surface have a long axis parallel to the direction of solder wave flow, which can reduce the solder bridging between the electrodes.

The large and small SMT components on the wave soldering surface cannot be arranged in a straight line, and they must be staggered so as to prevent the occurrence of welding and missing soldering due to the "shadow" effect of solder peaks during soldering.

2. SMT-PCB pad on the wave solder surface of the SMT components, the larger components of the pad (such as transistors, sockets, etc.) should be appropriately increased, such as the SOT23 pad can be extended 0.8-1mm, this can Avoid empty soldering due to component "shadow effect".

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 welding effect is best.

Avoid using a single large pad between two interconnected components, because the solder on the large pad will connect the two components to the middle, the correct approach is to separate the two components of the pad, in The middle of the two pads is connected with a thinner wire. If it is required that the wire passes a large current, several wires can be connected in parallel, and the wire is covered with green oil.

No vias on or near the pads of SMT components. Otherwise, during the REFLOW process, the solder on the pads will flow down through the vias, resulting in solder joints, less tin, and possibly flow to the board. The other side causes a short circuit.