SMT-PCB Design Principles
First, the component layout on the SMT-PCB
When the board is placed on a reflow oven conveyor, the long axis of the component should be perpendicular to the direction in which the device is driven, which prevents drift or "steep" components on the board during soldering.
The components on the PCB should be evenly distributed. In particular, high power devices should be dispersed to avoid stresses caused by local overheating of the PCB during operation, which can affect solder joint reliability.
For double-sided components, larger sized devices on both sides should be staggered to the mounting position. Otherwise, the increase in local heat capacity during the welding process will affect the welding effect.
Four-pin devices such as PLCC / QFP cannot be placed on the wave soldering surface.
The large SMT device mounted on the wave soldering surface has a long axis parallel to the flow direction of the solder wave, which reduces solder bridging between the electrodes.
Large and small SMT components on the wave soldering surface should not be aligned, and due to the "shadow" effect of the solder peaks during soldering, the locations should be staggered to prevent soldering and soldering.
Second, the pads on the SMT-PCB
For SMT components on the wave soldering surface, the pads of larger components (such as transistors, sockets, etc.) should be properly enlarged. For example, the SOT23 pad can be extended by 0.8-1mm to avoid the "shadow effect" of the component. The resulting empty weld seam.
The size of the pad is determined by the size of the component. The width of the pad is equal to or slightly larger than the width of the element electrode, and the soldering effect is optimal.
Avoid using a single large pad between the two interconnect components because the solder on the large pad will connect the two components in the middle. The correct way is to separate the pads of the two components. The middle of the two pads is connected by thinner wires. If the wire is required to pass a large current, multiple wires can be connected in parallel and the wire covered with green oil.
There should be no through holes on or near the pads of the SMT component. Otherwise, during the REFLOW process, the solder on the pads will melt along the vias, which will result in solder joints, less tin, and possibly flow to the board. The other side causes a short circuit.