In the manufacture of pcb manufacturing, the assembly of the double-sided pcb board is generally performed by first welding the bottom surface and then welding the top surface. Therefore, the bottom surface is usually referred to as an A-assembly welding surface, and the top surface (having a large number of components) is referred to as an assembly welding surface. When soldering the top component, the soldered bottom component is placed under the PCB and is fixed only by the bonding force of the molten solder. In order to prevent the components from falling under the force of gravity, a method is needed to judge whether the components will be dropped when the components are reflowed at the bottom (according to the quality of the components and the pad area).
Components that are generally easy to drop off have a common feature: fewer solder joints, heavy or very high. Such as encapsulated transformers, filters, point decoupling capacitors, coaxial connectors.
Its design principle: the components placed on the bottom surface need to meet (where C, is the component quality, P, is the total pad area below the requirements. (1) Under the lead process, the standard of the piece is not lost: C/P , < 0.048g/mm. (2) Under the lead-free process, the standard of C/P. < 0.038g/mm. In general, thin BGA, QFP type devices, etc. can be laid on the bottom surface.
There are several ways to prevent the bottom component from falling out. Method 1: The components that are easy to fall on the bottom surface are reinforced with red glue. Method 2: Weld the top surface component with a lower melting point solder. Method 3: During the second pass, slightly lower the temperature of the hot air in the heating unit at the lower part of the reflow soldering furnace.