The component lead hole taking tin is a more complicated problem. It is related to the package of the components, the aperture, the thickness of the board, the surface treatment of the PCB, and the thickness of the solder paste.
In the QFN package, since the heat sink pad and the soldering surface of the lead end are on the same surface (Stand off is 0), when the board is manufactured and soldered, the solder often has a serious tin under the gravity of the component, while the QFP and SOP are basically No.
The surface treatment of spray tin (HASL) has good wetting properties and is prone to tinning.
Due to the action of the hair suction, the smaller the pore diameter of the heat conduction hole, the easier it is to cause tin.
Solder paste printing cover holes are also prone to tinning.
Then for the board pad hole to lead tin, we give the following countermeasures:
1. Optimize the design and select the appropriate aperture and spacing
2. Optimize the design of steel mesh
The design of the thermal via of the board pad via hole and the tin pad has the following points
1. The single-sided layout of the board, the heat sink components are uniformly placed on the second reflow soldering surface, in order to eliminate the influence of the "tin ball"
2. No plug hole, suitable for the back of the component to allow flux or a plate that can add a flux scrubbing process.
3. No-ring plug hole, if the back surface has flatness requirements, such as a polished RF board, a ringless plugging process can be used. With the acyclic plug hole process, two points need to be noted: the thin plate is not suitable; the total area of the solder resist area cannot exceed 20% of the total area of the heat sink pad. as the picture shows: