Non-wetting phenomenon of orifice edge of PCB plug-in

- Mar 08, 2017-

The top surface (component surface) of the Pcb prototype board has no problem with the chip pad of the board during reflow soldering.

When the bottom surface of the wave soldering is used, the edge of the hole plate of the insert component is not wetted, and the edge is semicircular and concave. As shown in Fig. 1, the main features are as follows:

1. Non-wetting occurs around the soldering pad pads of the interposing components. As shown in Figure 2, the component pads are well wetted.

2. The pores are well wetted to form an intermetallic compound (IMC), as shown in Figure 3:

3. It is difficult to repair the soldering iron without soldering.

So why is there a phenomenon that the printed circuit board does not wet at the edge of the plug hole after wave soldering?

The reason for our analysis is not very clear, but mainly has the following characteristics:

(1) Mainly found in lead-free soldering of pcb prototype boards, but not found in lead soldering of pcb boards. Therefore, lead is probably a cause of this phenomenon.

(2) Non-wetting surface The Ni layer does not have the "crack" characteristic of a typical black disk. It cannot be simply understood as a black disk problem. If it is a black disk, it cannot explain why the reflow soldering point does not have this phenomenon.

(3) The PCB prototype board is not related to the solder used in wave soldering and the manufacturer's process.

(4) Related to a specific manufacturer. There seems to be a certain relationship with the potion used, and it will be studied in the future. It is generally believed that the MTO of the nickel plating bath needs to be controlled. If the MTO reaches 4 to 5 times, the solderability of the plating layer will be deteriorated, and the impermeability of the Au layer is added. After the reflow soldering, the Ni layer is oxidized by the flux atmosphere. The resulting solder pads are not solderable, but none of this explains the fact that only the edge of the solder pad is not wet.

The above-mentioned welding problems, FASTPCBA gives the following countermeasures:

(1) Inserted components with lead plating are prohibited under lead-free process

(2) Strengthen the quality control of ENIG process for PCB manufacturers (effective, the main MTO can be avoided no more than 2 times)

Due to the good soldering in the hole, this kind of defect is acceptable for some products with poor reliability requirements.

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