Via hole is also called through hole. In order to meet the requirements of customers, the through hole must be plugged in the process of PCB production. It is found that in the process of plug hole, if we change the traditional aluminum plug hole process and use white mesh to complete the solder mask and plug hole, the PCB production can be stable and the quality is reliable. The development of electronic industry promotes the development of PCB, and also puts forward higher requirements for PCB manufacturing process and surface mounting technology. Via hole plug hole technology emerges as the times require. At the same time, it should meet the following requirements: (1) copper in the through hole is enough, and solder mask can be plugged; （2） There must be tin and lead in the through hole, with certain thickness requirements (4 μ m), and no solder paste can enter into the hole, resulting in tin beads in the hole; (3) the through hole must have solder resist ink plug hole, which is not transparent, and has no tin ring, tin bead and leveling requirements; With the development of electronic products in the direction of "light, thin, short and small", PCB also develops to high density and high difficulty. Therefore, there are a large number of SMT and BGA PCBs. Customers require plug holes when mounting components, which have five main functions: （1） In order to prevent short circuit caused by tin passing through the through hole during PCB wave soldering, especially when the via hole is placed on BGA pad, plug hole must be made first, and then gold plating treatment shall be carried out to facilitate BGA welding; (2) flux residue in the through hole shall be avoided; (3) after the surface mounting and component assembly of electronic factory are completed, PCB shall be vacuum absorbed and negative pressure formed on the testing machine; （4） In order to prevent solder paste from flowing into the hole, causing false soldering and affecting the mounting; (5) preventing the solder bead from popping out and causing short circuit during wave soldering; the realization of conductive hole plug hole process. For the surface mount board, especially BGA and IC, the plug hole of the through hole must be flat, convex and concave plus or minus 1mil, and there should be no red tin on the edge of the through hole. Due to the variety of through hole plug technology, the process is very long and the process control is difficult. There are many problems such as oil dropping during hot air leveling and green oil solder resistance test, and oil explosion after curing. According to the actual conditions of production, this paper summarizes various plug hole processes of PCB, compares and expounds the process flow, advantages and disadvantages: Note: the working principle of hot air leveling is to remove the excess solder on the surface and hole of printed circuit board by using hot air, and the remaining solder is uniformly covered on the pad, the non blocking solder line and the surface packaging point, which is one of the surface treatment methods of printed circuit board. 1、 The technological process of plug hole after hot air leveling is as follows: resistance welding of plate surface → Hal → plug hole → solidification. Non plug hole process is adopted for production. After hot air leveling, aluminum screen plate or ink screen is used to complete all through hole plug holes required by customers. Plug ink can be photosensitive ink or thermosetting ink, in the case of ensuring that the wet film color is consistent, the plug ink is best to use the same ink as the board. This process can ensure that the through hole will not drop oil after hot air leveling, but it is easy to cause plug hole ink pollution and uneven surface. It is easy for customers to cause false soldering during mounting (especially in BGA). So many customers don't accept this approach. 2、 Plug hole technology before hot air leveling 2.1 use aluminum sheet to plug hole, solidify and grind plate, then transfer the figure With NC drilling machine, the aluminum sheet to be plugged is made into screen plate for plug hole, so as to ensure that the through hole plug hole is full. The plug hole ink can plug the hole ink. In addition, thermosetting ink can be used, but its characteristics must be large hardness, small resin shrinkage change, and good adhesion with hole wall. The technological process is as follows: pretreatment → plug hole → grinding plate → pattern transfer → etching → resistance welding on plate surface. This method can ensure that the plug hole of the through hole is flat, and there will be no oil explosion and oil dropping from the hole edge during hot air leveling. However, this process requires one-time thickening of copper to make the copper thickness of the hole wall meet the customer's standard. Therefore, the copper plating on the whole plate is required to be very high, and the performance of the plate grinder is also very high. It is necessary to ensure that the resin on the copper surface is completely removed and the copper surface is clean and free from pollution. Many PCB plants do not have one-time thickening copper process, and the performance of the equipment can not meet the requirements, resulting in this process is not used in PCB plants. 2.2 the process flow is as follows: pre treatment - plug hole - screen printing - pre - drying - exposure - Development - curing This process can ensure that the oil is well covered, the plug hole is flat and the color of wet film is consistent. Hot air leveling can ensure that the through hole is not covered with tin and there are no tin beads in the hole, but it is easy to cause the ink in the hole to stick on the pad after curing, resulting in poor solderability, and the edge of the through hole will blister and oil will drop after hot air leveling. It is difficult to control the plug hole with this process, and the special process and parameters are needed to ensure the quality of plug hole. 2.3 aluminum sheet plug hole, development, pre curing, grinding plate surface resistance welding With the help of NC drilling machine, the aluminum sheet with required plug hole is drilled out and made into screen plate. The plug hole must be full and protruded on both sides. After solidification, the plate is ground for surface treatment. The process flow is as follows: pretreatment plug hole pre drying development pre curing board surface resistance welding Because this process adopts plug hole solidification to ensure that there is no oil dropping and oil explosion in the via hole after Hal, but tin coating on the via hole tin bead and through hole is difficult to completely solve after Hal, so many customers do not accept it. 2.4 the resistance welding and plug hole on the plate surface are completed at the same time. In this method, 36t (43T) wire mesh is used, which is installed on the screen printing machine, and the backing plate or nail bed is used
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