The vacuum press made in Germany can help you sort out the significance of the plug hole process in circuit board production:

The hole through hole plays a role in connecting and conducting circuits. The development of the electronic industry also promotes the development of PCB, and also puts forward higher requirements for PCB manufacturing process and surface mount technology. At the same time, the following requirements shall be met:

（1） Copper in the through hole can be used, but plug can be used for resistance welding;  

（2） There must be tin lead in the through hole, with a certain thickness requirement (4 microns). Solder blocking ink is not allowed to enter the hole, causing tin beads to be hidden in the hole;  

（3） The through hole must be provided with solder blocking ink plug hole, which is light tight, without tin ring, tin bead, flatness and other requirements.

With the development of electronic products in the direction of "light, thin, short and small", PCBs are also developing towards high density and high defects, so a large number of SMT and BGA PCBs appear. Customers require plug holes when mounting components, which have five main functions:

（1） Prevent short circuit caused by tin inserted into component surface from through hole during PCB over wave soldering; Especially when we put the vias on the BGA alignment, we must first make the plug holes, and then gold plating to facilitate the BGA welding.

（2） Avoid flux residues in the through hole;  

（3） After the surface mounting and component assembly of the electronics factory are completed, the PCB must be vacuumed on the testing machine to form a negative pressure:

（4） Prevent the solder paste on the surface from flowing into the hole, causing false soldering and affecting the mounting;  

（5） Prevent solder beads from crystallizing during wave soldering, causing short circuit.    

For surface mounting plates, especially BGA and IC mounting, the through hole plug hole must be neat, plus or minus 1mil, and there must be no red tin on the edge of the through hole; In order to meet the customer's requirements, the through hole plug hole process can be described as varied, the process flow is particularly long, and it is difficult to control the process. Oil is often lost during hot air leveling and green oil solder resistance test; Oil explosion and other problems occur after curing. Now, according to the actual production conditions, various PCB plug hole processes are summarized, and some comparisons and decomposition are made in the process, advantages and disadvantages:

Note: The working principle of hot air leveling is to use hot air to remove the excess solder from the surface and hole of the printed circuit board, and the remaining solder is evenly covered on the substitute and non barrier solder lines and surface packaging points, which is a way of surface treatment of the printed circuit board.  

1、 Plug hole process after hot air leveling

The process flow is: resistance welding of plate surface → HAL → plug hole → curing. Non plug hole process shall be adopted for production. After hot air leveling, aluminum screen or ink screen shall be used to complete the through hole plug holes of all fortresses required by customers. The hole plug ink can be photosensitive ink or thermosetting ink. Under the condition that the wet film color is consistent, the hole plug ink should be the same as the ink on the board. This process flow can ensure that the through hole will not lose oil after hot air leveling, but it is easy to cause the ink of the plug hole to pollute the board surface and become uneven. It is easy for customers to cause faulty soldering (especially in BGA) during mounting. Therefore, many customers do not accept this method. 2、 Plug hole process before hot air leveling

2.1 Use aluminum sheet to plug holes, solidify, and perform pattern conversion after plate grinding

In this process, a numerical control drilling machine is used to drill the aluminum sheet that needs to be plugged, make it into a screen plate, and make the plug hole to ensure that the through hole plug hole is full. The plug hole ink, plug hole ink, or thermosetting ink can also be used. Its characteristics must be high hardness, small resin shrinkage change, and good adhesion with the hole wall. The process flow is: pretreatment → plug hole → grinding plate → pattern transfer → etching → resistance welding of plate surface. This method can ensure that the plug hole of the through hole is flat, and the hot air leveling will not have quality problems such as oil explosion and oil loss at the hole edge. However, this process requires one-time thickening of copper to make the copper thickness of the hole wall meet the customer's standards. Therefore, there are high requirements for copper plating of the whole plate, and there are also high requirements for the performance of the grinder to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and not polluted. Many PCB plants do not have a one-time copper thickening process, and the performance of the equipment does not meet the requirements, resulting in less use of this process in PCB plants.  

2.2 Resistance welding of direct screen plate surface after aluminum sheet plug hole

In this process, a CNC drilling machine is used to drill the aluminum sheet that needs to be plugged into the hole, which is made into a screen plate. It is installed on the screen printing machine to plug the hole. After the hole is plugged, it shall be parked for no more than 30 minutes. The 36T screen is used to directly block the welding of the screen plate surface. The process flow is: pretreatment - plug hole - screen printing - pre drying - exposure - development - curing

This process can ensure that the conductive hole cover is well oiled, the plug hole is flat, and the wet film color is consistent. After hot air leveling, it can ensure that the conductive hole does not have tin, and there is no tin bead in the hole, but it is easy to cause the ink in the hole to be as much as possible after curing, resulting in poor solderability; After the hot air is leveled, the edge of the through hole bubbles and loses oil. It is difficult to control the production with this process method. The process engineer must adopt special processes and parameters to ensure the quality of the plug hole.  

2.3 Aluminum sheet plug hole, development, pre curing, plate surface resistance welding after grinding.  

The numerical control drilling machine is used to drill the aluminum sheet with the required plug hole, make it into a screen plate, and install it on the displacement screen printing machine to make the plug hole. The plug hole must be full, and the two sides are preferred to protrude. Then, after curing, the grinding plate is processed for the plate surface. The process flow is: pre-treatment - plug hole - pre drying - development - pre curing - plate surface resistance welding. Because this process uses the plug hole curing, it can ensure that the hole after HAL does not lose oil or explode oil, but after HAL, It is difficult to completely solve the problem of tin coating on vias and through holes, so many customers do not accept it.  

2.4 The plate surface resistance welding and plug hole shall be completed at the same time.  

This method uses a 36T (43T) screen, which is installed on the screen printing machine. A pad or a nail bed is used. When the plate surface is completed, all the through holes are plugged. The process flow is as follows: pretreatment - screen printing - pre drying - exposure - development - curing. This process has a short time and a high utilization rate of equipment. It can ensure that the through holes will not lose oil and tin after air leveling. However, the through holes will not be plugged by screen printing, and the results will be stored in the through holes, After a lot of experiments, our company selected different types of inks and viscosities, which led to insufficient air. During curing, the air expanded and broke the solder mask, causing holes and irregularities. A small amount of conductive holes would be hidden in the hot air leveling, Adjust the pressure of silk screen, basically solve the problem of via hole and unevenness, and have adopted this process for batch production.  

Source: PCBWorld