Via hole is another name for via hole, and the through hole of circuit board needs to be plugged to meet the requirements of customers. After many practices, the standard aluminum sheet plugging process is changed, and the white grid is used to plug the hole on the surface of circuit board, which is reliable in production and quality.

Through holes can realize the connection and conduction of wires. The development of electronic industry also puts forward higher requirements for printed circuit board manufacturing technology and surface mount technology, which also promotes the development of PCB. Through-hole plugging technology came into being, and it is required to meet the following conditions at the same time:

(1) Only enough copper must be left in the through hole, and the solder resist can be plugged or not;

(2) There must be tin and lead in the through hole with a minimum thickness of 4 microns;

(3) The through hole must be filled with solder resist ink, which is opaque, Wuxi ring and tin bead, and uneven.

With the development of electronic products in the direction of "light, thin, short and small", PCB is also developing in this direction. Therefore, there are many SMT and BGA PCB, and customers need jacks when installing components. Five characteristics:

(1) To prevent short circuit caused by tin piercing the surface of components through the through hole during PCB wave soldering. This is especially important when the vias are located on BGA pads, because BGA can be soldered more easily.

(2) Avoid leaving flux residue in the through hole.

(3) Finally, after surface mounting and component assembly of the PCB in the electronics factory, the PCB needs to be put on the testing machine for vacuum pumping to generate negative pressure, so as to be completed.

(4) Prevent solder balls from popping up during wave soldering, causing short circuit.

(5) Prevent the solder paste on the surface from flowing into the hole to cause false welding and position deviation.

Conductive plug technology has been introduced.

In order to meet the requirements of customers, the plug holes of surface mount boards, especially those for BGA and IC mounting, must be flat, with bumps within plus or minus 1mil, and there should be no red tin on the edge of the via, and solder balls should be hidden in the via. The process of plugging the via hole is characterized by changeability, very long process flow and great difficulty in process control. According to the requirements, oil loss often occurs during hot air leveling and green oil welding resistance test, and oil explosion often occurs after curing. Based on the actual production situation, this paper compares and expounds different PCB hole plugging processes, and discusses their advantages and disadvantages:

As a surface treatment technology of printed circuit board, hot air leveling works by using hot air to blow off the redundant solder on the surface of printed circuit board and board holes, and at the same time, evenly smear the remaining solder on pads, non-solder resist lines and surface packaging points.

1. After hot air leveling, plug the opening.

The process flow consists of solder resist, tin spraying, hole plugging and curing. After hot air leveling, all the through holes required by customers are plugged with aluminum screen or ink screen without plugging holes. The ink used for plugging holes can be thermosetting or photosensitive. It is best to use the same ink as the board for plug hole ink to keep the color of wet film consistent. This process can prevent oil dripping from the through hole after hot air leveling, but the plug ink is easy to pollute and uneven the board surface, and it is easy for customers to place virtual solder unintentionally (especially in BGA), so a large number of customers refuse this practice.

2. Hot air shall be used for front hole plugging operation.

2.1 Cover the hole with aluminum plate to harden it, and then grind the plate to transfer the pattern.

When making the screen board, first drill the aluminum board with CNC drilling machine, and then fill the hole to ensure that the through hole is completely filled. Thermosetting ink can also be used to plug holes, and the ink must have high hardness. The resin has small shrinkage and good adhesion to the hole wall. Pretreatment, hole plugging, plate grinding, pattern transfer, etching, and plate surface solder resist constitute the process flow.

This process can make the plug hole of the through hole smooth, and there will be no product quality problems such as oil explosion and oil droplets on the hole edge after hot air leveling. However, in order to make the hole wall copper thickness meet the customer's requirements, the process needs to thicken the copper at one time. In order to ensure that the resin on the copper surface is completely removed and the copper surface is clean and pollution-free, the requirements for copper plating on the whole plate and the performance of the grinder are extremely high. Because many PCB companies lack a one-time copper thickening process, the equipment performance can not meet the standard, so this process is rare in PCB factories.

2.2 Immediately after covering the hole with aluminum sheet, screen print the solder mask on the board surface.

Cut the aluminum sheet that needs to be plugged with CNC drilling machine, and install it on a screen printing machine to plug the hole. After the hole plugging is completed, the downtime should not exceed 30 minutes. Use a 36T screen to screen print the solder directly on the board. The process is as follows: pretreatment → screen printing → hole plugging → pre-baking → exposure → development → repair.

This process can ensure smooth plug hole, good oiliness of through hole cover and consistent color of wet film. The solder pad causes poor solderability, and the edge of the via hole bubbles and oil drops after hot air leveling. This production control is difficult. In order to ensure the quality of plug hole, the process engineer needs to adopt specific procedures and guidelines.

2.3 After the hole position of aluminum plate is filled, the circuit board is developed, pre-cured and ground, and the board surface is welded.

The aluminum plate that needs to be plugged to make a screen is drilled with a numerical control drilling machine, installed on a shift screen printing machine to plug the hole, and then ground for surface treatment after the plate is cured. The plug hole needs to be full, and it is best to have protrusions on both sides. Pretreatment-hole plugging is the sequence of processes. Plate solder resist-pre-baking-developing-pre-curing

Because this process uses plug hole curing to ensure that the via hole after HAL does not leak oil or explode, many customers do not accept it, because after HAL, it is difficult to completely solve the buried tin beads in the via hole and the tin on the via hole.

2.4 The plug hole and the board are welded at the same time.

Using pad or nail bed, put 36T(43T) screen board on screen printing machine, and seal all the through holes while completing the board. Pretreatment, screen printing, pre-baking, exposure, development and curing constitute the process flow.

In this rapid process, the equipment utilization rate is extremely high. After hot air leveling, the air expands and penetrates the solder mask, resulting in cavities and bumps, which can prevent the vias from dripping oil or plugging tin. During hot air leveling, a small number of vias will be covered. After a lot of tests, the company selected several ink types and viscosities, adjusted the screen printing pressure, etc., and basically solved the problems of vias and bumps, and adopted this method for mass production.