Conductive hole Via hole is also known as via hole. In order to meet customer requirements, the circuit board via hole must be plugged. After a lot of practice, the traditional aluminum plugging process is changed, and the circuit board surface solder mask and plugging are completed with white mesh. hole. Stable production and reliable quality. 

Why are vias blocked on the PCB circuit board?

Via hole plays the role of interconnection and conduction of circuits. The development of the electronics industry also promotes the development of PCB, and also puts forward higher requirements on the printed board manufacturing process and surface mount technology. The Via hole plugging process came into being, and the following requirements should be met at the same time:

1. The via hole should have copper, and the solder mask can be plugged or not;

2. The via hole must have tin and lead with a certain thickness requirement ( 4 microns), no solder mask ink should enter the hole, causing tin beads to be hidden in the hole;

3. The through hole must have a solder mask plug hole, which is opaque, and must not have tin rings, tin beads, and flatness requirements.

With the development of electronic products in the direction of "light, thin, short, and small", PCBs have also developed to high density and high difficulty. Therefore, a large number of SMT and BGA PCBs have appeared, and customers require plugging when mounting components, mainly including Five functions:

1. Prevent short circuit caused by tin passing through the component surface from the via hole when the PCB is wave soldered; especially when we put the via hole on the BGA pad, we must first make the plug hole and then gold-plated, which is convenient for BGA Welding.

2. Avoid flux residues in the through holes;

3. After the electronics factory's surface mount and component assembly are completed, the PCB must be vacuumed on the testing machine to form a negative pressure before it is completed:

4. Prevent surface solder paste from flowing into the holes and causing false soldering. Affect placement;

5. Prevent tin balls from popping up during wave soldering, causing short circuits.

Realization of Conductive Hole Plugging Process

For surface mount boards, especially the mounting of BGA and IC, the via hole plug must be flat, convex and concave plus or minus 1mil, and there must be no red tin on the edge of the via hole; the via hole hides the tin ball, in order to reach the customer According to the requirements, the via hole plugging process can be described as diverse, the process flow is particularly long, the process control is difficult, and the oil is often dropped during the hot air leveling and the green oil solder resistance test; problems such as oil explosion after solidification occur. According to the actual conditions of production, we summarize the various plugging processes of PCB, and make some comparisons and explanations in the process and advantages and disadvantages: (Note: The working principle of hot air leveling is to use hot air to make the surface of the printed circuit board and the excess inside the hole The solder is removed, and the remaining solder is evenly coated on the pads, non-resistive solder lines and surface packaging points. This is one of the surface treatment methods of printed circuit boards.)

1. Hole plugging process after hot air leveling

This process is: board surface resistance Welding→HAL→plug hole→curing. Non-plugging process is adopted for production. After hot air leveling, aluminum sheet screen or ink blocking screen is used to complete the via hole plugging required by customers for all fortresses. The plug hole ink can be photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plug hole ink is best to use the same ink as the board surface. This process can ensure that the through holes will not lose oil after the hot air is leveled, but it is easy to cause the plugging ink to contaminate the board surface and uneven. Customers are prone to false soldering (especially in BGA) during mounting. So many customers do not accept this method.

2. Hot air leveling process for plugging holes before

2.1 Use aluminum sheet to plug the holes, solidify, and grind the board to transfer

the graphics. This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged to make a screen and plug the holes to ensure The via hole plug hole is full, and the plug hole ink plug hole ink can also be used with thermosetting ink. Its characteristics must be high hardness, small change in resin shrinkage, and good bonding force with the hole wall. The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → board surface solder mask

Using this method can ensure that the via hole plug hole is flat, and there will be no quality problems such as oil explosion and oil drop on the edge of the hole when leveling with hot air. 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 requirements for copper plating on the entire board are very high, and the performance of the plate grinding machine is also very high, to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and not contaminated. Many PCB factories do not have a one-time thickening copper process, and the performance of the equipment does not meet the requirements, resulting in not much use of this process in PCB factories.

2.2 After plugging the hole with aluminum sheet, directly screen-print the surface of the board.

This process uses a CNC drilling machine to drill the aluminum sheet that needs to be plugged to make a screen, install it on the screen printing machine for plugging. After the plugging is completed, the parking shall not exceed 30 Minutes, use 36T silk screen to directly screen the solder mask on the board surface. The process flow is: pretreatment-plug hole-silk screen-pre-bake-exposure one by one development-curing

This process can ensure good oil cover for the vias. , The plug hole is flat and the color of the wet film is the same. After the hot air leveling, it can ensure that the through hole is not tinned and the tin bead is not hidden in the hole, but it is easy to cause the ink in the hole to be on the pad after curing, resulting in poor solderability; hot air trimming After flattening, the edges of the vias are blistered and oil is removed. This process method is difficult to produce and control, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of the plug holes.

2.3 The aluminum sheet is plugged into holes, developed, pre-cured, and polished, and then solder resist is performed on the surface of the board.

Use a CNC drilling machine to drill out the aluminum sheet that requires plugging holes to make a screen, install it on the shift screen printing machine for plugging holes. The plugging holes must be full and protruding on both sides. The process flow is: pre-treatment-plug hole-pre-baking-development-pre-curing-board surface solder mask.

Because this process uses plug hole curing to ensure that the through hole after HAL does not drop or explode, but after HAL, It is difficult to completely solve the problem of tin beads hidden in via holes and tin on via holes, so many customers do not accept them.

2.4 The board surface solder mask and plug hole are completed at the same time.

This method uses a 36T (43T) screen, installed on the screen printing machine, using a pad or a bed of nails, and when completing the board surface, all the through holes are plugged. The process flow is: pretreatment-screen printing- -Pre-baking-exposure-development-curing.

The process time is short and the equipment utilization rate is high. It can ensure that the via holes will not lose oil after the hot air leveling, and the via holes will not be tinned. However, due to the use of silk screen for plugging the holes, there is a large amount of air in the via holes. , The air expands and breaks through the solder mask, resulting in cavities and unevenness. There will be a small amount of through holes hidden in the hot air leveling. At present, after a large number of experiments, choosing different types of ink and viscosity, adjusting the pressure of the screen printing, etc., the via holes and unevenness have basically been solved, and this process has been used for mass production.