PCB News - Why do the via holes on the PCB board have to be plugged?

Why do the via holes on the PCB board have to be plugged?

Conductive hole Viahole 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 plug hole process is changed, and the PCB board surface solder mask and plug hole are completed with white mesh. . Stable production and reliable quality.

Viahole through holes play the role of interconnection and conduction of lines. The development of the electronic industry also promotes the development of PCB boards, and also puts forward higher requirements on the production process of printed boards and surface mount technology. Viahole plugging technology came into being, and should meet the following requirements at the same time:

There is copper in the through hole, and the solder mask can be plugged or not plugged;

There must be tin-lead in the via hole, with a certain thickness requirement (4 microns), and no solder mask ink can enter the hole, causing tin beads to be hidden in the hole;

The through holes must have solder mask ink plug holes, 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:

Prevent the PCB circuit board from passing through the component surface through the through hole to cause short circuit when the PCB circuit board is wave soldered; especially when we put the via hole on the BGA pad, we must first make the plug hole, and then plate the gold to facilitate the BGA soldering.

Avoid the flux residue in the via holes;

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

Prevent surface solder paste from flowing into the hole, causing false soldering and affecting placement;

Prevent the solder balls from popping up during wave soldering, causing short circuits.

The 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. Now according to the actual conditions of production, the various plugging processes of PCB are summarized, and some comparisons and explanations are made in the process and advantages and disadvantages:

Note: The working principle of hot air leveling is to use hot air to remove excess solder from the surface and holes of the printed circuit board, and the remaining solder is evenly coated on the pads, non-resistive solder lines and surface packaging points, which is the surface treatment method of the printed circuit board one.

One, plug hole process after hot air leveling

The process flow is: PCB surface solder mask-HAL-plug hole-curing. The non-plugging process is adopted for production. After the hot air is leveled, the aluminum sheet screen or the ink blocking screen is used to complete the via hole plugging required by the customer for all the 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.

Two, hot air leveling front plug hole process

1, use aluminum sheet to plug the hole, solidify, and polish the board for pattern transfer

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

This method can ensure that the plug hole of the via 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 polluted. 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, plug the hole with aluminum sheet and directly screen-print the board surface solder mask

In this process, a CNC drilling machine is used to drill the aluminum sheet that needs to be plugged to make a screen, which is installed on the screen printing machine for plugging. After the plugging is completed, it should not be parked for more than 30 minutes. The process flow is: pre-treatment-plug hole-silk screen-pre-baking-exposure-development-curing

This process can ensure that the via hole is well covered with oil, the plug hole is flat, and the color of the wet film is consistent. After the hot air is leveled, it can ensure that the via 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 after curing. The soldering pads cause poor solderability; after the hot air is leveled, the edges of the vias bubble and lose oil. It is difficult to use this process to control production, and it is necessary for process engineers to use special processes and parameters to ensure the quality of the plug holes.

3, aluminum sheet plugging holes, developing, pre-curing, and polishing the PCB circuit board surface solder mask.

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-circuit board surface solder mask

Because this process uses plug hole solidification to ensure that the via hole does not lose oil or explode after HAL, but after HAL, it is difficult to completely solve the problem of tin bead in the via hole and tin on the via hole, so many customers do not accept it.

4, PCB circuit 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, our company has selected different types of inks and viscosity, adjusted the pressure of the screen printing, etc., and basically solved the voids and unevenness of the vias, and has adopted this process for mass production.