Tutorial For Conductive Hole Plugging Process In PCB Design
Jul 21,2021

With the development of electronic products to the direction of “light, thin, short and small”, PCB also develops to high density and high difficulty, so there is a lot of SMT and BGA PCB, and customers require plug holes when mounting components, what is the use of it?

This paper first answers the PCB why the conductive hole plug hole, then introduces the implementation of the conductive hole plug hole process, specific to follow the small makeup to understand in detail.


Why do PCB need to plug the conductive hole?

The conductive hole is also known as a through-hole, to meet customer requirements, the circuit board through-hole must plug the hole, after a lot of practice, change the traditional aluminum plug hole process, with white mesh to complete circuit board surface resistance welding and plug hole.


Stable production, reliable quality.

The conduction hole plays the role of connecting the circuit. The development of the electronic industry also promotes the development of PCB and puts forward higher requirements for the manufacturing process of printed board and surface mount technology.

The plug hole process arises as The Times require, at the same time, it should meet the following requirements:

(1) there is copper in the conduction hole, and welding resistance can be blocked or not blocked;

(2) there must be tin and lead in the conduction hole, there must be a certain thickness requirement (4 microns), there can be no solder ink into the hole, resulting in tin beads in the hole;

(3) the through-hole must have a soldier-proof ink plug hole, light-proof, no tin ring, tin bead, and leveling requirements.


With the development of electronic products to the direction of “light, thin, short and small”, PCB also develops to the direction of high density and high difficulty, so there are a lot of PCB, and customers require plug holes when mounting components, which have five main functions:

(1) to prevent PCB through the wave soldering tin through the lead hole through the component surface caused by a short circuit; Especially when we put the hole on the pad, we must first do the plughole, and then gold-plated treatment, easy to weld.

(2) to avoid residual flux in the conduction hole;

(3) After the surface mounting and component assembly of the electronics factory is completed, the PCB shall be vacuum absorbed and negative pressure formed on the test machine before completion:

(4) to prevent the surface solder paste from flowing into the hole, resulting in virtual welding and affecting the installation;

(5) to prevent the tin bead from popping up during wave soldering, resulting in a short circuit.



Conductive hole plugging process for PCB design

For surface mount board, especially for BGA and IC mount, it is required that the plughole of the guide hole must be flat, the convex and concave must be positive and negative 1mil, and there shall be no red tin on the edge of the guide hole; Conducting hole hiding tin beads, to meet customer requirements, conducting hole plug process is varied, the process is particularly long, the process control is difficult, often in hot air level and green oil solder resistance experiment oil drop; After curing, oil burst and other problems occur.

According to the actual conditions of production, various PCB hole processes are summarized, and some comparison and elaboration are made in the process and advantages and disadvantages:

Note: the working principle of hot air leveling is to remove the excess solder in the surface and hole of the printed circuit board by hot air, and the remaining solder is evenly covered on the solder pad, unhindered solder lines, and surface seal, which is one of the surface treatment methods of the printed circuit board.

1, hot air leveling after the plug hole process

The process is: plate resistance soldering →HAL→ plug hole → solidification.

Adoption of non-plug hole process for production, hot air leveling after aluminum plate or ink screen to complete the customer requirements of all the fortress through-hole plug holes.

Plugging ink available photosensitive ink or thermosetting ink, to ensure the same wet film color, the best use of the same printing ink with the plate.

This process can ensure that the hot air leveling after the conduction hole does not drop oil, but easy to cause the plug hole ink pollution of the surface, uneven.

The customer is easy to cause false soldering during mounting (especially in BGA).

So many customers do not accept this approach.


2, hot air leveling before the plug hole process

2.1. Packed holes with aluminum sheets, solidified and ground plates for the graphic transfer

This process with numerical control drilling machine, drilling out the aluminum to plug holes, made of mesh, plug holes, ensure the full hole through the plug hole, plug hole ink plug ink, also available thermosetting ink, its characteristics must be high hardness, resin shrinkage change is small, and the hole wall binding force is good.

The process flow is pretreatment → plug hole → grinding plate → graphic transfer → etching → plate resistance welding.

By this method can guarantee the smooth conduction of plug hole, hot air leveling there would be no oil, the side of hole blasting off the quality problems such as oil, but the process requirement of disposable thickening copper, make this hole wall copper thickness meet customer standard, so the whole board high demand for copper plating, and also has a high requirement on the performance of the grinding machine, to ensure that the resin on the surface of the copper thoroughly removes, such as 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 can not meet the requirements, resulting in the use of this process in PCB factories.

2.2. Direct screen printing surface welding resistance after aluminum plug holes

This process uses a CNC drilling machine to drill out the aluminum sheet that must plug the hole and makes the screen plate, which is installed on the screen printing machine to plug the hole. After the completion of the hole, the park shall not exceed 30 minutes, and the 36T screen is directly welded on the screen printing plate surface. The process is pre-treatment – plug hole – screen printing – pre-drying – exposure a developing – solidification.

This process can ensure that the oil is good, the plughole is flat, the color of the wet film is consistent, and the hot air can ensure that the tin is not on the through-hole and the tin beads are not hidden in the hole, but it is easy to cause the ink solder pad in the hole after curing, resulting in poor solderability;

After the hot air leveling, the edge of the through-hole foamed and the oil was removed. It was difficult to use this process to control the production, so the process engineers needed to adopt special processes and parameters to ensure the quality of the plughole.


2.3. Packing holes, developing, procuring, and grinding of aluminum plates are followed by surface resistance welding.

With a CNC drilling machine, drill out the required hole of aluminum, made into a screen plate, installed on the shift screen printing machine for the hole, the hole must be full, protruding on both sides of the better, after curing, grinding plate for surface treatment, the process flow is pre-treatment – hole pre-drying – development – pre-curing – surface resistance welding.

Because the use of plug hole curing in this process can ensure that the oil will not fall off or explosion through the hole after HAL, but after HAL, it is difficult to completely solve the tin beads hidden through the hole and Tin on the guide hole, so many customers do not accept it.


2.4. The welding resistance of the plate surface shall be completed at the same time as the plughole.

This method uses 36T (43T) screen, installed on the screen printing machine, the use of pad or nail bed, in the completion of the board at the same time, all the hole plugging, the process flow is pre-treatment – screen printing – pre-drying – exposure – development – solidification.

The processing time is short, high equipment utilization can guarantee after hole out oil, hot air leveling guide hole is not on the tin, but because of using screen printing to plug hole, in the hole memory with a large number of air, when curing, air inflation, to break through resistance welding membrane, have holes, uneven, hot air leveling will guide hole is a small amount of tin.

At present, after many experiments, our company choose different types of ink and viscosity, adjust the pressure of screen printing, etc., basically solved the hole and unevenness, has adopted the process of mass production.


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