Why do PCB circuit board vias have to be plugged?

As electronic products develop towards "light, thin, short and small", PCBs also develop towards high density and high difficulty. Therefore, a large number of SMT and BGA PCBs appear. Customers require plugging holes when mounting components. What are the uses?

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

Via hole conduction hole plays the role of interconnecting circuits. The development of the electronics industry also promotes the development of PCB, and also puts forward higher requirements for printed circuit board production process and surface mounting technology. The via hole plugging process came into being, and the following requirements should be met at the same time:

(i) There can be copper in the via hole, and solder mask can be plugged or not;

(ii) There must be tin-lead in the via hole, and a certain thickness requirement (4 microns) is required. Solder mask ink must not enter the hole, causing tin beads in the hole;

(iii) The via hole must be plugged with solder mask ink, opaque, and must not have tin rings, tin beads, and flatness.

As electronic products develop towards "light, thin, short, and small", PCBs are also developing towards high density and high difficulty. Therefore, a large number of SMT and BGA PCBs have appeared, and customers require plugging when mounting components. There are five main functions:

(i) Prevent tin from penetrating the component surface from the via hole during wave soldering of the PCB, causing a short circuit; especially when we put the via on the BGA pad, we must first plug the hole and then gold-plated it to facilitate BGA soldering.

(ii) Avoid flux residue in the vias;

(iii) After the surface mounting and component assembly of the electronics factory, the PCB must be vacuumed to form a negative pressure on the test machine before it is completed:

(iv) Prevent the surface solder paste from flowing into the hole to cause cold soldering and affect mounting;

(v) Prevent the tin beads from popping out during wave soldering and causing short circuits.

Implementation of the conductive hole plugging process

For surface mount boards, especially BGA and IC mounting, the via plugging requirements must be flat, with a convexity of plus or minus 1mil, and the via edge must not be red and tinned; the via hole must not have tin beads hidden. In order to meet customer requirements, the via plugging process can be said to be varied, the process flow is particularly long, and the process control is difficult. There are often problems such as oil loss during hot air leveling and green oil solder resistance experiments; oil explosion after curing. According to the actual production conditions, various PCB plugging processes 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 on the surface and inside the hole of the printed circuit board, and the remaining solder is evenly covered on the pads and unresisted solder lines and surface packaging points. It is one of the surface treatment methods of printed circuit boards.

1. Plug process after hot air leveling

This process flow is: board surface solder mask → HAL → plugging → curing. The non-plugging process is used for production. After hot air leveling, an aluminum sheet screen or an ink blocking screen is used to complete the plugging of all the vias required by the customer. The plugging ink can be photosensitive ink or thermosetting ink. Under the condition of ensuring the consistency of the wet film color, the plugging ink is preferably the same ink as the board surface. This process flow can ensure that the via does not drop oil after hot air leveling, but it is easy to cause the plugging ink to contaminate the board surface and make it uneven. Customers are prone to cause cold solder joints (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, grind the board and then transfer the pattern

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

This method can ensure that the via hole is flat, and hot air leveling will not have quality problems such as oil explosion and oil loss on 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, it has high requirements for the copper plating of the whole board, and also has high requirements for the performance of the grinding machine 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 this process not being used much in PCB factories.

2.2 Directly screen-print the board surface solder mask after plugging the hole with aluminum sheet

This process uses a CNC drilling machine to drill out the aluminum sheet to be plugged, make a screen, and install it on the screen printer for plugging. After the plugging is completed, it should not be parked for more than 30 minutes. Use 36T screen to directly screen-print the board surface solder mask. The process flow is: pre-treatment-plugging-screen printing-pre-baking-exposure-development-curing

This process can ensure that the via cover oil is good, the plugging is flat, and the wet film color is consistent. After hot air leveling, it can ensure that the via is not tinned and there are no tin beads in the hole. However, it is easy to cause the ink in the hole to be on the pad after curing, resulting in poor solderability; after hot air leveling, the edge of the via will bubble and lose oil. It is difficult to control the production using this process method, and the process engineering personnel must use special processes and parameters to ensure the quality of plugging.

2.3 After the aluminum sheet plugging, development, pre-curing, and grinding, the board surface solder mask is performed.

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

Since this process uses plugging hole curing to ensure that the vias will not drop oil or explode after HAL, but after HAL, it is difficult to completely solve the problem of tin beads in the vias and tin on the vias, so many customers do not accept it.

2.4 Board surface solder mask and plugging holes are completed at the same time.

This method uses a 36T (43T) screen, installed on a screen printer, and uses a pad or nail bed to plug all the vias while completing the board surface. The process flow is: pre-treatment--screen printing--pre-baking--exposure--development--curing

This process is short in time and has high equipment utilization. It can ensure that the vias do not lose oil and the conductive holes do not get tin after hot air leveling. However, due to the use of silk screen printing for plugging holes, a large amount of air is stored in the vias. During curing, the air expands and breaks through the solder mask, causing voids and unevenness. Hot air leveling will cause a small amount of tin in the conductive holes. At present, after a large number of experiments, our company has selected different types of inks and viscosities, adjusted the pressure of silk screen printing, etc., and has basically solved the voids and unevenness of the vias. This process has been used for mass production.