Causes of Via Blockage in PCB Circuit Boards

In order to meet the customer's requirements, the vias of the circuit board must be plugged. After a lot of practice, the traditional aluminum sheet plugging process was changed, and the white board was used to complete the soldering and plugging of the circuit board surface. Stable production and reliable quality.

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Via hole plays the role of interconnecting and connecting lines. The development of the electronics industry also promotes the development of PCBs. It also places higher requirements on printed board manufacturing processes and surface mount technology. The Via hole plugging process came into being, and it should meet the following requirements:

Copper can be used in the vias, soldering can be plugged or not;

The lead-through hole must have tin-lead and a certain thickness (4 microns). There must be no solder resist ink entering the hole, causing tin beads to be hidden in the hole;

The through hole must have a solder resist ink plug hole, it is opaque, and there must be no 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 holes when mounting components. Five roles:

Prevent short-circuiting of the tin from the through hole through the component surface during PCB over-wave soldering; especially when we place the via on the BGA pad, we must first plug the hole and then plate it with gold to facilitate BGA soldering.

Avoid flux residue in the vias;

After the surface mount and component assembly of the electronics factory are completed, the PCB must be vacuumed to form a negative pressure on the test machine:

Prevent surface solder paste from flowing into the hole and cause false soldering, which will affect the placement;

Prevents solder balls from popping out during over-wave soldering, causing short circuits.

Implementation of conductive hole plugging process

For surface-mount boards, especially for BGA and IC, the via hole plugging requirements must be flat, the convexity and concavity is 1 mil, and there must be no red on the edge of the via hole; tin holes are hidden in the via hole, in order to reach customers According to the requirements, the through-hole and plug-hole technology can be described as various, 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 and other problems occur. According to the actual conditions of production, the various plugging processes of the PCB are summarized, and some comparisons and elaborations are made in the process, advantages and disadvantages:

Note: The working principle of hot air leveling is to use hot air to remove excess solder on the surface of printed circuit boards and holes. The remaining solder is evenly covered on the pads and unresisted solder lines and surface packaging points. It is a surface treatment method for printed circuit boards. one.

I. Plugging process after hot air leveling

This process flow is: board surface soldering → HAL → plug hole → curing. Adopt non-plugging process for production. After hot air leveling, use aluminum sheet screen or ink screen to complete the via hole plug holes required by customers. The plugging ink can be a photosensitive ink or a thermosetting ink. Under the condition that the color of the wet film is consistent, it is best to use the same ink as the board surface. This process can ensure that the via hole does not lose oil after the hot air is leveled, but it is easy to cause plugging ink to contaminate the board surface and cause unevenness. Customers are likely to cause false soldering during placement (especially in BGA). So many customers do not accept this method.

Second, hot air leveling front plugging process

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

In this process, a numerical control drilling machine is used to drill the aluminum sheet that has to be plugged to make a screen. The plugging is performed to ensure that the via hole is full. , Resin shrinkage change is small, and the binding force with the hole wall is good. The process flow is: pre-processing → plug hole → grinding plate → pattern transfer → etching → board surface solder resist

This method can ensure that the via hole plug hole is flat, and hot air leveling will not have quality problems such as oil explosion and oil drop at the edge of the hole. Therefore, the entire plate is required to be plated with copper, and the performance of the plate mill is also very high. 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 this process not being used in PCB factories.

2.2 Soldering directly on the screen printing board surface after plugging the hole with aluminum sheet

This process uses a numerical control drilling machine to drill out aluminum sheets that need to be plugged into a screen, and install it on a screen printing machine to plug the hole. After the plugging is completed, it should be parked for no more than 30 minutes. The process flow is: pre-treatment-plug hole-silk screen-pre-bake-exposure-development-curing

This process can ensure that the via holes are covered with good oil, the plug holes are flat, and the color of the wet film is consistent. After the hot air is leveled, the via holes are not tinned, and no tin beads are hidden in the holes. The pads cause poor solderability. After the hot air is leveled, the edges of the via holes blister and oil is removed. This process is difficult to control production. Process engineers must use special processes and parameters to ensure plug quality.

2.3 After the aluminum sheet is plugged, developed, pre-cured and ground, the surface of the board is soldered.

Using a CNC drilling machine, drill out the aluminum sheet that requires plugging, make a screen, and install it on a shift screen printer to plug the plug. The plug must be full, and the two sides should protrude, and then cured. The process is as follows: pre-treatment-plug hole pre-baking-development-pre-curing-board surface solder resist

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

2.4 Board soldering and plugging are completed at the same time.

This method uses a 36T (43T) screen and is installed on a screen printer. A pad or nail bed is used to plug all the through holes at the same time as the surface of the board is completed. The process is as follows: pretreatment-silk screen- -Pre-baking-exposure-development-curing.

This process has a short time and a high utilization rate of the equipment. It can ensure that the vias do not lose oil and the vias are not tinned after the hot air is leveled. The air expands and breaks through the solder resist film, causing voids and unevenness. There will be a small number of via holes to hide tin in the hot air leveling.