PCB Technical - Causes of clogging of circuit board vias

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 sheet plugging process is changed, and the circuit board surface solder mask and plugging are completed with white mesh.Stable production and reliable quality.

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. Via hole plugging technology came into being, and should meet the following requirements at the same time:

There is copper in the via 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 should 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:

To prevent the tin from passing through the via hole to the component surface and cause a short circuit when the PCB is wave soldered; especially when we put the via on the BGA pad, we must first plug the hole and then plate it with gold to facilitate the soldering of the BGA.

Avoid flux residue in the vias;

After the surface mounting of the electronics factory and the assembly of the pcb 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 tin balls from popping up during wave soldering, causing short circuits.

Realization of Conductive Hole Plugging Process

For surface mount boards, especially BGA and IC mounting, the via hole plugs 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.

1.Hole plugging process after hot air leveling

The process flow is: board 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.

2.Hot air leveling and plug hole technology

2.1Use aluminum sheet to plug the hole, solidify, and polish the board to transfer the graphics

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: pre-treatment - plug hole - grinding plate - pattern 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.

2.2After plugging the hole with aluminum sheet, 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.

2.3The aluminum sheet is plugged into the hole, 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 via hole does not lose oil or explode after HAL, but after HAL, it is difficult to completely solve the problem of tin bead storage in the via hole and tin on the via hole, so many customers do not accept it.

2.4The 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 to mass produce PCBs.