Via hole is also called via hole. In order to meet customer requirements, the via holes must be plugged in the
PCB process. Through practice, it has been found that in the process of plugging, if the traditional aluminum sheet plugging process is changed, and the white mesh is used to complete the board surface solder mask and plugging, the
PCB production can be stable and the quality is reliable. The development of the electronics industry also promotes the development of PCB, and also puts forward higher requirements on the production process of printed boards and surface mount technology. The Via hole plugging process came into being, and the following requirements should be met at the same time:
(1) It is sufficient if there is copper in the through hole, and the solder mask can be plugged or not plugged;
(2) There must be tin-lead in the through 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;
(3) 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",
PCB 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:
(1) Prevent the tin from passing through the component surface through the through hole to cause a short circuit when the
PCB is wave soldered; especially when the via is placed on the BGA pad, the plug hole must be made first, and then gold-plated, which is convenient for BGA soldering;
(2) Avoid flux residue in the vias;
(3) After the surface mounting and component assembly of the electronics factory are completed, the
PCB must be vacuumed on the testing machine to form a negative pressure to complete;
(4) Prevent surface solder paste from flowing into the hole, causing false soldering and affecting placement;
(5) Prevent the tin beads from popping up during wave soldering, causing short circuits;
The realization of the conductive hole plugging process. For surface mount boards, especially BGA and IC mounting, they must be flat, convex and concave plus or minus 1mil, and there must be no red tin on the edge of the via hole. . Since the via hole plugging process can be described as diverse, the process flow is particularly long, and the process control is difficult. There are often problems such as oil drop during hot air leveling and green oil solder resistance experiments, and oil explosion after curing. 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 This process is: board surface solder mask→HAL→plug hole→curing. Non-plugging process is adopted for production. After hot air leveling, aluminum sheet screen or ink screen is used to complete all the via hole plugging required by customers. 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
2.1 Use aluminum sheet to plug the hole, solidify, and grind the board to transfer the graphics. This process uses a CNC drilling machine to drill the aluminum sheet that needs to be plugged into a screen and plug the hole to ensure that the via hole is full and the hole is plugged. Ink plugging ink, thermosetting ink can also be used, but its characteristics must be high hardness, small change in resin shrinkage, and good adhesion to the hole wall. The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → board surface solder mask. Using this method can ensure that the via hole plug 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 plate are very high, and the performance of the plate grinding machine is also very high. It is necessary 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 not much use of this process in PCB factories.
2.2 After plugging the hole with aluminum sheet, directly screen-print the surface of the board. This process uses a CNC drilling machine to drill the aluminum sheet that needs to be plugged into a screen, install it on the screen printing machine for plugging. After the plugging is completed, the parking shall not exceed 30 Minutes, use 36T silk screen to directly screen the solder mask on the board surface. The process flow is: pretreatment-plugging-screen printing-pre-bake-exposure-development-curing. This process can ensure that the via hole is covered with good oil. The plug hole is flat and the color of the wet film is consistent. After hot air leveling, it can ensure that the via holes are not tinned and no tin beads are hidden in the holes, but it is easy to cause the ink in the hole to be on the pad after curing, resulting in poor solderability. After the hot air leveling, the edges of the via holes will bubble and oil . It is difficult to use this process method to control the production, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of plug holes.
2.3 The aluminum sheet is plugged, developed, pre-cured, and polished. After the board is ground, the board surface solder mask is used. Drill the aluminum sheet that requires plugging to make a screen. Install it on the shift screen printing machine for plugging. The plugging must be Plump, protruding on both sides is better, and then after curing, grinding the board for surface treatment, the process flow is: pre-processing-plug hole-pre-baking-development-pre-curing-board surface solder mask because this process uses plugs Hole curing can ensure that the via hole does not lose oil or explode after HAL. However, after HAL, it is difficult to completely solve the problem of tin beads in the via hole and tin on the via hole, so many customers do not accept it.
2.4 The board surface solder mask and plug hole are completed at the same time. This method uses 36T (43T) screen, installed on the screen printing machine, using a backing plate or nail bed, while completing the board surface, plug all the through holes, its The process flow is: pre-treatment-screen printing-pre-baking-exposure-development-curing. This process takes a short time and has a high utilization rate of the equipment. However, due to the use of silk screen to plug the holes, there is a large amount of air in the vias. During curing, the air expands and breaks through the solder mask, resulting in cavities and unevenness. Hot air leveling will cause a small amount of through holes to hide tin.
