Via hole is also known as a via hole. In order to meet customer requirements, the via holes must be plugged in the PCB fabrication process. It has been found through practice that in the process of plugging, if the traditional aluminum sheet plugging process is changed, the white surface is used to complete the surface solder mask and the plug hole, which can make the PCB production stable and reliable in quality.
The development of the electronics industry, while promoting the development of PCBs, also puts higher requirements on the manufacturing process and surface mount technology of printed boards. Via hole plugging process came into being, and the following requirements should be met:
(1) There is copper in the via hole, and the solder resist can be plugged;
(2) There must be tin-lead in the via hole, which has a certain thickness requirement (4 micrometers), and no soldering ink is allowed to enter the hole, resulting in the inclusion of tin beads in the hole;
(3) The through hole must have a soldering ink plug hole, which is opaque, and must not have tin ring, tin bead and leveling requirements;
With the development of electronic products in the direction of “light, thin, short and small”, PCBs are also developing at a high density and high difficulty. Therefore, a large number of SMT and BGA PCBs are present, and customers require plugging holes when mounting components. Five roles:
(1) Preventing the short circuit of the tin from the via hole through the component surface during the over-wave soldering of the PCB; especially when the via hole is placed on the BGA pad, the plug hole must be made first and then gold-plated to facilitate the BGA soldering;
(2) avoiding flux remaining in the via hole;
(3) After the surface mount and component assembly of the electronics factory is completed, the PCB must be vacuumed on the test machine to form a negative pressure to complete;
(4) Preventing the surface solder paste from flowing into the hole to cause solder joints and affecting the mounting;
(5) Preventing the soldering of the solder balls from over-wave soldering, causing a short circuit;
Implementation of conductive hole plug hole process
For surface mount boards, especially BGA and IC mounts, they must be flat on the via hole, and the bumps are plus or minus 1 mil. There must be no red tin on the edges of the vias. Due to the variety of the via hole plugging process, 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 test, and oil explosion after solidification. 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, advantages and disadvantages:
Note: The working principle of hot air leveling is to remove the excess solder on the surface of the printed circuit board and the holes by using hot air. The remaining solder is evenly covered on the pads and the unresisted solder lines and surface package points, which is the way of processing the surface of the printed circuit board. one.
I. Hot air leveling and plugging process
The process flow is: plate surface soldering→HAL→plug hole→curing. The non-plugging process is used for production. After the hot air is leveled, the aluminum mesh screen or the ink screen is used to complete all the via holes required by the customer. The plug ink can be made of photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plug ink preferably uses the same ink as the surface of the board. This process can ensure that the through hole does not drop oil after the hot air is leveled, but it is easy to cause the plug hole ink to pollute the surface and uneven. Customers are prone to soldering during placement (especially within the BGA). So many customers do not accept this method.
Second, the hot air leveling front plug hole process
2.1 Using aluminum sheet to plug holes, solidify, and transfer the pattern after pattern transfer.
Using a CNC drilling machine, the aluminum piece with the plug hole is drilled to make a screen plate, and the plug hole is made to ensure that the hole of the through hole is full, the ink of the plug hole is filled with ink, and the thermosetting ink can also be used, but the characteristics must be high hardness. The resin shrinks little and has good adhesion to the pore walls. The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → plate surface solder mask.
This method can ensure that the through hole of the through hole is flat, the hot air leveling will not have quality problems such as oil explosion and oil drop on the hole, but this process requires thickening copper once, so that the copper thickness of the hole wall reaches the customer’s standard. Therefore, the whole plate copper plating is very demanding, 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 polluted. Many PCB factories do not have a one-time thick copper process, and the performance of the equipment does not meet the requirements, resulting in the use of this process in the PCB factory is not much.
2.2 Direct screen printing surface soldering after plugging with aluminum
This process uses a CNC drilling machine to drill the aluminum piece with the plug hole, and make a screen plate. It is installed on the screen printing machine to carry the plug hole. After the plug hole is completed, it should not be parked for more than 30 minutes. The screen is directly screen printed with 36T screen. The process is: pre-treatment – plug hole – silk screen – pre-bake – exposure – development – curing
This process can ensure that the through-hole cover oil is good, the plug hole is flat, and the wet film color is uniform. After the hot air is leveled, the through hole is not tinned, and the tin beads are not hidden in the hole, but it is easy to cause the ink on the pad in the hole after curing, resulting in poor solderability, and the edge of the through hole is bubbled off after the hot air is leveled. . It is difficult to use this process for production control. Process engineers need special procedures and parameters to ensure plug quality.
2.3 aluminum plate plug hole, development, pre-curing, after plate grinding
Using a CNC drilling machine, the aluminum piece required to be plugged is drilled to make a screen, which is installed on the shift screen printing machine to make the plug hole, the plug hole must be full, the two sides protrude better, and then the solidified plate is processed for the surface treatment. The process is: pre-treatment – plug hole – pre-bake – development – pre-cure – plate surface solder resist
Since the process uses plug hole curing to ensure that the HAL does not drop oil and oil after the HAL, after the HAL, the tin in the via hole and the tin on the via hole are difficult to completely solve, so many customers do not receive it.
2.4 plate surface soldering and plugging at the same time
This method uses a 36T (43T) screen, installed on the screen printing machine, using a pad or a nail bed, and plugs all the through holes while completing the board surface. The process flow is: pre-processing – silk screen – -Prebaking–exposure–development–curing
The process time is short, the utilization rate of the equipment is high, the hole can be ensured that the hot air is not drained, and the via hole is not tinned. However, since the screen hole is used for plugging, a large amount of air is stored in the via hole, and when solidified, The air expands and breaks through the solder mask, causing voids and unevenness. The hot air leveling will have a small amount of conductive vias.
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