Conductive hole Via hole is also known as conduction 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 is completed with white mesh. hole. The production is stable and the quality is reliable.
Via holes play the role of interconnection and conduction of lines. The development of the electronics industry also promotes the development of PCB, and also puts forward higher requirements for printed board manufacturing technology and surface mount technology. The Via hole plugging process came into being, and the following requirements should be met at the same time:
(1) There is only enough copper in the via hole, and the solder mask can be plugged or not;
(2) There must be tin-lead in the via hole, with a certain thickness requirement (4 microns), and there must be no solder resist ink entering the hole, causing tin beads to be hidden in the hole;
(3) The via holes must have solder resist ink plug holes, which are opaque, and must not have tin rings, tin beads, and flatness.
With the development of electronic products in the direction of “light, thin, short and small”, PCBs are also developing towards high density and high difficulty, so there are a large number of SMT and BGA PCBs, and customers require plug holes when mounting components. Five functions:
(1) To prevent short circuit caused by tin penetrating through the component surface through the via hole when the PCB is over wave soldering; especially when we put the via hole on the BGA pad, we must first make the plug hole and then gold-plate it to facilitate BGA soldering.
(2) Avoid flux residues in the via holes;
(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 before it is completed:
(4) Prevent the solder paste on the surface from flowing into the hole to cause false soldering and affect the placement;
(5) Prevent tin beads from popping out during wave soldering, causing short circuits.
Realization of Conductive Hole Plug Technology
For surface mount boards, especially BGA and IC mounting, the via hole plug hole must be flat, with a bump of plus or minus 1mil, and there must be no red tin on the edge of the via hole; tin beads are hidden in the via hole, in order to achieve customer satisfaction According to the requirements of the requirements, the via hole plug hole technology can be described as varied, the process flow is extremely long, and the process control is difficult. There are often problems such as oil loss during hot air leveling and green oil solder resistance tests; oil explosion after curing.
Now, according to the actual production conditions, we will summarize the various plugging processes of PCB, and make some comparisons and elaborations on 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 of the printed circuit board and in the holes. It is one of the surface treatment methods of printed circuit boards.
- Plug hole process after hot air leveling
The process flow is: board surface solder mask → HAL → plug hole → curing. The non-plug hole process is used for production, and the aluminum sheet screen or ink blocking screen is used to complete the through hole plug holes of all the fortresses required by the customer after hot air leveling. The plugging ink can be photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plugging ink uses the same ink as the board surface. This process can ensure that the via hole does not drop oil after hot air leveling, but it is easy to cause plugging ink to contaminate the board surface and make it uneven. It is easy for customers to cause virtual soldering (especially in BGA) during placement. So many customers do not accept this method.
- Hot air leveling front plug hole process
(1) Use aluminum sheets to plug holes, solidify, and grind the board for pattern transfer
This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged to make a screen, and then plug the hole to ensure that the via hole is full. Plugging ink can also be thermosetting ink, which must have high hardness. , The shrinkage of the resin changes little, and the binding force with the hole wall is good. The process flow is: pretreatment → plug hole → grinding plate → graphic transfer → etching → solder mask on the board surface. This method can ensure that the through-hole plug hole is flat, and hot air leveling will not cause quality problems such as oil explosion and oil drop at the edge of the hole. However, this process requires thicker copper to make the copper thickness of the hole wall meet the customer’s standard, so The requirements for copper plating on the whole board are very high, and the performance of the 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 free from pollution. Many PCB factories do not have permanent copper thickening process, and the performance of the equipment cannot meet the requirements, resulting in that this process is not used much in PCB factories.
(2) After plugging the hole with aluminum sheet, directly screen the solder mask on the surface of the board
This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged to make a screen, install it on the screen printing machine for plugging, and stop it for no more than 30 minutes after completing the plugging. Use a 36T screen to directly screen the solder on the board. The process flow is: pre-treatment – plugging – silk screen printing – pre-baking – exposure – development – curing This process can ensure that the oil on the via hole cover is good, the plug hole is smooth, the color of the wet film is consistent, and after hot air leveling It can ensure that the via hole is not filled with tin, and no tin beads are hidden in the hole, but 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 hole is foamed and oil is removed. This process is adopted The production control of the method is relatively difficult, and process engineers must adopt special processes and parameters to ensure the quality of plug holes.
(3) After plugging holes in aluminum sheet, developing, pre-curing, and grinding the board, solder masking on the board surface
Use a CNC drilling machine to drill out the aluminum sheet that requires the plug hole to make a screen, install it on the shift screen printing machine for the plug hole, the plug hole must be full, and it is better to protrude on both sides, and then after curing, the plate is ground for surface treatment. The process flow is: pre-treatment – plug hole – pre-baking – development – pre-curing – board surface solder mask Since this process uses plug hole curing to ensure that the via hole does not drop oil or explode after HAL, but after HAL, Tin beads hidden in via holes and tin on via holes are difficult to completely solve, so many customers do not accept them.
(4) The solder mask and the plug hole are completed at the same time
This method uses a 36T (43T) screen, installed on the screen printing machine, using a backing plate or a nail bed, and plugging all the via holes while completing the board surface. The process flow is: pre-processing – silk screen – -Pre-baking–exposure–development–curing This process takes a short time and has a high utilization rate of the equipment, which can ensure that the via hole does not drop oil and the via hole is not tinned after the hot air is leveled. However, due to the use of silk screen for plugging , There is a large amount of air in the via hole. When curing, the air expands and breaks through the solder mask, causing voids and unevenness. There will be a small amount of via hole hidden tin in the hot air leveling. At present, after a lot of experiments, our company has selected different types of inks and viscosities, adjusted the pressure of silk screen, etc., basically solved the hole and unevenness of the via, and has adopted this process for mass production.