Conductive hole Viahole is also known as conduction hole. In order to meet customer requirements, PCB circuit board via hole must be plugged. hole. The production is stable and the quality is reliable.
Viahole via holes play the role of interconnection and conduction of lines. The development of the electronics industry also promotes the development of circuit boards, and also puts forward higher requirements for printed circuit board manufacturing technology and surface mount technology. The Viahole plugging process came into being, and the following requirements should be met at the same time:
(1) It is enough that there is copper in the via hole, and the solder resistance 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, 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", PCB boards are also developing towards high density and high difficulty. Therefore, there are a large number of SMT and BGA PCB circuit boards, and customers require plug holes when mounting components. There are five main functions:
(1) Avoid flux residues in the via holes;
(2) Prevent tin beads from popping out during wave soldering, causing short circuits;
(3) Prevent the solder on the surface from flowing into the hole to cause false soldering and affect the placement;
(4) After the surface mounting and component assembly of the electronics factory are completed, the PCB must be vacuumed to form a negative pressure on the testing machine;
(5) To prevent short circuit caused by tin penetrating through the component surface through the via hole when the PCB circuit board passes through the 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 the soldering of the BGA .
Realization of Conductive Hole Plug Technology
For surface mount boards, especially for BGA and IC mounting, the via hole plug hole must be flat, with a convex and concave 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 According to 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. According to the actual conditions of production, we will summarize the various plugging processes of PCB, and make some comparisons and elaborations in 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 PCB and in the holes, and the remaining solder evenly covers the pads, non-resistance solder lines and surface packaging points, which is one of the surface treatment methods of printed circuit boards.
一、Plug hole process after hot air leveling
The process flow is: solder mask on the board surface → HAL → plug hole → curing. The non-plugging process is used for production hot air leveling, and then the aluminum sheet screen or the ink blocking screen is used to complete the through hole plugging of all the fortresses required by the customer. The plugging ink can be photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, it is better to use the same ink as the board surface for the plugging ink. This process can ensure that the via hole does not drop oil after the hot air is leveled; but it is easy to cause the plug hole ink to contaminate the board surface and make it uneven. It is easy for customers to cause false soldering (especially in BGA) during placement. So many customers do not accept this method.
二、Front plug hole process of hot air leveling
①. Use aluminum sheets to plug holes, solidify, and grind the board to transfer graphics
This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen, and make a plug to ensure that the via hole is full. Plugging ink can also be thermosetting ink, which must have high hardness. , The resin shrinkage changes little, and the binding force with the hole wall is good. The process flow is: pre-treatment → 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, but this process requires one-time thickening of copper to make the copper thickness of the hole wall meet the customer's standard. Copper plating on the whole board has high requirements, and also has high requirements on the performance of the grinding machine, to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and free from pollution. Many circuit board factories do not have a one-time thickening copper process, and the performance of the equipment does not meet the requirements, resulting in that this process is not used much in circuit board 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 the plugging is completed. Use a 36T screen to directly screen the solder on the board surface. The process flow is: pre-treatment - plugging - silk screen - pre-baking - exposure - development - curing
This process can ensure that the oil on the via hole cover is good, the plug hole is smooth, and the wet film color is consistent. After hot air leveling, it can ensure that the via hole is not tinned, and there is no tin ball hidden in the hole, but it is easy to cause soldering on the ink in the hole after curing. After hot air leveling, the edge of the via hole foams and loses oil. It is difficult to control the production with this process. Process engineers must use special processes and parameters to ensure the quality of the plug hole.
③. After plugging the holes of the aluminum sheet, developing, pre-curing, and grinding the board, the board surface is soldered.
Use a CNC drilling machine to drill out the aluminum sheet that requires plug holes, make a screen, install it on a shift screen printing machine for plug holes, the plug holes must be full, and it is better to protrude on both sides. After curing, the plate is ground for surface treatment and its process The process is: pre-treatment - plug hole - pre-baking - development - pre-curing - board surface solder mask.
Since this process adopts plug hole curing, it can ensure that the through hole after HAL will not drop oil or explode; but after HAL, it is difficult to completely solve the problem of tin beads hidden in the via hole and tin on the via hole, so many customers do not accept it.
④. PCB circuit board surface solder mask and plug holes 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 bed of nails, and plugging all the via holes while completing the board surface. The process flow is: pretreatment-screen printing-pre-baking- Exposure-development-curing.
The process time is short and the utilization rate of the equipment is high. It can ensure that the via hole does not drop oil and the via hole is not filled with tin after the hot air is leveled. Expansion breaks through the solder mask, resulting in voids, uneven hot air leveling, and a small amount of tin hidden in via holes. At present, after a lot of experiments, our company has select 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.
