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 plugging process is changed, and the circuit board surface solder mask and plugging are completed with white mesh. hole. Stable production and reliable quality.
Via hole plays the role of interconnection and conduction of lines. The development of the electronic 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:
(1) There is copper in the via 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”, 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:
(1) Prevent the short circuit caused by the tin passing through the component surface from the via hole when the PCB is wave soldered; especially when we put the via hole on the BGA pad, we must first make the plug hole and then gold-plated to facilitate the BGA soldering.
(2) Avoid flux residue in the via holes;
(3) 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 to complete:
(4) Prevent surface solder paste from flowing into the hole, causing false soldering and affecting placement;
(5) Prevent the tin balls from popping up during wave soldering, causing short circuits.
Realization of conductive hole plugging process
For surface mount boards, especially the mounting of BGA and IC, the via hole plug 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 customers According to the requirements, the via hole plugging process can be described as diverse, the process is particularly long, the process is difficult to control, and the oil is often dropped during the hot air leveling and the green oil solder resistance test; problems such as oil explosion after curing. 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. 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. 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 plugging ink can be photosensitive ink or thermosetting ink. Under the condition that the color of the wet film is consistent, the plugging 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 plug hole 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 polish the board for graphic transfer
This 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 hole to ensure that the via hole is full. The plug hole ink can also be used with thermosetting ink, and its characteristics must be strong. , 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 → 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 plate 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 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.
1. 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 plugging ink can be photosensitive ink or thermosetting ink. Under the condition that the color of the wet film is consistent, the plugging 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 plug hole 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 polish the board for graphic transfer
This 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 hole to ensure that the via hole is full. The plug hole ink can also be used with thermosetting ink, and its characteristics must be strong., 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 → 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 plate 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 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 board surface solder mask
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 to plug the hole, and park it for no more than 30 minutes after completing the plugging, and use 36T screen to directly screen the surface of the board. The process flow is: pretreatment-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 wet film color is consistent. After the hot air is flattened, 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 pads cause poor solderability; after the hot air is leveled, the edges of the vias bubbling and oil is removed. It is difficult to control the production with this process method, and the process engineers must use special processes and parameters to ensure the quality of the plug holes.
2.2 After plugging the hole with aluminum sheet, directly screen-print the board surface solder mask
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 to plug the hole, and park it for no more than 30 minutes after completing the plugging, and use 36T screen to directly screen the surface of the board. The process flow is: pretreatment-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 wet film color is consistent. After the hot air is flattened, 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 pads cause poor solder ability; after the hot air is leveled, the edges of the vias bubbling and oil is removed. It is difficult to control the production with this process method, and the process engineers must use special processes and parameters to ensure the quality of the plug holes.