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 plug hole process has been changed, and the circuit board surface solder mask and plug are completed with white mesh. hole. Stable production and reliable quality.
Via holes play the role of interconnecting and conducting lines. The development of the electronics industry also promotes the development of PCBs, and also puts forward higher requirements for printed board manufacturing technology and surface mount technology. Via hole plugging technology came into being, and the following requirements should be met at the same time:
(1) There is only copper in the via hole, and the solder mask can be plugged or not;
(2) There must be tin and lead in the via hole, with a certain thickness requirement (4 microns), and no solder resist ink should enter the hole, causing tin beads to be hidden in the hole;
(3) The via holes must have solder resist ink plug holes, opaque, and must not have tin circles, tin beads, and leveling requirements.
Why do PCB circuit boards need to block vias?
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. Therefore, a large number of SMT and BGA PCBs have appeared, and customers require plug holes when mounting components, mainly including Five functions:
(1) Prevent tin from penetrating through the component surface through the via hole to cause a short circuit when the PCB passes through the wave soldering; especially when we place the via hole on the BGA pad, we must first make a plug hole, and then gold-plated to facilitate BGA soldering.
(2) Avoid flux residue 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 testing machine to form a negative pressure before it is completed:
(4) Prevent the surface solder paste from flowing into the hole to cause virtual welding, which affects the mounting;
(5) Prevent tin beads from popping out during wave soldering, causing short circuit.
Realization of Conductive Hole Plugging Technology
For surface mount boards, especially for BGA and IC mounting, the via holes must be flat, with a convex and concave plus or minus 1 mil, 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 The requirements of the via hole plugging process can be described as various, the process flow is particularly long, and the process control is difficult. There are often problems such as oil loss during hot air leveling and green oil solder resistance experiments; oil explosion after curing. Now according to the actual production conditions, the various plug hole 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 use hot air to remove the excess solder on the surface of the printed circuit board and in the holes, and the remaining solder is evenly covered on the pads, non-resistance solder lines and surface packaging points, which is the surface treatment method of the printed circuit board. one.
1. Plug hole process after hot air leveling
The process flow is: board surface solder mask → HAL → plug hole → curing. The non-plugging process is used for production. After the hot air is leveled, the aluminum screen or the ink blocking screen is used to complete the via 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, the plugging ink is best to use the same ink as the board surface. 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 be uneven. It is easy for customers to cause virtual soldering (especially in BGA) when mounting. So many clients do not accept this method.
2. Plug hole process before hot air leveling
2.1 Use aluminum sheet to plug holes, cure, and grind the plate for pattern transfer
In this process, a CNC drilling machine is used to drill out the aluminum sheet that needs to be plugged, make a screen plate, and plug the holes to ensure that the via holes are full, and the plugging ink is used to plug the hole. , the resin shrinkage change is small, and the bonding force with the hole wall is good. The process flow is: pretreatment → plug hole → grinding plate → pattern transfer → etching → board surface solder mask
This method can ensure that the via hole plug hole is flat, and the hot air leveling will not have quality problems such as oil explosion and oil loss at the edge of the hole, but this process requires one-time thickening of copper, so that the copper thickness of the hole wall can meet the customer's standard. Therefore, the requirements for copper plating on the whole plate are very high, and there are also high requirements for 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 PCB factories do not have a one-time thickening copper process, and the performance of the equipment does not meet the requirements, resulting in this process is not used much in PCB factories.
2.2 After plugging the holes with aluminum sheets, directly screen the solder mask on the board surface
In this process, a CNC drilling machine is used to drill out the aluminum sheet that needs to be plugged to make a screen plate, which is installed on the screen printing machine for plugging. After the plugging is completed, it should not be parked for more than 30 minutes. The process flow is: pretreatment - plug hole - silk screen - pre-baking - exposure - developing - curing
This process can ensure that the via hole is well covered with oil, the plug hole is flat, and the color of the wet film is the same. Pads, resulting in poor solderability; after hot air leveling, the edge of the via hole bubbles and oil is removed. It is difficult to control the production using this process method, and the process engineer must adopt special processes and parameters to ensure the quality of the plug hole.
Why do PCB circuit boards need to block vias?
2.3 After the aluminum sheet plugs the holes, develops, pre-cures, and grinds 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 plate, and install it on a shift screen printing machine for plug holes. The plug holes must be full, and both sides are preferably protruding. The process flow is: pre-treatment - plug hole - pre-baking - development - pre-curing - board surface solder mask
Because this process adopts plug-hole curing to ensure that the via hole will not lose oil or explode oil after HAL, but after HAL, it is difficult to completely solve the problem of tin bead in the via hole and tin on the via hole, so many customers do not accept it.
2.4 The solder mask on the board surface and the plug hole are completed at the same time.
This method uses 36T (43T) screen mesh, which is 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: pretreatment--screen printing- -Pre-bake--Exposure--Development--Cure
This process has a short time and a high utilization rate of equipment, which can ensure that the via holes will not lose oil and the via holes will not be tinned after hot air leveling. , The air expands and breaks through the solder mask, causing voids and unevenness. There will be a small amount of via holes hidden in tin during hot air leveling. At present, our company has basically solved the hole and unevenness of the via hole after a lot of experiments, choosing different types of ink and viscosity, adjusting the pressure of silk screen printing, etc., and this process has been adopted for mass production.
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