PCB circuit board through hole plugging process
The information comes from:the Internet posted on:2023-04-19
The copper foil lines between conductive patterns in different layers of a PCB circuit board are connected or connected through these holes, but cannot be inserted into copper plated holes of component leads or other reinforcing materials. Printed circuit boards (PCBs) are formed by stacking and accumulating many layers of copper foil. The reason why copper foil layers cannot communicate with each other is because each layer of copper foil is covered with an insulation layer, so they need to rely on through holes (via) for signal connection between them, hence the name of through holes.
Drilling is very important in the production process of printed circuit boards (PCBs). Drilling is simply understood as drilling the required through holes on the copper clad plate, which has the function of providing electrical connections and fixing devices. If the operation is not correct and causes problems in the through-hole process, the device cannot be fixed on the circuit board, which can affect the use of the circuit board at a light level and cause the entire board to be scrapped at a heavy level. Therefore, the drilling process is quite important.
Blind hole refers to the use of electroplated holes to connect the outermost circuit and adjacent inner layers in a printed circuit board (PCB). Due to the inability to see the opposite side, it is called blind hole. In order to increase the utilization of space between board circuit layers, blind holes come in handy. A blind hole is a conductive hole on the surface of a printed circuit board. Blind holes are located on the top and bottom surfaces of the circuit board, with a certain depth, used for connecting the surface circuit with the inner circuit below. The depth of the holes generally has a specified ratio (aperture). This production method requires special attention, and the drilling depth must be appropriate. Failure to pay attention can cause difficulties in electroplating inside the hole. Therefore, few factories adopt this production method. In fact, it is possible to drill holes in individual circuit layers that need to be connected beforehand, and then glue them together, but more precise positioning and alignment devices are required.
Buried hole refers to the connection between any circuit layer inside a printed circuit board (PCB), but without conduction to the outer layer, which means there is no conduction hole extending to the surface of the circuit board. This production process cannot be achieved by bonding the circuit board and then drilling holes. It is necessary to drill holes at individual circuit layers, first partially bonding the inner layer, then electroplating treatment, and finally fully bonding. Due to the more laborious operation process compared to the original through and blind holes, the price is also the most expensive. This production process is usually only used for high-density circuit boards, increasing the space utilization of other circuit layers.
Conductive holes, also known as through holes, play a role in connecting and conducting circuits. The development of the electronics industry also promotes the development of PCBs, and puts forward higher requirements for printed circuit board manufacturing processes and surface mount technology. The Via hole plug hole process has emerged. In order to meet customer requirements, the circuit board through holes must be plugged. After a lot of practice, the traditional aluminum sheet plug hole process has been changed to use white mesh to complete the circuit board surface resistance welding and plug hole. Stable production and reliable quality. At the same time, the following requirements should be met:
1. If there is copper in the through-hole, it is sufficient, but solder resistance can be plugged or not;
2. There must be tin and lead inside the conducting hole, with a certain thickness requirement (4 microns), and there must be no solder blocking ink entering the hole, causing tin beads to be hidden inside the hole;
3. The conducting hole must have solder blocking ink plug holes, which are not transparent, and must not have tin rings, solder beads, or flatness requirements.
With the development of electronic products in the direction of "light, thin, short, and small", PCBs are also developing towards high density and difficulty, resulting in a large number of SMT and BGA PCBs. Customers require plug holes when mounting components, which mainly serve five purposes:
1. Prevent short circuits caused by tin penetrating the component surface through the through hole during wave soldering of PCB boards; Especially when we place the via hole on the BGA pad, we must first make a plug hole and then gild it to facilitate BGA welding.
2. Avoid flux remaining in the through hole;
3. After the surface mounting and component assembly of the electronic factory are completed, the PCB needs to be vacuumed on the testing machine to form a negative pressure before it can be completed:
4. Prevent surface solder paste from flowing into the hole, causing false soldering and affecting installation;
5. Prevent solder beads from popping out during peak soldering, causing short circuits.
Implementation of conductive hole plugging process
For surface mount boards, especially BGA and IC mounting, the plug holes for the through holes must be flat, with a convex and concave positive and negative 1mil, and there must be no red or tin on the edges of the through holes; The guide hole contains tin beads. In order to meet customer requirements, the plug hole process of the guide hole can be described as diverse, with a particularly long process flow and difficult process control. Oil often falls off during hot air leveling and green oil solder resistance experiments; Problems such as oil explosion occur after solidification. Based on the actual production conditions, various PCB plug hole processes are summarized, and some comparisons and explanations are made in terms of processes, 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 printed circuit boards, and the remaining solder is evenly covered on the solder pads, unobstructed solder lines, and surface packaging points. It is one of the surface treatment methods for printed circuit boards.
Plug hole process after hot air leveling
This process flow is: board surface resistance welding → HAL → plug hole → curing. Production is carried out using a non plug hole process, and after hot air leveling, aluminum mesh or ink blocking mesh is used to complete the plug holes of all fortresses required by customers. Plug hole ink can be photosensitive ink or thermosetting ink, and it is best to use the same ink as the board surface to ensure consistent color of the wet film. This process flow can ensure that the guide hole does not lose oil after hot air leveling, but it is easy to cause ink pollution and unevenness on the board surface. Customers are prone to false soldering during installation (especially within BGA). So many customers do not accept this method.
Hot air leveling front plug hole process
The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → plate surface resistance welding. After using aluminum sheets to plug holes, solidify, and grind the plate, the graphics transfer process is carried out using a CNC drilling machine. The aluminum sheets that need to be plugged are drilled into a mesh plate, and the holes are plugged to ensure that the conductive hole plug hole is full. The plug hole ink can also be used as a thermosetting ink, and its characteristics must be high hardness, small resin shrinkage changes, and good adhesion with the hole wall. This method can ensure that the plug hole of the guide hole is flat, and there will be no quality issues such as oil explosion or oil drop at the hole edge during hot air leveling. However, this process requires one-time thickening of copper to ensure that the copper thickness of the hole wall meets the customer's standard. Therefore, there are high requirements for copper plating on the entire board and the performance of the grinding machine, ensuring that the resin on the copper surface is completely removed, 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 the limited use of this process in PCB factories.
After plugging the holes with aluminum sheets, directly screen the surface of the board for resistance welding
The process flow is: pre-treatment → hole plugging → screen printing → pre drying → exposure → development → curing. This process involves using a CNC drilling machine to drill aluminum sheets that need to be plugged, making a mesh plate, which is installed on a screen printing machine for plugging. After the plugging is completed, the plate should not be parked for more than 30 minutes. The 36T silk screen is used to directly screen the surface of the plate for welding resistance. This process can ensure that the conductive hole cover is well oiled, the plug hole is flat, and the wet film color is consistent. After hot air leveling, it can ensure that the conductive hole does not have tin and there are no tin beads inside the hole, but it is easy to cause the ink in the hole to be soldered onto the pad after solidification, Causing poor weldability; After hot air leveling, the edge of the guide hole bubbles and oil falls off. It is difficult to control production using this process method, and special processes and parameters must be adopted by process engineering personnel to ensure the quality of the plug hole.
Plug holes in aluminum sheets, develop, pre cure, and perform plate surface resistance welding after grinding
The process flow is: pre-treatment → hole plugging → pre drying → development → pre curing → board surface resistance welding. Using a CNC drilling machine, drill out aluminum sheets that require plug holes, make a screen, and install them on a displacement screen printing machine for plug holes. The plug holes must be full, with both sides protruding, and then undergo curing and grinding the board for surface treatment. Due to the use of plug hole curing in this process, it can ensure that the hole does not fall off or explode oil after HAL, but after HAL, it is difficult to completely solve the problem of tin beads in the hole and tin on the through hole, so many customers do not accept it.
Simultaneous completion of board surface resistance welding and plug hole
The process flow is: pre-treatment - screen printing - pre drying - exposure - development - curing. This method uses a 36T (43T) silk screen installed on a screen printing machine, using a pad or nail bed. While completing the surface of the board, all the through holes are plugged. This process has a short time and high equipment utilization rate, which can ensure that the through holes do not fall off oil or tin after hot air leveling. However, due to the use of silk screen for plugging, there is a large amount of air in the through holes. During solidification, the air expands and breaks through the solder mask, causing voids, Uneven, hot air leveling may result in a small amount of tin hidden in the through holes. At present, a large number of experiments have been conducted to select different types of ink and viscosity, adjust the pressure of screen printing, and basically solve the problem of through holes and unevenness. This process has been adopted for mass production. (iPcb ®) We are a professional manufacturer of high-precision PCB circuit boards, capable of mass production of 4-46 layers of PCB boards, circuit boards, circuit boards, high-frequency boards, high-speed boards, HDI boards, PCB circuit boards, high-frequency high-speed boards, double-sided, multi-layer circuit boards, HDI circuit boards, mixed voltage circuit boards, high-frequency circuit boards, soft and hard combination boards, etc
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