Via hole also known as a conduction hole. In order to meet customer requirements, in the process of PCB production, the through hole must be plugged. Through practice, it is found that in the process of plugging, if the traditional aluminum sheet plugging process is changed, the use of white mesh to complete the plate face welding and plugging, the PCB production can be stable and the quality is reliable. The development of the electronics industry, while promoting the development of PCB, also put forward higher requirements for printed board production process and surface mount technology, Via hole plug process came into being, and should meet the following requirements:
1. through the hole can be copper, solder can plug or not plug;
2. there must be tin lead in the through hole, there is a certain thickness requirement (4 microns), no solder resistance ink into the hole, resulting in tin beads in the hole;
3. the pass-through hole must have solder resistance ink plug hole, opaque, no tin ring, tin bead and flat requirements;
With the development of electronic products to the direction of "light, thin, short, small", PCB also to high-density, difficult development, so there are a large number of SMT, BGA PCB, and customers require jack holes when mounting components, there are five main roles:
1. to prevent PCB over wave soldering tin through the through hole through the component surface caused by short circuit; Especially when the hole is placed on the BGA pad, the plug hole must be made first, and then gold-plated to facilitate the welding of BGA;
2. avoid flux residue in the through hole;
3. after the surface mounting and component assembly is completed, the PCB should absorb vacuum to form negative pressure on the test machine to complete;
4. prevent the surface solder paste into the hole caused by virtual welding, affecting the installation;
5. to prevent over wave soldering tin bead pop out, resulting in short circuit;
The realization of the plugging technology of conductive hole
For the surface mount board, especially the mount of BGA and IC, they must be flat, concave and concave plus or minus 1mil, and there must be no red tinning on the edge of the pilot hole. Due to the variety of the through hole plug process, the process 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 experiment, and oil explosion after curing. Now, according to the actual conditions of production, various PCB plug processes are summarized, and some comparisons and explanations are made in the process and advantages and disadvantages:
Note: The working principle of hot air smoothing is to use hot air to remove excess solder on the surface of the printed circuit board and in the hole, and the remaining solder is uniformly covered on the solder pad and the open solder line and the surface sealing decoration, which is one of the ways of surface treatment of the printed circuit board.
1. hot air leveling after plugging process
The process flow is: surface solder →HAL→ plug → curing. Non-jack process is used for production. After hot air smoothing, aluminum screen plate or ink screen is used to complete all the through-hole plugs required by the customer. The hole ink can be photosensitive ink or thermosetting ink, in the case of ensuring the same color of the wet film, the hole ink is best used with the same ink as the board. This process can ensure that the hot air leveling through the hole does not drop oil, but easy to cause the plug hole ink pollution and uneven surface. It is easy for customers to cause virtual welding during installation (especially in BGA). Therefore, many customers do not accept this method.
2. hot air leveling before plugging process
2.1. After plugging, curing and grinding the aluminum sheet, the process of graphic transfer is carried out
With a CNC drilling machine, drill out the aluminum sheet that needs to plug holes, make a screen plate, plug holes, ensure that the through hole plug holes are full, plug ink plug ink, and thermosetting ink can also be used, but its characteristics must be high hardness, small change in resin shrinkage, and good bonding force with the hole wall. The process flow is as follows: pretreatment → plug → grinding → pattern transfer → etching → plate face resistance welding.
This method can ensure that the through-hole plug hole is smooth, hot air smoothing will not have oil explosion, hole edge oil and other quality problems, but this process requires a one-time thickening of copper, so that the copper thickness of the hole wall can meet the customer's standard, so the whole plate copper plating requirements are very high, and the performance of the grinding machine also has high requirements, to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and not polluted. 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 use of this process in PCB factories is not much.
2.2. use the aluminum sheet to plug the hole and then directly screen the plate face welding
This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen plate, install it on the screen printing machine to plug the hole, park it for not more than 30 minutes after the completion of the plug hole, and use 36T screen directly to block welding on the screen plate surface. The process flow is: pre-treatment -- plug hole -- screen printing -- pre-drying -- exposure -- development -- curing
This process can ensure that the oil cover of the through-hole is good, the plug hole is smooth, and the color of the wet film is consistent. After hot air leveling, it can ensure that the through hole is not tin, and there is no tin bead in the hole, but it is easy to cause the solder pad on the ink in the hole after curing, resulting in poor weldability. After hot air leveling, the edge of the through hole bubbles off oil. It is difficult to use this process to control the production, and process engineers need to use special flow and parameters to ensure the quality of the plug hole.
2.3. aluminum sheet plug hole, development, pre-curing, grinding plate after the plate face welding
With a CNC drilling machine, drill out the aluminum sheet required to plug the hole, made of screen plate, installed in the shift screen printing machine to plug the hole, the plug hole must be full, both sides prominent is better, and then after curing, grinding plate surface treatment, the process is: pre-treatment - plug hole a pre-drying - development - pre-curing - plate surface welding
Because this process uses plug curing to ensure that no oil is dropped and oil is exploded after HAL, but after HAL, the tin bead in the hole and the tin in the through hole are difficult to be completely solved, so many customers do not accept.
2.4. the plate face welding and the plug hole are completed at the same time
This method uses 36T(43T) screen, installed on the screen printing machine, using a cushion plate or nail bed, while completing the plate surface, all the through holes are plugged, the process is: pre-treatment - screen printing - pre-drying - exposure - development - curing
The process time is short and the utilization rate of the equipment is high, which can ensure that the oil does not drop through the hole after the hot air leveling and the through-hole does not get tin. However, due to the use of screen printing to plug the hole, there is a lot of air in the through-hole. During curing, the air expands and breaks through the solder resistance film, resulting in holes and uneven, and a small amount of tin will be stored in the through-hole after hot air leveling.
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