Via hole is also called through hole. In order to meet the requirements of customers, the through hole of circuit board must be plugged. After a lot of practice, the traditional aluminum plug hole process is changed, and the resistance welding and plug hole of circuit board surface are completed with white mesh. Stable production and reliable quality.
Why are vias blocked on PCB?
Via hole plays a role in connecting and conducting circuits. The development of electronic industry also promotes the development of PCB, and puts forward higher requirements for PCB manufacturing process and surface mount technology. The via hole plug process came into being and should meet the following requirements:
1. If there is copper in the through hole, it can be plugged but not plugged for resistance welding;
2. There must be tin and lead in the through hole, with certain thickness requirements (4 microns). No solder resist ink shall enter the hole, resulting in tin beads in the hole;
3. The through hole must be provided with solder resist ink plug hole, which is opaque, and there shall be no tin ring, tin bead, flatness and other requirements.
With the development of electronic products in the direction of "light, thin, short and small", PCB is also developing to high density and high difficulty. Therefore, there are a large number of SMT and BGA PCBs, and customers require plug holes when installing components, which mainly has five functions:
1. Prevent short circuit caused by tin penetrating through the element surface from the through hole during PCB wave soldering; In particular, when we put the via on the BGA pad, we must first make the plug hole and then gold plating to facilitate the welding of BGA.
2. Avoid residual flux in the through hole;
3. After the surface mounting and component assembly of the electronic factory are completed, the PCB should absorb vacuum on the testing machine to form negative pressure:
4. Prevent the surface solder paste from flowing into the hole, resulting in false welding and affecting the installation;
5. Prevent tin beads from popping out during over wave soldering, resulting in short circuit.
Realization of conductive hole plugging process
For the surface mounting plate, especially the mounting of BGA and IC, the plug hole of the through hole must be flat, convex and concave plus or minus 1mil, and there shall be no red and tin on the edge of the through hole; Tin beads are stored in the through hole. In order to meet the requirements of customers, there are various processes for plug holes in the through hole. The process flow is particularly long and the process control is difficult. Oil often falls out during hot air leveling and green oil solder resistance test; Oil explosion and other problems occur after curing. Now, according to the actual production conditions, 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 remove the excess solder on the surface of printed circuit board and in the hole by using hot air, and the residual solder is evenly covered on the solder pad, unimpeded solder lines and surface packaging points, which is one of the methods of printed circuit board surface treatment.)
1、 Plug hole technology after hot air leveling
The process flow is: plate surface resistance welding → Hal → plug hole → curing. Non plug hole process is adopted for production. After hot air leveling, aluminum plate screen or ink screen is used to complete the through hole plug holes of all fortresses required by customers. The plug hole ink can be photosensitive ink or thermosetting ink. Under the condition of ensuring the consistency of wet film color, the plug hole ink is best to use the same ink as the plate surface. This process can ensure that the oil will not fall out of the through hole after hot air leveling, but it is easy to cause the plug hole ink to pollute the plate surface and uneven. It is easy for customers to cause false soldering during mounting (especially in BGA). Therefore, many customers do not accept this method.
2、 Hot air leveling front plug hole technology
2.1 use aluminum sheet to plug the hole, solidify and grind the plate, and then transfer the graphics
In this process, a numerical control drilling machine is used to drill the aluminum sheet with plug holes, make it into a screen, and plug holes to ensure that the plug holes of the through holes are full. Plug hole ink, plug hole ink and thermosetting ink can also be used. It must be characterized by large hardness, small resin shrinkage change and good adhesion with the hole wall. The process flow is: pretreatment → plug hole → grinding plate → pattern transfer → etching → plate surface resistance welding
This method can ensure that the plug hole of the through hole is flat and the hot air leveling will not have quality problems such as oil explosion and oil drop at the hole edge. However, this process requires one-time thickening of copper to make the copper thickness of the hole wall meet the customer's standard. Therefore, it has high requirements for the copper plating of the whole plate and the performance of the plate grinding machine, so as 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 one-time copper thickening process, and the performance of the equipment can not meet the requirements, resulting in little use of this process in PCB factories.
2.2 use aluminum sheet to plug the hole and then directly screen the plate surface for resistance welding
In this process, a CNC drilling machine is used to drill the aluminum sheet to be plugged into a screen, which is installed on the screen printing machine for plugging. After the plugging is completed, it shall be parked for no more than 30 minutes, and the 36t screen is used to directly screen the plate surface for resistance welding. The process flow is: pretreatment - plugging - 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 flat, and the wet film color is consistent. After hot air leveling, it can ensure that the through hole is free of tin and there are no tin beads in the hole, but it is easy to cause the solder pad on the ink in the hole after curing, resulting in poor solderability; After hot air leveling, the edge of the through hole bubbles and drops oil. It is difficult to control the production by this process method. The process engineers must adopt special processes and parameters to ensure the quality of the plug hole.
2.3 aluminum sheet plug hole, development, pre curing and plate surface resistance welding after grinding.
Use the numerical control drilling machine to drill the aluminum sheet requiring plug holes, make it into a screen, and install it on the shift screen printing machine for plug holes. The plug holes must be full, and the two sides are better to protrude. After curing, grind the plate for plate surface treatment. The process flow is: pretreatment - plug holes - pre drying - Development - pre curing - plate surface resistance welding
Because this process adopts plug hole solidification, it can ensure that there is no oil drop and oil explosion in the through hole after Hal, but it is difficult to completely solve the tin on the through hole tin beads and through holes after Hal, so many customers do not accept it.
2.4 plate surface resistance welding and plug hole shall be completed at the same time.
This method uses 36t (43T) wire mesh, which is installed on the silk screen printing machine, and uses a backing plate or nail bed to plug all the through holes while completing the plate surface. The process flow is: pretreatment - silk screen printing - pre drying - exposure - Development - curing.
This process has the advantages of short time and high utilization rate of equipment, which can ensure that there is no oil loss in the through hole and tin on the through hole after hot air leveling. However, due to the use of silk screen for plug hole, there is a large amount of air in the through hole. During solidification, the air expands and breaks through the solder resist film, resulting in holes and unevenness. There will be a small amount of tin in the through hole after hot air leveling. At present, after a large number of experiments, selecting different types of ink and viscosity, adjusting the pressure of silk screen printing, basically solving the through-hole cavity and unevenness, and this process has been used for mass production.