Via hole is also known as the conduction hole. In order to meet the customer’s requirements, the through hole must be plugged in the PCB process. Through practice, it is found that in the process of plug hole, if the traditional aluminum sheet plug hole technology is changed and the white mesh is used to complete the plate surface soldering and plug hole, the PCB production can be stable and the quality is reliable.
The development of the electronic industry and the promotion of the development of PCB also put forward higher requirements for the manufacturing process and surface mount technology of printed board. The Via hole technology emerged at the right moment and should meet the following requirements:
(1) there is copper in the guide hole, but plug or not plug the soldering resistance;
(2) there must be tin lead in the conduction hole, with a certain thickness (4 microns), and there must be no soldering ink into the hole, causing the hole to contain tin beads;
(3) the through hole must have a soldering ink plug hole, no light transmission, no tin ring, tin beads and other requirements for leveling;
With the development of electronic products to the direction of “light, thin, short and small”, PCB also develops to the high density and high difficulty, so there are a large number of SMT and BGA PCB, and customers require plug holes in the mounting of components, which mainly have five functions:
(I) prevent the soldering of PCB through wave soldering when tin penetrates through the through hole to cause short circuit; In particular, when the through-hole is placed on the BGA welding plate, the plug hole must be made first, and then the gold plating treatment, so as to facilitate the welding of BGA;
(2) avoid residual flux in the through hole;
(iii) after the surface mount and component assembly of the electronic factory are completed, the PCB can only be completed by vacuuming and forming negative pressure on the test machine;
(4) prevent surface solder paste from flowing into the hole to cause virtual welding and affect the mounting;
(5) prevent the solder bead from popping out during wave soldering and causing short circuit;
Realization of conductive hole plug technology
For the surface mount plate, especially the BGA and IC mount, they must be flat on the guide hole plug hole requirements, convex and concave plus or minus 1mil, there must be no lead hole edge red tin phenomenon. Due to various through hole plug technology, the process flow is particularly long, the process control is difficult, often in hot air leveling and green oil resistance to solder experiment oil drop, after curing oil explosion and other problems. According to the actual production conditions, various PCB plug hole processes are summarized, and some comparisons and elaborations 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 and hole of printed circuit board. The remaining solder is evenly covered on the solder plate, the line of unblocked solder and the surface sealing point, which is one of the ways to treat the surface of printed circuit board.
1. Plug hole technology after hot air leveling
The process flow is as follows: plate resistance welding, HAL, plug hole and curing. The non-plug hole process is adopted for production. After hot air leveling, aluminum mesh plate or ink screen is used to complete all through hole plug holes required by customers. Plug hole ink can be photosensitive ink or thermosetting ink, in order to ensure that the wet film color is consistent, plug hole ink had better use the same ink as the plate. This process can ensure that hot air leveling through hole does not drop oil, but easy to cause plug hole ink pollution plate surface and uneven. It is easy for customers to cause virtual welding (especially in BGA) during mounting. So many customers don’t accept this approach.
Two, hot air leveling before plug hole technology
2.1 the process flow of pattern transfer is carried out after aluminum sheet plug hole, solidification and grinding plate is used
With numerical control drilling machine, drill out the aluminum sheet that needs to plug hole, make mesh plate, plug hole, ensure that the guide hole plug hole is full, plug hole ink plug hole ink, in addition, can also be used thermosetting ink, but its characteristics must be large hardness, resin shrinkage change is small, and the hole wall binding force is good. The process flow is: pretreatment, plug hole, grinding plate, graphic transfer, etching, plate surface welding resistance.
By this method can guarantee the smooth conduction of plug hole, hot air leveling there would be no oil, the side of hole blasting off the quality problems such as oil, but the process requirement of disposable thickening copper, make this hole wall copper thickness meet customer standard, so the whole board high demand for copper plating, and also has a high requirement on the performance of the grinding machine, the need to ensure that the copper on the surface of the resin thoroughly remove, such as 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 this process is not used in PCB factories.
2.2 direct screen printing plate surface soldering after aluminum sheet plug hole
This process flow with CNC drilling machine, drilling out of the aluminum plug hole, made of mesh plate, installed on the screen printing machine plug hole, plug hole should not be left for more than 30 minutes, with 36T screen direct screen printing plate surface soldering, process flow is: pre-treatment — plug hole — screen printing — pre-drying — exposure a development — curing
This process can ensure that the oil of the through hole cover is good, the plug hole is flat and the wet film color is consistent. Hot air leveling can ensure that the through hole is not on tin, tin beads are not hidden in the hole, but it is easy to cause curing in the hole on the ink pad, resulting in poor solderability, hot air leveling after the through hole edge bubble oil. It is difficult to control the production with this process method, and special process and parameters need to be adopted by the process engineer to ensure the quality of the plug hole.
2.3 aluminum plate plug hole, development, precuring and surface welding shall be carried out after grinding the plate
With CNC drilling machine, drilling out of the aluminum requirements plug hole, made of mesh plate, installed in the shift screen printing machine for plug hole, plug hole must be full, prominent on both sides is better, and then through curing, grinding plate surface treatment, the process flow is: pre-treatment — plug hole a pre-drying — development — pre-curing — plate surface resistance welding
Many customers do not accept HAL due to the fact that the plug hole curing process can ensure that the oil will not drop or explode after HAL, but the tin beads and the tin on the conduction hole can not be completely solved after HAL.
2.4 the plate surface welding shall be completed simultaneously with the plug hole
This method USES 36T(43T) screen, installed on the screen printing machine, the use of pad or nail bed, in the completion of the board surface at the same time, all the through hole plug, the process flow is: pre-treatment — screen printing — pre-drying — exposure — development — curing
The process time is short, high equipment utilization, can guarantee after hole out oil, hot air leveling guide hole is not on the tin, but because of using screen printing to plug hole, in the hole memory with a large number of air, when curing, air inflation, to break through resistance welding membrane, have holes, uneven, hot air leveling will guide hole is a small amount of tin.