Why are PCB vias blocked?

Conductive holes are also called conductive holes. In order to meet the requirements of customers, the through hole of the circuit board must be a jack. After a lot of practice, the traditional aluminum plate jack technology has been changed, and the resistance welding and jack on the surface of the circuit board have been completed by using white net. Stable production and reliable quality.

Through holes are used to connect wires to each other. The development of electronic industry also promotes the development of PCB. Higher requirements are put forward for the manufacturing process and surface mounting technology of printed circuit boards. Through-hole plugging technology came into being, which should meet the following requirements-

(1) There is copper in the through hole to avoid blockage.

(2) There must be tin and lead in the conductive hole, and there must be a certain thickness requirement (4 microns). No solder resist ink can enter the hole, resulting in solder beads in the hole; (2) There must be tin and lead in the conductive hole, and there must be a certain thickness requirement (4 microns);

(3) The through hole must be blocked by solder resist ink, opaque, Wuxi ring, tin beads and flatness.

With the development of electronic products in the direction of light, thin, short and small, PCB also develops in the direction of high density and high difficulty. Therefore, there are a large number of PCB, SMT and BGA, and customers need jacks when installing components. The jacks mainly have five functions:

(1) Prevent the solder from short-circuiting through the surface of the component when the PCB is over-soldered; Especially when we place through holes on BGA pads, we must first make sockets, and then carry out BGA welding with gold plating.

(2) avoid residual flux in the through hole;

(3) After the surface installation and component assembly of the electronic factory are completed, the PCB board must be vacuumed to form negative pressure on the tester, and then it can be completed:

(4) Prevent the solder paste on the surface from flowing into the hole to cause virtual welding, thus affecting the installation;

(5) To prevent solder balls from bursting during wave soldering, resulting in short circuit.

Realization of plug hole technology of conductive hole

For surface mount board, especially BGA and IC installation, the plug hole must be flat, plus or minus1mills, and there should be no red tin on the edge of the through hole; In order to meet the requirements of the customer's hole plugging process, the tin beads in the through hole can be described as various and extremely long, which is difficult to control. The welding resistance of oil drops and green oil is often tested during hot air leveling. The problem of oil explosion occurs after curing. According to the actual production situation, various sealing processes of PCB are summarized, and their advantages and disadvantages are compared:

Note: the principle of hot air leveling is to use hot air to remove excess solder from the surface and holes of printed circuit boards. The remaining solder is evenly coated on the bonding pad, the non-resistance bonding wire and the surface package point. This is a method of surface treatment of printed circuit board.

1。 Hot air leveling post-plug hole technology

Its technological process is: resistance welding on the surface of steel plate, HAL plug hole curing. Production adopts plug-free technology, hot air leveling is completed by aluminum mesh or ink mesh, which meets all requirements of customers, and conductive holes are plugged. Jack ink can be photosensitive ink or thermosetting ink. In order to ensure the consistency of wet film color, it is best to use the same ink as the layout. This process can ensure that the conductive hole does not lose oil after hot air straightening, but it is easy to cause ink insertion to pollute the surface of the plate and make it uneven. It is easy for customers to cause virtual soldering during installation (especially in BGA). Many customers don't accept this way.

2. Plug hole technology before hot air leveling

2. 1 plug the hole with an aluminum plate, cure and grind the plate for pattern transfer.

This process uses numerical control drilling machine to drill pinhole aluminum plate to make sieve plate and plug hole to ensure full plug hole. Plug ink can also be used to plug holes. It is characterized by high hardness, small shrinkage change of resin and good adhesion to the hole wall. The process flow is as follows: pretreatment plug hole grinding plate pattern transfer etching plate surface resistance welding.

This method can ensure the smooth plugging of conductive holes, and there will be no quality problems such as oil explosion and oil droplets at the hole edge during hot air leveling, but this process needs to thicken copper to make the hole wall copper thickness meet the user's requirements. So the whole copper-plated plate is very high. But also has high requirements on the performance of the grinder. In order to ensure that the resin on the copper surface is completely removed, the copper surface is clean and pollution-free. Many PCB factories do not have a copper concentration process, and the performance of the equipment can not meet the requirements, so this process has not been widely used in PCB factories.

2.2 Direct resistance welding of screen printing board after aluminum plug hole.

In this process, a plug hole is drilled on an aluminum plate by a numerical control drilling machine to make a sieve plate, and the plug hole is installed on a screen printing machine. After the plug hole is completed, the plug hole shall be placed for no more than 30 minutes, and the surface of the sieve plate shall be directly welded with a 36T screen. The process flow is as follows: pretreatment-hole plugging-screen printing-pre-drying-exposure-curing.

This process can ensure that the through hole cover oil is good, the plug hole is smooth and the wet film color is consistent. Hot air leveling can ensure that the conductive hole is not tinned, and tin beads will not be hidden in the hole, but it is easy for ink to enter the hole after curing. The bonding pad leads to poor solderability; After hot air leveling, the edge of the through hole bubbles and oil is lost. It is difficult to use this process for production control, and process engineers must use special procedures and parameters to ensure the quality of embolization.

2.3 After the surface of aluminum plate is polished, plug, unfold, pre-cure and weld.

Using numerical control drilling machine, the aluminum sheet that needs plug is drilled to make mesh plate, and the plug hole is installed on the line changing printing machine. The plug hole should be full, with both sides protruding, then solidified, and the grinding plate should be processed. The technological process is as follows: pretreatment-hole plugging-.

Because the process adopts plug hole curing, it is difficult to completely solve the problem of tin beads and tin on the through hole after HAL, so many customers do not accept it.

2.4 Resistance welding and hole plugging on the surface of steel plate are completed at the same time.

This method uses a 36T(43T) screen installed on a screen printing machine, uses a pad or a nail bed, and inserts all the through holes when the board is finished. The technological process is: pretreatment-screening-pre-baking-exposure-development-curing.

Short process time and high equipment utilization rate. It can ensure that the through hole does not lose oil, and the through hole does not stick to tin after hot air leveling. However, because screen printing uses the method of plugging holes, there is a lot of air in the through holes. During the curing process, air expands and penetrates the solder mask, resulting in pores, irregularities and a small number of through holes, thus storing tin in hot air. At present, after a lot of experiments, our company has selected different types of inks and viscosities, adjusted the pressure of screen printing, basically solved the problems of holes and unevenness, and has adopted this process for mass production.