Realization of conductive hole plugging process

For surface mount boards, especially BGA and IC mounting on the through-hole plug hole requirements must be flat, convex and concave plus or minus 1 mil, there shall be no through-hole edge of the red on the tin; through-hole hidden tin beads, in order to meet customer requirements, through-hole plug hole process can be described as a variety of processes, the process is particularly long, the process of controlling the difficult, and from time to time, in the hot air leveling and green oil resistance to soldering experiments when the oil; curing burst oil and other problems occur. The process is very long and difficult to control. Now according to the actual conditions of production, PCB various plug hole process is summarized in the process and the advantages and disadvantages of some comparison and elaboration:

Note: The working principle of hot air leveling is the use of hot air to PCB surface and hole excess solder removed, the remaining solder uniformly overlay in the pads and non-resistive solder lines and surface encapsulation points, is one of the ways of surface treatment of printed circuit boards.

一、 hot air leveling after plug hole process.

This process is: plate surface welding → HAL → plug holes → curing. The use of non-hole-plugging process for production, hot air leveling with an aluminum screen or ink-blocking network to complete the customer's requirements to plug all the holes to plug the through-hole plugging. Plugging ink can be used photographic ink or thermosetting ink, in order to ensure that the color of the wet film is consistent, plugging ink is best to use the same ink with the board surface. This process can ensure that no oil is removed from the guide hole after hot air leveling, but it is easy to cause the plug hole ink to pollute the board surface and uneven. It is easy to cause soldering (especially in BGA) when the customer is mounting. So many customers do not accept this method.

二、 hot air leveling before the plug hole process.

1.Aluminum plug holes, curing, grinding board after the transfer of graphics

This process with a CNC drilling machine, drilling the aluminum sheet to be plugged holes, made of mesh, plug holes to ensure that the guide hole plug holes full, plug holes ink plug holes ink, can also be used thermosetting ink, which must be characterized by a large hardness, resin shrinkage changes are small, and the wall of the holes with a good combination of force. Process flow is: pretreatment → plug holes → grinding plate → graphic transfer → etching → plate surface welding. With this method can ensure that the through-hole plug holes flat, hot air leveling will not have burst oil, hole edge oil and other quality issues, but this process requires a one-time thickening of copper, so that the hole wall thickness of copper to meet the customer's standards, so the whole board copper plating requirements are very high, and the performance of the grinding machine also has high requirements to ensure that the copper surface of the resin and other thoroughly removed, the copper surface is clean, and not be contaminated. Many PCB factories do not have a one-time copper thickening process, as well as the performance of the equipment does not meet the requirements, resulting in the use of this process in the PCB factory is not much.

2.Aluminum plug holes directly after screen printing board resistance welding.

This process with CNC drilling machine, drilling holes to be plugged into the aluminum sheet, made of screen, installed in the screen printing machine for plugging holes, complete plugging holes after the parking shall not exceed 30 minutes, with 36T screen direct screen printing board soldermasking, the process is: pretreatment - plug holes - - screen printing - pre-drying - screen printing - screen printing - pre-drying - pre-drying - screen printing -Screen printing - pre-drying - exposure a development - curing. With this process to ensure that the through-hole cover oil, plug holes flat, wet film color is consistent, hot air leveling to ensure that the through-hole not on the tin, the hole does not hide the tin beads, but easy to cause the curing of the hole ink on the pads, resulting in poor weldability; hot air leveling of the through-hole edge of the blistering off the oil, the use of the production control of this process is difficult to use this technology, must be process engineers using a special process and parameters in order to ensure that the quality of the plug holes.

3.Aluminum plate hole plugging, developing, pre-curing, grinding plate after the plate resistance welding.

With CNC drilling machine, drilling requirements plug holes in the aluminum, made of screen, installed in the shift screen printing machine for plug holes, plug holes must be full, both sides of the protruding for the best, and then after curing, grinding plate for the plate surface treatment, the process is: pre-treatment - plug holes in a pre-drying - - developing - pre-curing - - plate surface resistance welding. The process is as follows: pre-treatment - hole plugging a pre-drying - developing - pre-curing - board surface welding resistance. Due to this process using plug hole curing to ensure that HAL after the hole does not fall off the oil, oil explosion, but after HAL, the hole hidden tin beads and guide holes on the tin is difficult to completely solve, so many customers do not receive.

4.board solder resist and hole plugging at the same time.

This method uses 36T (43T) screen, installed in the screen printing machine, using pads or bed of nails, in the completion of the board at the same time, all the guide holes plugged, the process is: pre-treatment - screen printing - pre-drying - exposure - developing - curing. This process is short, the utilization rate of equipment is high, can ensure that the hot air leveling after the hole does not fall off the oil, the guide hole does not tin, but because of the use of silk screen for plugging holes, in the hole memory with a large amount of air in the curing, the air expands, breaking through the soldermask, resulting in hollow, uneven, hot air leveling will have a small number of guide hole hidden tin. At present, our company after a lot of experiments, choose different types of ink and viscosity, adjust the pressure of screen printing, basically solved the hole hole and uneven, has been using this process of mass production.