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PCB circuit board via hole plug hole process

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Published by: Keyou June 20,2019

For surface mount boards, especially for BGA and IC mounting, the via hole must be flat, positive and negative 1 mil, no red tin on the edge of the via hole; tin beads in the via hole, in order to reach the customer The requirements of the via hole plug hole process can be described as various, the process flow is particularly long, the process control is difficult, and often there is oil drop during the hot air leveling and green oil solder resistance test; the problem of oil explosion after curing occurs. According to the actual conditions of production, the process of plugging holes in PCB circuit board is 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 the printed circuit board and the holes by using hot air. The remaining solder is evenly covered on the pads and the unresisted solder lines and surface package points, which is the way of processing the surface of the printed circuit board. one.

                

       PCB circuit board via hole plug hole process

 

    I. Hot air leveling and plugging process

    The process flow is: plate surface soldering→HAL→plug hole→curing. The non-plugging process is used for production. After the hot air is leveled, the aluminum plate or the ink-blocking net is used to complete the via hole of all the plugs required by the customer. The plug ink can be made of photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plug ink preferably uses the same ink as the surface of the board. This process can ensure that the through hole does not drop oil after the hot air is leveled, but it is easy to cause the plug hole ink to pollute the surface and uneven. Customers are prone to soldering during placement (especially within the BGA). So many customers do not accept this method.

    Second, the hot air leveling front plug hole process

 

    2.1 Using aluminum sheets to plug holes, solidify, and grind the board for pattern transfer

    This process uses a CNC drilling machine to drill the aluminum sheet of the plug hole to make a screen plate, to make the plug hole, to ensure the plug hole is full, the plug ink plug hole ink, and the thermosetting ink can also be used, and the characteristics must be hard. The resin shrinks little and has good adhesion to the pore walls. The process flow is: pre-treatment → plug hole → grinding plate → pattern transfer → etching → surface solder mask

    This method can ensure that the through hole of the through hole is flat, the hot air leveling will not have quality problems such as oil explosion and oil drop on the hole, but this process requires thickening copper once, so that the copper thickness of the hole wall reaches the customer's standard. Therefore, the whole plate copper plating is very demanding, and the performance of the plate grinding machine is also very high, ensuring that the resin on the copper surface is completely removed, the copper surface is clean and not polluted. Many PCB factories do not have a one-time thick copper process, and the performance of the equipment does not meet the requirements, resulting in the use of this process in the PCB factory is not much.

 

   2.2 Direct screen printing surface soldering after plugging with aluminum

    This process uses a CNC drilling machine to drill the aluminum sheet of the plug hole to make a screen. It is installed on the screen printing machine to carry the plug hole. After the plug hole is completed, it should not be parked for more than 30 minutes. The screen is directly screen printed with 36T screen. The process is: pre-treatment - plug hole - silk screen - pre-bake - exposure - development - curing

    This process can ensure that the through-hole cover oil is good, the plug hole is flat, the wet film color is consistent, the hot air leveling can ensure that the conductive hole is not tinned, the tin beads are not hidden in the hole, but it is easy to cause the ink in the hole after curing. The pad causes poor solderability; after the hot air is leveled, the edge of the via hole is foamed and oil is lost. It is difficult to use this process for production control, and the process engineer must use special procedures and parameters to ensure plug quality.

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