Realization of Conductive Hole Plugging Technology
For surface mount boards, especially for BGA and IC mounting, the via holes must be flat, with a convex and concave plus or minus 1 mil, and there must be no red tin on the edge of the via hole. Tin beads are hidden in the via hole. In order to achieve customer satisfaction, the requirements of the via hole plugging process can be described as various, the process flow is particularly long, and the process control is difficult. There are often problems such as oil loss during hot air leveling and green oil solder resistance experiments, oil explosion after curing. Now according to the actual production conditions, the 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 use hot air to remove the excess solder on the surface of the printed circuit board and in the holes, and the remaining solder evenly covers the pads, non-resistance solder lines and surface packaging points, which is one of the surface treatment method of the printed circuit board.
1. Plug hole process after hot air leveling
The process flow is: board surface solder mask → HAL → plug hole → curing. The non-plugging process is used for production. After the hot air is leveled, the aluminum screen or the ink blocking screen is used to complete the via hole plugging of all the fortresses required by the customer. The plugging ink can be photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plugging ink is best to use the same ink as the board surface. This process can ensure that the via hole does not drop oil after the hot air is leveled, but it is easy to cause the plug hole ink to contaminate the board surface and be uneven. It is easy for customers to cause virtual soldering (especially in BGA) when mounting. So many clients do not accept this method.
2. Plug hole process before hot air leveling
2.1 After plugging holes with aluminum sheets, curing, and grinding the plate, the pattern is transferred.
In this process, a CNC drilling machine is used to drill out the aluminum sheet that needs to be plugged, make a screen plate, and plug the holes to ensure that the via holes are full. , the resin shrinkage change is small, and the bonding force with the hole wall is good. The process flow is: pretreatment → plug hole → grinding plate → pattern transfer → etching → board surface solder mask
This method can ensure that the via hole plug hole is flat, and the hot air leveling will not have quality problems such as oil explosion and oil loss at the edge of the hole, but this process requires one-time thickening of copper, so that the copper thickness of the hole wall can meet the customer’s standard. Therefore, the requirements for copper plating on the whole plate are very high, and there are also high requirements for the performance of the grinding machine to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and free from pollution. 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 much in PCB factories.
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