Why PCB circuit board vias must be plugged. What is worth studying?(Part Ⅱ)
1. 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.
2. 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.
2.3 Aluminum plate plug hole, development, pre-curing, after the plate is polished.
Using a CNC drilling machine, the aluminum piece required to be plugged is drilled to make a screen, which is installed on the shift screen printing machine to make the plug hole, the plug hole must be full, the two sides protrude better, and then the solidified plate is processed for the surface treatment. The process is: pre-treatment - plug hole - pre-bake - development - pre-cure - plate surface solder resist.
Since the process uses plug hole curing to ensure that the HAL does not drop oil and oil after the HAL, after the HAL, the tin in the via hole and the tin on the via hole are difficult to completely solve, so many customers do not receive it.
2.4 Plate surface soldering and plugging are completed at the same time.
This method uses a 36T (43T) screen, installed on the screen printing machine, using a pad or a nail bed, and plugs all the through holes while completing the board surface. The process flow is: Pr-processing - silk screen - - Prebaking - exposure - development - curing.
The process time is short, the utilization rate of the equipment is high, the hole can be ensured that the hot air is not drained, and the via hole is not tinned. However, since the screen hole is used for plugging, a large amount of air is stored in the via hole, and when solidified, The air expands and breaks through the solder mask, causing voids and unevenness. The hot air leveling will have a small amount of conductive vias. At present, our company has carried out a large number of experiments, choose different types of ink and viscosity, adjust the pressure of silk screen, etc., basically solve the hole void and unevenness, has been mass-produced by this process.