Discussion on PCB Inner Layer Plug Hole Manufacturing Technology (2)

The main investment of roller scraping and filling is the special machine for plugging. Its construction method is different from that of screen printing plugging. Its operation method is to fill the ink into the hole diameter of the plug hole to carry out the operation. The layer plate enters between the two rollers. During the process, the plug hole plate and the rollers located below and below the plug hole plate produce mutual compression and push effects, forcing the ink-containing roller below to fill the ink into the hole diameter of the plug hole. Part of it is immersed in the ink storage tank, and the required plugging ink can be continuously replenished during the operation. Finally, when the plugging plate continues to advance, it will pass through the pre-set scraper to scrape the excess and protruding ink for recovery.

The advantages of roller scraping and filling are:
1. Can quickly fill orifice plate.
2. No need for printed stencils.
3. Fewer process parameters.
4. It is easy to obtain a relatively flat grinding surface.

The disadvantages of roller scraping and filling holes are:
1. The hole diameters that do not need plug holes to need to be covered separately.
2. Has a high operational risk (such as thin-plate pallets).
3. The amount of ink required for one operation is large, and the ink needs to have a good operating cycle.
4. There are fewer types of inks to choose from.

Screen printing plug holes and roller squeegee filling holes have their own advantages and disadvantages and their scope of application. For example, screen printing plug holes are suitable for plugging plates with small samples or small batches due to low production efficiency. It is suitable for the plugging plate with thin plate thickness, and the roller scraping and filling is suitable for the plugging plate with a large batch number because of its high production efficiency.

Whether the ink used in the inner plug hole is screen printing plug hole or roller scraping hole filling, based on the above considerations, it must have the following characteristics:

1. 100% solid content, no solvent is allowed and low CTE is required to prevent cracking or delamination during heating.
2. The ink hardness after hardening should be at least 6H pencil hardness.
3. After the plug hole is ground, it must have a flat surface without any depressions.
4. Good adhesion to the copper-plated hole wall is required.
5. The metallization (copper plating) ability and adhesion of the ink after hardening should be quite good.
6. The Tg point should be above 140℃.
7. The CTE below the Tg point must be less than 50 PPM.
8. It is easy to grind, and no hole depression can be left after grinding.

4. Plug Hole Ink Characteristics Brief Introduction

IPC-6012A stipulates in 3.6.2.15 Glue Filling Specifications for Blind and Buried Vias: Blind holes do not have glue filling requirements, and ClassV2 professional electronic products and ClassV3 high-reliability electronic product PCB boards must be filled with film during lamination The amount of glue to 60%. Class 1 general electronic products are allowed to the level of complete voids. If the product needs to be applied to a special structure such as Stack Via, in addition to being required to fill 100% of the inner plug hole, it also needs to be easy to grind, and the hole recess after grinding must be less than 100%. Below 5um to avoid damage to the integrity of the signal at high frequencies.

The inner plug hole ink can be roughly divided into three types according to the hardening method:

1. One-stage heat-baking hardening plug-hole ink.
2. Two-stage heat-baking hardening plug-hole ink.
3. UV exposure heating and baking hardening plug hole ink.

The baking conditions for one-stage hot-baking hardening type plug ink are about 150°C, 30~45 minutes, and the optimal baking conditions need to be adjusted to different degrees depending on the aspect ratio of individual plug hole diameters. Although the hardened plug hole ink has a high baking efficiency, it will reach a pencil hardness of 8-9H after baking, which will also cause difficulties in grinding. If there is any occurrence of grinding pits, it is difficult to achieve the above requirements without good and stable grinding equipment.