Discussion on PCB Inner Layer Plug Hole Manufacturing Technology (1)

The term plug hole is not a new term in the printed circuit board industry. In the early etching process of the outer layer circuit, in order to avoid dry-film tenting on the ringside of the PTH hole that is too small, the hole cannot be completely covered and the plating layer on the hole wall is etched. The problem of becoming open appeared. At that time, the plughole method was used to fill the temporary ink to protect the hole wall. Later, this method was gradually eliminated because the Tin Tenting process became mainstream in the market. Even so, the current multi-layer board is also required to resist soldering green paint plug holes. But the above process is applied to the plughole operation of the outer layer, the theme to be discussed in this article is the inner layer buried hole plug hole technology.

I. Introduction

In the era of HDI high-density connection technology, the line width and line spacing will inevitably develop towards a smaller and denser trend, and as a result, different types of PCB structures will appear, such as Via on Pad, Stack Via, etc. Under this premise, the inner layer buried holes are usually required to be completely filled and ground to increase the wiring area of ​​the outer layer. The market demand not only tests the process capability of the PCB industry, but also forces the raw material suppliers to develop higher Hi-Tg , Low CTE, low water absorption, solvent-free, low shrinkage, easy grinding and other characteristics of plug-hole ink to meet the needs of the industry. The main processes of the plugging section are drilling, electroplating, hole wall roughening (pre-plugging), plugging, baking, grinding, etc. This section will give a more detailed introduction to the resin plugging process.

2. Inner Layer Plug Hole Purpose

In addition to the above-mentioned wiring area as the main consideration, there are also requirements for uniform thickness of the dielectric layer. The purpose of the inner layer plug holes is:
1. Avoid damage to the outer line signal.
2. As the base of the upper stacked hole structure.
3. Meet the customer’s characteristic impedance requirements.

3. Current Inner Layer Plugging Methods and Capabilities

Common inner layer plugging methods include press-filling and filling (which can be divided into RCC and HR, PP with high glue content, etc., in this article, RCC pressing and filling are used as an example) and resin ink plugging. Generally speaking, if the inner layer has a small aperture, buried vias with low aspect ratio and a small number of holes can be plugged by natural filling by lamination; while the buried vias with large aperture, high aspect ratio and large number of holes will be filled due to the glue content of RCC is not enough to fill buried holes with larger and deeper diameters, so it is not suitable for plugging holes in this way. The occurrence of problems will also affect the overall reliability of the product. The resin (glue) contained in RCC also has a relatively high coefficient of thermal expansion CTE (Coefficient of Thermal Expansion), which is the characteristic of the resin contained in typical RCC. Cracks or delaminations occur during reliability tests such as impact, thermal stress, etc.; the CTEs that are very different between the two materials and the inclusion of plugged air bubbles are the main causes of the above defects.

The advantages of screen printing plug holes are:
1. The printing machine has a wide range of uses and can be used in processes such as solder mask and text printing.
2. For the common plug hole method, the process arrangement is relatively easy.
3. Blocking points can be set on the screen without the aperture of the plug hole to avoid ink.
4. There is no need to purchase additional plugging equipment, which is suitable for the existing process in the industry.

Screen printing plug holes have the following disadvantages:
1. Operators need to accumulate considerable operating experience before they can become proficient.
2. The operation parameters are cumbersome and complicated.
3. It is difficult to apply to the requirement of different plug hole diameters in the same inner layer.
4. For each inner plug hole plate, a corresponding screen plate needs to be made separately.
5. Poor production efficiency.