The term plug hole is not new in the printed circuit board (PCB) industry. During the early etching process of the outer layer circuit, there was a concern about dry-film tenting on the ringside of the plated-through hole (PTH) that could lead to an open circuit if the hole was not completely covered. The plughole method was introduced to fill temporary ink and protect the hole wall. Over time, the Tin Tenting process became mainstream, but multi-layer boards still require solder resist paint plug holes on the outer layer. However, this article will focus on the inner layer buried hole plug hole technology.
In the era of HDI high-density connection technology, line width and spacing are becoming smaller and denser, leading to different PCB structures like Via on Pad and Stack Via. Inner layer buried holes need to be completely filled and grounded to increase the wiring area on the outer layer. This demand not only tests the process capabilities of the PCB industry but also pushes raw material suppliers to develop plug-hole inks with specific characteristics like Hi-Tg, low CTE, low water absorption, solvent-free, low shrinkage, and easy grinding. The main processes involved in plugging sections are drilling, electroplating, hole wall roughening, plugging, baking, and grinding.
Apart from increasing the wiring area, there is also a requirement for a uniform dielectric layer thickness. The purpose of inner layer plug holes is to avoid damaging outer line signals, serve as the base for stacked hole structures, and meet customer’s impedance requirements.
Common methods for inner layer plugging include press-filling and resin ink plugging. For small apertures and low aspect ratio buried vias, natural filling by lamination can suffice. However, for larger apertures, high aspect ratio buried vias, and a greater number of holes, a higher glue content like RCC pressing and filling might be necessary. The resin in RCC has a high coefficient of thermal expansion CTE, leading to potential reliability issues like cracks or delaminations.
Screen printing plug holes have advantages like wide usage, easy process arrangement, and no need for additional equipment. However, operators need experience, operation parameters can be complicated, and it may not be suitable for different plug hole diameters within the same inner layer. Additionally, a separate screen plate is needed for each inner plug hole plate, leading to lower production efficiency.
In the era of HDI high-density connection technology, line width and spacing are becoming smaller and denser, leading to different PCB structures like Via on Pad and Stack Via. Inner layer buried holes need to be completely filled and grounded to increase the wiring area on the outer layer. This demand not only tests the process capabilities of the PCB industry but also pushes raw material suppliers to develop plug-hole inks with specific characteristics like Hi-Tg, low CTE, low water absorption, solvent-free, low shrinkage, and easy grinding. The main processes involved in plugging sections are drilling, electroplating, hole wall roughening, plugging, baking, and grinding.
Apart from increasing the wiring area, there is also a requirement for a uniform dielectric layer thickness. The purpose of inner layer plug holes is to avoid damaging outer line signals, serve as the base for stacked hole structures, and meet customer’s impedance requirements.
Common methods for inner layer plugging include press-filling and resin ink plugging. For small apertures and low aspect ratio buried vias, natural filling by lamination can suffice. However, for larger apertures, high aspect ratio buried vias, and a greater number of holes, a higher glue content like RCC pressing and filling might be necessary. The resin in RCC has a high coefficient of thermal expansion CTE, leading to potential reliability issues like cracks or delaminations.
Screen printing plug holes have advantages like wide usage, easy process arrangement, and no need for additional equipment. However, operators need experience, operation parameters can be complicated, and it may not be suitable for different plug hole diameters within the same inner layer. Additionally, a separate screen plate is needed for each inner plug hole plate, leading to lower production efficiency.
