The electronics industry is growing rapidly. The demand of PCBs with small form factor with higher circuit density is consequently increasing. These types of PCBs are formed with smaller sizes while including circuitry much more complex in small PCB area. These types of PCBs are called High Density Interconnect (HDI). The manufacturing of HDI Boards is achieved by using the technique of Blind Hole Vias and Buried Holes Vias.
Types of Via Holes:
Plated Through Hole (PTH):
The plated through hole is the most common and simplest form of via hole. It connects the top and bottom layer and it is relatively cheap. It simply uses the drill to create a bore through the PCB to create a PTH
The Blind via Hole:
The blind via hole is the one which connects the top or bottom layer with the adjacent inner layer with plated through hole. We cannot see the other end of via so it is called “Blind Via”.
The Buried via Hole:
The buried via hole connects the inner layer with other inner layer and does not go through top and bottom layers. The via fabrication process is not the same as bonding after drilling like for blind via and PTH but the individual circuit layer drilling then plating and finally bonding all layers.
The blind and buried vias can downsize the production cost, increase the quality of HDI, faster on time delivery and optimizing the PCB space that can be used by other circuits.
Methods of Plating the Blind Hole Via:
Point Plating Blind Hole Via:
The point plating process is complex and copper on the blind hole is thicker. This extra copper needs to be rubbed away using abrasive belt that could damage the circuit or scrap it.
Panel Plating the Blind hole Vias:
The panel plating process is less complex than point plating and copper is homogenously spread in the hole and the process of abrasive rubbing, file stripping and plating graphics can be eliminated.
Panel Plating Technology:
There are three types of agents used in the panel plating technology.
The agent that is responsible for the formation of tougher structure of grain is called carrier.
This increases the thickness of diffusion layer at the cathode surface hence giving better definition to via.
The agent accelerator is responsible for smooth, strong and bright deposit of copper fine grain in the bottom of the trench/via.
The suppressor or carrier is engrossed in the side walls and accelerator goes in bottom.
The leveler is the polarizing agent. It will be absorbed in the side walls of via and inhibits extra imposition of copper during plating procedure. It also remains on top of the surface to ensure uniformity in deposition of copper.
Applications of Panel Plating Blind hole filling in Inner Planes:
There are two main types of designs of multilayer PCB. First is stacking micro via (MV) holes design and other is Non-Stacking or staggered micro via (MV) hole design. These are called HDI boards with blind holes in inner planes only.
HDI Board with only Blind hole in inner plane:
For stacked version, blind holes do not need complete fill and level up but plating copper is done enough. For non-stack or staggered case, the holes must be completely filled and leveled up.
The inner bronze thickness is ensured to be in the range of 17.1um to 34.3um in the case of stack MV and blind holes do not need filling and level up. Here the blind hole copper is met with the requirements specified by plating parameters. For the case of staggered MV, the plating parameters ensure the filling, leveling and inner bronze thickness beyond 34.3 um. In this case copper elimination process flow is not needed.
Non-Stacking Blind Holes Process Flow:
Stacking Blind Holes Process Flow:
HDI Board with both Blind hole and buried hole in inner plane:
They can be classified as
1. Non-stacking blind and buried holes
2. Stacking blind hole and non stacking buried hole
3. Stacking buried hole and non stacking blind hole
4. Stacking blind and buried holes
The HDI boards with blind and buried holes can have thickness to radius ratio up-to 6:1. Panel plated blind hole filling is used. For thickness to radius ratio greater than 6:1 hole plating must be done to meet requirement of blind hole copper, that is blind holes must be leveled and filled prior to the buried hole plating
Blind holes are all leveled and filled irrespective of stacking / non-stacking. The process flow will be same as long as it is determined that buried hole is stacking or non stacking
1). Thickness-to-radius < 6:1 (buried holes non-stacking)
2). Thickness-to-radius < 6:1 (buried holes stacking)
3). Thickness-to-radius > 6:1 (buried holes non-stacking)
4). Thickness-to-radius > 6:1 (buried holes stacking)