High-Density PCB Layouts

Key Takeaways

• High-density PCB design arranges the vias properly for improved routing. 

• Micro vias are more suitable than through-hole vias, as they don’t introduce any coefficient of thermal expansion mismatches. 

• The material characterization, interconnecting structure, micro via formation, selection of design rules, etc. are regulated by IPC 2226.

High-density PCB layouts use blind vias, micro vias, vias-in-pad, fine traces, etc. to increase the component density for the given board dimension

The demand for miniaturized circuits has persuaded manufacturers to pack more capabilities and functionalities into smaller packages. High-density PCB design is best suited for smaller spaces. The component density of high-density PCBs is relatively high compared to conventional boards. High-density PCB layouts use blind vias, micro vias, vias-in-pad, fine traces, etc. to increase the component density for the given board dimension. 

How Are High-Density PCBs Different From Conventional PCBs?

High-density PCBs are designed when the product needs to be compact, with a high density of components per unit area. In order to achieve a compact board, the design of high-density PCBs differs from conventional PCBs. The main three board facts that differentiate high-density boards from standard PCBs are:

1. Routing improved with the use of vias.
2. Layer stack-up alternatives are provided to incorporate micro vias.
3. Through-holes are replaced by micro vias where high component density is expected.

Via Arrangements for Routing Improvements

High-density PCB design arranges vias properly for improved routing. By using vias, the routing space in the inner layers is improved. The routing space improvement achieved through the arrangement of vias provides better signal integrity.

Eliminating Through-Hole Vias With Layer Stack-Up Alternatives

The high-density PCB board uses various stack-up arrangements in its structure to reduce the inner layers and through-holes. In most cases, vias are drilled into the ground plane and power plane. In high-density PCBs, the ground and power planes are usually placed at the top of the stack-up.

The component placement is exclusively made at the top and bottom layers. This stack-up structure enables the replacement of through-hole vias with micro vias. The ground and power planes can be separated from each other by thin dielectric layers of less than .005 inches. Separation of this kind is provided to lower the power supply impedance. There is a provision that also helps in skipping the intermediate layers.

Replacing Through-Hole Vias With Micro Vias

The use of micro vias instead of through-hole vias is preferred whenever high precision is required. Micro vias are more suitable than through-hole vias, as they don’t introduce any coefficient of thermal expansion mismatches. Micro vias have short barrels, which help in diverting problems due to different coefficients of thermal expansion of the copper and substrate.

Micro vias are restricted from going around the entire thickness of the high-density board and help in giving more space to other layers. In this way, the presence of micro vias improves the routing density of the inner layers, and the total number of signal layers is also reduced. The table below gives a description of different types of micro vias.