HDI Rigid-flex PCBs Layer Stackup

High-density interconnects (HDI) refers to the use of blind, micro, and buried vias to create printed circuit boards. The technology is driven by the strong economic incentive to increase processing speeds into smaller form factors. PCB designers use HDI stackup technology to pack even more components into smaller boards.

The Basics of HDI PCB Stackups

Besides the obvious advantage of packing more features and functions into a smaller form factor, an HDI rigid flexible circuit board does nearly everything better when compared to conventional multilayer designs including:

● High-quality signal integrity
● Much lower power consumption
● Fewer layers required (and therefore, fewer points of failure)
● Improved electrical performance

At the core of these benefits is the improvement in signal integrity. This is achieved by reducing throughout the Rigid-Flex PCB board to improve electrical performance, provided the designer is able to account for electromagnetic compatibility and electromagnetic interference.

The most important features that make HDI technology possible are:

Blind vias: blind vias connect the outer layer of the Rigid Flex PCB to its inner layers without having to go through the entire board.
Buried vias: buried vias connect two or more inner layers but do not go through the outer layer.
Microvias: They are very small vias equal to or smaller than 0.15mm and enable connections between each layer of the PCB.
HDI any-layer: the next evolution of HDI microvia that enables free electrical connections between all layers with the help of filled plating technology and laser drilling technology. Drilling allows the use of microvias for more components to be used on the PCB surface.

Different Stackups for HDI PCB Boards

PCI board designers use three standard types of stacks for assembling HDI packages. These are:
● Standard lamination with plated-thru holes or vias
● Sequential lamination with plated-thru hole, buried vias and blind vias
● The laminated substrate consists of microvias

Prot tip: Lamination substrates that use microvias are especially useful for use in PCBs that have a high pin-count of ball grid arrays and fine-pitch boards.

Each type has its pros and cons. For example, standard lamination with plated-thru holes can be affordable to manufacture for 28 layers and under, but it is difficult to route for fine pitched boards. Similarly, sequential lamination with buried and blind vias is more costly to produce because of various procedures. They can only be used with a maximum of two or three layers, which limits their practical limitations.

Due to these limitations, most HDI PCB designers are switching to laminated substrates with microvias. Advantages of microvias HDI PCB design include high route density with fewer layers because of the smaller dimensions of the vias and traces.

Design Considerations for HDI Rigid Flex PCB Stackups

Designing an HDI rigid-flex stackup can be complicated because you have to factor in various features such as:

EMC and EMI: HDI is often used for high-speed signal designs and therefore, the design must account for EMI and EMC. This can be done by following good PCB design guidelines.
Impedance Control: The PCB tolerances for any dielectric material must be within tight tolerances (+/- 10%). This will ensure that the material’s thickness, spacings, and trace widths do not affect signal integrity.
Thermal stability: Although HDI PCB design can simplify thermal management, designers will still need to improve the thermal stability of microvias in high-speed board designs.

HDI PCB Stackups - Standards from the Institute of Printed Circuits (IPCs)

The Institute of Printed Circuits (IPC) has established six categories of HDI PCB stackups named after Roman numerals. These are Types I, II, III, IV, V, and VI.

It is worth noting that Types IV, V, and VI are more expensive to manufacturers and may not be ideal for use in high-density PCBs.

We can define the HDI types as follows:

HDI Type I
This board has a laminated core with at least one layer of microvias on one or both sides. This type may also use plated thru holes, vias, and blind vias, but it does not make use of buried vias.

HDI Type II
This board uses blind, buried, and micro vias on a laminated core with at least one layer of microvias on one or both sides. Designers can also use a staggered design during the fabrication process. Although Type II is ideal for use in denser boards, it shares the same limitations as Type I due to the limitation on the number of laminated core layers.

HDI Type III
Type III also uses blind, buried, and micro vias on a laminated core similar to Type II. The difference is that it uses at least two layers of microvias on one or both sides. A Type III HDI PCB is best used for dense multilayer PCBs, but they share limitations on the number of layers, especially when they use thin FR-4 dielectrics and plated-through holes.

Furthermore, the use of microvias in the inner layers of Type III boards allows the outer layers to be used for power planes, leaving more layers for signal routing. Fabricators can use stacked vias to achieve a higher routing density, but at a much higher cost.

HDI Type IV
Type IV is an HDI board that uses microvia layers over a passive substrate that is pre-drilled. Fabricators can use additional microvia layers

HDI Type V
Type V is an HDI board that uses both plate microvias and conductive paste interconnections using a co-lamination process.

HDI Type VI
Type VI is a coreless HDI board in which the mechanical structure and electrical connections are developed simultaneously. The layers may be formed co-laminated or sequentially, and the conductive interconnections can be formed by other means such as electroplating.

Wrapping Up

Understanding the design guidelines of HDI stackups, limitations, and cost factors are important for good results. This is why it is recommended to consult with the fabricator to learn about their design guidelines.

At Hemeixin, we offer ultra-thin flexible HDIs for use in small-form electronics such as smartphones, smartwatches, and other applications that are subjected to harsh environmental conditions. We also offer Every Layer Interconnect, specialized microvias, and buildup flexibility. Contact us to learn more.