How to choose HDI PCB material

What is an HDI stackup?

HDI stands for High Density Interconnect. An HDI printed circuit board is characterized by a high density of components and routed interconnections. HDI design is inherently a high-performance design.

It features fine lines and spaces (≤100µm), small vias (<150µm), small capture pads (<400µm) and high connection pad density (>20 pads/cm2). The small size and lightweight of HDI PCBs make them ideal for small consumer applications.

Read more about choosing a smaller footprint for HDI designs here.

The composition of laminates and their functions. Image source

In an HDI stack-up, a resin matrix provides dielectric properties and resistance, separating highly conductive layers such as copper foil.

Select dielectric material

Selecting the correct dielectric material or resin is important for HDI performance. They generally require higher quality than traditional multilayer PCB materials. The following properties are critical:

Glass transition temperature (Tg): The temperature at which a material changes from a solid to a viscous state. This is a key parameter in PCB material selection.

Decomposition temperature (Td): The temperature at which a material chemically decomposes.

Dielectric constant (Dk): It is the ratio of the electrical conductivity of a material to the electrical conductivity of free space (i.e. vacuum). Dielectric constant is also a measure of the potential energy stored in a given volume of material under the action of an electric field.

Dielectric constant or relative permeability equation

Coefficient of Thermal Expansion (CTE): CTE is the rate of expansion of the PCB material when heated. It is expressed in parts per million (ppm) of expansion per degree Celsius of heating.

Loss tangent (tanδ): The power loss when a signal passes through a transmission line on a dielectric material.

HDI Material Cost and Performance

The higher the performance, the more expensive the material is. Below is a cost versus performance chart of common dielectrics, along with typical applications:

Performance and Cost of HDI PCB Materials.

To learn more about PCB material selection, watch the webinar PCB Material Selection: Electrical and Manufacturing Considerations.

The right HDI material type for your application

Signal energy loss considerations at high frequencies require PCB materials to have a low dielectric loss tangent or dissipation factor (Df) and a flatter Df versus frequency response curve. There are four categories of materials suitable for HDI:

Normal speed, normal loss

These are the most common PCB materials – the FR-4 series. Their dielectric constant (Dk) versus frequency response is not very flat and they have higher dielectric losses. Therefore, their suitability is limited to a few GHz digital/analog applications. An example of this material is Isola 370HR.

Medium speed, medium loss

Medium speed materials have a flatter Dk versus frequency response curve. The dielectric loss is about half that of normal speed materials. These are suitable for use up to ~10GHz. An example of this material is Nelco N7000-2 HT.

High speed, low loss

These materials also have flatter Dk vs. frequency response curves and low dielectric losses. They also produce less unwanted electrical noise than other materials. An example of this material is Isola I-Speed.

Very high speed, very low loss (RF/Microwave)

Materials used in RF/microwave applications have the flattest Dk versus frequency response and the smallest dielectric losses. They are suitable for applications up to ~20GHz. An example of this material is Isola Tachyon 100G.

For better signal transmission performance in high-speed digital applications, use materials with lower Dk, Df, and better SI properties. For RF and microwave applications, use materials with the lowest possible Df material. When signal attenuation is important, use low-loss high-speed materials. If crosstalk is a problem, reduce it by using materials with lower Dk. BT materials are more suitable when dealing with microelectronic substrates with smaller PCB size and layout features.

Keep in mind that these materials are more difficult to work with and are not suitable for every layup. For more information on HDI stacking, check out our tech talk: HDI Considerations for Manufacturability and Cost.

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HDI Cabling Challenges and Tips

What is HDI cabling

HDI (High Density Interconnects) wiring refers to the use of the latest design strategies and manufacturing technologies to achieve denser designs without affecting circuit functions. In other words, HDI involves the use of multiple routing layers, smaller trace sizes, vias, pads, and thinner substrates to fit complex and often high-speed circuits in previously impossible footprints .

As manufacturing technology evolved, HDI cabling began to appear in many designs, such as motherboards, graphics controllers, smartphones, and other space-constrained devices. If implemented properly, HDI routing not only significantly reduces design space, but also reduces EMI issues on the PCB. Reducing costs is an important goal for companies, and HDI cabling can achieve just that.

HDI routing and microvias

It is important to understand that HDI cabling is more complex than typical multilayer cabling strategies. We may have designed 8-layer or 16-layer PCBs, but there are still some completely new concepts involved in HDI routing that we need to learn.

In a typical PCB design, the actual printed circuit board is considered a single entity and is divided into multiple layers. However, HDI routing requires design engineers to think in terms of integrating multiple ultra-thin layers of a PCB into a single functional PCB.

It can be said that the key driving force for realizing HDI routing is the development of via technology. Vias are no longer plated copper holes drilled into various layers of a PCB. Traditional via mechanisms reduce routing area in PCB layers that are not used by signal lines.

Traditional vias have no place in HDI routing

In HDI routing, microvias are the driving focus, responsible for integrating multiple layers of dense routing. For ease of understanding, micro vias can be thought of as consisting of blind holes or buried holes, but with different structural methods. Traditional vias are drilled with a drill after combining the layers. However, microvias are drilled with a laser into each layer before they are stacked. Laser-drilled microvias allow interconnection between layers with minimal aperture and pad size. This facilitates a fan-out layout of the BGA part, where the pins are arranged in a grid pattern.

HDI routing strategies

With the use of microvias, PCB design engineers are able to implement complex routing on any layer of the PCB. This approach is called any-layer HDI or per-layer interconnect. Thanks to space-saving microvias, densely packed components can be placed on both outer layers since most of the routing is done on the inner layer.

Low impedance ground plane is critical for HDI routing

However, the denser density of components and traces in multilayer designs also increases the risk of EMI radiation and magnetic susceptibility. When we are working on an HDI design, it is important to ensure that the PCB stackup has the proper structure. We need to have enough ground plane to get a low impedance return path.

Internal wiring layers should be placed between ground or power planes to reduce cross-coupling or crosstalk. Keep high-speed signal paths as short as possible, including return paths. Proper planning and use of microvias can help limit the signal path to a small area and reduce the risk of EMI.

Of course, it also helps to use the right software to simulate the HDI PCB just to be on the safe side.