Choosing the right dielectric material for a PCB is important no matter what application you’re working. However, the stakes are higher with High-Density Interconnect (HDI) technologies.
They’re small, light, and powerful, yet they have specific construction requirements. When using lead-free, solder you must choose materials that have a higher decomposition temperature (Td).
So how do you choose? Here’s an overview of considerations when selecting an HDI PCB material. For a crash course in PCB material properties, check out our article How to Choose PCB Materials and Laminates for Fabrication. You just have to plug in the property values you require and get a list of compatible materials in an easy-to-compare format.
We will be covering the following topics in this article:
- What is an HDI stackup?
- Dielectric material
- Types of HDI materials suitable for your application
- PCB material properties and recommended application areas
- Sierra Circuits’ preferred materials
What is an HDI stackup?
HDI stands for High-Density Interconnect. An HDI PCB is characterized by its high density of components and routing interconnections. An HDI design is by its very nature a high-performance design.
It has 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 an HDI PCB makes it perfect for small consumer applications.
In an HDI stackup, the resin matrix provides the dielectric properties and resistance that separates highly conductive layers, like copper foil.
Selecting dielectric material
Picking the right dielectric material or resin is important for HDI performance. They generally need to be higher quality compared to traditional multilayer PCBs materials. The following properties are critical:
Glass transition temperature (Tg): The temperature at which the material transforms from a solid-state to a viscous state. This is a critical parameter for PCB material selection.
Decomposition temperature (Td): The temperature at which the material decomposes chemically.
Dielectric constant (Dk): It is the ratio of the electric permeability of the material to the electric permeability of free space (i.e., vacuum). The dielectric constant is also a measure of the amount of electric potential energy stored in a given volume of material under the action of an electric field.
Coefficient of Thermal Expansion (CTE): CTE is the rate of expansion of a PCB material as it heats up. It is expressed in parts per million (ppm) expanded for every degree Celsius that it is heated.
Loss tangent (tanδ): The power loss of a signal as it passes through a transmission line on a dielectric material.
HDI materials cost vs. performance
The higher the performance, the costlier the material. Here is a chart of common dielectrics comparing the cost to performance, along with typical applications:
Types of HDI material suitable for your application
Signal energy loss considerations at high frequencies require PCB materials that have a low dielectric loss tangent or dissipation factor (Df) and a flatter Df versus frequency response curve. There are four categories of HDI-suitable materials:
Normal speed and loss
These are the most common PCB materials—the FR-4 family. Their dielectric constant (Dk) versus frequency response is not very flat and they have a higher dielectric loss. 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 about half that for normal speed materials. These are suitable for up to ~10GHz. An example of this material is Nelco N7000-2 HT.
High speed, low loss
These materials also have flatter Dk versus frequency response curves and low dielectric loss. They also generate less unwanted electrical noise compared to other materials. An example of this material is Isola I-Speed.
Very high speed, very low loss (RF/microwave)
Materials for RF/microwave applications have the flattest Dk versus frequency response and the least dielectric loss. They are suitable for up to ~20GHz applications. An example of this material is Isola Tachyon 100G.
To get better signal transmission performance in high-speed digital applications use materials with lower Dk, Df, and better SI features. For RF and microwave applications, use materials with the lowest possible Df materials. When signal attenuation is important, use a low-loss high-speed material. If crosstalk is an issue, reduce it by using a material with a lower Dk. When working with microelectronic substrates where the PCB size and layout features are small, BT materials are more suitable.
Keep in mind these materials are much harder to process and not suitable for every stack-up. For more information on HDI stack-ups, check out our tech talk 9 HDI Considerations for Manufacturability and Cost.
Proper material selection is important since materials will affect the electrical performance of the signal traces. Sierra circuits’ material selector helps you determine what of material will best suit your design needs. It provides a list of 12 materials, with their most important properties, that you can compare. We chose these specific materials out of a whole list of PCB materials because they are suitable for HDI PCB. These materials cover the entire range of HDI applications.
Source: HDI Handbook by Happy Holden
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