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HDI PCB Materials: Choosing the right dielectric material for your PCB is important no matter what application you're working on, however, the stakes are higher with high-density interconnect (HDI) technology. They're small, they're light, they're strong, but they have specific structural requirements, When using lead-free solder, you have to choose a material with a higher decomposition temperature (Td) and an overall higher quality.
So, how do you choose?
Here's an overview of what to consider when selecting HDI PCB materials. For a crash course in PCB material properties before diving into HDI applications, check out Choosing a PCB Substrate: Understanding Dielectric Material Properties. Or, get instant help with Sierra Circuits' Material Selector—plug in your desired property values and get a list of compatible materials in an easy-to-compare format.
HDI PCBs have a higher circuit density per unit area than traditional PCBs. It features fine lines and spacing (≤100 µm), small vias (<150 µm) and capture pads (<400 µm), and high connection pad density (>20 pads/cm2).HDI PCBs are small and lightweight, making them ideal for small consumer applications such as mobile phones or medical devices.
In an HDI stack-up, the resin matrix provides dielectric properties and resistance to separate highly conductive layers such as copper foil. For additional information on different HDI stack-ups, watch our HDI Cost Considerations video.
The resin matrix in the PCB provides the resistive properties to the separated conductive layers.
Choosing the right dielectric material or resin is critical to HDI performance. They typically require higher quality than traditional multilayer PCB materials, and the following properties are crucial:
Glass transition temperature (Tg)
Decomposition temperature (Td)
Coefficient of thermal expansion along the Z axis (CTEz)
Delamination time
Generally, the higher the performance, the more expensive the material.Here is a chart comparing cost to performance and typical applications for common dielectrics:
The cost of dielectric materials increases as performance increases. Via: HDI Handbook
Given signal energy loss at high frequencies, PCB materials must have a low dielectric loss tangent or dissipation factor (Df) and a flatter Df vs. frequency response curve.
Medium-speed and lossy: Medium-speed materials are the most common PCB materials - the FR-4 family. Their dielectric constant (Dk) vs. frequency response is not quite as flat and they have higher dielectric losses. Therefore, their suitability is limited to a few GHz digital/analog applications.
High-speed, low-loss:High-speed materials have a flatter Dk vs. frequency response curve, and dielectric losses are about half
those for medium-speed materials. These are suitable for frequencies up to ~0 GHz.
High-speed, low-loss,high-signal integrity: These materials have a flatter Dk vs. frequency response curve and lower dielectric losses than other materials, they also generate less unwanted electrical noise.
High Speed, Low Loss, High Signal Integrity, RF and Microwave:
The materials used for RF/microwave applications have the flattest Dk concerning frequency response and the lowest dielectric loss. They are suitable for applications up to 20 GHz.
Note that these stacking materials are difficult to process and are not suitable for use with every HDI stacking. For more information, check out our HDI Materials video.
In general, 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. Use the lowest Df materials, and where signal attenuation is important, use low-loss, high-speed materials. If crosstalk is an issue, use materials with lower Dk to reduce crosstalk.BT materials are suitable when using microelectronic substrates with small board sizes and layout features.
Remember that these materials are difficult to process and are not suitable for use with every stacking. For more information on HDI stacking, check out our Tech Talk on HDI Manufacturability and Cost.
Here is a chart of common materials and recommended application areas, along with key property values such as Tg, Dk, Tf, and CTEz.
Highqualitypcb SMT processing: DK values can be found in the above data tables; however, they will be different from various cores and prepregs used in actual PCB construction.