HDI PCBs: Choosing The Right Material
Choosing the right dielectric material for a PCB is important no matter what application you’re working on, but the stakes are higher with High-Density Interconnect (HDI) technologies. They’re small, they’re light, they’re powerful; yet they have specific construction requirements and when using lead-free solder you must choose materials that have a higher decomposition temperature (Td) and are generally higher quality overall.
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 before moving on to HDI applications, check out Choosing your PCB substrate: Know your dielectric material’s properties. Or, get instant help with the Sierra Circuits Material Selector—plug in the property values you require and get a list of compatible materials in an easy-to-compare format.
What Is An HDI Stack-up?
An HDI PCB has a higher circuit density per unit area than a traditional PCB. It has fine lines and spaces (≤ 100 µm), small vias (<150 µm) and capture pads (<400 µm), and high connection pad density (>20 pads/cm2). The small size and lightweight of an HDI PCB makes it perfect for small consumer applications like cell phones or medical equipment.
In an HDI stack-up, the resin matrix provides the dielectric properties and resistance that separates highly conductive layers, like copper foil. For additional information on different HDI stack-ups, watch our HDI Cost Considerations video.
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, and the following properties are critical:
- Glass transition temperature (Tg)
- Decomposition temperature (Td)
- Coefficient of thermal expansion (CTE) along the Z axis (CTEz)
- Time to delamination
Generally, the higher the performance, the more costly the material. Here is a chart of common dielectrics comparing 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:
Medium speed and loss: Medium-speed materials are the most common PCB materials—the FR-4 family. Their dielectric constant (Dk) versus frequency response is not very flat and they have higher dielectric loss. Therefore, their suitability is limited to a few GHz digital/analog applications.
High speed, low loss: High-speed materials have a flatter Dk versus frequency response curve, and have a dielectric loss about half that for medium speed materials. These are suitable for up to ~0 GHz.
High speed, low loss, high signal integrity: These materials also have flatter Dk versus frequency response curves and low dielectric loss, and they also generate less unwanted electrical noise compared to other materials.
High speed, very low loss, high signal integrity, RF and microwave:
Materials for RF/Microwave applications have the flattest Dk versus frequency response and the least dielectric loss. They are suitable for up to ~20 GHz applications.
Be aware that these stack-up materials are much harder to process and are not suitable for every HDI stack-up. For more information, check out our HDI materials video.
In general, to get better signal transmission performance in very 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. use lowest Df materials, and 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 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 on HDI manufacturability and cost.
Here is a chart of common materials and recommended areas of application, as well as key property values like Tg, Dk, Tf, and CTEz.
PCB Material Properties and Recommended Application Areas
Dk values can be seen on the above datasheet; however, they will differ for various cores and prepregs used in actual PCB constructions.
Source: HDI Handbook by Happy Holden
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