How to Choose HDI PCB Materials?

High-density Interconnect PCB Material: Choosing the correct dielectric material is
crucial for any PCB, but the stakes increase with HDI technology. The PCBs are small, light and strong
but have certain structural requirements. When using lead-free, you need to select a material that has a
higher Td (decomposition temperature) and a better overall quality.

How Do You Decide?

This is an overview of the factors to be considered when choosing HDI materials for PCB. Before diving
into HDI applications you can get a crash-course in PCB materials by reading Choosing a PCB substrate:
Understanding Dielectric Materials Properties. You can also get immediate help using Sierra Circuits
Material Selector. Simply enter your desired properties and you will be presented with a list of
materials that are compatible.

What Is HDI Stackup?

HDI PCBs are smaller and have less vias (150 um) than traditional PCBs. It has fine lines (=100um),
small vias and capture pads (400um), as well as a high density of connection pads (>20pads/cm2).

The resin matrix in an HDI stack provides dielectric properties as well as resistance to separate highly
conductive layers, such a copper foil. Watch our HDI Cost Considerations Video for more information
about different HDI stacks.

The PCB matrix provides resistive properties for the separated conductive layer.

Dielectric Materials

HDI performance is dependent on the dielectric resin or material chosen. These materials are typically
of higher quality than multilayer PCBs, and have the following important properties:

Glass transition temperature (Tg).

Temperature of decomposition (Td).

Coefficient of thermal Expansion along the Z-axis (CTEz).

Delamination Time

In general, the better the dielectric material performs, the more it costs.

As performance increases, so does the cost.

What Is the Best HDI Material for Your Application?

Due to the signal energy loss that occurs at high frequencies, it is important for PCB materials to have
a dielectric loss factor or dissipation factor(Df), and a flat Df vs. Frequency response curve.

Four categories of materials are suitable for HDI:

Lossy: The most common PCB materials are medium-speed materials, the FR-4 Family. They
have a less flat dielectric constant response (Dk) and higher dielectric losses. Their suitability is
therefore limited to a handful of digital/analog GHz applications.

Low-loss, high-speed:High speed materials have a flatter Dk curve versus frequency
response curve and dielectric loss is about half.

These are suitable for materials with medium speed. These are designed for frequencies as low as 0 GHz.

High speed, low loss,high signal integrity: These materials are characterized by a
flatter Dk curve versus frequency response and have lower dielectric losses. They also produce less
unwanted electrical noise.

High speed, low loss,High signal integrity, RF, and Microwave:Materials used in
RF/microwave application have the flattest Dk regarding frequency response, and the lowest dielectric
losses. These materials are suitable for applications of up to 20 GHz.

These stacking materials can be difficult to handle and may not be suitable for all HDI stackings. Check
out our HDI Materials Video for more information.

Use materials with better SI and lower Dk for high-speed digital applications. Use materials with the
lowest Df possible for RF and microwave applications. When signal attenuation matters, choose materials
with low loss and high speed. Use materials with a lower Dk to reduce the crosstalk.

These materials can be difficult to work with and may not be suitable for every stacking. Check out Tech
Talk about HDI Manufacturing and Cost for more information.