HDI PCB Materials Selection

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No matter the application, selecting the suitable dielectric material for a PCB is crucial. But with an HDI PCB, the risks are far greater. Despite being compact, lightweight, and powerful, they have unique building needs. You must select materials with a greater decomposition temperature, especially if you plan to use lead-free solder to mount electrical components.

The electrical characteristics of the materials used in an HDI PCB vary depending on the bonding materials used. The material formulation used to bond the high-frequency multilayer film might also change significantly.

Glass woven fiber reinforced bonding material is normally thermoset and includes fillers to improve high-frequency performance. In contrast, a non-reinforced bonding material is typically a thermoplastic polymer film.

With that in mind, here are a few things you must consider when selecting dielectric and bonding materials for an HDI PCB.

HDI PCB Stack-Up

An HDI PCB is distinguished by its extensive routing links and high component density. By definition, an HDI design is a high-performance design. It has small vias (150 m), small capture pads (400 m), fine lines and gaps (100 m), and a high connection pad density (greater than 20 pads/cm2). An HDI PCB is ideal for small consumer applications due to its lightweight design and compact size.

What to Consider When Selecting the Dielectric Material for Your HDI PCB

The appropriate dielectric resin or materials must be used for optimal HDI performance. In comparison to materials used in conventional multilayer PCBs, they often need to be of better quality. On that note, listed below are a few material properties you should know about:

Loss Tangent (tanδ)

The loss tangent is the measure of the signal power loss as it passes through a dialectical resin or material.

Coefficient of Thermal Expansion (CTE)

The Coefficient of thermal expansion is the rate of expansion of a PCB when heat is applied to it. It is typically denoted by PPM (parts per million).

Dielectric constant (DK)

The dielectric constant is the proportion of the permeability of free space (vacuum) to a material's electric permeability. It also gauges how much electric potential energy is held in a particular volume of material when an electric field is present.

Decomposition Temperature (Td)

The decompositions temperature is the temp at which the dielectric material deteriorates when undergoing chemical reactions.

Glass transition temperature (Tg)

The glass transition temperature is the temperature at which the dielectric turns into a viscous state from a solid state.

Delamination Time

A method for assessing the performance of dielectric materials is the delamination time. It is the measure of time needed for a dielectric resin or material to delaminate. In general, the longer it takes for an HDI stack-up to delaminate, the better.

Categories of HDI PCB Materials

HDI PCB materials must have a flatter Df versus frequency response curve and a low dielectric loss tangent or dissipation factor (Df) to account for signal energy loss issues at high frequencies. These materials are broken down into four categories. These include:

Very Low-Loss, Very High-Speed Materials

The flattest Dk vs. frequency response and the least amount of dielectric loss is seen in materials used in RF/microwave applications. They are appropriate for applications using up to 20GHz. Isola Tachyon 100G is a prime example of a high-speed and low-loss HDI PCB material.

Low-Loss, High-Speed Materials

These materials also exhibit minimal dielectric loss and flatter Dk vs. frequency response. In comparison to other materials, they also produce less unwanted electrical noise. Isola I-Speed is an excellent example of a low-loss, high-speed material.

Medium-Loss, Medium-Speed Materials

The Dk versus frequency response is flatter for such materials operating at medium speed. Furthermore, the dielectric loss is around half that of materials with standard speeds. These work well for frequencies up to 10GHz. The Nelco N7000-2 HT is a prime example of a medium-loss, medium-speed HDI PCB material.

Normal-Loss, Normal-Speed Materials

The FR-4 family of materials is the most commonly used PCB material. These materials have a flatter dielectric constant (Dk) vs. frequency response and a larger dielectric loss. Their usefulness is therefore restricted to a handful of digital and analog applications that utilize few GHz in frequencies. The Isola 370HR is a common example of normal-loss, normal-speed materials.

In the end, utilize materials with lower Dk, Df, and superior SI characteristics for higher-speed digital applications to enhance signal transmission performance. On the other hand, use materials with the lowest Df feasible for RF PCBs.

Moreover, use low-loss high-speed materials in applications where signal attenuation is a priority. If you're facing cross-talk issues, use a material with a lower dielectric constant to lessen it. Lastly, BT materials are more appropriate in an HDI PCB with microelectronic substrates or when its size and layout characteristics are less important.

Cost vs. Performance of HDI PCB Materials

HDI PCB materials with lower Df vs. Dk values have excellent SI performance. They are excellent for obtaining outstanding HDI performance. However, the factors mentioned above are sometimes challenging to understand and are not necessarily relevant for all HDI Stack-ups.

Aside from that, it would be best to prepare yourself to spend more money on higher-quality materials. Low-loss, high-speed, high signal integrity, microwave, and RF materials typically cost a lot of money. Ultimately, the better the performance, the costlier the HDI PCB materials. So, keep that in mind the next time you select material for your HDI PCB stack-up!

Wrapping Up

Given the widespread adoption of HDI technology in today's electronics industry, the future is anything but gloomy. Even if HDI PCB technology can be improved, there are too many benefits and uses for it to be overlooked. In fact, the use of HDI PCBs across almost all industries, including the military, has further boosted its acceptance. So, follow the tips in this article to ensure you select the right materials for your HDI PCB design. It will do wonders for your circuit's application and performance!

For more HDI PCB design tips and materials, visit Hemeixin today. We are leaders in PCB design and can help you optimize and improve your circuits!

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