What is HDI PCB material?

02 August 2022
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PCB Material selection plays an important role in PCB fabrication and application functionality. Designers must carefully choose materials when working with High density interconnectors that feature micro vias, capture pads, and fine pitch BGAs.

Working with HDI PCB boards often imposes a number of challenges, such as:

Limited space to work with
Tighter spacing and smaller components such as micro via PCB
More trace routes to complete the PCB
A large number of components on either side of the HDI PCB stackup
Complex trace routes with longer signal flight times
Working with excessive thermal cycling

HDI circuits are small and light yet extremely powerful with very specific material requirements. These circuits are also prone to heat problems, which is why you should choose materials with a high decomposition temperature (Td).

Selecting HDI Circuit Board Material

The following properties must be considered when choosing an HDI material:

Decomposition Temperature (Td)

This is the temperature at which the material chemically decomposes due to heat. It is often determined using the IPC ™ 650 Method and is expressed in degrees Celsius (℃). This important parameter is used to estimate the thermal survivability of the circuit.

Changes to a material’s properties are inevitable if the temperature exceeds Td. The HDI PCB material should have a Td that is well above the temperature range of its application. The solder temperatures during HDI PCB assembly are in the 250 ℃ to 300 ℃ range, so make sure that the

material’s Td is higher than this range.

Glass Transition Temperature (Tg)

The glass transition temperature (Tg) is the range at which a material changes from a hard glassy state to a rubbery state. It’s the temperature at which the carbon chains begin to move and exhibit undesirable properties that can disrupt a circuit’s functions.

Precisely measuring Tg is hard because of several factors, including the molecular structure of the material.

Exceeding an HDI PCB’s Tg for an extended period of time will affect its functions. However, staying within the Tg will ensure the proper functioning and mechanical stability of the circuit during its lifetime.

Coefficient of Thermal Expansion (CTE)

CTE is an important property of materials that indicates the extent to which they expand after heat is applied. In other words, the CTE specifies how much a Circuit will expand or contract when heated and cooled. CTE is measured in parts per million degrees Celsius or PPM/ °C.

This sudden expansion and contraction of the circuit can have devastating consequences on components, especially large silicon chip packages (LBGAs). Excessive thermal cycling result in the failure of solder joints because the circuit expands at a faster rate than the silicon chip is rated to tolerate. Furthermore, this will result in shear forces that create micro tears over time.

The resulting damage to a circuit is not acceptable in applications where functionality is important such as medical devices.

Dielectric Constant (Dk)

The dielectric constant, also known as relative permittivity, is the ratio of the material’s permittivity to the permittivity of air. Most fabricators use this parameter to test the substance.

The dielectric constant also measures a material’s electric potential energy under the action of an electric field.

Using Copper Foils with a Low Profile

An excellent way of making flexible HDI circuits is by using fine-grained copper foils with a relatively thin and low profile. The thickness of these copper foils should lie within the range of most flexible circuits, which is about 1 ounce.

Common Materials for HDI PCBs

Materials that are best suited for HDI PCBs have a lower dielectric loss tangent and a relatively flat dissipation factor versus frequency response curve. These materials can be divided into four categories that will be discussed below.

Normal Speed and Loss

The dielectric constant versus frequency response curve of normal speed materials is not flat. This means that they exhibit undesirable properties such as a high dielectric loss. As a result, these materials are not suited to advanced applications where no dip in performance can be tolerated.

Normal speed and loss materials are best suited to digital devices that are limited to a few GHz. A popular example of such a material is Isola 370HR.

Medium Speed and Medium Loss

Medium speed materials have a relatively flat dielectric constant versus frequency response curve. As a result, they experience half the dielectric loss compared to normal speed substances. Medium speed materials are best suited to applications that are limited to 10 Ghz but not any higher. The Nelco N7000-2 is a popular example from this category of materials.

High Speed, Low Loss

High speed, low loss materials exhibit a very flat dielectric constant versus frequency curve. As a consequence, the dielectric loss is extremely low in these materials. Another advantage of using these materials is that they don’t generate a lot of electrical noise. These high performance materials have a Tg of nearly 180°C.

A popular example of high speed, low loss material is Isola’s I-Speed.

Very High Speed, Very Low Loss

Very High Speed, Very Low Loss materials have the most flat dielectric constant versus frequency response curve possible. They also have the least amount of dielectric loss. This makes them well suited to applications that go up to 100 Ghz and higher. The Isola Tachyon 100G is a popular material that belongs to this category.

These materials have excellent electrical properties and are very stable over a broad range of temperatures and frequencies. They are designed for high speed digital devices.

The Expenses Involved in HDI PCB Materials

In general, materials with lower Df and Dk values are the best candidates for HDI circuits. They do require a higher budget compared to other materials. High speed, low loss signal materials are usually the most expensive to produce.

Wrapping Up

HDI PCBs are playing an important role in the miniaturization of technology in electronics. The coming years will result in even more improvements with endless applications and innovations. Nearly every industry is leveraging HDI PCBs to some capacity.

Need more guidelines on your project? Get in touch with the experts at Hemeixin PCB for more information.