What Type of PCB Substrate Material Is Right for Your PCB?

It has been widely known that fundamental attributes of PCBs (Printed Circuit Boards) depend on the performance of their substrate material. Therefore, to improve the performance of your circuit boards, you have to optimize the performance of substrate material first. Up to now, numerous new types of materials are being developed and coming into applications so as to meet requirements compatible with new technologies and market trends.

This Gold Phoenix PCB Knowledge Center article discusses how to select the right PCB substrate material. Recent years have witnessed a transformation in the printed circuit board market, with its focus shifting from traditional hardware products such as desktop PCs to wireless communications like servers and mobile terminals. Mobile communication devices represented by smartphones drive the progress of PCBs towards high density, lightness and multiple functions. Printed circuit technology can never be achieved without substrate material, whose technological requirement is closely related with the performance of the PCB. Thus, substrate material selection plays a crucial role in contributing to the quality and reliability of PCBs and the end products they will serve.

Requirement Catering to High Density and Fine Lines

Requirement on Copper Foil

All PCB boards are developing towards higher density and finer lines, especially HDI PCBs (High Density Interconnect PCBs). One decade ago, HDI PCB was defined as a PCB whose line width (L) and line space (S) is 0.1 mm or below by IPC. At present, however, the standard value of L and S in the electronic industry can be as tiny as 60 µm, and in advancing situations their value can reach as low as 40 µm.

The traditional method of circuit pattern formation lies in the process of imaging and etching, as a result of which the lowest value of L and S reaches 30 µm with the application of thin copper foil substrate (thickness in the range from 9 µm to 12 µm).

Since thin copper foil CCL (copper clad laminate) features high cost and stacking with many defects, many PCB manufacturers tend to use the etching minus copper foil method instead, with copper foil thickness set to 18 µm. This method is not actually recommended because it contains too many procedures, with thickness difficult to control, and leads to higher cost. As a result, thin copper foil is the better option. Additionally, standard copper foil does not work when the value of L and S of the board is less than 20 µm. Finally, ultra-thin copper foil is advised due to its copper thickness being regulated within the range from 3 µm to 5 µm.

Apart from the thickness of copper foil, current fine circuits also require low roughness on the copper foil surface. To improve bonding capability between copper foil and substrate material and to ensure peel strength of conductors, roughing treatment is implemented on the copper foil plane, and ordinary roughness of copper foil is more than 5 µm.

Embedding humps on copper foil into substrate material aims to increase its peel strength. However, to control lead accuracy and avoid over-etching during circuit etching, hump contaminant tends to be caused so that short circuits between lines may occur or insulating capacity decreases, which particularly affects fine circuits. Therefore, copper foil with low roughness (less than 3 µm or even 1.5 µm) is required.

In spite of roughness reduction of copper foil, the peel strength of conductors still needs to be maintained, which calls for special surface finish on the surface of copper foil and substrate material that will help ensure peel strength of conductors.

Requirement on Insulating Dielectric Laminates

One of the major technological properties of HDI PCBs lies in the building-up process. RCC (Resin Coated Copper) that is ordinarily applied, or prepreg epoxy glass cloth and copper foil lamination, seldom fails to lead to fine circuits. Now SAP (Semi-Additive Process) and MSPA (Modified Semi-Additive Process) tend to be applied, which means the application of insulating dielectric film lamination with chemical copper plating to generate a copper conducting plane. Fine circuits can be produced owing to the thin copper plane.

One of the key points of SAP lies in lamination dielectric material. In order to meet the requirement of high-density fine circuits, some requirements of lamination material have to be met, including dielectric performance, insulation, anti-heat capacity and bonding, coupled with technological adaptation compatible with HDI PCBs.

Among global semiconductor packages, IC package substrates have converted from ceramic substrate to organic substrate. The pitch of FC (Flip Chip) package substrates is becoming increasingly small, so that the present typical value of L and S is 15 µm and it will become even smaller.

Multi-layer substrate performance should emphasize low dielectric property, low coefficient of thermal expansion (CTE) and high heat resistance, which refers to substrate catering to low cost with performance objectives fulfilled. Nowadays, MSPA technology of insulating dielectric lamination coupled with thin copper foil is applied in volume production of fine circuits. SAP is applied to manufacture circuit patterns whose value of both L and S are less than 10 µm.

High density and thinness of PCBs leads to the conversion of HDI PCBs from lamination with core to any layer without core. For HDI PCBs with the same functions, the area and thickness of those with interconnection at any layer decreases by 25% compared to those with core lamination. A thinner dielectric layer with better electrical performance has to be applied in both of these HDI PCBs.

Requirement Deriving from High Frequency and High Speed

Electronic communication technology has been developing from wired to wireless, from low frequency and low speed to high frequency and high speed. Smartphone performance has been progressing from 4G to 5G with demanding needs for faster transmission speed and larger transmission capacity.

The advent of the global cloud computing era leads to a multiple-fold increase in data traffic, with an obvious trend of high frequency and high speed in communication devices. To meet the requirement of high-frequency and high-speed transmission, high-performance material is the most essential element apart from reduction of signal interference and consumption, signal integrity and manufacturing compatible with design requirements in terms of PCB design.

Up to now, commonly used high-frequency circuit board substrate materials come primarily in three types: fluorine-series resin, PPO or PPE resin, and modified epoxy resin. Fluorine-series dielectric substrate, PTFE for example, featuring the lowest dielectric performance, is usually applied for products with a frequency of 5 GHz or more. Modified epoxy FR-4 or PPO substrate is applied for products with a frequency in the range from 1 GHz to 10 GHz.

Comparing the three types of high-frequency substrate materials, epoxy resin features the lowest price while fluorine-series resin the highest. In terms of dielectric constant, dielectric loss, water absorption and frequency characteristic, fluorine-series resin performs the best while epoxy resin performs the worst. When the frequency applied by products is higher than 10 GHz, only fluorine-series resin works. Disadvantages of PTFE include high cost, poor rigidity and high coefficient of thermal expansion.

For PTFE, massive inorganic matters (such as silicon dioxide) can be applied as filling material or glass cloth to strengthen substrate material rigidity and to reduce the coefficient of thermal expansion. Furthermore, because polyflon molecules are difficult to combine with copper foil as a result of the inertness of polyflon molecules, it is necessary to implement special surface finish compatible with copper foil. The treatment method lies in either chemical etching on the surface of polyflon so as to increase surface roughness, or adding a bonding film so as to increase adhesion capability. With the application of this method, dielectric performance will possibly be influenced, and further development has to be carried out for the whole fluorine-series high-frequency circuits.

Unique insulating resin composed of modified epoxy resin or PPE combined with TMA, MDI and BMI, coupled with glass cloth, is applied more widely. Similar to FR-4 CCL, it also features excellent heat resistance and dielectric features, mechanical strength, together with manufacturability of PCB, all of which makes it more popular than PTFE-type substrate boards.

Besides the requirement on performance of insulating material such as resin mentioned above, surface roughness of copper as a conductor is also an important element influencing signal transmission loss, which is the result of the skin effect. Put basically, the skin effect is that electromagnetic induction generated at leads in high-frequency signal transmission causes inductance to become so focused at the center of the cross-sectional area of the lead, driving current or signal to be focused at the surface of the lead. Surface roughness of conductors plays a key role in affecting transmission signal loss, and low roughness leads to little loss.

At the same frequency, high surface roughness of copper leads to high signal loss. Therefore, roughness of surface copper has to be controlled in practical manufacturing and it should be as low as possible in situations where adhesion is not influenced. Much attention must be paid to signals within the category of 10 GHz or more. Roughness of copper foil is required to be less than 1 µm, and it is better to use ultra-surface copper foil with roughness of 0.04 µm. Surface roughness of copper foil has to be coupled with suitable oxidation treatment and adhesive resin system. In the near future, there will possibly be a type of copper foil that has no outline coated by resin and features higher peel strength with dielectric loss prevented from being influenced.

Requirement for High Heat Resistance and Dissipation

With the development trend towards miniaturization and high function, electronic devices tend to generate a larger amount of heat, so that thermal management of electronic devices calls for increasingly higher requirements. One of the solutions to this issue lies in the research and development of thermal-conducting PCBs. The primary condition for a PCB to perform well in heat resistance and dissipation is the heat resistance and dissipation capacity of the substrate. Present improvement in terms of thermal-conducting capability of PCBs lies in improvement through resin and filling additives, but this only works within a limited range. A typical method is to apply IMS (Insulated Metal Substrate) or metal core PCBs that serve as a heating component. Compared with traditional radiators and fans, this method leads to advantages such as small volume and low cost.

Aluminum is a very attractive material with advantages of rich resources, low cost, excellent heat-conducting property and strength. Moreover, it is so environment-friendly that it is applied in most metal substrates or metal cores. Owing to advantages including economy, reliable electrical connection, heat conductivity and high strength, solder-free and lead-free capability, aluminum-based circuit boards have been applied in consumer goods, automobiles, military goods and aerospace products. There is no doubt about the heat-resistance and dissipation property of metal base boards, and the key point lies in the performance of adhesion between the metal board and the circuit plane.

How to Determine Substrate Materials for Your PCBs?

In the modern electronics era, miniaturization and thinness of electronic devices leads to the necessary advent of rigid PCBs and flex/rigid PCBs. Then what type of substrate material is suitable for them?

Increasing application fields of rigid PCBs and flex/rigid PCBs bring about new requirements in terms of number and performance. Polyimide film, for example, can be classified into multiple categories including transparent, white, black and yellow, with high heat resistance and low coefficient of thermal expansion, in order to be applied in different situations. Similarly, mylar substrate with high cost effectiveness will be accepted by the market as well, owing to its advantages including high elasticity, size stability, film surface quality, photoelectric coupling and environmental resistance, so as to meet the changeable requirements of users.

Similar to rigid HDI PCBs, flex PCBs have to adapt to the demands of high-speed, high-frequency signal transmission; the dielectric constant and dielectric loss of flex substrate material have to be focused on as well. Flex circuits can be composed of polytetrafluoroethylene and advanced polyimide substrate. Inorganic dust and carbon fiber can be added to polyimide resin to lead to the generation of three-layer flex thermal-conducting substrate. Inorganic filling material can be either aluminum nitride, aluminum oxide or hexagonal boron nitride. This type of substrate material features a thermal conductivity of 1.51 W/mK and is capable of resisting a voltage of 2.5 kV and curvature of 180 degrees.

Flex PCBs are mainly applied in smartphones, wearables, medical equipment and robotics, which calls for new requirements on flex PCB structure. Up to now, some new products containing flex PCBs have been developed, such as ultrathin flex multi-layer PCBs whose thickness has been reduced to 0.2 mm from 0.4 mm. High-speed transmission flex PCBs can reach a transmission speed of 5 Gbps with the application of polyimide substrate material with low Dk and Df. Flex PCBs with large power apply conductors whose thickness is more than 100 µm so as to meet the requirement of circuits with high power and large current. All these special flex PCBs naturally require unconventional substrate material.

When it comes to selecting substrate material for printed circuit boards, attributes such as dielectric constant (Dk), dissipation factor (Df), surface roughness, thermal decomposition temperature and CTE should all be carefully evaluated to arrive at the most suitable substrate material for your specific application.

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