As we all know, the basic properties of PCB (printed circuit board) depend on the performance of its substrate material. Therefore, in order to improve the performance of the circuit board, the performance of the substrate material must first be optimized. So far, various new materials are being developed and applied to meet the requirements of new technologies and market trends.

In recent years, printed circuit boards have undergone a transformation. The market has mainly shifted from traditional hardware products such as desktop computers to wireless communications such as servers and mobile terminals. Mobile communication devices represented by smart phones have promoted the development of high-density, light-weight and multi-functional PCBs. If there is no substrate material, and its process requirements are closely related to the performance of the PCB, printed circuit technology will never be realized. Therefore, the choice of substrate material plays a vital role in providing the quality and reliability of the PCB and the final product.

Meet the needs of high density and fine lines

•Requirements for copper foil

All PCB boards are moving towards higher density and finer circuits, especially HDI PCB (High Density Interconnect PCB). Ten years ago, HDI PCB was defined as PCB, and its line width (L) and line spacing (S) were 0.1mm or less. However, the current standard values ​​of L and S in the electronics industry can be as small as 60 μm, and in advanced cases, their values ​​can be as low as 40 μm.

How to determine your PCB substrate material

The traditional method of circuit diagram formation is in the imaging and etching process. With the application of thin copper foil substrates (with a thickness in the range of 9μm to 12μm), the lowest value of L and S reaches 30μm.

Due to the high cost of thin copper foil CCL (Copper Clad Laminate) and many defects in the stack, many PCB manufacturers tend to use the etching-copper foil method, and the copper foil thickness is set to 18μm. In fact, this method is not recommended because it contains too many procedures, the thickness is difficult to control and leads to higher costs. As a result, thin copper foil is better. In addition, when the board’s L and S values ​​are less than 20μm, the standard copper foil does not work. Finally, it is recommended to use ultra-thin copper foil, because its copper thickness can be adjusted in the range of 3μm to 5μm.

In addition to the thickness of the copper foil, current precision circuits also require a copper foil surface with low roughness. In order to improve the bonding ability between the copper foil and the substrate material and ensure the peel strength of the conductor, rough processing is performed on the copper foil plane, and the general roughness of the copper foil is greater than 5μm.

Embedding hump copper foil as the base material aims to improve its peel strength. However, in order to control the lead precision away from over-etching during circuit etching, it tends to cause hump pollutants, which may cause a short circuit between lines or a decrease in insulation capacity, which particularly affects fine circuits. Therefore, copper foil with low roughness (less than 3 μm or even 1.5 μm) is required.

Although the roughness of the copper foil is reduced, it is still necessary to retain the peel strength of the conductor, which causes a special surface treatment on the surface of the copper foil and the substrate material, which helps to ensure the peel strength of the conductor.

• Requirements for insulating dielectric laminates

One of the main technical characteristics of HDI PCB lies in the construction process. The commonly used RCC (resin coated copper) or prepreg epoxy glass cloth and copper foil lamination rarely lead to fine circuits. It is now inclined to use SAP and MSPA, which means the application of insulating dielectric film laminated electroless copper plating to produce copper conductive planes. Because the copper plane is thin, fine circuits can be produced.

One of the key points of SAP is to laminate dielectric materials. In order to meet the requirements of high-density precision circuits, some requirements must be put forward for laminate materials, including dielectric properties, insulation, heat resistance and bonding, as well as technical adaptability compatible with HDI PCB.

In global semiconductor packaging, IC packaging substrates are converted from ceramic substrates to organic substrates. The pitch of FC package substrates is becoming smaller and smaller, so the current typical value of L and S is 15 μm, and it will be smaller.

The performance of multi-layer substrates should emphasize low dielectric properties, low coefficient thermal expansion (CTE) and high heat resistance, which refers to low-cost substrates that meet the performance targets. Nowadays, MSPA insulation dielectric stacking technology is combined with thin copper foil to be used in the mass production of precision circuits. SAP is used to manufacture circuit patterns with both L and S values ​​less than 10 μm.

The high density and thinness of PCBs have caused HDI PCBs to transition from lamination with cores to cores of any layer. For HDI PCBs with the same function, the area and thickness of PCBs interconnected on any layer are reduced by 25% compared to those with core laminates. It is necessary to apply a thinner dielectric layer with better electrical properties in these two HDI PCBs.

Requires export from high frequency and high speed

Electronic communication technology ranges from wired to wireless, from low frequency and low speed to high frequency and high speed. The performance of smartphones has evolved from 4G to 5G, requiring faster transmission speeds and greater transmission volumes.

The advent of the global cloud computing era has led to a multiple increase in data traffic, and there is a clear trend for high-frequency and high-speed communication equipment. In order to meet the requirements of high-frequency and high-speed transmission, in addition to reducing signal interference and consumption, signal integrity and manufacturing are compatible with the design requirements of PCB design, high-performance materials are the most important element.

The main job of an engineer is to reduce the properties of electrical signal loss to increase PCB speed and solve signal integrity problems. Based on PCBCart’s more than ten years of manufacturing services, as a key factor affecting the choice of substrate material, when the dielectric constant (Dk) is lower than 4 and the dielectric loss (Df) is lower than 0.010, it is regarded as an intermediate Dk/Df laminate When Dk is lower than 3.7 and Df is lower than 0.005, it is considered a low Dk/Df laminate. Currently, a variety of substrate materials are available on the market.

So far, there are mainly three types of commonly used high-frequency circuit board substrate materials: fluorine-based resins, PPO or PPE resins and modified epoxy resins. Fluorine series dielectric substrates, such as PTFE, have the lowest dielectric properties and are usually used for products with a frequency of 5 GHz or higher. The modified epoxy resin FR-4 or PPO substrate is suitable for products with a frequency range of 1GHz to 10GHz.

Comparing the three high-frequency substrate materials, epoxy resin has the lowest price, although fluorine resin has the highest price. In terms of dielectric constant, dielectric loss, water absorption, and frequency characteristics, fluorine-based resins perform best, while epoxy resins perform worse. When the frequency applied by the product is higher than 10GHz, only the fluorine-based resin will work. The disadvantages of PTFE include high cost, poor rigidity, and high thermal expansion coefficient.

For PTFE, bulk inorganic substances (such as silica) can be used as filler materials or glass cloth to enhance the rigidity of the substrate material and reduce the coefficient of thermal expansion. In addition, due to the inertness of the PTFE molecules, it is difficult for the PTFE molecules to bond with the copper foil, so a special surface treatment compatible with the copper foil must be realized. The treatment method is to perform chemical etching on the surface of the polytetrafluoroethylene to increase the surface roughness or to add an adhesive film to increase the adhesion ability. With the application of this method, the dielectric properties may be affected, and the entire fluorine-based high-frequency circuit must be further developed.

Unique insulating resin composed of modified epoxy resin or PPE and TMA, MDI and BMI, plus glass cloth. Similar to FR-4 CCL, it also has excellent heat resistance and dielectric properties, mechanical strength, and PCB manufacturability, all of which make it more popular than PTFE-based substrates.

In addition to the performance requirements of insulating materials such as resins, the surface roughness of copper as a conductor is also an important factor affecting signal transmission loss, which is the result of the skin effect. Basically, the skin effect is that the electromagnetic induction generated on the high-frequency signal transmission and the inductive lead becomes so concentrated in the center of the cross-sectional area of ​​the lead, and the driving current or signal is focused on the surface of the lead. The surface roughness of the conductor plays a key role in influencing the loss of the transmission signal, and low roughness leads to very small loss.

At the same frequency, the high surface roughness of copper will cause high signal loss. Therefore, the roughness of surface copper must be controlled in actual manufacturing, and it should be as low as possible without affecting adhesion. Great attention must be paid to signals in the frequency range of 10 GHz or higher. The roughness of copper foil is required to be less than 1μm, and it is best to use ultra-surface copper foil with a roughness of 0.04μm. The surface roughness of the copper foil must be combined with a suitable oxidation treatment and bonding resin system. In the near future, there may be a copper foil with no profile-coated resin, which has a higher peel strength to prevent the dielectric loss from being affected.

Requires high thermal resistance and high dissipation

With the development trend of miniaturization and high functionality, electronic equipment tends to generate more heat, so the thermal management requirements of electronic equipment are becoming more and more demanding. One of the solutions to this problem lies in the research and development of thermally conductive PCBs. The basic condition for PCB to perform well in terms of heat resistance and dissipation is the heat resistance and dissipation capability of the substrate. The current improvement in the thermal conductivity of PCB lies in the improvement of resin and filling addition, but it only works in a limited category. The typical method is to use IMS or metal core PCB, which act as heating elements. Compared with traditional radiators and fans, this method has the advantages of small size and low cost.

Aluminum is a very attractive material with the advantages of abundant resources, low cost and good thermal conductivity. And intensity. In addition, it is so environmentally friendly that it is used by most metal substrates or metal cores. Due to the advantages of economy, reliable electrical connection, thermal conductivity and high strength, solder-free and lead-free, aluminum-based circuit boards have been used in consumer products, automobiles, military supplies and aerospace products. There is no doubt that the key to the heat resistance and dissipation performance of the metal substrate lies in the adhesion between the metal plate and the circuit plane.

How to determine the substrate material of your PCB?

In the modern electronic age, the miniaturization and thinness of electronic devices has led to the emergence of rigid PCBs and flexible/rigid PCBs. So what type of substrate material is suitable for them?

Increased application areas of rigid PCBs and flexible/rigid PCBs have brought new requirements in terms of quantity and performance. For example, polyimide films can be classified into various categories, including transparent, white, black and yellow, with high heat resistance and low coefficient of thermal expansion for application in different situations. Similarly, the cost-effective polyester film substrate will be accepted by the market due to its high elasticity, dimensional stability, film surface quality, photoelectric coupling and environmental resistance, to meet the changing needs of users.

Similar to rigid HDI PCB, flexible PCB must meet the requirements of high-speed and high-frequency signal transmission, and attention must be paid to the dielectric constant and dielectric loss of the flexible substrate material. The flexible circuit can be composed of polytetrafluoroethylene and advanced polyimide substrate. Inorganic dust and carbon fiber can be added to the polyimide resin to result in a three-layer flexible thermally conductive substrate. The inorganic filler material may be aluminum nitride, aluminum oxide or hexagonal boron nitride. This type of substrate material has a thermal conductivity of 1.51W/mK, can resist a voltage of 2.5kV and a curvature of 180 degrees.