How to determine the PCB printed circuit boards

As we all know, the basic properties of (PCBS) depend on the properties of their substrate materials. Therefore, to improve the performance of the circuit board, we must first optimize the performance of the substrate material. To date, many new types of materials are being developed and put into use in order to meet the requirements that are compatible with new technologies and market trends.

The market for printed circuit boards has shifted in recent years, shifting its focus from traditional hardware products such as desktop computers to wireless communications such as servers and mobile terminals. Mobile communication devices represented by smartphones have promoted the development of PCBS in the direction of high density, lightweight and multi-functional. Without the substrate material, whose technical requirements are closely related to the performance of the PCB, it is impossible to achieve printed circuit technology. Therefore, the choice of substrate materials plays a crucial role in improving the quality and reliability of the PCB and its final product.

All PCB boards are moving in the direction of higher density and thinner lines, especially HDI PCBS (high-density Interconnect PCBS). A decade ago, HDI PCBS were defined by IPC as PCBS with a line width (L) and line spacing (S) of 0.1mm or less. However, at present, the standard values of L and S in the current electronics industry can be as small as 60μm, and in advanced cases, their values can be as low as 40μm.

The traditional circuit pattern formation method lies in the imaging and etching process, which results in a minimum L and S value of 30μm when using a thin copper foil substrate (thickness in the range of 9μm to 12μm).

Due to the high cost, many stacks, and many defects of thin copper foil clad copper sheet (CCL), many PCB manufacturers tend to use the etching copper foil reduction method to replace copper foil with a copper foil thickness of 18μm. This method is not actually recommended because it contains too many processes and the thickness is difficult to control, resulting in higher costs. As a result, thinner copper foil is better. In addition, when the L and S values of the plate 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 should be controlled in the range of 3μm to 5μm.

In addition to the thickness of the copper foil, current fine circuits also require a low-roughness copper foil surface. In order to improve the bonding ability between the copper foil and the base material and ensure the stripping strength of the conductor, the coarser treatment is carried out on the copper foil plane. The roughness of the ordinary copper foil is greater than 5μm.

The insertion of a bump on the copper foil into the substrate material is designed to increase its peel strength. However, in order to control lead accuracy away from excessive etching during circuit etching, it tends to cause bump contamination, which may lead to short circuits or reduced insulation between lines, which particularly affects fine circuits. Therefore, a copper foil with low roughness (less than 3μm or even 1.5μm) is required.

Although the roughness of the copper foil is reduced, it is still necessary to maintain the peel strength of the conductor, which causes special surface forming on the surface of the copper foil and the base material, which will help to ensure the peel strength of the conductor.

- Requirements for insulating dielectric laminates

One of the main technical features of HDI PCBS is the build process. Lamination of commonly used RCC (resin-coated copper) or prepreg epoxy glass cloths and copper foils rarely results in fine circuits. There is now a tendency to use SAP and MSPA, which means that an insulating dielectric film is laminated with electroless copper plating to create a copper conductive plane. Thanks to the thin copper plane, fine circuits can be produced.

One of the key points of SAP is the layer of piezoelectric dielectric material. In order to meet the requirements of high-density fine circuits, a number of requirements must be placed on the laminate material, including dielectric properties, insulation, heat resistance capacity and adhesion, as well as technical adaptability compatible with HDI PCBS.

In semiconductor packaging worldwide, IC packaging substrates have been converted from ceramic substrates to organic substrates. The pitch of the FC package substrate is getting smaller, so the current typical value of L and S is 15μm and will be smaller.

Multilayer substrate performance should emphasize low dielectric properties, low coefficient of thermal expansion (CTE) and high heat resistance, which refers to a low-cost substrate that meets performance objectives. Today, through the mass production of fine circuits, MSPA technology is applied to combine layers of insulating dielectrics with thin copper foils. SAP is used to manufacture circuit patterns with both L and S values less than 10μm.The high density and thinness of PCBS cause HDI PCBS to change from laminating with core to any layer without core. For an HDI PCB with the same function, the area and thickness of a PCB with interconnect function on any layer are 25% less than that of a PCB with core layer. Both HDI PCBS must use a thinner dielectric layer with better electrical properties.

Demand for high frequency and high speed

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

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

The engineer's main job is to rely on the properties of electrical signal loss to improve PCB speed and deal with signal integrity issues. Based on more than a decade of PCB manufacturer manufacturing services, as a key factor affecting substrate material selection, when the dielectric constant (Dk) is less than 4 and the dielectric loss (Df) is less than 0.010, it is considered an intermediate Dk/Df laminate, when the Dk is less than 3.7 and the Df is less than 0.005, it is considered a low Dk/Df laminate. At present, there are many types of substrate materials available on the market.

So far, the commonly used high-frequency circuit board substrate materials are mainly the following three kinds: fluorine resin, PPO or PPE resin and modified epoxy resin. Fluorine series dielectric substrates with the lowest dielectric properties, such as PTFE, are typically used for products with frequencies of 5GHz or higher. Modified epoxy FR-4 or PPO substrates are suitable for products in the frequency range of 1GHz to 10GHz.

Comparing these three types of high-frequency substrate materials, the price of epoxy resin is the lowest, while the price of fluorine series resin is the highest. In terms of dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorinated resin showed the best performance, while epoxy resin showed poor performance. When the frequency applied by the product is higher than 10GHz, only the fluorine series resin works. The disadvantages of PTFE include high cost, poor rigidity and high coefficient of thermal expansion.

For PTFE, bulk inorganic materials such as silica can be used as filling materials or glass cloths to enhance the rigidity of the substrate and reduce the coefficient of thermal expansion. In addition, due to the inertia of polyvinyl fluoride molecules, it is difficult for polyvinyl fluoride molecules to bind to copper foil, so it is necessary to achieve special surface forming compatible with copper foil. The treatment method consists of chemically etching the surface of the polyethylene to increase surface roughness, or adding a bonding film to increase adhesion. With the application of this method, the dielectric properties may be affected, and the entire fluorine series of high-frequency circuits must be further developed.

Unique insulating resins consisting of modified epoxy resin or PPE combined with glass cloth of TMA, MDI and BMI are being used more frequently. Similar to FR-4 CCL, it also has excellent heat resistance and dielectric properties, mechanical strength, and the manufacturability of PCBS, all of which make it more popular than PTFE-type substrates

In addition to the requirements for the performance of insulating materials such as the above resins, the surface roughness of copper as a conductor is also an important factor affecting the signal transmission loss, which is the result of skin effect. In short, the skin effect is that the electromagnetic induction and inductance generated by the wire in high-frequency signal transmission become so concentrated in the center of the cross-sectional area of the wire that the driving current or signal is concentrated on the surface of the wire. The surface roughness of the conductor plays a key role in the loss of the transmitted signal, and the loss caused by low roughness is very small.

At the same frequency, the high surface roughness of copper leads to high signal loss. Therefore, the roughness of the surface copper must be controlled in actual manufacturing, and it should be made as low as possible without affecting the adhesion. Special attention must be paid to signals in the frequency range of 10 GHz or higher. The roughness of the copper foil is required to be less than 1μm, and it is best to use a metasurface copper foil with a roughness of 0.04μm. The surface roughness of the copper foil must be combined with appropriate oxidation treatment and bonding resin system. In the near future, there may be a copper foil shape that is not coated with resin, and it has a higher peel strength and does not affect the medium

High heat resistance and heat dissipation requirements

With the development trend of miniaturization and high functionalization, electronic devices tend to generate a lot of heat, so the thermal management requirements of electronic devices are becoming more and more demanding. One of the ways to solve this problem is the research and development of thermal conductive PCB. The main condition for good heat resistance and heat dissipation of PCB is the heat resistance and heat dissipation of the substrate. Current improvements in the thermal conductivity of PCBS lie in improvements through the addition of resins and fillers, but only work in limited categories. The typical method is the application of IMS or metal core PCB that acts as a heating component. Compared with traditional radiators and fans, this method has the advantages of small size and low cost.

Aluminum is a very attractive material with abundant resources, low cost, excellent thermal conductivity and strength. In addition, it is very environmentally friendly and can be used on most metal substrates or metal cores. Due to the advantages of economical, reliable electrical connection, thermal conductivity and high strength, solder - and lead-free, aluminum-based circuit boards have been used in consumer goods, automotive, military products and aerospace products. There is no doubt that the heat resistance and heat dissipation of the metal substrate is the key, and the key is the adhesion between the metal plate and the circuit board.

How to determine the PCB substrate material?

In the modern electronic age, the miniaturization and thinness of electronic devices have led to the necessity of rigid PCBS and flexible/rigid PCBS. So which type of substrate is suitable for them?

The application areas of rigid and flexible/rigid PCBS are constantly increasing, putting new requirements in terms of quantity and performance. For example, polyimide membranes can be divided into several 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, highly cost-effective polyester film substrates are also accepted by the market due to their advantages of high elasticity, dimensional stability, film surface quality, photocoupling and environmental resistance to meet the variable needs of users.

Similar to rigid HDI PCBS, flexible PCBS must adapt to the requirements of high-speed and high-frequency signal transmission, and the dielectric constant and dielectric loss of flexible substrate materials must also be paid attention to. Flexible circuits can be composed of polytetrafluoroethylene and advanced polyimide substrates. Inorganic dust and carbon fiber can be added to the polyimide resin to produce three layers of flexible thermal conductivity substrate. Inorganic filling materials can be aluminum nitride, alumina or hexagonal boron nitride. This type of substrate material has a thermal conductivity of 1.51W/mK and is able to resist a voltage of 2.5kV and a curvature of 180 degrees.

Flexible PCBS are mainly used in smart phones, wearable devices, medical devices and robotics, which puts new requirements on flexible PCB structures. So far, some new products containing flexible PCBS have been developed, such as ultra-thin flexible multi-layer PCBS, whose thickness is reduced from 0.4mm to 0.2mm. By using polyimide substrate materials with low Dk and Df, high-speed transfer flexible PCBS can achieve transfer speeds of 5Gbps. High-power flexible PCBS use conductors with thickness greater than 100μm to meet the requirements of high-power, high-current circuits. All these special flexible PCBS naturally get unconventional substrate materials.