How to choose high speed HDI PCB material

How to choose high-speed PCB material

When the signal reaches a certain rate or more, and a long path needs to be taken on the PCB, especially the backplane, if you use ordinary plates, you may not be able to meet the requirements. At this time, you must use low-loss plates. We also call high-speed plates. .

Common high-speed plates are:

Rogers RO4003, RO3003, RO4350 (PTFE), etc.

Tuc 862, 872SLK, 872SLK-SP, TU883, TU933, etc.

Panasonic Megtron4 / Megtron6 etc.

Isola FR408HR / 408 etc.

ShengYi S1141 / S1000-2 etc.

How to choose so many high-speed plates? What parameters should be concerned about high-speed sheet? When do I need to use high-speed plates? This topic "How to choose high-speed plates" will focus on the material parameters and factors affecting high-speed performance, using simulation and testing as a means to comprehensively explain how we choose high-speed plates in engineering to meet the best cost performance of the product.

With the rapid development of electronic design and communication fields and the application of cloud computing, products have higher and higher data volume requirements and faster and faster data transmission rates. At the same time, the performance of PCBs that carry data transmission is also getting higher and higher. The application of the board directly affects the stability and cost of the PCB and the system, which requires us to have a comprehensive understanding of the material; this article from the PCB composition, the general classification of the board and conventional characteristics, factors affecting high-speed performance and simulation test comparison, and finally by example Explain how to choose high-speed plates suitable for design requirements in engineering applications.

Key words:High-speed sheet, Dk, Df, surface roughness, glass fiber effect, simulation verification

introduction:

When the signal reaches a certain rate or more, and a long path needs to be taken on the PCB, especially the backplane, if you use ordinary plates, you may not be able to meet the requirements. At this time, you must use low-loss plates. We also call high-speed plates. What exactly is high-speed plate, what parameters does high-speed plate focus on, and when is high-speed plate required? This article will focus on the parameters of the materials and the factors that affect the high-speed performance, and use simulation and testing as a means to comprehensively explain how we choose high-speed plates in the project to meet the best cost performance of the product.

1. PCB sheet and parameter characteristics

printed circuit board (PCB) is an important mechanism component of electrical appliances or electronics made with copper-clad substrate (CCL) as raw material.

1.1 PCB sheet classification

There are many types of PCBs, which can be divided into various types according to different classification methods. The most common ones are based on substrate type, glass fiber type, supplier resin system, loss and flame retardant level. Of course, there are other classifications, this article does not do Highlights.

According to the type of substrate, it is divided into rigid substrate and flexible substrate. This article mainly introduces the rigid substrate, as shown in Figure 3 below.

Rigid substrate: An important variety of general rigid substrate materials is copper clad laminate. It is made of reinforced material (Reinforeing Material), impregnated with resin adhesive, dried, cut and laminated into blanks, covered with copper foil, and made of steel plate as a mold, processed by high temperature and high pressure in a hot press of.

Flexible substrate: Flexible printed circuit board (Flexible printed circuit board) is made of polyimide or polyester film as a substrate with high reliability, excellent flexible printed circuit.

According to the type of glass fiber cloth, there are 106, 1067, 1080, 1078, 2116, 2113, 3313, 7628, etc .;

According to the resin systems used by suppliers and their performance classification, there are the following types:

Rogers RO4003, RO3003, RO4350 (PTFE), etc.

Tuc 862, 872SLK, 872SLK-SP, TU883, TU933, etc.

Panasonic Megtron4 / Megtron6 etc.

Isola FR408HR / 408 etc.

ShengYi S1141 / S1000-2 etc.

Classified by loss level:

Ordinary loss sheet, medium loss sheet, low loss sheet, ultra-low loss sheet, etc .;

Low-loss and ultra-low-loss plates are usually called high-speed plates and are classified according to the level of loss.

Classification according to flame retardant performance:

Flame retardant (UL94-VO, UL94-V1) and non-flame retardant (UL94-HB grade)

.2. Material parameters of PCB

Whether it is a normal plate or a high-speed plate, we need to pay attention to some of the characteristics of the material itself, because they will eventually affect the performance and applicability of the PCB.

Dk & Df

Dk is the dielectric constant, also called dielectric constant, dielectric coefficient or permittivity, it is a coefficient that represents the characteristics of the insulation capacity, expressed by the letter ε, in engineering applications, the dielectric constant is often in the form of relative dielectric constant Be expressed, not absolute. Common applications include calculating impedance and delay.

Formula 2

Df, also known as loss factor, damping factor or internal dissipation or loss tangent, is the tangent of the phase difference angle between the strain and the stress cycle of the material under the action of the alternating force field, which is also equal to the loss modulus of the material Ratio with storage modulus. Generally, loss is closely related to Dk & Df. The following is the approximate calculation formula of loss.

Glass transition temperature (Tg)

The Tg value of the FR-4 board is generally 130-140 degrees. In the PCB manufacturing process, there are several process problems that will exceed this range, which will have a certain impact on the processing effect and final state of the product. Therefore, increasing Tg is a major method to improve the heat resistance of FR-4. One of the important means is to increase the relative density of the curing system or increase the content of aromatic groups in the resin formulation. In general FR-4 resin formulations, some epoxy resins with three functional groups and multifunctional groups are introduced or some phenolic epoxy resins are introduced to increase the Tg value to about 160-200 degrees.

Coefficient of thermal expansion (CTE)

With the development of precision, multi-layer and BGA, CSP and other technologies of printed boards, higher requirements have been placed on the stability of the size of copper clad laminates. Although the dimensional stability of the CCL is related to the production process, it mainly depends on the three raw materials that make up the CCL: resin, reinforcement, and copper foil. The usual methods are:

(1) Modify the resin, such as modified epoxy resin; (2) Reduce the resin content ratio, but this will reduce the electrical insulation performance and chemical performance of the substrate; copper foil has little effect on the dimensional stability of the copper clad laminate .

CAF resistance

Conductive Anodic Filament (CAF) was first discovered by Bell Labs researchers in 1955. It refers to the electromigration chemical reaction of metal ions in a non-metallic medium under the action of an electric field. A conductive channel is formed between the anode and the cathode of the battery, causing a short circuit. With the rapid development of the electronics industry, electronic products are lighter, thinner, shorter, and smaller, the hole spacing and line spacing of PCBs will become smaller and smaller, and the lines will become more and more dense, so that the PCB is resistant to ion migration Performance becomes more and more important.

The harm of ion migration to electronic products

1) The signal of electronic products becomes worse, the performance decreases, and the reliability decreases.

2) The service life of electronic products is shortened.

3) Increased energy consumption.

4) Insulation damage, short circuit may occur and fire safety problems may occur.

Environmental requirements

EU RoHS Directive:

Mainly used to standardize the material and process standards of electronic and electrical products, making it more conducive to human health and environmental protection. The purpose of this standard is to eliminate lead (Pb), cadmium (Cd), mercury (Hg) in electrical and electronic products Hexavalent chromium (Cr6 +), polybrominated biphenyls (PBBs) and polybrominated diphenyl ethers (PBDEs) are a total of 6 substances, and focus on the lead content must not exceed 0.1%.

The standard has been formally implemented since July 1, 2006.

WEEE directive:

Waste Electrical and Electronic Equipment Directive

EU REACH Regulation-No SVHCs

It is the EU's regulations for the preventive management of all chemicals entering its market. It was officially implemented on June 1, 2007.