PCB Technical - PCB impedance calculation formula and PCB substrate

1. What is PCB impedance

In a circuit with resistance, inductance, and capacitance, the obstacle to alternating current is called impedance. At a certain frequency, in the transmission signal line of electronic devices, relative to GND&VCC, the resistance of the high-frequency signal or electromagnetic wave during the propagation process is called the characteristic impedance, which is a vector sum of electrical impedance, inductive reactance, and capacitive reactance. . The unit of impedance is ohms, which is often represented by Z.

PCB impedance

2. Impedance calculation formula

Generally speaking, the influencing factors of impedance are signal line width w, signal line thickness t, dielectric layer thickness h, and dielectric constant εr. Generally speaking, the impedance is inversely proportional to the dielectric constant, directly proportional to the thickness of the dielectric layer, inversely proportional to the line width, and inversely proportional to the copper thickness.

Impedance calculation formula:

Z0=87/SQRT(εr+1.41)*ln[(5.98h)/(0.8w+t)]

Z0: Characteristic impedance of printed wire

εr: the dielectric constant of the insulating material

h: The thickness of the medium between the printed wire and the reference surface

w: the width of the printed wire

t: the thickness of the printed wire

With the upgrading of customers' products, they are gradually developing in the direction of intelligence, so the requirements for PCB board impedance are becoming more and more stringent, which also promotes the continuous maturity of the impedance design technology of Shenlian.

PCB substrates that are becoming more and more environmentally friendly

With the development of electronic equipment in the direction of miniaturization, light weight, multi-function and environmental protection, the printed circuit board as its foundation has also developed in these directions, and the PCB substrate material used for the printed board should naturally be adapted. These needs.

PCB substrate

Environmentally friendly materials

Environmentally-friendly products are required for sustainable development, and environmentally-friendly printed boards require environmentally-friendly materials. For the copper clad laminate, the main material of the printed board, the toxic polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) regulations are prohibited in accordance with the EU RoHs decree, which involves the elimination of bromine-containing flame retardant copper clad laminates Agent. At present, advanced countries in the world have begun to adopt a large number of halogen-free copper clad laminates, while domestic halogen-free copper clad laminate products have only been successfully developed in large foreign-funded enterprises. Many small and medium-sized copper clad laminates are still stuck in the traditional The production of copper clad laminates failed to meet the requirements of environmental protection bans.

In addition to being non-toxic, environmentally-friendly products also require the products to be recycled and reused after being discarded. Therefore, the insulating resin layer of the printed board substrate is considering changing from a thermosetting resin to a thermoplastic resin, which facilitates the recycling of waste printed boards. After heating, the resin is separated from the copper foil or metal parts, and each can be recycled and reused. In this regard, foreign countries have successfully developed high-density interconnect printed boards that have been successfully applied to the build-up method, but there has been no news in China.

Solderable coating materials on the surface of printed circuit boards, the most traditionally used tin-lead alloy solder, now the EU RoHS ban prohibits lead, and the alternative is tin, silver or nickel/gold plating. In the past few years, foreign electroplating chemical companies have developed and launched chemical nickel/immersion gold, chemical tin, and chemical silver plating drugs, but domestic suppliers of the same type have not seen similar new materials launched.

Cleaner production materials

Cleaner production is an important means to realize the sustainable development of environmental protection, and to achieve cleaner production requires supplementary cleaner production materials. The traditional printed board production method is the subtraction method of copper foil etching to form patterns, which consumes chemical corrosion solutions and also produces a lot of waste water. Foreign countries have been developing and applying non-copper foil catalytic laminate materials, using the additive process of direct electroless copper to form circuit patterns, which can eliminate chemical corrosion and reduce waste water, which is conducive to clean production. The development of this type of laminate material used in the additive process is still blank in China.

The cleaner inkjet printing wire pattern technology that does not require chemical syrup and water cleaning is a dry production process. The key to this technology is inkjet printers and conductive paste materials. Nowadays, nano-scale conductive paste materials have been successfully developed abroad, which enables inkjet printing technology to enter practical applications. This is a revolutionary change for printed boards towards cleaner production. Domestically, there is a lack of micron-level conductive paste materials that meet the use of printed board cross-line and through-holes, and nano-level conductive paste materials are even more shadowless.

In clean production, we also look forward to non-cyanide gold electroplating process materials, chemical copper deposition process materials that do not use harmful formaldehyde as a reducing agent, etc., it is necessary to speed up the development and application of printed board production.