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Control characteristic impedance PCB design technology

Posted:02:22 PM September 17, 2018 writer: G

In the past, printed circuit boards only functioned as electrical interconnections between the electronic components mounted on it. Now, many new functions are required. For example, the high-speed and large-capacity of the whole machine requires the stabilization of the signal transmission of the PCB. The PCB is required to have corresponding countermeasures against EMI/EMC. Because of the MCM of the component (Multi Chip Module), high-density mounting of SiP (System in a Package), and high concentration of heat sources in electronic components require PCBs to have better heat resistance and heat dissipation. The appearance of other product forms gives the PCB a new function. From the viewpoint of improving the operability, the processability, and the reduction of the manufacturing cost, the substrate used in the IC package is oriented from an inorganic PCB material such as ceramics to an organic PCB substrate material such as an epoxy resin or a polyimide resin. Conversion.

1. Aspects of PCB design techniques with controlled characteristic impedance in high frequency electronic products.

PCB manufacturing with high-precision control characteristics and impedance is guaranteed by the overall design technology. The overall design technology of this system mainly includes the dielectric properties, component characteristics, design methods, PCB manufacturing characteristics, assembly methods and the like of the substrate material. The high-precision control of the resistive PCB has three key elements (high-precision lamination technology, high-precision plating technology, high-precision pattern forming technology).

The characteristics of a general high-frequency printed circuit board substrate material include small signal propagation loss (having low dielectric constant, low dielectric loss factor), high signal transmission speed, and frequency in terms of dielectric characteristics. Contents such as high stability exhibited by changes in temperature and humidity.

To select a high-frequency printed circuit board substrate material, it must first consider its signal propagation loss characteristics on the high-frequency circuit PCB. There is also a problem of conductor loss caused by the "skin effect" (also known as "skin effect" in the field above 1 GHz.

It should also be recognized that there are slight deviations (especially variations in interlayer thickness, dielectric constant, conductor thickness, conductor width) on the substrate material, in the PCB fabrication, and in the assembly. The unconformity of the characteristic impedance of the substrate material increases the amount of reflection and attenuation.

2. About copper foil

Electrolytic copper foil is often used in the manufacture of rigid printed circuit boards. The peel strength between the resin and the copper foil in the substrate material is related to the surface treatment profile of the roughened surface of the copper foil. In general, a copper foil having a large processing layer of the treated surface has a high peeling strength. In the case of a printed circuit board in which a high-frequency signal is present, only the surface of the conductive line has a signal flow due to the influence of the "skin effect", so that when the contour of the copper-treated surface treatment layer is large, it is reflected and attenuated. The amount of performance. This will cause an increase in signal transmission loss. Therefore, it is desirable to reduce the profile of the roughened surface treatment layer for the substrate for high frequency circuits.

At present, a copper foil having a profile of 4 μm or less is referred to as a low profile copper foil (referred to as a VLP copper foil). In high frequency circuits, the use of low profile and extremely thin foils has become a trend. Since the rolled copper foil is characterized by a low profile, a variety of rolled copper foils having a high peel strength performance are currently being actively developed.

In the high-frequency circuit substrate, not only the thickness direction of the copper foil is required to be reduced in size, but also the dimensional accuracy of the low surface of the copper foil (the surface of the base resin) is also improved. Low profile copper foil is easy to achieve the performance requirements of the above two copper foils. Moreover, the low profile copper foil is used, and since the copper powder remains little (or not) on the substrate after the etching of the circuit pattern, the voltage resistance of the PCB and the long-term electrical insulation can be improved. effect.

3. Control of graphic dimensional accuracy in PCB manufacturing

There are six aspects (including ten parameters) of the control of the characteristic impedance precision in the manufacture of printed circuit boards. They include: 1 insulation layer thickness (ie PCB layer thickness) (h) and its thickness accuracy (△h); 2 conductor width (w) and its width accuracy (△w); 3 etching factor (ef); 4 conductor Thickness (t) and its thickness accuracy (Δt); 5 dielectric constant value (ε) and its accuracy (Δε); 6 solder mask film thickness (mh). These factors have a direct impact on the characteristic impedance of the control PCB and its accuracy, and the realization of the high-frequency characteristics of the printed circuit board.

In the above-described constituent elements of the characteristic impedance, the accuracy of the dielectric constant value is closely related to the uniformity of the resin content of the substrate material (prepreg). The etch factor and conductor width accuracy of the wire are directly related to the profile of the copper foil.

The technical index of the resin content of the prepreg is determined by the ability of each substrate material manufacturer according to the actual molding process of the user's factory (printed circuit board manufacturing plant) and the production level. Due to the difference in the amount of the resin, there is a difference in the melt viscosity of the prepreg and a difference in the lamination process. These will cause the PCB to differ in the thickness of the insulation layer and its accuracy. Therefore, the multilayer board produced by the prepreg material of different manufacturers and different resin quantity indexes exhibits different height and precision in its dielectric properties, especially dielectric constant values. It can be seen that if it is desired to improve the high-precision control of the characteristics of the PCB, the substrate material manufacturer must have a good cooperation with the printed circuit board manufacturer in controlling the index of the resin amount of the prepreg.

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