With the increasing complexity and high-speed circuit design , how to ensure that the various signals ( especially high-speed signal ) integrity , which is to ensure signal quality , become a problem . At this point , it needs to analyze the transmission line theory , the characteristic impedance control signal lines becomes critical , not strictly impedance control , will lead to considerable signal reflections and signal distortion , leading to design failures. Common signals, such as the PCI bus , PCI-E bus , USB, Ethernet , DDR memory , LVDS signals are required for impedance control . Impedance control PCB design through final implementation process for PCB board also put forward higher requirements , through communication with the PCB plant , combined with the use of EDA software , I had some superficial understanding of this issue , and is willing to share .
Multilayer structure:
In order to perform well on the PCB impedance control , we must first understand the structure of the PCB :
Usually we are talking together by a multilayer core board and laminating the prepreg made with each other , is a hard core board , there is a specific thickness , two copper plates of bread , is the basis of material PCB . And a so-called prepreg wetting layer , the core plate acts as an adhesive , although a certain initial thickness, but the thickness of the pressing process will be changed .
Usually two dielectric multilayer outermost layers are the wetting layer , using a separate copper foil as the outer layer of these two layers on the outside . The outer and inner copper foil thickness of the original specifications , generally 0.5OZ, 1OZ, 2OZ (1OZ about 35um or 1.4mil) three kinds , but after a series of surface treatment, the final thickness of the outer layer of copper foil general will increase by nearly 1OZ. The inner sides of the copper foil is copper-clad core sheet , and its final thickness away from the original thickness is small , but the etching reasons as reduction of several um.
Outermost layer of the multilayer solder layer is that we often say " green oil ", of course it can also be yellow or other colors. The thickness of the solder layer is generally not easy to accurately determine the surface area than there is no copper foil to be slightly thicker region , but because of the lack of thickness of the copper foil , copper foil or appear to be more prominent so that when we use be able to feel your fingers touch the PCB surface.
When making the thickness of a particular printed circuit board , on the one hand requires a reasonable choice of parameters of various materials , on the other hand , the thickness of the final molded prepreg will be smaller than the initial thickness. Here is a typical 6-layer laminate structure :
PCB parameters :
Different PCB factory , PCB parameters have subtle differences , through communication with the circuit board on Kerry InfoQuick technical support , get some parameter data of the plant :
Copper surface :
The surface of the copper foil thickness of the material can be used in three ways : 12um, 18um and 35um. The final thickness after the completion of processing is about 44um, 50um and 67um.
Core: we used plate is S1141A, standard FR-4, two- sided copper , optional specifications may contact the manufacturer to determine .
Prepreg :
Specifications ( original thickness ) has 7628 (0.185mm), 2116 (0.105mm), 1080 (0.075mm), 3313 (0.095mm), the thickness of the actual suppression is usually done smaller than the original value of about 10-15um. With a wetting layer can use up to three prepreg and prepreg thickness of 3 can not be the same, at least you can use only one prepreg , but some manufacturers required to use at least two . If the thickness of the prepreg is not , both sides of the copper core plate can be etched away , and then the prepreg sheet with adhesive on both sides , this can be achieved relatively thick wetting layer .
Solder layer :
Foil thickness at the solder resist layer C2 ≈ 8-10um, the surface area of the solder resist layer without the thickness of the copper foil depending on the copper surface C1 of different thickness , when the thickness of the copper surface 45um C1 ≈ 13-15um, when the surface of the copper 70um thick when C1 ≈ 17-18um.
Conductor cross-section:
I always thought that the cross-section is a rectangular wire , but in fact it is a trapezoid. To TOP layer , for example, when 1OZ copper thickness , the trapezoidal lower than the bottom edge of the bottom edge of the short 1MIL. For example, linewidth 5MIL, then on the bottom about 4MIL, under the bottom edge 5MIL. Differences between the bottom and the top and bottom copper thickness relevant , the following table is a different relationship between the upper and lower trapezoidal bottom .
Dielectric constant : Dielectric constant and the thickness of the prepreg on the table below for the different types of prepreg thickness and dielectric constant parameters :
The dielectric constant of the resin sheet material on its use , FR4 dielectric constant of 4.2-4.7 plate , and decreases with increasing frequency .
Dielectric loss factor : a dielectric material in alternating electric field , the energy consumed due to heat dissipation call it that , usually in dielectric loss factor tanδ said . S1141A is typically 0.015.
To ensure that the processing of the minimum line width and line spacing : 4mil/4mil
Impedance Calculation Tools Description:
When we understand and master the multilayer structure of the required parameters , you can calculate the impedance through the EDA software. Allegro can be used to calculate , but here I would like to recommend another tool Polar SI9000, this is a good tool to calculate the characteristic impedance , many PCB factories are using this software.
Whether it is a single-ended or differential line cord, when calculating the characteristic impedance of the inner signal , you will find Polar SI9000 calculation results with Allegro there is only a slight gap, which deal with some of the details about , for example, the cross section of the wire shape. However, if the surface is calculated characteristic impedance of the signal , I suggest you choose Coated model, rather than Surface model, because such models to consider the existence of the solder resist layer , so the results will be more accurate. The figure is part of the case with the Polar SI9000 screenshot calculated considering the surface of the solder layer differential line impedance :
Due to the thickness of the solder layer is difficult to control , so it can be based on recommendations mill using an approximate way : subtract a specific value on the results of Surface model , I suggest subtracting 8 ohm differential impedance , single-ended impedance minus 2 ohms.
