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PCB Technology

PCB Technology - How to control impedance PCB

PCB Technology

PCB Technology - How to control impedance PCB

How to control impedance PCB
2023-03-23
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Author:IPCB

Without Impedance control, it will cause considerable signal reflection and distortion, leading to design failure. Common signals such as PCI bus, PCI-E bus, USB, Ethernet, DDR memory, LVDS signals, etc. all require impedance control. Impedance control ultimately needs to be achieved through PCB design, PCB technology also puts forward higher requirements, after communication with the PCB factory, and combined with the use of EDA software, in accordance with the requirements of signal integrity to control the impedance of the alignment.


The following different types of piping can be calculated to get the corresponding impedance value.


Microstrip line

A microstrip line consists of a strip of conductor wire with a ground plane and a dielectric in the middle. The dielectric constant of the dielectric of the microstrip line, the width of the line, and its distance from the ground plane are controlled, and so is its characteristic impedance, which is within ±5% of the accuracy.

Description of Microstrip Line

Description of Microstrip Line

Stripline

Stripline is a copper strip of dielectric material placed between two conductive planes. The thickness and width of the stripline, the dielectric constant of the dielectric, and the distance between the two ground planes are all controllable, and the characteristic impedance of the wire is also controllable to within 10%.

Regarding the structure of multilayer PCB, in order to have a good impedance control of PCB, we should first understand the structure of PCB.


Usually we say that the multi-layer PCB is made of core board and semi-cured sheet laminated and pressed into each other, the core board is a kind of hard, with a specific thickness, the two packets of copper sheet, is the basic information of the PCB. And semi-cured sheet constitutes the so-called immersion layer, play the role of bonding core board, although there is also a certain initial thickness, but in the pressing process its thickness will occur some changes.


Usually, the two outermost dielectric layers of a multilayer PCB are immersion layers, and a separate layer of copper foil is used on the outside of these two layers as the outer layer of copper foil. The original thickness of the outer and inner copper foils are generally 0.5OZ, 1OZ, 2OZ (1OZ is about 35um or 1.4mil), but after a series of surface treatments, the final thickness of the outer copper foils is generally increased by about 1OZ. The inner layer of copper foil is the copper cladding on both sides of the core board, and its final thickness is very small compared to the original thickness, but due to etching, it is generally reduced by a few um.


The outermost layer of a multi-layer PCB is the soldermask, which is often referred to as green paint, but of course it can also be yellow or other colours. The thickness of the soldermask is generally not easy to determine accurately, in the surface of the area without copper foil is slightly thicker than the area with copper foil, but because of the lack of thickness of the copper foil, so the copper foil is still more prominent, when we touch the surface of the PCB with a finger can be felt.


When producing a PCB with a specific thickness, on the one hand it is necessary to choose the parameters of the various data wisely, on the other hand, the final thickness of the semi-cured sheet will be smaller than the initial thickness. Below is a 6-layer PCB stack structure:

Multi-layer PCB Stacking Structure

Multi-layer PCB Stacking Structure

PCB Parameters

PCB parameters may vary slightly from one PCB manufacturer to another. By communicating with the PCB manufacturer's technical support, we can get some parameter data from the PCB manufacturer.


Surface Copper Foil: There are three thicknesses of surface copper foil that can be used: 12um, 18um and 35um. The final thickness after processing is about 44um, 50um and 67um.


Core board: Our commonly used board is IT180, standard FR-4, copper clad on both sides, optional specifications can be determined by contacting the PCB manufacturer.


Semi-cured sheet: Specifications (original thickness) are 7628 (0.185mm), 2116 (0.105mm), 1080 (0.075mm), 3313 (0.095mm), the actual thickness of the finished press is usually smaller than the original value of about 10-15um. A maximum of 3 semi-cured sheets can be used for the same impregnation layer, and the thickness of all 3 semi-cured sheets cannot be the same, at least one semi-cured sheet can be used, but some manufacturers require at least two sheets to be used. If the thickness of the semi-curing sheet is not enough, you can etch off the copper foil on both sides of the core board, and then use the semi-curing sheet to stick on both sides, so that a thicker immersion layer can be realised.


Solder Resist Layer: The thickness of the solder resist layer on top of the copper foil is C2 ≈ 8-10um, the thickness of the solder resist layer in the area without copper foil on the surface is C1 which varies according to the thickness of the copper on the surface, when the thickness of the copper on the surface is 45um, the thickness of the solder resist layer on the surface is 13-15um, when the thickness of the copper on the surface is 70um, the thickness of the solder resist layer on the surface is C1 ≈ 17-18um.


Lead wire cross-section: We would think that the cross-section of the lead wire is a rectangle, but it is actually a trapezoid. Take TOP layer as an example, when the thickness of copper foil is 1OZ, the upper bottom edge of the trapezoid is 1MIL shorter than the lower bottom edge. For example, if the width of the wire is 5MIL, then the upper bottom edge is about 4MIL, and the lower bottom edge is 5MIL, and the difference between the upper and lower bottom edges is related to the thickness of copper.


Dielectric constant: The dielectric constant of semi-cured sheet is related to the thickness: the dielectric constant of the plate is related to the resin data used, the dielectric constant of FR4 plate is 4.2-4.7, and it will decrease with the increase of frequency.


Dielectric Loss Factor: The energy consumed by the heating of dielectric material under the action of an alternating electric field is called dielectric loss, and is usually expressed as the dielectric loss factor, tan δ. Typical value of S1141 is 0.015.


Line Tolerance: Minimum line width and line spacing to ensure processing: 3mil/3mil.


Introduction to Impedance Calculation Tools


Once we understand the structure of a multi-layer PCB and have the required parameters, we can calculate the impedance using EDA software. You can use Polar SI9000, which is a very good tool to calculate the characteristic impedance, now many PCB manufacturers are using this software.


Whether it is a differential line or single-ended line, when calculating the characteristic impedance of the inner layer of the signal, you will find that the Polar SI9000 calculation results and Allegro there is only a small difference, which is related to some of the details of the processing, such as the shape of the cross-section of the wire. However, if you are calculating the characteristic impedance of a surface signal, I recommend that you choose the Coated model instead of the Surface model, because this model takes into account the presence of the soldermask, so the results will be more accurate. Below is a partial screenshot of the impedance of a surface differential line calculated with Polar SI9000 taking into account the presence of a soldermask:


Since the thickness of the soldermask is not easy to control, an approximation can also be used according to the board manufacturer's recommendations: Subtract a specific value from the result of the Surface model calculation, suggesting a differential impedance of 8 ohms and a single-ended impedance of 2 ohms.


Differential PCB requirements for alignment:


  1. Determine the alignment pattern, parameters and impedance calculation. Differential pair alignment of the outer microstrip line differential mode and the inner layer of the strip line differential mode, through a reasonable set of parameters, impedance can be calculated using the relevant impedance calculation software (such as POLAR-SI9000) can also be calculated using the impedance calculation formula.

  2.  Take parallel isometric lines. Determine the alignment line width and spacing, in the alignment should be strictly in accordance with the calculated line width and spacing, the distance between the two lines should always remain unchanged, that is, to maintain parallel. There are two types of parallel pipework: one is where the two lines run on the same level (side-by-side) and the other is where the two lines run on two levels (over-under). Generally avoid using the latter, i.e., interlayer differential signals, because in the actual processing of PCB boards, due to the precision of lamination alignment between layers is much lower than the precision of same-layer etching, as well as the loss of dielectrics in the process of lamination, which does not guarantee that the spacing of the differential lines is equal to the thickness of the interlayer dielectrics, and this will result in the differential impedance of the differential pairs between the layers to be varied. It is recommended to use differentials within the same layer as much as possible.


The above is our explanation of how to control the impedance of PCB, as well as the multi-layer PCB stacked structure and the parameters that affect the impedance.