A brief discussion on the impedance control of pcb fabrication and assembly

A brief discussion on the impedance control of PCB fabrication and assembly

As circuit design becomes increasingly complex and high-speed, how to ensure the integrity of all kinds of signals, especially high speed signals, is to ensure the quality of signal ,it become a problem. At this time, it need to analyze by using transmission line theory, the characteristics of the control signal impedance matching becomes the key, there is not strict impedance control, will lead to considerable signal reflection and signal distortion, cause the failure of design.


Common signals, such as PCI bus, PCI-E bus, USB, Ethernet, DDR memory, LVDS signal, etc., need to be controlled by impedance. The impedance control ultimately needs to be realized through PCB design, and higher requirements for PCB technology are also proposed and combined with the use of EDA software.


The structure of Multi-layer board:

In order to control the impedance of PCB fabrication and assembly well, we should first understand the structure of PCB:

What we usually call multilayer board is composed of core board and semi-solidified plates that stack together with each other, core board is a kind of hard, has a certain thickness, copper plate, two sides of the PCB covered with copper. The semi-solidified plate constitutes the so-called infiltration layer, which ACTS as the bonding core plate, although it has some initial thickness, but the thickness can change during the suppression process.

Usually the two dielectric layers in the outer layer of multi-layer board are the infiltration layer, use a separate copper foil layer outside the two layers as the outer copper foil. The original thickness specification of the outer copper foil and the inner copper foil, generally speaking, there are 0.5 OZ, 1 OZ, 2 OZ (1 OZ of about 35 um or 1.4 mil) three, but after a series of surface treatment, outer final thickness of copper foil will generally increase nearly 1 OZ. The inner copper foil is the copper cover both sides of the core board. The final thickness of the copper is very small difference compared to the original thickness. However, due to the etching, it is generally reduced a few um.

The outer layer of the multilayer board is the resistance layer, which is often referred to as "green oil", but it can also be yellow or other colors. The thickness of resistance welding layer is generally not too easy to determine accurately, The area with no copper on the surface is slightly thicker than the area with copper foil, but because of the lack of the copper foil thickness, so the copper foil appears more prominent, when we use finger to touch the PCB surface can feel it.

When making a printing board of a certain thickness, on the one hand, it is required to choose the parameters of various materials reasonably, on the other hand, the final thickness of the semi-solidified plate is also smaller than the initial thickness.Here is a typical layer of 6 laminated layers:

6 laminated layers datasheet.jpg

      The parameters of PCB:

For different PCB plants, the parameters of PCB will be slightly different. Here are some reference data:


       The surface layer of copper foil

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


       Core board:

Our commonly used the material of board is S1141A, the standard is fr-4, two sides cover with copper, the optional specification can be contacted with the manufacturer to confirm.


       Semi-solidified plates:

The specifications (original thickness) are 7628(0.185 mm), 2116(0.105 mm), 1080 (0.075mm), 3313 (0.095mm), and After the actual suppression, the thickness is usually 10-15um smaller than the original value. The same infiltration layer can use at most three semi-solidified plates, and the thickness of the three semi-solidified plates should not be the same, can use at least one semi-solidified plates, but some manufacturer require must use at least two. If the thickness of the semi-solidified plate is not enough, the copper foil on both sides of the core board can be etched, and then it can be adhered on both sides with a semi-solidified plate, which can achieve thicker infiltration layer.


       Resistance welding layer:

The thickness of resistance welding layer on the copper foil is C2≈88-10um, and the thickness of the resistance welding layer in the non-copper foil area is different according to the thickness of surface copper. When the thickness of surface copper is 45um, C1 is approximately13-15um,When the thickness of surface copper is 70um, C1 is approximately 17-18um.


       Dielectric constant:

The dielectric constant of the semi-solidified plate is related to the thickness, and the following picture is the thickness and dielectric constant parameters of different types of semi-solidified plates:

The dielectric constant of the board is related to the material of resin, the dielectric constant of the FR4 is 4.2-4.7, and will decreases with the increase of frequency.


Dielectric loss factor: the dielectric material is under the action of alternating electric field, the energy consumed by heat is called dielectric loss,which is usually represented by tanδ. The typical value of S1141A is 0.015.


Can ensure the minimum line width and spacing of the machining: 4mil/ 4mil.


      Tool introduction to impedance calculation:


When you understand the structure of the multilayer board and master the required parameters, you can calculate the impedance through EDA software. You can use Allegro to calculate, Polar SI9000, which is a great tool for calculating the characteristic impedance, and now a lot of PCB factories are using this software.


No matter the difference line and single end line, when you are calculating the characteristic impedance of the inner signal, you will find that Polar SI9000 calculation results and the Allegro there are only small gap, it has to do with the processing on some details, such as the shape of the conductor cross section. But if it is calculating the characteristic impedance of the Surface signal, I suggest that you choose the Coated model instead of the Surface model, Because such models consider the existence of resistance welding layers, the results are more accurate. The image below is a partial screenshot of the surface difference line impedance in the case of Polar SI9000 calculation considering the resistance welding layer:


As a result of the resistance welding layer thickness is not easy to control, so we can also accept the suggestion of  the PCB factory, using an approximate way: on the results of model calculation of Surface minus a certain value, I suggest that the differential impedance minus 8 ohms, single-ended impedance minus 2 ohm.