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Stack Up - Impedance Control

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Standard PCB Layer Stack Up

Stackup_Planning_AN2011_2

PCB Stack ups (Ott)

Stack up

For multilayer pcbboards send your requested stack up as our capacities we will send our recommended stack up to you.

2- PCB Stack-Up

Part 1. Introduction

PCB stack-up is an important factor in determining the EMC performance of a product. A good stack-up can be very effective in reducing radiation from the loops on the PCB (differential-mode emission), as well as the cables attached to the board (common-mode emission). On the other hand a poor stack-up can increase the radiation from both of these mechanisms considerably.

Four factors are important with respect to board stack-up considerations:

1. The number of layers,
2. The number and types of planes (power and/or ground) used,
3. The ordering or sequence of the layers, and
4. The spacing between the layers.

Usually not much consideration is given except as to the number of layers. In many cases the other three factors are of equal importance. Item number four is sometimes not even known by the PCB designer. In deciding on the number of layers, the following should be considered:

1. The number of signals to be routed and cost,
2. Frequency,
3. Will the product have to meet Class A or Class B emission requirements,
4. Will the PCB be in a shielded or unshielded enclosure, and
5. The EMC engineering expertise of the design team.

Often only the first item is considered. In reality all the items are of critical importance and should be considered equally. If an optimum design is to be achieved in the minimum amount of time and at the lowest cost, the last item can be especially important and should not be ignored.

Multi-layer boards using ground and/or power planes provide significant reduction in radiated emission over two layer PCBs. A rule of thumb, that is often used, is that a four-layer board will produce 15 dB less radiation than a two-layer board, all other factors being equal. Boards containing planes are much better than those without planes for the following reasons:

1. They allow signals to be routed in a microstrip (or stripline) configuration. These configurations are controlled impedance transmission lines with much less radiation than the random traces used on a two-layer board.
2. The ground plane decreases the ground impedance (and therefore the ground noise) significantly.

Although two-layer boards have been used successfully in unshielded enclosures at 20 to 25 MHz, these cases are the exception rather than the rule. Above about ten or fifteen MHz, multi-layer boards should normally be considered.

When using multi-layer boards there are five objectives that you should try to achieve. They are:

1. A signal layer should always be adjacent to a plane.
2. Signal layers should be tightly coupled (close) to their adjacent planes.
3. Power and Ground planes should be closely coupled together.
4. High-speed signals should be routed on buried layers located between planes. In this way the planes can act as shields and contain the radiation from the high-speed traces.
5. Multiple ground planes are very advantageous, since they will lower the ground (reference plane) impedance of the board and reduce the common-mode radiation..

Often we are faced with the choice between close signal/plane coupling (objective #2) and close power plane/ground plane coupling (objective #3). With normal PCB construction techniques, there is not sufficient inter-plane capacitance between the adjacent power and ground planes to provide adequate decoupling below about 500 MHz. The decoupling, therefore, will have to be taken care of by other means and we should usually opt for tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal (trace) layers and the current return planes will more than outweigh the disadvantage caused by the slight loss in interplane capacitance.

An eight-layer board is the fewest number of layers that can be used to achieve all five of the above objectives. On four and six layer board some of the above objectives will have to be compromised. Under those conditions you will have to determine which objectives are the most important to the design at hand.

The above paragraph should not be construed to mean that you can't do a good EMC design on a four- or six-layer board, because you can. It only indicates that all the objectives cannot be met simultaneously and some compromise will be necessary. Since all the desired EMC objectives can be met with an eight-layer board, there is no reason for using more than eight layers other than to accommodate additional signal routing layers.

Another desirable objective, from a mechanical point of view, is to have the cross section of the board symmetrical (or balanced) in order to prevent warping. For example, on an eight-layer board if layer two is a plane, then layer seven should also be a plane. Therefore, all the configurations presented here use symmetrical, or balanced, construction. If a non-symmetrical, or unbalanced, construction is allowed additional stack-up configurations are possible.

3- How to design PCB stackup

Posted on August by Robert Feranec

There are two options:

1) Leave your PCB manufacturer to design PCB stackup for you (Recommended)

Why?

  • You don't need to spend your time by designing stackup. Leave it on PCB manufacturer – they are professionals. They do it every day. Also, by doing it this way, you can work on design in parallel of handling issues with PCB stackup.
  • Many times PCB manufacturer is not able to build PCB stackup designed by other company or a person outside their company. The main reason is unavailability of material used in some PCB stackups. Selection of material is the best to leave on them – they will choose stocked material.
  • PCB manufacturers use different track geometry calculators and they will not guarantee track impedance if track geometry is calculated by someone else. Many times their calculated numbers and your calculated numbers will be different.
 

2) Design PCB stackup by yourself

Basic information about PCB stackups:

  • PCB is build from three basic materials: Copper foil, Prepreg, Core
  • Standard Copper foil thickness: 5um, 12um, 18um, 35um, 70um
  • Standard prepreg thickness: 65um, 100um, 180um
  • Standard core thickness: 0.15mm, 0.20mm, 0.36mm, 0.46mm, 0.56mm, 0.71mm, 1mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm, 3.2mm. Core is supplied with copper foil on both sides. For some cores you need to add copper foil thickness (18um or 35um) to the core thickness.

There are some rules how to build your stackup. Not every combination is possible. Check it out with your PCB manufacturer.

PCB impedance calculation and Track geometry design

Example of Microstrip (the tracks on TOP and BOTTOM layers) impedance calculation
90 Ohms Differential / 55 Ohm Single ended for:
Track width: 11mil / Copper foil 18um (0.7mil) / Track Gap 8 mil / Dielectric thickness 2x 0.1 prepreg = 0.2mm (7.8mil) / material FR-4 (dielectric constant 4.8)

PCB impedance calculator – Single ended / Differential pair


 Stack-Up-Impedance-Control-001

4- PCB stackup example – minimum track, clearance, VIA …

Here is the PCB stackup what I normally start with + requirements for minimum VIA, track, clearance and impedances.

  • 12 Layers
    • L1 – Signal
    • L2 – GND
    • L3 – Signal
    • L4 – Signal
    • L5 – GND
    • L6 – Powers
    • L7 – Powers
    • L8 – GND
    • L9 – Signal
    • L10 – Signal
    • L11 – GND
    • L12 – Signal
  • Required impedances
    • Single ended: 50 OHMs (all signal layers)
    • Differential: 70, 90 and 100 OHMs (all signal layers)
  • Other requirements
    • minimum through-hole VIA: 0.45mm (pad) / 0.2mm (drill)
    • minimum track / clearance: 0.1 mm / 0.1 mm
      (more expensive PCB: 0.075mm / 0.075mm)

I also found useful information about HDI stackups in this Mentor document:
 http://communities.mentor.com/mgcx/servlet/JiveServlet/previewBody/1128-102-1-1183/hdi%20_layer_stackups_for_large_dense_pcbs.pdf

HDI PCB technology comparison (picture from the Mentor document)
 Stack-Up-Impedance-Control-002

1-020 Finished Thicknesses

The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric."

Stack-Up-Impedance-Control-004

Click Here to View a Larger image of .020" 4 Layer Stack Up

ETAG Circuits builds Single-Sided through 10 layer prototype and production circuit boards. The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric". We process four layer boards with final thickness of 0.020", 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125". You may select ½, 1 or 2 ounce inner layer copper foil for your board.

While our 4, 6, and 8 layer boards are limited to 16X22 inches the 10 layer maximum size is 14x20.

We have several inner layer cores available using Isola's FR406 materials - they include 5, 8, 10, 14, 18, 21, 28, 35, 39, 59 and 93 mil cores.

-Six layer boards are produced in thickness of 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125" with the same inner layer foil options.

-Both our eight and ten layer boards are available in finished thickness of 0.062", 0.093" and 0.125".

ETAG Circuits can supply you with a RoHS compliant multi-layer board using laminate materials that can be matched up with the exposure to high temperatures during your assembly processes. It is important to keep in mind that some lead free assembly processes will require the laminate base material to withstand temperatures in excess of 260 degrees C or 500 degrees F for extended periods of time. To resolve this, we have high temperature laminates in our inventory so that our customers will be able to meet the higher temperature cycling requirements of some lead free assembly applications.

In high temperature materials we currently stock Isola's 370HR in cores of 5, 8, 10, 14, 21, 28, 39, 59, 93 and 125 mils. ETAG Circuits has UL approval for processing high thermal capacity materials manufactured by Isola and Nelco among others in producing your multi-layer board. These materials met the UL testing for 130°C maximum operating temperature, solder limits of 288°C for 20 seconds, 94-V0 flame rating and direct support of current carrying parts. Each of these laminate systems meets the minimum requirements of IPC 4101B. Individual manufacturer's material types may exhibit variations in electrical, thermal and physical properties. Normally a single manufacturer's material will be stocked for use. If you have specific questions please contact your ETAG Circuits sales representative

062 Finished Thickness on PCB's

The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric."

Stack-Up-Impedance-Control-006

Click Here to view a Larger Image of .062" 4 Layer Stack Up

Stack-Up-Impedance-Control-007

Click Here to view a Larger Image of .062" 6 Layer Stack Up

Stack-Up-Impedance-Control-009

Click Here to view a Larger Image of .062" 8 Layer Stack Up

Stack-Up-Impedance-Control-011

Click Here to vew a Larger Image of .062" 10 Layer Stack Up

ETAG Circuits builds Single-Sided through 10 layer prototype and production printed circuit boards. The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric."We process four layer boards with final thickness of 0.020", 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125". You may select ½, 1 or 2 ounce inner layer copper foil for your board.

While our 4 and 6 layer boards are limited to 16X22 inches the 8 layer is 12x18 and 10 layer maximum size is 10x16.

We have several inner layer cores available using Isola's FR406 materials - they include 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59 and 93 mil cores.

-Six layer boards are produced in thickness of 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125" with the same inner layer foil options.

-Both our eight and ten layer printed circuit boards are available in finished thickness of 0.062", 0.093" and 0.125".

ETAG Circuits can supply you with a RoHS compliant multi-layer board using laminate materials that can be matched up with the exposure to high temperatures during your assembly processes. It is important to keep in mind that some lead free assembly processes will require the laminate base material to withstand temperatures in excess of 260 degrees C or 500 degrees F for extended periods of time. To resolve this, we have high temperature laminates in our inventory so that our customers will be able to meet the higher temperature cycling requirements of some lead free assembly applications.

In high temperature materials we currently stock Isola's 370HR in cores of 8, 10, 14, 22 and 39 mils. ETAG Circuits has UL approval for processing high thermal capacity materials manufactured by Isola and Nelco among others in producing your multi-layer board. These materials met the UL testing for 130°C maximum operating temperature, solder limits of 288°C for 20 seconds, 94-V0 flame rating and direct support of current carrying parts. Each of these laminate systems meets the minimum requirements of IPC 4101B. Individual manufacturer's material types may exhibit variations in electrical, thermal and physical properties. Normally a single manufacturer's material will be stocked for use. If you have specific questions please contact your ETAG Circuits sales representative.

093 Finished Thickness

The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric."

Stack-Up-Impedance-Control-013

Click Here to view a Larger Image of .093" 4 Layer Stack Up

Stack-Up-Impedance-Control-015

Click Here to view a Larger Image of .093" 6 Layer Stack Up

Stack-Up-Impedance-Control-017

Click Here to view a Larger Image of .093" 8 Layer Stack Up

Stack-Up-Impedance-Control-019

Click Here to view a Larger Image of .093" 10 Layer Stack Up

ETAG Circuits builds Single-Sided through 10 layer prototype and production circuit boards. You can use one of our "standard" dielectric builds or specify your own controlled stack-up. We process four layer boards with final thickness of 0.020", 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125". You may select ½, 1 or 2 ounce inner layer copper foil for your board.

While our 4 and 6 layer boards are limited to 16X22 inches the 8 layer is 12x18 and 10 layer maximum size is 10x16.

We have several inner layer cores available using Isola's FR406 materials – they include 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59 and 93 mil cores.

Six layer boards are produced in thickness of 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125" with the same inner layer foil options.

Both our eight and ten layer boards are available in finished thickness of 0.062", 0.093" and 0.125".

ETAG Circuits can supply you with a RoHS compliant multi-layer board using laminate materials that can be matched up with the exposure to high temperatures during your assembly processes. It is important to keep in mind that some lead free assembly processes will require the laminate base material to withstand temperatures in excess of 260 degrees C or 500 degrees F for extended periods of time. To resolve this, we have high temperature laminates in our inventory so that our customers will be able to meet the higher temperature cycling requirements of some lead free assembly applications.

In high temperature materials we currently stock Isola's 370HR in cores of 8, 10, 14, 22 and 39 mils. ETAG Circuits has UL approval for processing high thermal capacity materials manufactured by Isola and Nelco among others in producing your multi-layer board. These materials met the UL testing for 130°C maximum operating temperature, solder limits of 288°C for 20 seconds, 94-V0 flame rating and direct support of current carrying parts. Each of these laminate systems meets the minimum requirements of IPC 4101B. Individual manufacturer's material types may exhibit variations in electrical, thermal and physical properties. Normally a single manufacturer's material will be stocked for use. If you have specific questions please contact your ETAG Circuits sales representative.

125 Finished Thickness

The stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these stackups are not "Guaranteed" unless you so specify in your files used when placing your order. Additionally, for these or any specific stackup that you require, be sure to order as "Custom" and select "Controlled Dielectric."

Stack-Up-Impedance-Control-021

Click Here to view a Larger Image of 0.125" 4 layer Stack Up

Stack-Up-Impedance-Control-023

Click Here to view a Larger image of 0.125" 6 Layer Stack Up

Stack-Up-Impedance-Control-025

Click Here to view a Larger Image of 0.125" 8 Layer Stack Up

Stack-Up-Impedance-Control-027

Click Here to view a Larger Image of 0.125" 10 Layer Stack Up

ETAG Circuits builds Single-Sided through 10 layer prototype and production circuit boards. You can use one of our "standard" dielectric builds or specify your own controlled stack-up. We process four layer boards with final thickness of 0.020", 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125". You may select ½, 1 or 2 ounce inner layer copper foil for your board.

While our 4 and 6 layer boards are limited to 16X22 inches the 8 layer is 12x18 and 10 layer maximum size is 10x16.

We have several inner layer cores available using Isola's FR406 materials – they include 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59 and 93 mil cores.

Six layer boards are produced in thickness of 0.031", 0.040", 0.047", 0.062", 0.093" and 0.125" with the same inner layer foil options.

Both our eight and ten layer boards are available in finished thickness of 0.062", 0.093" and 0.125".

ETAG Circuits can supply you with a RoHS compliant multi-layer board using laminate materials that can be matched up with the exposure to high temperatures during your assembly processes. It is important to keep in mind that some lead free assembly processes will require the laminate base material to withstand temperatures in excess of 260 degrees C or 500 degrees F for extended periods of time. To resolve this, we have high temperature laminates in our inventory so that our customers will be able to meet the higher temperature cycling requirements of some lead free assembly applications.

In high temperature materials we currently stock Isola's 370HR in cores of 8, 10, 14, 22 and 39 mils. ETAG Circuits has UL approval for processing high thermal capacity materials manufactured by Isola and Nelco among others in producing your multi-layer board. These materials met the UL testing for 130°C maximum operating temperature, solder limits of 288°C for 20 seconds, 94-V0 flame rating and direct support of current carrying parts. Each of these laminate systems meets the minimum requirements of IPC 4101B. Individual manufacturer's material types may exhibit variations in electrical, thermal and physical properties. Normally a single manufacturer's material will be stocked for use. If you have specific questions please contact your ETAG Circuits sales representative.

PrePreg Thickness Chart

Click Here to to view our PrePreg Thickness Chart

Impedance control

notes_25

Controlled_Impedance

PCB Controlled Impedance

Click Here to see our Controlled Impedance Best Price Guarantee + FREE Shipping! and all of our other specials.

Impedance is the sum of the resistance and reactance of an electrical circuit expressed in Ohms. The resistance being the opposition to current flow present in all materials. The reactance is the opposition to current flow resulting from the effect of the inherent capacitance and inductance of the conductor interacting with changes in voltage and current. In DC circuits there is no reactance and the resistance of copper conductors is typically insignificant. However in high speed AC circuits (those with sharp changes in voltage and/or current) the reactance and thus the impedance can become very significant. This can become critical to a design's functionality because of the effects that changes in the impedance along the signals path from transmitter to receiver will have on the efficiency of power transfer as well as signal integrity. While a circuit's speed is often expressed as the frequency of the wave form: the critical concern is the speed at which the voltage and/or current is required to change.

Controlled Delectric

The typical design considerations involved in the determination of the requirement for controlling the impedance are the strength of the signals involved, the susceptibility of the circuit to noise and signal distortion, the criticality of signal timing and the speed at which the signal's source is attempting to force a change in voltage and/or current.
The design considerations for setting the value of the impedance for the conductor is typically the output impedance of the transmitter and the input impedance of the receiver. The impedance of other conductors (i.e. coaxial cables) in the circuit path will also need to be considered. The acceptable range (tolerance) for the impedance will need to be determined and taken into account during the design phase as well as when specifying the PCB parameters. In many cases merely by using software models to determine the anticipated impedance with particular dielectric materials and spacing, followed by requesting these parameters be followed when the PCB's are fabricated will suffice. This is what we call "controlled dielectric". For more critical applications you will need to specify "controlled impedance" and supply the actual impedance requirements for the conductors and we will fine tune the dielectrics and conductors to meet these requirements. The impedance requirements will have to be specified based on the layer and the conductor widths.

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