PCB Design Challenges for 1.6mm 6 layer PCB

November 4, 2019 Cadence PCB Solutions

A layer of a 6 layer PCB

 

There are a few famous ‘6’s out there: Toronto, the Philadelphia 76ers, 6-pack abs, and the 1.6mm 6 layer PCB. While it’s unlikely I’ll ever regain the abs I had in my youth, and perhaps equally unlikely I join a professional basketball team, Toronto seems alright but the 6 layer PCB is really where my heart lays in the matter. 

Designing a 6 layer PCB with 1.6 mm thickness is certainly within my ability. With that said, I’ll stress that a 6 layer PCB stackup design offers more challenges than it appears to be. You’ll need to be ‘disciplined,’ at least in weighing each aspect of the design and paying close attention to the best practices to avoid unwanted problems. 

Why Do You Need A 1.6 mm 6 Layer PCB 

Unlike my ‘has-been’ 6-packs abs which grow by the year, electronics have been shrinking for the past decade. With smart wearables and IoT revolutionizing industries, PCB designers are pressured to deliver smaller PCB footprints without compromising on functionalities.

Naturally, this calls for increasing the layers between the standard 1.6 mm layer PCB. For most designers, a 4-layer PCB stackup wouldn’t be an issue, but adding a couple more layers means some tough decisions need to be made.

The additional layers help to decrease the board area of the PCB. Also, when you have mixed signals, the inner layers can be used to separate high-frequency from analog or power signals. A 6 layer PCB can decrease EMI susceptibility and emission if done right. 

 

 Circuit board heavily laden with components

Densely-populated PCB calls for a 6 Layer stackup

 

Typical 1.6 mm 6 Layer PCB Stackup

A 6 layer PCB is achieved by adding a couple of cores, which are dielectric substrate coated with copper in between the 1.6 mm PCB. These cores are held together with prepregs, which are epoxy-based materials. 

There are a few variants of 6-layer PCB stackups that are commonly used. Each comes with its advantages and challenges. 

 

6 Layer PCB Stackup Variant 1 

  • Signal (Top Layer)

  • Ground

  • Signal (Inner Layer 1)

  • Signal (Inner Layer 2)

  • Power

  • Signal (Bottom Layer)

 

This setup offers the most conductive layers for signal routing. However, having 2 adjacent routing layers means the possibility of crosstalk amongst the traces. The return path for signals on Inner Layer 2 and Bottom Layer will pass through the sole ground plane and may cause ground noise.

 

6 Layer PCB Stackup Variant 2 

  • Signal (Top Layer)

  • Ground

  • Signal (Inner Layer 1)

  • [Large Gap]

  • Signal (Inner Layer 2)

  • Power

  • Signal (Bottom Layer)

 

Introducing a larger gap between Inner Layer 1 and Inner Layer 2 helps to decrease the chance of cross-coupling between signals of both layers. This can be done by increasing the core thickness that separates both layers. 

 

6 Layer PCB Stackup Variant 3 

  • Signal (Top Layer)

  • Ground

  • Power 

  • [Large Gap]

  • Signal (Inner Layer 1)

  • Ground

  • Signal (Bottom Layer)

 

This stackup, which has only 3 signal layers, is notably better in terms of noise immunity. There are no adjacent layers with risks of cross-coupling, and the presence of two ground planes decreases the risk of ground noise. It is also easier to plan for the return paths for the signal layers. Of course, the downside of this variant is the reduction of the signal layers. 

Important Considerations For 1.6mm 6 layer PCB Design

Regardless of which stackup you chose for a 6 layer PCB design, return paths must take priority. If a high-speed signal travels a great distance, it will affect the integrity of adjacent signals. In such cases, having an additional ground plane helps. With that said, you’ll need to ensure that the ground plane is continuous to provide the shortest return path

 

Monitor measure frequency for EMI board testing

EMI on a 6 layer PCB can be reduced with the right techniques.

 

You’ll also want to be smart in managing the layer stackup. Apparently, having an equally distributed layers isn’t the best option when it comes to creating a low EMI susceptibility 6 Layer PCB. This means you’ll need a thicker prepreg in the middle and thinner ones on the outer part of the PCB stack.

Having  good PCB design software helps you to create a 6 layer stackup easily on a 1.6m PCB. Cadence cross-section editor gives you the flexibility to customize all the parameters involved in a multilayer PCB. Utilizing all that Allegro PCB Designer can offer you for your layout and design analysis will be sure to make any challenging board design a breeze. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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