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Layer Stackup in Rigid and Rigid-Flex PCBs

Created: July 16, 2018
Updated: November 27, 2020

Digital TV decoder on green PCB

Building multilayer PCBs is like building with Legos. The all fit together easily, but you still need to follow directions if you want your design to stay together. Nowadays, only the simplest PCBs use a single layer, or a pair of top and bottom layers. Multilayer PCBs are now the norm rather than the exception, and manufacturers can fabricate PCBs with up to 30 layers. Layer stackup strategies in these boards are important for a number of applications.

The PCB must be equipped for any shift in circuit board strategy. And what’s the PCB designer’s main equipment? Their CAD tools of course. Any printed circuit is going to need a strong supporting PCB design software to back it up. This goes double for flexible and rigid flex circuits wherein the design rules get a bit harder to keep track of. Don’t drown among your layer stack, keep your flex circuit boards printing smoothly.

Multilayer Stackup Strategies

In a perfect world, your layer stackup strategy should allow you to arrange traces and ground planes to perfectly block EMI and crosstalk, it should be adaptable to flex and rigid-flex design, and what the heck, let’s say that it won’t heat up past room temperature. Obviously, that’s an impossible order, and no stackup strategy will be able to perfectly accommodate every requirement simultaneously.

High pin density boards will generally require more signal layers, and there are some general guidelines on the number of layers that should be included in a standard PCB. Depending on your routing density during layout, you may find that some signal layers were unnecessary and can be removed. Once you have chosen the appropriate number of layers, you need to arrange them while paying attention to how the arrangement affects EMI and crosstalk.

A typical multilayer stackup will alternate between signal and power/ground planes, with each layer separated by either dielectric core or prepreg. The recommended stackup arrangement will be depend on the number of layers in the board, but the above guideline is always followed in order to suppress EMI and crosstalk between layers. Addressing thermal issues requires its own design considerations that can also be included in multilayer boards.

Devices with analog and digital elements should use separate ground layers in a multilayer stackup. The two ground planes should only be connected at a single point. The same applies to analog and digital signal layers. Separating your analog and digital signal layers and crossing them at only a single point is a good strategy to avoid noise coupling. An alternative is to use a single layer for ground, and simply split the ground plane into digital and analog sections.

Colorful microchip on PCB with blue traces

Routing doesn’t have to be as much of a challenge in flex circuit boards

Separating the analog and digital signal layers with their respective ground planes is also a good strategy to prevent EMI. Placing the ground planes between the analog and digital signal layers creates effective shielding between the two layers. The analog ground plane should be placed next to the analog signal layer, and the same applies to the digital signal layer. This ensure that each signal layer will only induce a return current in its respective ground plane.

Rigid-flex Stackup

Once you move on to rigid flex PCB design, you’ll need to define your flex ribbon using a similar stackup as your rigid PCB. Flex ribbons are thinner than the boards with which they interface, and the flex ribbon will need to have the same stackup as the inner layers in the rigid printed circuit board. Flex ribbons typically need to carry signal between boards, and they will also need to have a ground return connection.

If you intend to extend a power or ground plane all the way across the flex ribbon, use a cross-hatched copper pattern. Cross-hatched copper provides improved flexibility over a solid copper film or copper foil. Cross-hatched copper should also be used if you need shielding for signal layers on the flex ribbon.

Flex ribbons do not need to have the signal layer placed directly below the coverlayer on the flex ribbon unless you plan to place components directly on the ribbon. You can place SMT components placed directly on the ribbon as long as there is a signal layer below the coverlay. This is becoming a more popular design choice.

Solder lands need to be placed in the signal layer directly below the coverlay, and the coverlay needs to have holes punched so that SMT components can access the signal layer. Always check with your fabricator and make sure that their capabilities will accommodate your design choice. Always avoid placing these features and components directly in the bend area, and place components so that the length of the component is parallel with the bend.

Flex ribbon with surface mount components

SMT components can be managed effectively with flex ribbon

An alternative method for placing components on a ribbon involves button plating. The signal layer still needs to be placed below the coverlay in the stackup, and punched holes in the coverlay are also used to access the signal layer. A through hole via is placed in the mounting pad, and the structure is then plated up to a specific thickness. If the via holes are left open, through hole components can then be placed on the flex ribbon.

Your PCB board design software should allow you to define rigid flex stackup without requiring extra design steps or specialized tools. The advanced CAD, layout, and simulation tools in Altium Designer® can make it easy to define your layer stackup. Download a free trial and find out if Altium Designer is right for you.

To resolve and accomplish any of your designed flexible PCB, equip yourself with the software to do more. With the right tools backing you, you can design anything. Talk to an Altium Designer expert today.

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