A Must-Know Flex PCB Tip

Today, in what will be the first of many flex tips, we will be discussing optimal flex stackups and materials. One of our customers recently sent us a four-layer stackup that needed a little tweaking. We talked it over with our design engineers and came up with solutions and alternatives to all the issues at hand. It’s amazing how a few changes to your stackup design can ensure durability and manufacturability on your flex board.

The Board:

The Issues:

The original stackup given to Sierra Circuits by a customer.

The original stackup given to Sierra Circuits by a customer.

The Solution:
We embedded the flex layers in the center of the stackup. This protected the layers during the manufacturing process and ensured that the less-durable flex layers were not exposed to outer-layer plating. This is how most rigid-flex stackups are designed. When the flex layers are on the outside, panels are harder to handle and harder to process. This made the board more durable and easier to manufacture. It also allowed for better impedance and better control around the flex finger area.

Because the flex layer is a separate process, putting the flex layers inside allows flex manufacturers the ability to etch away from the design while protecting the flex layers. Putting the rigid material on the outside also allows us to manufacture what is essentially a rigid panel. The flex layers are also protected by our surface plating because it should brittle the material. The material used also played a large part in making this board rigid-flex instead of flex. Rigid AP material was used, allowing for better impedance and reliability. It was a much better option than the original FR-4 material.

Sierra’s rigid-flex manufacturing process is as follows: first we process the flex layer as a two-layer flex board. Then we laminate the flex layers in between the rigid layers. The last step is milling the layers so the flex becomes visible.

Sierra Circuits's flex stackup.

Sierra Circuits’s flex stackup.

Working on a flex design? Use our HDI Stackup Planner and Material Selector to save time on your next design.



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