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Flex Circuit Assembly Process (Sitiffeners)

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#62790

Flex Circuit Assembly Process (Sitiffeners) | 29 September, 2010

Hi,

We are placing a Surface Mount connector on a flex circuit, we are planing on using SMT carriers to support the flex through the SMT process.

Underneath the connector a stiffener needs to be adhere, I'm not sure if we should have the flex manufacturer adhere the stiffener or for us to adhere after the SMT process. I'm not sure if the stiffener is on the flex before will cause any problems through the SMT process. We don't have experience on flex circuit assembly, any suggestions or ideas are greatly appreciated.

Thank you, David

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#62793

Flex Circuit Assembly Process (Sitiffeners) | 29 September, 2010

You might try assembly carrier tape such as Tacsil on your fixtures. Beware - arrays are very diffult to run in SMT flex unless you have no fine pitch.

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#62795

Flex Circuit Assembly Process (Sitiffeners) | 29 September, 2010

SWAG, Thank you for your suggestion, you brought a very good point.

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#62816

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

Is the stiffener a piece of FR4, or a component that needs to be installed?

We've done a number of flex assemblies taht have a rigid stiffener, generally a piece of rigid flex, or FR4 that has the components mounted on it. This type of stiffener is a part of the assembly performed by the flex manufacturer, and, as such, was received to our process that way. The stiffeners that we have worked with have shown no impact on the processing, except to give us a nice rigid surface to place the parts on (which has aided in repeatability). We observed no impact to reflow profiling.

The only note is to ensure that the baking process that is incorporated for processing the flex circuits is extended a bit to allow the rigid material to be baked completely dry.

Cheers, ..rob

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#62818

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

Rob, thank you for input.

I notice you mention a baking process for the flex circuits, this is our first time with flex assembly, do the flex circuits need to be baked prior SMT, Moisture?

Thanks, David

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#62819

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

Yes, they should be baked prior. We pre-bake all polyimide fabs, flex or not. They are prone to delam.

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#62821

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

You don't necessarily have to bake the flex circuits, but you do have to worry about moisture. Polyimide is very hydroscopic, and depending on the flex construction, any moisture in the product can lead to delamination. Adhesive based flex circuits are very sensitive to this.

We run only flex circuits in our process and we place all flex in a "dry room" for at least 48 hours before using. This room is a controlled humidy area where the rH < 25%. The lower the rH, the less time needed to pull all the moisture out of the material.

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#62822

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

Thank you Noid,

Is the baking process product specific are they any guidelines/standard procedures? Any recommendations are appreciated.

Thanks, David

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CL

#62823

Flex Circuit Assembly Process (Sitiffeners) | 1 October, 2010

Good Afternoon,

How is the stiffener adheared? If it is laminated to the flex, it may help you with the surface planarity through reflow. If it put on with PSA, it may cause more problems then it would solve. PSA gets soft through the reflow. It may loose adheasion and cause planarity issues.

Best of luck

Chris

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