Flexi Circuits Assembly

We've talked about assembling on flex previously. Search the fine SMTnet Archives for background.

Issues that come to mind [which is very risky, given that this Saturday morning and last night was at Jimbo's] are: * Use carriers that can flow though the process, so that you don't need to remove the flex between or during processing. * Don't bend the flex GT minimum bend radius. 10X thickness is a commonly quoted radius, but truly this varies with contruction type and materials. * Be careful with absorbed moisture when soldering. * Be aware of the temperature limits of the material you are using when soldering.

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My first reaction would be to run! FAST!

Flex -- well, it... flexes... and isn't flat. While you assemble it, While you transport it, While it goes through the oven...

SMT Connectors typically need to be held down during reflow. Fiducial types are critical depending on how the top layer was built. Moisture is a big issue, typically a bake out is recommended. Fixturing is critical depending on flex complexity. Don't rely on panel fiducials too much. Love to get stuck/lost in the washers (use baskets). Stencil designs are different, can't use rigid board rules (you'll typically want 80% apature size of vs. lan size due to inherent misregistration), tooling pins get in the way of the squeegee (over 80% of the flex I worked with required hand printing to get around the tooling pins - what a mess). Depanaling can EASILY damage these critters. Stiffners are a real source of problems. Any PSA that is applied could affect your close loop system. Carriers are recommended for shipment (there's a shop on Lompoc, CA that does this).

Fabricating the flex circuits is more of an art than what you see in the bare board industry. You could easily get a bad batch of flex that would show up as assembly defects without knowing how to inspect for flex defects and how the fab process can determine how the defects appear (delamination is one that really sticks in my mind).

Just a few things off the top of my head, it's been almost a year since I left the company that specialized in flex and I just hit a few of the biggies.

Hope this helps, Scott

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