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I need to design some tiny flex-rigid boards, about 40 x 30 mm, that have small radius bends - space is extremely limited in the enclosure the circuit has to fit into.  I expect I'm going to push the envelope with this application, but I need to know how far is too far because the application is potentially cost-sensitive if the boards wind up being exhorbitant.  I'm aiming for a four layer flex-rigid board (two internal flex layers, two component sides), but I'm open to suggestions.

Can someone tell me what the tightest radius a right-angle bend could be?

Also, what is the smallest track width/drill-size I can realistically hope to implement in flex before cost becomes outlandish?

I'm aiming for 0.2 mm tracks and spacing, but I don't think this will be achievable.  Is 0.1 mm going to cause me headaches with tight bends, or is this likely to be ok?

Thanks for your help!

You should be able to bend the flex to a .045" radius (approx 5:1 bend ratio) one time without problems.  This assumes that the bend is constrained after installation and not allowed to repeatedly flex.   

Feature size depends a lot on copper weight.  The internal layers without plating can be produced with feature sizes approaching .1 mm without a lot of trouble.  The outer layers will have additional plated copper added which  makes the small feature more difficult to define.  If you can split the difference and design for .15 mm, you should be in pretty good shape.

When it comes to drill size, bigger is better.  We would like to see at least .015" drill for a finished hole size around .010".  We can go smaller, but it will start to impact the cost.

Following the above recommendations will keep the rigid-flex cost to a minimum, but if you compare only raw circuit costs, the rigid-flex may still seem exhorbitant compared to a small rigid circuit cost that you are used to. The real value in the rigid-flex design becomes more apparent when you are in tight spaces and need to take advantage of your 3-dimensional package space. The rigid-flex allows the design to fold over itself for increased packaging density without the need for additional connectors which add cost and take up space.

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Thanks!  Given those specifications, I shouldn't have any trouble.  Flex-rigid will certainly pay for itself, since actual connectors would be impossible.  Unfortunately, since I need components both sides, rigidised flex isn't practical.