My employer specializes in rigid flex.   The industry accepted practice is 
to put a stress relief fillet of ECCO Bond flexible formulation at the rigid 
to flex transition.  Without it if you bend the flex sharply at this point 
the copper may crack even if it is rolled annealed.

Ted Edwards
Dynaco Corp
[log in to unmask]

----- Original Message ----- 
From: "Wayne Thayer" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, February 22, 2007 6:53 AM
Subject: Re: [TN] Disconnections in transition zone in rigid-flex boards


> Anna-
>
> That's a very high stress point, and there are a whole bunch of factors
> which come into account.
>
> Obviously, you can place some stiff RTV at that point to make the bend
> radius larger.  Hot melt glue might work for some apps.  This can be a
> sloppy process and adds time.
>
> Alternatively, you can introduce a separate forming process, which
> bends the flex section as a very controlled process.  Following this the
> assembly would be placed in a fixture to prevent inadvertent extra
> bending during assembly.
>
> The circuit board production process can also probably be re-engineered
> to improve the crack resistance of the conductors.  For example, is the
> metal on the flex Rolled Annealed?  Is the roll direction controlled to
> be in line with the conductors?  How thick is the metal?  Can the
> dielectric within the flex layer be increased in thickness to help
> control the bend radius?  Can the metal thickness on the flex be reduced
> to make it less likely to crack?  Is the metal on the flex
> electro-plated during the manufacturing process (this vastly reduces
> crack resistance)?
>
> I have seen most products move from rigid/flex constructions to
> separate rigid and flex boards.  This is made possible because of cheap,
> dependable flex connectors which mount on the rigid boards.  With
> independent flex construction its a lot easier to control stiffness as
> well as problems with the assembly process.
>
> Wayne Thayer
>
>>>> [log in to unmask] 2/22/2007 6:55:25 am >>>
>
> Hello Techneters,
>
> We have some boards disconnected in transition zone: between rigid and
> flex segments. This happens during an assembly process. What is
> recommended to add to manufacture process in order to strengthen the
> area.
>
> Thank you,
>
> Anna Seredovaya
>
> Kodak IL Ltd.
>
>
>
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