B e nd r a diu s ( f le xi b i l ity ): Single-layer:6 × circuit thickness (minimum) Double-layer:12 × circuit thickness (minimum) Multilayer:24 × circuit thickness (minimum)Circuit thickness is approximately 0.006" (0.150mm) per layer.

Standard Design Recommendations

How to improve flexibility and bend radius

Single-layer circuits are the best choice for dynamic (flex-in-use)applications. Two or more layer circuits are best suited to static applications, with flexes only during installation.Several problems can arise when a circuit is bent sharply.Compression can cause wrinkles in the cover coat on the inside of the bend.Stretching can result in tears in the cover material and/or broken conductors on the outside of the bend.Start the mechanical design by establishing the bend radius. Ifthe radius is at least ten times the thickness of the material, there is a good chance that the circuit will function reliably. The minimum allowable bend radius depends on a number offactors, and is best defined by IPC-2223. Overall circuit thick- ness is slightly less than the sum of the individual insulator, adhesive and foil layer thicknesses.

Incorpor a t e th ese f ea tur es into mu l ti la y e r a nd r e duc e d be nd r a tio d es i g n s to incr ease r el i ab i l ity

1. R e duc e o ve r all thic k n ess in th e f le x a r ea
•Reduce the base copper weight (and the corresponding adhe-sive thicknesses) or reduce the dielectric thickness.•Use adhesiveless base materials. Adhesiveless materials willusually reduce the starting thickness of each substrate by 1-2 mils (25-50µm) when compared to adhesive based substrates.•Eliminate copper plating on the conductors in the flexing areaby utilizing selective (pads-only) plating or adding outer pads- only layers to the circuit. 2. M ake th e circuit ro b u s t to with s t a nd f le xin g
•Balance the conductor weights and material thicknesses oneach side of the neutral bend axis.•Conductors should be staggered from layer to layer and notstacked on top of each other to increase flexibility.•Conductors should always be routed through bend areas asclose to perpendicular as possible.•Conductor thickness and width should remain constant inbend areas.•Plated through holes should be kept out of the bend areas. •If the circuit will be bent within 1" (25.4mm) of terminationpads, fillets should be placed at each conductor/pad interface. Stresses from a bend are not isolated to the immediate bend area and residual stresses can radiate out from the bend. See page 17 for more information on fillets.•If shields and/or ground planes are required on the circuit, usea crosshatched pattern rather than solid copper. Another shielding option is a screened-on conductive coating such as silver epoxy, which is much more flexible than copper. •Incorporate tear stops orreliefs for slits in the circuit. The end of the slit represents a vulnerable point for a tear to start and to propagate.•Avoid any discontinuities in the cover coat or substrate near abend.•The circuit outline should be designed so there are no twistsin the finished assembly. Any burr or irregularity from the blanking operation could potentially propagate into a tear.•Consider factory forming. Reliable bend radii tighter than 10:1are possible if the circuit is formed using specialized tooling and will only be flexed one time.•If bend reliability is still a concern, consider "unbonding" theflexible substrates from each other. Since each of the sub- strates in the unbonded area has a much lower thickness than the total circuit, they are able to bend tighter than if they were fully bonded. For a mor e in-d e pth l oo k a t thi s s u b j e ct p lease see
VS.BADGOOD D es i g nin g for F le xi b i l ity a nd R el i ab i l ityApp l ic a tion Aid FAA31 - a t www.minco.com/f le x/
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