Flex circuits/flexible circuits design guide

Designing flex circuits

Do NOT look up a manufacturer’s capabilities and design your flex circuits to their absolute limits.
A circuit with every aspect at the limits of capability is a manufacturing nightmare. You might save a few bucks by reducing the layer count to an absolute minimum, but you will lose hundreds when the manufacturability yield factor is multiplied into the costing equation.

Artwork and layout for flex circuits

Pad staking
The low mass of flexible circuits, with its weight and space advantages, becomes a disadvantage when soldering components: solder pads should be as large as possible and mouse-eared to give good anchorage. Tracks should be blended into pads at all times to prevent weakness at the transition point and to improve heat dissipation when soldering. See Figure 1. 

Coverlay apertures should be round, oval, or rectangular with rounded corners so that they can be drilled or pecked: apertures with sharp corners require more expensive tooling. The coverlay aperture of the flexible printed circuits should be small enough to stake the corners of solder pads. See Figure 2 

Trace Routing of flex circuits

Bending of flexible circuits
Bend where there is biggest gap between traces and use holes in between tracks to aid bending

Bend areas radii should be approximately 10 times overall thickness as shown in Figure3. A bend radius of 6 times overall thickness can be used for single sided flex circuits and selective plated parts where there is no low ductility plated copper in the bending area.

Screening of flex circuits

Cross hatching that allows bending

The Minimum annular Ring on flexible circuits

 1 = Coverlay     2 = Solder pad ;

 3 = PTH allowance

 4 = Minimum Annular ring   5 = Adhesive squeeze out

 6 = desired hole size

Although squeeze out only affects external layers, internal annular rings in flex rigid and flex multilayers are often compromised especially where tight hole to pad ratios are demanded. This is mainly due to the dimensional stability (1000ppm) of the flexible material. It is common to allow zero breakout of the hole from the internal pad and on some commercial parts there is sometimes agreement to allow a 270° minimum contact ring. The annulus permitted on both the external and internal layers should be clearly identified on the drawings or detail specifications. REMEMBER when designing flexible printed circuits to allow for some miss-registration between the internal pads and the drilled hole, you must also consider the minimum space between the tracks and the drilled holes, especially if you plan to run tracks in between pads of a connector layout.

Mechanical considerations for flex circuits

 Prevent rips and tears

 Stiffeners
Single-sided and double-sided flex circuits can be stiffened in certain areas by the addition of a rigid reinforcement material. This provides areas for component mounting, as well as additional mechanical strength and stiffness. Common reinforcement materials are 0.005" (125µm) or thicker polyimide films and epoxy glass; less common materials are polyester film, and sheet metal These stiffeners are normally bonded to the flexible circuit with an adhesive similar to that used in the construction of the flex circuit itself. Alterna­tive adhesives such as heat-activated or pressure-sensi­tive adhesives can be used depending on the particular application.
Polyimide stiffeners have the advantage that they can be blanked at the same time as the circuit profile and meet tight tolerances. They are routinely used to provide added thickness under the track fingers to meet ZIF connector requirements.
Stiffener and bared coverlay should overlap by 0.030" (0.76mm) to avoid stress points. See Figure 1

When an epoxy glass material is used as a stiffener, and the flex circuit is to be bent at or near the rigid, it is beneficial to add a bead of strain relief to provide a radius between the rigid material and the flex. This assists in the transition from the rigid to the flexible sections and prevents damage during flexing operations. This also applies to rigid flex. See Figure 1

Holes drilled in the stiffener material should be drilled a minimum of 0.012" (300µm) larger in diameter that the corresponding holes in the flexible circuit. This over sizing of the hole diameters will allow for any miss-registration of the stiffener to the pads on the circuitry.

Fixing flex circuits

Flexible circuits are commonly affixed using Pressure Sensitive Adhesive (PSA) tapes. The PSA tapes can be pre-profiled and finally profiled along with the circuit.
Parts which have vibration requirements are normally secured using the rigidised areas. Flex -rigid’s are normally secured using fixings attached to the rigid board area. N.B. Plated holes should never be used for the mounting of stand-offs, eyelets, rivets or other fixings which compress the hole. This will damage the PTH and can even cause barrel cracking. 

Differences to be remembered when designing flex circuits

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before solder                                      after solder
 

Workmanship, Design and Material standards
for use with flex circuits

 The following is a list of specifications commonly used in the design and manufacturing of flexible printed circuits.

Legal Notice

This information is supplied in good faith by Flexible Technology Limited. We hope you find the above information helpful. It is by no means a complete guide to flex circuits and is only offered as an aid to flex designers. The use of any information contained in this site at your own risk.

For information on where to purchase flex circuits visit flexible circuits manufacturer Flexible Technology Limited 

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