DESIGN COMPARISON: FLEX CIRCUIT WITH STIFFENERS VS. RIGID-FLEX PCB

Functionality: Flex PCB With Stiffeners

Flexible circuits manufactured with stiffeners serve the following functions in a flex PCB design:

1. Mechanical support to specific areas of the design that contain SMT and/or PTH components. Prevents the flex circuit from being bent at or near components which if allowed could compromise the integrity of the solder joints.
2. Localized increase in thickness required to meet ZIF connector specifications
3. Heat dissipation

Stiffeners do not have plated holes and have no electrical interconnect to the circuitry in the flex layers.

For flexible circuit board designs that have stiffeners on both sides, but in different areas due to component placement requirements, this will add significant complexity and cost at assembly. Stiffeners on the same side as SMT components will prevent solder past stencils from lying flat in the SMT areas and may necessitate a rigid-flex construction.

Flexible circuit board designed with stiffeners

Functionality: Rigid-Flex PCB

Rigid-flex PCBs provide the same functionality and capability of rigid PCB technology.

1. Added benefit of fully integrated connections between rigid areas. Eliminates the need for added connectors, which reduces the real estate requirements, and all the additional points of interconnect required by the connectors. This, in turn, improves the reliability of the design by eliminating potential points of failure.
2. Allows for SMT components on both sides in the same area(s).
3. Blind and buried via capable.
4. Full depth PTH holes for higher reliability PTH component interconnect.
5. Allows for Press fit connectors.

Rigid-flex circuit board designed without stiffeners

Cost Impact

A flexible circuit with stiffener design will, with few exceptions, be more cost effective than that of a comparable rigid-flex at the individual part level. This is due to the added materials and manufacturing process steps required by rigid-flex technology. This cost difference can range easily from 2x or greater.

This, however, does not define that a rigid-flex solution will be overall more expensive than that of a flex with stiffeners once all additional cost factors are accounted for. The added benefits of higher-level integration, reduced component requirements, improved highspeed signal performance, and reduced higher level assembly requirements can result in a rigid-flex based solution being more cost effective.

Design: Flex With Stiffeners

Stiffeners can be incorporated in flex design to meet a wide variety of requirements:

1. Can be applied to one or both sides of the design.
2. Can vary in thickness and material within the same design.
3. Attached with either thermo setting flex adhesive or pressure sensitive adhesive (PSA), ZIF connector applications require thermo WHERE set adhesive.
4. Silkscreen and PSA allowed on external layer of stiffeners.
5. Electrical shielding layers, if required, cannot extend under stiffeners due to slip priorities of the external layer of the shielding film materials.
6. Advanced FR4 stiffeners can have circuit layers but without electrical interconnect to the flex layers.

Common applications are:

1. Component mounting pads to allow for soldering of connector mounting lugs from both sides for added strength.
2. SMT pads for edge mounted connectors.
3. Additional localized circuit requirements.

Example of 3-layer flex circuit with fr4 and polyimide stiffeners both sides

Design: Rigid-Flex PCB

The primary advantage is the integration of both rigid PCB and flex technology which allows for designs that can achieve significantly tighter packaging requirements than any other solution. Also provides for higher level performance and reliability by eliminating the points of interconnect required by connectors and wiring.

Stiffeners are also commonly utilized in rigid-flex designs, in the flex areas, serving the same purposes as in flex only designs. Designs can incorporate flex sections, or tails, that terminate in ZIF contacts and, as such, require a polyimide stiffener to meet the ZIF connector specifications. Flex areas may also have components which then require FR4 stiffeners for support and reliability.

Example of a 6-layer rigid-flex PCB with 2-layer ZIF flex tail

Summary

The capabilities of flex with stiffeners and rigid-flex allow for an extremely wide variety of combinations and solutions. When combined with the overlap in capabilities, this can complicate the decision-making process early in the design. Rigid-flex designs can always meet the capabilities of flex with stiffeners though often, but not always, come with an unnecessary cost premium. Flex with stiffeners, while generally more cost effective, may incur additional costs elsewhere in the finished assembly.

Our recommendation to all our customers is to engage with our engineering at the early stages. Once we understand the design parameters, we can assist in narrowing down the options to a few configurations that merit further investigation and provide as accurate as possible preliminary costing to factor in to the decision-making process.