Cadence Paper: Automating Inter-Layer In-Design Checks in Rigid-Flex PCBs

Estimated reading time: 10 minutes

Inter-Layer Checks

Performing inter-layer checks allows the designer to check a variety of areas in the rigid-flex PCB design: 

• Layer-to-layer checks to assess stack-up mask layers
• Coverlay to pad
• Mask to pad
• Precious metal to coverlay
• Bend area/line to stiffener, component, pin, and via
• Gaps, such as edge-to-edge spacing in areas such as the bend line to the component, the via to the bend line, and the stiffener to the bend area
• Inside areas, such as gold mask to coverlay, pin to coverlay, and stiffener adhesive to stiffener
• Overlaps when two geometries overlay by a minimum or more, such as soldermask overlay into the transition zone

Typically, designers have had to perform design rule checks (DRCs) manually, or write their own software to automate the process. There are also tools on the market that support rigid-flex design, but they are not particularly comprehensive in terms of the breadth of inter-layer checks now needed. A useful tool also needs to be able to address various design considerations, which we will outline in the next section. 

Rigid-Flex Design Considerations

MCAD-ECAD Co-Design

All electronics have to fit into enclosures, making MCAD-ECAD co-design a necessity. However, rigid-flex PCBs call for additional scrutiny with the bending of the flex inside the enclosure. The mechanical engineer needs to provide the bend area, bend line, and bend radius to the PCB designer, who must create and adhere to various rules: 

• Do not place vias in bend areas to avoid cracking the substrate over time
• Do not put pads too close to the bend area, as the pads can eventually peel off
• Avoid overlapping bend areas with stiffeners, or else there could be peeling or restriction of the full bend
• Avoid placing stiffeners too close to vias or pins to avoid shorting

Mechanical engineers must also define the specific boundaries for zones, where the thicknesses are different across the entire design structure. In return, mechanical engineers need to get additional data about layer structures and thickness for the zones, including above and below the top and bottom layers to calculate accurate thickness and accurate collision detection before handing the design to manufacturing. These layers include paste mask, coverlay, stiffeners, external copper, and other materials that impact overall height, thickness, and bend performance. See Figure 4 for a table showing ECAD-MCAD data transfer.

Component Placement

Due to various advances, CAD tools can now intelligently auto-drop components as they are moved across rigidflex substrate boundaries. This capability eliminates the tedious steps of moving the components to the right surface layers. But, are the results good enough? In most cases, component packages used for flex zones will differ from the ones used in rigid zones. For example, padstacks for flex zones tend to be longer to support the bending action of the material. Therefore, the CAD system should be able to “retarget” the package with the proper alternate symbol for the respective technology zone.

Interconnect

Routing flex vs. rigid generally comes down to one word: arcs. The nature of all geometry residing in a flex zone, whether it’s the board outline, teardrops, or routing, involves arcs and tapered transitions. CAD tools need to support group routing functions to carry a bus across the flex while locking to the contour of the board outline. Line-width transitions should be tapered and all pin/via junctions should be tear-dropped to reduce stress at the solder joints. Advances in CAD tools over the years have resulted in a better ability to push and shove traces during the edit commands. However, this has, for the most part, been a challenge with arc routes. Change, even daily change, is a given in PCB design. But adding an additional signal to a routed bus structure should not require designers to delete routes followed by the group reroute.

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