Working with flex has its rewards, if you follow a few rules.

In a world of mobile flip-phones and miniature video cameras, the ability to bend or shape circuitry in three dimensions within a product's package is becoming critical. When more space yields lower margins, there is little wonder why electronics designers are feeling the pressure. To meet these demands, many are turning to flexible circuits, otherwise known as flex or rigid-flex (part regular rigid board, part flex) PCBs. But, as with most design issues, when one problem is solved, new ones are created.

The trace routing, current-carrying and SMT component population requirements of today's flex circuits are similar to those of regular FR-4 printed circuit boards. But they're flexible. This not only implies that they can bend and wrap around components, but that they have the ability to be used in applications where regular movement of sections of the complete product is required, as in a flip-phone. It also allows the entire circuitry of a product to be created using just one flexible circuit instead of multiple rigid PCBs connected with countless plugs and sockets.

Creating abstract-shaped flex circuits may reduce the interconnects required, achieve engineering goals of saving space and meet the designer's aspirations of having a totally ergonomic shape. It also means electronics designers have a complex task at hand. Unusual shapes result in complex routing. Planning the layout prior to actually mapping the routes is therefore a critical task; as modifications down the line are exceptionally odious and time-consuming.

The two key concepts behind successful 3D flex circuit design are thorough documentation and consideration of essential flex and 3D design rules. See Figure 1.

Key areas of the flex circuit must be documented as part of the flex layer cross-section or stack up, such as the cover layer and stiffener areas. The stack-up should also indicate base material and copper thickness, as well as stiffener material. For the latter, the X and Y dimensions, tolerance of overall thickness of the stiffener area (which should not be less than +/- 10%), and details of whether the composition is of Kapton or FR-4 should be documented. Component placement should be clearly illustrated, and when indicating the copper weight, this measurement should be clearly marked as either the base material copper or finished copper weight.

Here is a short list of the essential 3D flex design rules:

Vias. Do not place vias by the stiffener area or at bends in the flex, as this may cause cracking. Create "all layer via inhibit areas" to avoid encroaching on these critical zones.

Holes. On stiffeners, hole openings should be + 0.10 cm over the finished pad size to avoid shorting. Also ensure to observe the correct hole tolerances as set by the manufacturer.

Bending. Flex circuits are not intended to bend to extremes. Bending beyond 90ö will crease the flex. Bends should not exceed a minimum radius set by the manufacturer.

Board edges. Ensure an overall 0.15 cm clearance to the edge of the board.

Pad stacks. Create specific pad stacks for flex design to prevent copper from lifting off of the dielectric layer. This may occur as a result of an undersized copper layer (solder mask).

Routing. Observe proper layer biasing; where possible, try to route perpendicular to a curve.

Although flex circuits have been in existence for over 20 years, they have been used in relatively few applications until recently. Increasing pressures to miniaturize (thus impacting design space) and market forces driving high quality and low-cost products are two reasons why use of this technology will increase in the coming years. This uptake will be further boosted as EDA vendors continue to refine and customize their tools to reduce the effort required to complete complex designs.   PCD&M

Andy Buja is an applications engineer for Zuken USA. He can be reached at andy.buja@zuken.com.