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PCBs Constrained? Try Flexing Your Circuits
December 31, 1969 |Estimated reading time: 7 minutes
Consumers demand smaller and faster products that deliver more bang for the buck while still expecting reliability and long battery life. Smaller footprints are driving the need to repackage electronics in these highly integrated systems. Flex-rigid PCBs can help maximize available space.
By Ian Gabbitas
As consumers continue to demand smaller, faster products that deliver more features and functionality at a lower cost, they still expect reliability and extended battery life. The convergence of communications and computing technology in mobile consumer products with smaller footprints is driving the need to repackage the electronics. This leads to an ever-increasing need to maximize available space within the electronic assembly using flex-rigid PCBs.
Flexible circuits are exactly that - flexible. The substrate allows engineers to design electronic sub-systems that can be bent, twisted or otherwise articulated, but still have improved reliability over interconnected, rigid PCBs without degrading system performance. The flip phone is one example of flexible circuitry.
The great thing about a flip phone is that the large display screen doesn’t compromise keypad size. The challenge is to connect both reliably. By using flex-rigid technology, the circuit assembly does not comprise multiple rigid PCBs joined by a small flexible interconnect. Instead, it is a single PCB with rigid areas where the keypad and display are mounted, and the flexible areas provide a hinge to carry the interconnecting traces. The use of flex-rigid printed circuits allows multiple rigid PCBs joined with cables or wires to be repackaged as a single circuit.
Why Flex-rigid?
The benefits of flex-rigid PCBs can be significant; and knowing them will determine if this is the most suitable choice for your product. Some expected benefits include:
- Increased reliability through the elimination of interface connections using dynamic flex;
- Reduced manufacturing costs using a single board means less to assemble. Therefore, less can go wrong during assembly;
- Reduced packaging weight because there is no need for separate connector components;
- Better impedance control;
- A single CAD design instead of individual PCB databases.
There are, however, some challenges engineers and designers must address when considering the application of flex-rigid PCBs. For example, flex-rigid designs cannot be completed in the same manner as standard PCBs. There must be a thorough understanding of how flex-rigid PCBs are fabricated and how they operate during the lifetime of a product.
Flex Application
The application of a product using flex technology is an important consideration when defining design requirements. There are many types of flex applications, including static flex and dynamic flex. With static flex technology, the circuit is installed flat in the product assembly, or bent once to install and not bent again throughout the life of the product. With dynamic flex circuits, the circuit typically is connected to a moving part of the product and is subjected to continuous bending throughout the product’s life.
Once the decision has been made to use flex-rigid technology, it is highly recommended that the manufacturer be involved as early as possible to ensure design guidelines are implemented correctly. It is also important that the manufacturer know the application and usage so appropriate materials and fabrication processes are chosen.
Laying Out the Flex
Flex-rigid circuits can be more expensive than their rigid counterparts, but they offer many benefits. For the board designer, there are challenges to overcome - not just in design complexity, but in CAD tools used.
Specialized capabilities. The CAD system must support specialized capabilities required for flex-rigid boards, such as:
- Material properties for layer stack-up (flex material, cover layers and openings).
- True-arced traces - not segmented arcs. Using arcs relieves stress along the trace.
- Trace tapering and corner filleting improve flex properties in dynamic-bend applications, avoiding sudden change in trace width to relieve stress.
- Flex-bend regions define bend and fold lines, including slide applications.
A CAD architecture that accommodates these design constraints to provide seamless interaction between the rigid and flexible areas must support these features.
Heterogeneous constraint regions. Flex-rigid circuits not only require constraints by layer, but also by region. If we were to trace the route path from a component pin on one end of the circuit, through the layer stack, and along the flex layer to the terminating component, we would encounter varying constraints that dictate interconnect topology. For example, the designer routes a signal from a component pin on rigid board A, through flex interconnect B, to the termination on board C (Figure 1). The actual path is straightforward; however, the rules that govern its passage change considerably. We start at the keypad where the base substrate is rigid, so routing constraints allow for 90° and 45° bends in traces on the top surface of the board. Traveling through the layer stack, we get to the inner flex layer where the rules change.
Figure 1. PCB editor showing flex sub-circuit connected to two rigid sections. The bend regions can be seen clearly.
No matter how carefully the board is manufactured, allowance must be made for the flexible material during fabrication. Any sharp angles in the copper potentially can cause fractures or peeling before assembly. To avoid this, all sharp angles must be filleted, corners changed to curves, track spacing increased and trace-width changes tapered. The layout tool must recognize different constraints applied to the flex layer. Even in areas where flex material is sandwiched between rigid layers, things may change again when the trace travels out of the rigid section and onto the exposed flex region where mechanical stresses are more severe, especially with a dynamic hinge such as a sliding keypad.
The flex part of the substrate plays an active role in the slide mechanism that extends the keypad from the main body of the device. Bend regions in the flex may stretch over an extended part of the flex material as the keypad slides back and forth with respect to the main processor board/display panel. With such a complicated mechanism, component placement could be restricted. To ensure parts on the board do not interfere, careful placement rules are required.
Mechanical collaboration. Flex-rigid PCBs have one unusual benefit over their rigid counterparts in that they allow designers to work in 3-D. The board can be rolled, folded, twisted and wrapped to follow the contours of an enclosure or hinge mechanism. Traditionally, this meant laboriously creating board profiles, flex contours and placement constraints derived from mechanical assembly drawings or, if more fortunate, by reading the layout-design data directly from the MCAD system. In this scenario, data is translated first from the MCAD design as it exports the board profile and placement constraint fixtures; and second, at various times when the PCB designer transfers the board back to the mechanical designer (Figure 2).
Figure 2. Viewing the flex-rigid database in a 3-D viewer allows the board designer to verify the flex layout as a flattened view (shown) or folded into its assembled state with the mechanical enclosure.
This process is not usually a problem if the interaction between the two CAD tools is infrequent. But when design cycles are compressed down to a few weeks, the process breaks down rapidly. With such a compression of project time scales, keeping the two designs in-sync becomes problematic, as there could be many file versions to manage.
More often, the 3-D design becomes a cardboard mock-up that is bent and twisted to evaluate the correct 2-D shape of the flex profile - reverse-engineered by taping the card strips to the inside of the housing. Once the desired 3-D layout is obtained, the card strips are removed from the housing and laid flat to show the 2-D profile.
What is required is a transparent, dynamic collaboration between the two design disciplines where data exchange is performed frequently throughout the project life. This should not be complicated or time-consuming because only the changes need to be transferred between systems. With such an infrastructure in place, the board designer could also look at the design in a 3-D viewer; the flex circuit read directly from the ECAD system and the mechanical housing loaded from the MCAD database. With real-time data synchronization, the board designer could use the 3-D viewer as a placement aid and tool to verify part placement, routing paths or flex-bend lines. The viewer would also read in the mechanical assembly and show the flex in place. This would not be a replacement for the MCAD tools, but instead an easy-to-use visualization tool.
Conclusion
Flex-rigid PCBs pose a challenge to the board designer on two fronts. Compact designs have highly constrained routing rules and use more complex fabrication processes and materials. Flex-rigid designs also bring the PCB designer closer to the mechanical design world, necessitating collaboration between the two disciplines. To help overcome these challenges, designers require a more integrated, concurrent design environment that can assimilate (without database translation) the ECAD design database with the MCAD design database and assembly. In facilitating a collaborative design flow, aggressive time-to-market goals can be met more easily.
Ian Gabbitas, product marketing manager, AutoActive Technology, Mentor Graphics Corporation, may be contacted at (720) 494-1282; e-mail: ian_gabbitas@mentor.com.