Form factor is a driving force in electronics adoption, with the classic example being the invention of the microprocessor. Before the microprocessor, computers took up entire rooms due to the space requirements of vacuum tube technology. They were only accessible by a tiny subset of students at a few universities. However, the advent of the new technology significantly reduced the cost and size of a computer, increasing its adoption rate and opening the door to many new businesses, services, and products that would’ve otherwise been restricted or severely impeded. Not every device has the transformational potential of a personal computer. Still, it is true that the more accessible a product is in terms of availability, size, and ease of operation, the more consumers it will attract.
With the smartphone pushing portable computers into the hands and pockets of the population, the next burgeoning market seems to be that of wearable devices that hold the promise of augmented reality and passive tracking of bioinformatics. Meeting the complex needs of modern electronics in unobtrusive packages is only possible with advancements in flex PCB design, allowing assemblies to convert rigid PCBs and bulky cable harnesses to a more free-flowing 3D optimization of space.
Flex PCB Design Starts with a Stackup
Flex PCB design is not a new technology, but miniaturization and other space-conscious design factors have further increased its prominence. Flex PCB functions as a stand-in for both rigid boards and cables, dramatically reducing area and volume within builds and allowing more granular control over traditional board-to-board connections. The ability to customize connections is a welcome addition to any designer’s tool belt, but it comes with the additional undertaking of how these flex connections operate mechanically. While designers often encounter situations where heat or vibration must be accounted for, the solutions are typically more passive. Flexing, bending, and other similar mechanical deformation modes necessitate a greater understanding of the material properties, particularly given the anisotropy of most designs.
Flexing is an inherent and desired feature of these printed circuits. However, it is still possible to stress the materials to the point that plastic deformation or complete material failure sets in. Flexibility is at its highest when there’s minimal material due to the complementary tension-compression matrix that arises at any deformation area. Engineers typically quantify this with a bend radius, the minimum curve for which a particular flex PCB thickness is rated. Bend radii must be larger to accommodate thicker stackups and dynamic flexing operations.
Much like a cable, the long-term reliability of the flex PCB itself and the greater assembly depends on the long-term viability of the connection and, wherever space allows, room for an ample bend radius.
Like standard rigid boards, the stackup provides the material backdrop for electrical performance. However, the general model can differ because of the additional freedom allotted in flex or a rigid-flex hybrid.
Stackup examples for 4-layer flex and rigid-flex.
Since flex arose as a solution to design confinements, it would only make sense that it offers many potential methods to address space and connectivity issues. A rigid-flex design with additional signal/plane layers can exist as one continuous section or unbounded. The latter would allow internal layer pairs to be routed and terminated through the assembly separately.
These separate internal layers can even be designed so that the flex layers are not stacked but instead have consecutive layer routes that form nonparallel pathways through the printed circuit. An additional wrinkle of the stackup is the relative difficulty in removing multilayer flex from the panel compared to single- or double-sided flex. One- or two-layer flex is thus extremely well-suited to complex shapes with multiple breakout routes when applicable.
Changes to Layout Practices in Flex
While the stackup defines the bulk of the physical shape and characteristics, the layout will also have to contend with how standard rigid features respond to flex and rigid-flex substrates that allow for a high amount of variance in construction.
While some of the discussion of features will not apply to every design, it is valuable to understand the large number of potential pratfalls that arise due to the possibilities offered by flex:
- Cross-hatching ground planes: Removing copper from plane layers is almost a nonoccurrence in rigid boards. Not only does this affect the impedance values, but the copper coverage helps prevent board warpage by approaching a constant coefficient of thermal expansion (CTE) between layers. However, cross-hatching across flex layers is generally an easier way to improve flexibility than altering the thickness of the layer.
- Flexible layer lengths: A multilayer flex design will encounter an issue along the bend radii. This scenario means the layers closest to the bend inside will require less compression slack than the outer layers, which will, conversely, need more length while being placed in tension. Outer layers will be subject to some scaling factor that depends on the bend radii and accommodating angle.
- Drilling on flex: Pads for drilled holes on outer layer flex stackups require additional reinforcement points to prevent peel-off. Through-holes exclusive to flex regions are generally discouraged as they are difficult to manufacture and act to concentrate stress in fleed regions due to the material void.
- HDI and high-layer counts: The electronics tend to reduce the size, increase performance, and expand functionality; flex circuits are not exempt from this trend. Importantly, layer density negatively correlates with flexibility, making it difficult to balance the two factors at the end of the former. Overall, accommodating high layer counts in flex circuits requires far more careful processing than pure rigid – already a difficult undertaking.
Your Contract Manufacturer Can Optimize Assemblies with Flex
The subfield of flex PCB design will only continue to grow in the coming years as available space shrinks. In contrast, demands for performance and features are also increasing. Flex serves a unique niche between rigid boards and cables, fulfilling both functions while minimizing volume and unlocking formerly impossible assembly configurations. Maximizing the performance of your PCB with flex is crucial to meeting the complexities of assembly, if not an outright necessity in many cases.
In the current landscape, you need a manufacturer that can leverage knowledge and experience with flex (among other assembly techniques) to deliver a superior product. At VSE, the difficult task of PCB manufacturing is boiled down to a simple creed: we’re a team of engineers who build electronics for our customers. Alongside our manufacturing partners, we surmount the most challenging board designs for life-saving applications.
