Rigidity is not always a bad thing in the design world, but it can be limiting. Historically, rigid PCBs provided the requisite functionality for a wide range of electronic applications, but form always follows function. Flexible printed circuits can accommodate designs and operate where simpler rigid PCBs cannot. A flex circuit board assembly overlaps with that of a rigid PCB, but there are stark differences that designers and manufacturers must address for optimal performance.
Flex Circuit Board Assembly vs. Standard Options
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Flex Circuit Board Assembly Advantages
Compared to traditional PCBs, flex circuit board assemblies provide many advantages:
- Streamlined assembly—Flex assemblies only fit one way. Since there is no ambiguity about how to connect the system, manufacturers save time and improve throughput during installation and testing processes. There is virtually no possibility for an incorrect installation (which can produce difficult-to-diagnose run-time errors) as in a traditional wire harness setup.
- Lower mass – By itself, lower mass could be mission-critical for an objective (think aerospace applications), but weight savings also contribute to better shock and vibration resistance. During rapid acceleration events, the substrate and solder joints reverberate against each other. Higher-weight boards induce greater stress at the substrate-solder junction, leading to more rapid wear. Finally, bulk shipping costs can be considerably cheaper for flex.
- Better enclosure fit – Because flex circuits can bend and twist (statically and dynamically), they can better fill 3D space. This feature reduces the size (and weight) of the enclosure and may be a necessity in certain design applications like wearables and other portable devices where
How Flex Circuits Alter Manufacturing Practices
Depending on the stackup, there are multiple methods to integrate the flex system. Broadly, there are two general types of rigid-flex assemblies: full and partial homogeneity. As the name suggests, homogeneous rigid-flex stackups are identical throughout the flex (and potentially rigid) layers. Partially homogeneous stackups possess different flex and rigid sub-sections that feature a connector between the rigid and rigid-flex sub-assemblies. Full homogeneous stackups fully integrate the flex and rigid sections together during manufacturing – they are, practically, a single assembly.
Full vs. Partial Homogeneous Stackups
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Compared to standard rigid PCBs, there are considerably more design aspect decisions for flex designs. While some techniques would be impermissible for rigid boards, they may be viable, practical, or even necessary for flex (and vice versa):
- Cross-hatching ground planes – Removing copper from plane layers affects the impedance values and can lead to warpage due to a less uniform coefficient of thermal expansion (CTE). However, cross-hatching across flex layers is generally an easier way to improve flexibility than altering the thickness of the layer.
- Drilling – 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 due to the material void.
- HDI and high-layer counts – The electronics tend to reduce 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 Flex
Flex circuit board assembly is an avenue for system integration that offers many advantages over traditional rigid PCBs, primarily size and weight reduction. Complex dsigns also benefit from a reduction in assembly and testing, as the straightforward nature of a flex assembly provides a uniquely singular method for system integration. Whatever the needs of your electronic system integration, VSE is here to help. Our team of engineers are committed to building custom electronics for our customers, including complex system integrations for PCBs, wire harness/cable assemblies, test fixtures, and enclosures. We’ve been partnering with our valued manufacturing partners for over forty years to realize life-changing and life-saving devices.
