Our electronic devices are becoming increasingly compact and more and more sophisticated, the demand for high-density interconnect, or HDI, has pushed multilayer rigid-flex PCB technology to the forefront of product development. Whether it be cutting-edge wearable devices, aerospace, electronics or miniaturized medical devices, you’ll nearly always find these multilayer rigid-flex circuits at the center with their triptych qualities of flexibility, durability and functionality. Key to this evolution are the recent innovations in blind and buried via structures, essential tools for engineers pushing the boundaries of modern PCB design, and the subject of our article for today.

Vias

Let's first revisit the tripartite categorization of the vias that we'll be seeing with rigid-flex designs:

• Through Vias are the ones that traverse the entire length of the board, from the top to the bottom, the vast majority in rigid-flex PCB boards of fewer layers
• Blind Vias are like blind alleys that come out on the surface, half-through and half-buried
• Buried Vias are uniquely interior

Arrangements of these three categories of vias allow engineers to set up dense interconnects while maintaining a formal compactness. The localization of connections to specific layers allows designers to do more complicated set-ups of circuits without increasing board size. Blink and buried vias are especially valuable routing signals between the rigid and flex sections, and the IPC-2223 guidelines estimates a 25% decrease of board area with appropriate use of blind and buried vias.

Challenges

But these advanced vias are not without challenges. Rigid-flex stack-ups alternate between rigid and flexible substrates, adhesive and adhesiveless layers, and this alternation causes specific issues:

Polyimide, the material usually used for the flex layers, has a higher coefficient of thermal expansion than its rigid core. During the process of lamination this mismatch can cause stress. This stress can make it harder to get perfect adhesion and flatness, resulting in warping, micro cracks or even delamination.

Blink vias and buried vias also require laser drilling with high precision and tight tolerances. The risk of voids and of incomplete copper plating is further increased by high aspect ratios, that is, the depth-to-diameter ratio.

Accurate rigid-flex via placements are all the more critical when you are traversing rigid and flex areas, and vias of miniscule sizes are far less tolerant of misalignment.

Exposed to dynamic bending and environmental stress, a bad via can suffer micro cracks, barrel fractures and even delamination. This is why most next-gen wearable devices will have microvias as their most common failure.

Innovations

But recent years have seen waves of innovations in the technologies of these rigid-flex vias.

Sequential Build-Up, or SBU, is the process by which engineers build complex layers by increments, forming blind and buried vias in multiple stages. This procedure optimizes each via’s aspect ratio and plating quality, and is now the standard for HDI and rigid-flex PCB.

Lazor drilling, a game-changer for rigid-flex via technology, can create intricately-patterned microvias with careful precision on thin flex layers, often ones of less than 100 micrometers. We can thank this technology for an improvement of some 15% on the yield rate compared to traditional mechanical drilling.

Better via filling techniques, such as high-conductivity copper plating and conductive plates, can ensure low resistance on blind and buried vias all the while maintaining good mechanical strength. These techniques also prevent voids and reduce the risk of via fatigue on your rigid-flex product.

Manufacturers have also been employing hybrid lamination for rigid-flex applications, combining both adhesive and adhesiveless bonding techniques. This hybrid execution can improve the mechanical integrity of your rigid-flex stack-up all the while enhancing via reliability, particularly at the interface between the rigid and flex parts where the stresses are most acute.

The production stage of quality control should take advantage of newer inspection systems. AOI, now a norm, as well as x-ray computed tomography let manufacturers scan hidden via structures and inspect for defects such as voids or misalignments. This early detection can especially help a rigid-flex project whose products are going to be much more delicate than other PCB technologies.

Practices

Here are several good practices and design concepts to take into consideration when you are working on your rigid-flex project:

• Keep robust annular rings when planning the layer sequence so as to accommodate the variances of manufacture
• Communicate with your substrate supplier and ask for their minimum via diameter and spacing. Many boast of microvias of 75 micrometers, not always of the same tolerance
• Use stress-relief features in the flex region and avoid via-in-pad designs for areas of high mechanical stress
• Employ Finite Element Analysis, or FEA, and via reliability modeling tools to predict possible failure points before proceeding to the fabrication of your rigid-flex prototype

Manufacturability

The complexity of advanced via structures, along with all of the problems they engender, are justified by the gains they provide in miniaturization, functionality and reliability. But held against rigid-flex manufacturing costs, these benefits should be reconsidered:

• An increased via density can reduce yield rates when the process controls are not stringent, ultimately resulting in higher per-panel costs
• A fabricator may lack the expertise or the equipment to execute advanced blind and buried vias of your multilayer rigid-flex project. Always audit suppliers and examine their capabilities and ask if they have rigid-flex experience similar to your project
• Plan your via structure and your penalization with the idea of efficiency. This will reduce material waste as well as improve cost competitiveness

Thanks to these innovations, blind and buried via structures have pushed miniaturization and reliability further and further in the world of multilayered rigid-flex design. Remember the tips we provided in this article, and we look forward to seeing you next time!