Multifunctional system boards are already state of the art today. Different base materials, methods and manufacturing processes enable highly specialized circuit carriers: densely packed and/or highly integrated, HF-capable, high-current capable, heat-optimized and three-dimensional. The current technology roadmap shows what drives and controls the development of PCB technology and where the journey is heading.
The development of PCB technology has been and continues to be driven by the development of electronic components, whose electrical and physical properties are being further perfected while their dimensions are becoming ever smaller. This process is geared towards the functions that are to be achieved by the end products. Added to this are cost pressure, reliability and service life as well as ever stricter environmental regulations, summarizes Ralph Fiehler, Head of Development at KSG. The KSG Group was actively involved in the ZVEI's technology roadmap. Ralph Fiehler coordinated the twelve-strong editorial team for the PCB chapter and presented the chapter to experts.
The requirements for the development of printed circuit boards in the coming years are: Miniaturization by increasing integration density, signal integrity and RF suitability, thermal management and flexible electronic systems that overcome physical and mechanical limits. The new highly specialized PCBs require not only process management at the PCB manufacturer, but also improved base materials and suitable system design tools for hardware developers.
Embedding: Integrated functions in the printed circuit board
Miniaturization has driven embedding, i.e. the embedding of passive and active components in the PCB. Embedding enables the short and impedance-matched connections required for signal integrity. ICs, passive components and sensors are integrated as a system in package (SiP) on a PCB substrate. The SiP itself is later assembled on a PCB.
This development requires adequate, technological 3D integration solutions. At the same time, the demands on the system design are increasing, which must take into account both electrical and thermo-mechanical reliability aspects. The following trends are emerging in embedding: use of thinner substrates, reduction of conductor spacing, reduction of via diameters, increase in thermomechanical requirements and an increase in the embedding of active components.
HDI/SBU PCBs with > 10 layers and BGA spacing equal to or less than 0.5 mm
Experts predict more layers and ultra-fine conductor structures for HDI/SBU printed circuit boards (HDI: High Density Interconnect, SBU: Sequential Build Up). This technology accounts for around 13% of the printed circuit boards produced in Europe. HDI multilayers with a line spacing of 100 µm or less and 4 to 10 layers are typical in Europe today.
Optimized signal integrity requires an even higher integration density. This forces PCB designers to combine impedance-controlled multilayers with layer structures >10 layers with complex SBU structures 3+x+3 and ultra-fine conductor patterns <75/75 µm line/space. This development will continue and gain momentum.
For the PCB, this means a reduction in the inner layer and PCB thickness, an increase in hole density and aspect ratio, a minimization of mechanical tolerances as well as PCB and stop varnish tolerances. In addition, the experts expect impedance-controlled structures and the use of mixed structures. The base material, the foundation of every printed circuit board, is one of the decisive factors. In concrete terms, this means an increase in the proportion of temperature-resistant, halogen-free and CAF-resistant base materials.
When routing fine-pitch BGA structures, rewiring strategies with resin or copper-filled staggered or stacked via arrangements are becoming increasingly important. If a BGA structure with a 0.8 mm pitch can still be routed using a dog-bone connection and a through-hole, a BGA structure with a 0.65 mm pitch already requires SBU placement with staggered micorvias. The BGA connection grid of 0.5 mm or less, which will become increasingly popular in the future, requires more complex solutions. "Rewiring in the SBU structure with copper-filled stacked-via or microvia on buried-via solutions is unavoidable," says Ralph Fiehler.
HF applications require optimized laminates
PCB technologies for high frequency applications require base material manufacturers to develop new PTFE material systems for 80 to 100 GHz requirements, laminates with low dissipation factor/dielectric constant, limited tolerance range and less copper treatment.
High-speed digital circuits with complex routing, e.g. FPGAs, are a challenge for designers and PCB manufacturers. While today's circuits process signals at around 12.5 Gb/s, in future there will be data streams of 25 Gb/s, 50 Gb/s or more, estimates Helmut Schmucker, segment manager for PCB production at Rohde & Schwarz.
The routing in backplanes and motherboards can extend over very long differential signal paths, multiple layers and high pin count connectors. This makes the signal paths particularly critical in terms of insertion loss and differential offset.
Very high layer structures, increasingly with HDI layers, are required for unbundling. While 20-layer multilayers are currently used, the trend is moving towards 30 layers. For cost reasons, the cheaper laminates are being stretched to the limits of what is electrically and technically feasible. At the same time, thinner laminate thicknesses ≤ 50 µm are required to keep the overall thickness and via length of the structures as low as possible. Critical signal vias with connections to inner layers are increasingly being back-drilled in order to achieve the necessary signal quality.
The HF expert points out another important aspect: Current processors today generate power losses of 130 W and more, which simultaneously requires supply currents of 140 A. Five years ago, the power loss was still 30 W.
Technical solutions for thermal management
PCB manufacturers already offer a wide range of technical solutions for drive electronics, lighting technology and power supplies in order to transmit high electrical power while taking thermal management into account.
In the coming years, embedded solutions for integrating device functions will also be increasingly used here. In most applications, copper-filled or unfilled thermovias are used to dissipate heat from hotspots on printed circuit boards. The heat loss from the power section is dissipated via these thermal paths and transferred to passive or active cooling concepts via heat spreading. Where this standard concept reaches its limits, inlay technology is usually used. Copper inlays partially embedded in the PCB and connected via thermovias can reduce the thermal resistance in the thermal path by a factor of 20 and specifically avoid hotspots.
In future, according to the experts, special thermally conductive materials will increasingly be combined in a hybrid structure with integrated copper inserts. In addition, system solutions in the form of overmolding an assembled circuit board with thermally conductive plastic (thermoplastic) turn the assembly into a complete system.
Flex and Flex-Rigid for smart textiles or wearables
While flexible or rigid-flexible standard technologies meet the requirements today, foldable or rolled circuit carriers that overcome mechanical and physical limits will also be needed in the future. The requirements for stretchability, flexibility and skin compatibility call for new materials such as polyurethane. The soft and highly flexible material adapts to different shapes and contours and thus enables applications in medical technology directly on the human skin.
Lightweight, stretchable and semi-transparent circuit carriers laminated directly onto textiles offer a high level of comfort. The new technology combines the advantages of rigid printed circuit boards, manufacturing options, assembly and robustness with the properties of stretchability, softness and biocompatibility of polyurethane films.
Experts have identified four trends for flexible printed circuit boards in the coming years. Firstly, an increase in the number of layers in the rigid and flexible area, secondly, an increase in embedded solutions with embedded ICs and inlays, thirdly, a minimization of mechanical tolerances and fourthly, smaller conductor pattern and stop varnish tolerances.
Additive PCB manufacturing
A lot is also happening beyond conventional PCB production. Digital and additive manufacturing technologies such as laser direct exposure and solder mask inkjet processes are fundamentally changing the technical basis of PCB manufacturers. In addition, the latest developments in the field of additive manufacturing technologies with 3D printing show new opportunities and possibilities for the additive fully digitized production of a printed circuit board outside the known standard processes. This requires printable materials with comparable or better final properties as well as machines and systems in order to bring the process costs into an economic corridor.
ZVEI technology roadmap: valuable orientation for the industry
In the 330-page ZVEI technology roadmap "Next Generation", experts from all areas of the electronics industry show the technological trends and innovation fields of the future in electronics. The large-scale analysis provides decision-makers with valuable orientation for identifying opportunities and risks in business areas and markets at an early stage. Ralph Fiehler, KSG, led the editorial team for chapter 8, Printed circuit board, and explains the most important technological developments.
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