Multifunctional system boards are already state of the art today. Different starting materials, methods and manufacturing processes enable highly specialized circuit carriers: densely packed and/or highly integrated, RF-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 headed.

The development of printed circuit board technology has been and continues to be driven by the development of electronic components, whose electrical and physical properties are being further perfected, while dimensions are becoming smaller and smaller. This process is oriented towards the functions to be achieved by the final products. Added to this are cost pressures, reliability and service life, as well as increasingly stringent environmental regulations, sums up Ralph Fiehler, head of development at KSG. The KSG Group has actively participated in the ZVEI technology roadmap. Ralph Fiehler coordinated the twelve-member editorial team for the PCB chapter and presented the chapter to the professional community.

The requirements for the development of printed circuit boards in the coming years are: Miniaturization by increasing integration density, signal integrity and RF capability, 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 appropriate system design tools for the hardware developers.

Embedding: Integrated functions in the printed circuit board

Miniaturization has driven embedding, which is the process of embedding passive and active components into the printed circuit board. Embedding enables the short and impedance-matched connections required for signal integrity. ICs, passive components and sensors are integrated as System in Package (SiP) on a PCB substrate. The SiP will later be assembled on a printed circuit board itself.

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. In embedding, the following trends are emerging: use of thinner substrates, reduction in lead spacing, reduction in via diameters, increase in thermomechanical requirements, and increase in embedding of active components.

HDI/SBU PCBs with > 10 layers and BGA pitches equal to or smaller than 0.5 mm

Experts predict more layers and the finest conductor structures for HDI/SBU PCBs (HDI: High Density Interconnect, SBU: Sequential Build Up). This technology accounts for about 13% of the printed circuit boards produced in Europe. Typical in Europe today are HDI multilayers with a line spacing of 100 µm or less and 4 to 10 layers.

Optimized signal integrity requires even higher integration density. To achieve this, PCB designers are forced 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 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, experts expect impedance-controlled structures and the use of mixed structures. The base material, the foundation of any printed circuit board, is one of the decisive factors. Specifically, this means an increase in the proportion of temperature-resistant, halogen-free and CAF-resistant base materials.

In the unbundling of fine-pitch BGA structures, rewiring strategies using resin- or copper-filled staggered or stacked via arrays are becoming increasingly important. If the routing of a BGA structure with 0.8 mm pitch is still possible via a dog-bone connection and a through-hole, an SBU assembly with offset micorvias is already necessary for a BGA structure with 0.65 mm pitch. The BGA connection pitch of 0.5 mm or less, which will become increasingly common in the future, requires more complex solutions. “Rewiring in the SBU structure with copper-filled stacked-via or microvia on buried-via solutions is inevitable,” 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 lower copper treatment.

A challenge for designers and PCB manufacturers are the high-speed digital circuits with complex unbundling, e.g. FPGAs. While today’s circuits process signals at about 12.5 Gb/s, in the future there will be data streams of 25Gb/s, 50Gb/s or more, estimates Helmut Schmucker, segment manager for printed circuit board manufacturing at Rohde & Schwarz.

Unbundling in backplanes and motherboards can extend over very long, differential signal paths, over 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 the unbundling process. While 20-layer multilayers are currently used, the trend is moving towards 30 layers. For cost reasons, the cheaper laminates are 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 back-drilled to achieve the necessary signal quality.

The RF expert points out another important aspect: Current processors today generate power losses of 130 W and more, which at the same time requires supply currents of 140 A. 5 years ago, the power dissipation was still 30 W.

Technical solutions for thermal management

For drive electronics, lighting technology and power supplies, PCB manufacturers already offer a wide range of technical solutions to transmit high electrical power while taking thermal management into account.

In the coming years, embedded solutions for the integration of device functions will also be increasingly used here. In most applications, copper-filled or unfilled thermovias are used to dissipate heat from hot spots on printed circuit boards. The heat loss of 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, the inlay technique is usually used. Copper inlays partially embedded in the PCB and connected via thermovias can reduce thermal resistance in the heat path by a factor of 20 and specifically avoid hotspots.

In the future, experts say, 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 printed circuit board with thermally conductive plastic (thermoplastic) turn the assembly into a complete system.

Flex and Flex-Rigid for smart textiles or wearables

If 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, enabling medical technology applications directly on 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 board, manufacturing possibilities, assembly, robustness with the properties of stretchability, softness, biocompatibility of polyurethane films.

For flexible printed circuit boards, the experts formulate four trends for the coming years. Firstly, an increase in the number of layers in the rigid and flexible range, 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 printed circuit board manufacturing

There is also a lot going on beyond conventional PCB production. Digital and additive manufacturing technologies such as laser direct exposure and solder mask inkjet processes are fundamentally changing the technical base of PCB manufacturers. In addition, the latest developments in additive manufacturing technologies with 3D printing show new opportunities and possibilities for additive fully digitalized manufacturing of a PCB outside the known standard processes. This requires printable materials with comparable or better end properties as well as machinery and equipment to bring 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 guidance for identifying opportunities and risks of business areas and markets at an early stage. Ralph Fiehler, KSG, has led the editorial team of the 8th chapter PCB and explains the most important technological developments.