Substrates for Advanced PCB Technologies: What Will the Future Hold?

Estimated reading time: 9 minutes

Developments in ultra-thin flexible integrated circuits were opening up opportunities for introducing intelligence and interactivity into everyday items, enabling smart packaging, labels, and objects. The proprietary PragmatIC technology utilised thin-film metal oxides on a polymer substrate with a total thickness of fewer than 10 microns at a fraction of the cost of equivalent silicon devices, and the capital cost of the manufacturing plant was far less than that for silicon semiconductors. Fujikora’s WABE hybrid die technology could mass-produce multilayer polyimide PCBs embedded with background ICs and low-profile passive components through a roll-to-roll process. The thin flexible body of the WABE package favoured applications in medical and wearable electronics.

Tremlett concluded his presentation with a brief overview of “substrateless” circuits and moulded interconnect devices with automotive application examples where the circuit was created directly on an existing substrate, and in wearable applications where the circuit was deposited directly on to a piece of fabric.

Martin Wickham then introduced Jim Francey, sales manager Northern Europe for Optiprint and well-known for his expert knowledge on low-loss materials for microwave and RF applications, to discuss organic substrates for PCBs and the factors influencing substrate development and user selection criteria.

Francey began with a broad overview of the available classes of organic substrate: paper, polyester films, FR-4 epoxy, high-Tg epoxy, polyimide, and PTFE. Although paper-phenolic laminates had been used since the early 1960s, there was growing interest in the use of paper coated with biodegradable polyimide as a low-cost PCB substrate. Polyesters such as polyethylene terephthalate (PET) and polyethylene naphthenate (PEN) were well-established flexible-circuit substrates, especially in high-volume reel-to-reel applications, and were being used as substrates for emerging near-field communication (NFC) smart labels with printed memory and printed sensors.

FR-4 woven-glass-reinforced thermoset epoxy resin laminates and prepregs were the established substrates of choice for multilayer PCBs, and blends with resins such as bismaleimide triazine, cyanate ester, and polypropylene ethers had given improved electrical and mechanical properties. Lead-free assembly requirements had driven a transition from di-functional to multifunctional epoxy for improved temperature capability. The addition of thermally conductive inorganic fillers conferred thermal dissipation properties.

Woven-glass-reinforced thermoset polyimide laminates and prepregs had become the industry standard for applications where operating temperatures exceeded the capability of multifunctional epoxy. For many military and aerospace applications, non-reinforced polyimide film was used as the basis of flexible and rigid-flex circuits. Further, adhesiveless materials were increasingly used where reduced thickness, increased thermal robustness, and improved high-frequency electrical properties were required.

Woven-glass-reinforced and non-reinforced PTFE substrates were used predominantly in RF and microwave designs and increasingly in millimetre-wave applications. These materials combined a low dissipation factor with a stable dielectric constant through a wide frequency range. Volume markets were cellular base-station power amplifiers, base-station antennae, and increasingly in automotive radar antennae. Inorganic fillers could be used to modify dielectric constant and thermal conductivity. Woven-glass-reinforced laminates based on thermoset hydrocarbon resins with inorganic fillers were being widely used in microwave and high-speed digital applications, and new hydrocarbons were seen as cost-effective replacements for PTFE in the automotive safety electronics market. Non-reinforced liquid crystal polymer (LCP)—a thermoplastic with negligible water absorption—was increasing in popularity as a substrate in microwave and millimetre-wave applications. Cyclic olefin copolymer was a crystal-clear plastic frequently used in medical applications and in conjunction with additive technology.

Moving on from this comprehensive survey of established and emerging substrate materials, Francey discussed the topic of satisfying PCB transmission requirements in some depth, beginning with some comments on miniaturisation. Thin-core dielectrics gave the opportunity to reduce plated via diameter and increase packaging density. Adhesiveless polyimide flex substrates available in thicknesses down to 12.5 microns, and an ultra-light-weight glass fabric style—1017, only 15 microns—enabled the manufacture of 30-micron laminates and prepregs. Francey showed an example of a 6-layer sequentially laminated rigid-flex with 12-micron single-sided polyimide cores and 12-micron bond plies, and 50-micron stacked vias laser-drilled and copper-filled. Thin-core rigid organic substrates with low-expansion woven glass and copper-filled vias were increasingly being used as alternatives to ceramic substrates in semiconductor packaging.

Francey considered basic requirements for maintaining high-speed signal integrity: low-loss polymers with stable dielectric constant through a range of frequencies, low-profile copper foil, and spread-glass fabrics to minimise the effect of glass-weave skew. He also demonstrated the importance of good layer-to-layer registration in minimising signal losses.

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