Editor’s Note: To read “Global Impact on European PCB Fabrication: EIPC Summer Conference 2017, Day 1”, click here.
The second day saw the conference room full once more, the late networking session in the bar having resulted in no significant casualties, and I was pleased to accept the invitation to moderate the first session, on processes and materials for flexible PCBs.
Thomas Michels got proceedings off to a flying start with an insight into a new range of cast polyimide flexible laminates where the polyimide resin, with or without filler, was coated and cured directly onto the foil to make single-sided material, and two single-sided could be bonded together with the same resin system to make double-sided. The laminates showed remarkably good dimensional stability, heat resistance, flexibility and flexural endurance. In anticipation of 5G requirements, the material was already available with polyimide thickness as low as nine microns with 9-micron copper, and even lower thicknesses shortly to be released. A modified polyimide resin system had been developed for low-loss applications, with Dk of 2.8 and Df of 0.005 at 10GHz. And whereas conventional coverlays required a separate adhesive layer, a versatile 25-micron one-layer coverlay had been developed with excellent thermal resistance and filling performance, which enabled thinner, lighter and more flexible constructions to be fabricated.
Well-known for his work on stretchable circuits, Professor Jan Vanfleteren, from IMEC, the Interuniversity Micro Electronics Centre in Belgium, introduced a new dimension in flexible PCB-based technology for randomly shaped circuits. He explained that here was a growing opportunity for shape-retaining free-form circuits that could conform to 3-dimensional surfaces beyond what could be achieved with flexible circuits, for example in ergonomic man/machine interfaces and free-form light sources.
Conventional flex-rigid assemblies were expensive and offered limited design freedom. Laser-structured moulded 3D interconnection devices were difficult to fabricate in more than a single layer, component placement was slow compared with 2D assemblies, and only a limited range of high-end plastics could withstand soldering with SAC alloys. Against this background, Professor Vanfleteren had set out to develop technologies for 3D circuits that would allow easy lab-to-fab transfer, using normal 2D fabrication and assembly procedures with off-the-shelf components and standard lead-free soldering.
He reviewed the principles established in his work on stretchable circuits, particularly the meander-pattern design rules for conductors, and showed how these principles had been adapted by replacing the elastomeric carrier with a flat, rigid thermoplastic polymer carrier. Once the circuit had been fabricated and assembled, it was subjected to a one-time deformation from flat to 3D using thermoforming techniques, resulting in rigid 3D-shaped thermoplastic objects with embedded electronics. Repetitive strain on components and interconnects was avoided, and components were additionally protected by the embedding polymer. There was great potential for industry take-up, and he showed examples of applications, including an omni-directional LED light source for homogeneous three-dimensional light distribution, and a three-dimensional touch-sensitive control panel for a washing machine. Remaining challenges for industrialisation of rigid, free-form 3D circuits were laser structuring of the meanders, high throughput technology based on punching, dedicated circuit lay-out tools and the precise positioning of components after 3D forming.
Next came a variation on two-dimensional flex, but with an effectively infinite X-axis. Philip Johnston, managing director at Trackwise in the UK gave an intriguing presentation on length-unlimited multilayer flex. The background to his story was that Trackwise had been approached by a large UK aero engine manufacturer with a requirement for single-piece flexible circuit more than eight metres in length. After a world-wide search for a supplier, where the rest of the industry had said “No,” Trackwise said “Why not…” and after some development work produced a proof-of-concept 6-layer flex five metres long, by a process they subsequently patented. Johnston stressed that this was a seamless continuous circuit design, not a step-and-repeat.
Having successfully manufactured and delivered the 8-metre order, Trackwise set out on a mission to change the way aeroplanes are wired with their “Improved Harness Technology™”—not an easy task in an industry characterised by its obsession with reliability and cautious about the possible negative consequences of any change from long-established standard practice—and many challenges had to be overcome along the way. Materials had to be sourced in roll format, and equipment suppliers had to be willing to adapt conventional machinery to meet these exceptional requirements. Manufacturing was a combination of roll-to-roll PCB and electrolaminate production techniques. Quality assurance could be a conundrum: “How do you verify the integrity of a PCB 25 metres long? And what about accreditation?” Whatever, the first flight qualification was about to take place, with a flexcircuit 26 metres long in a large U.S. unmanned aerial vehicle and there were many opportunities in the automotive, industrial and scientific sectors.
Whenever the patterns on two or more circuit layers were required to be aligned in manufacture, some form of registration system was required. Bernd Gennat, VP of sales and marketing for DIS in Germany presented a detailed account of lay-up technologies and bonding processes prior to the pressing of the multilayer PCBs, with an emphasis on state-of-the-art direct optical registration. He discussed the pros and cons of riveting and pin-laminating systems before going into detail on optical lay-up technology and coupled induction welding. Listing the benefits of the system developed by DIS, he explained that once the cameras had moved to a particular panel size, they stayed stationary for the entire process. Positioning and clamping took place in the same station, and layers were held in position by a clamping mechanism whilst in view of the vision system. The system easily handled thin cores without damage or distortion, and an arrangement of unique targets per layer ensured idiot-proof lay-up. A vision system maintained the position of the targets during the complete alignment and clamping cycle and alignment was always to the camera, never to the previous layer. A coupled induction welding system allowed aligned panels to be handled horizontally and an additional benefit was that the same lamination and separator plates could be used for a range of panel sizes.
EIPC board member and past chairman Paul Waldner, managing director of Multiline International Europa, gave a more general overview of registration systems, beginning by defining registration as “making things fit each other according to the requirements set forth by the end-user,” and listing the main registration challenges encountered in making printed circuit boards: aligning layers to one another, aligning pads on one layer to pads on other layers, aligning holes to pads on every layer, aligning outer layer images to innerlayer images, aligning solder mask images to primary imaged circuits and aligning components to the bare board. He then produced an equally long list of factors that could affect registration: material characteristics, environmental influences, mechanical process influences, mechanical process accuracy, mechanical process precision and, probably most important, good design based on an in-depth knowledge of materials, environmental influences and mechanical capabilities. He also suggested that a statistical process control system that tracked environmental and process changes and correlated those changes to changes in dimensions of the various images during the manufacture of a completed circuit should be an important tool in the manufacture of multilayer circuit boards.