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An increasingly popular solution for the efficient printing of flexible substrate materials is continuous web processing, or ‘Reel-to-Reel’ printing. Reel-to-Reel offers an excellent solution for a wide range of products from mobile phones and laptops, through to biosensors, rope lights, automotiveinstruments and RFID tag technologies. Flexible PCB demand is growing continuously. While miniaturization has long been a driver for progress in electronic subassembly design, recently portability has become the most significant industry trend. Advantages of flexible circuits include weight and space savings, higher circuit densities and the elimination of bulky connectors and wiring. Today, design and styling are recognized as crucial to differentiate products and brands. Its here that flexible circuitry becomes an enabler to overcome conventional limitations and deliver the design elegance that translates tocompetitive advantage, or success, or sometimes survival! So it’s clear that continuous fl exible substrates now play an important role in meeting the complex challenges of sub miniature assemblies. And the use of such substrates will only continue to grow – both in electronic circuitry applications and more diverse materials applications like biosensors. However, fixturing flex substrates for printingis a challenge. Fixturing non-rigid substrates in individual (singulated) formats is complex and expensive – sometimes prohibitively so. A popular solution is to panelize the individual substrate elements and implement large area, ‘multi-up’ processing, but this doesn’t eliminate the fi xturing and material handling challenge. An optimum solution is to choose a continuous flex substrate running off one reel, through the printer, and onto a collecting reel. This cleverly engineered process is known as Reel-to-Reel. There’s still fixturing to consider but now you have an automated process capable of high print speeds with minimal operator intervention to boost productivity and effectively drive down the cost per printed image. Reel-to-Reel is compatible with a broad range of substrate types including flexible printed circuits (FPCs), organic LED substrates, non-conductive materials, and fuel cell membranes as well as a wide range of foils, papers and composites. Let’s take a look at some of the key users of flexible substrates for whom Reel-to-Reel maydeliver productivity benefits. Cars, vans, trucks, buses, motorcycles
The automotive sector deploys an increasingly large amount of flexible circuitry in its electronics applications. Here, reduced bulk and weight are critical to automotive engineering, especially as electronics continue to pervade almost every part of every new vehicle design. So shrinking the wiring loom and reducing connectors and interconnections between subsystems becomes increasingly important. Now, sensors can be readily integrated into the wiring harness itself, and as more parameters need to be monitored around the vehicle for features like collision avoidance, automatic parking, lane change warnings, etc., more flex circuits are being designed into car parts like thesteering wheel and control levers for lights and wipers. Lighting the way
LED technology has been revolutionizing virtually all areas of lighting, including automotive exterior lighting where, combined with flexible circuitry, compelling new design styling is enabled. In addition, sports and children’s clothing designs often incorporate flexible circuitry and new LED technologies to provide safety lighting far superior to simple reflective strips. Some premium clothing and sportswear brands are even incorporating wearable flexible circuitry to enhance the experience of personal media and entertainment equipment, as well as interactive gaming and integrated biometric monitoring for athletes, cyclists, joggers, rowers, and so on. Expect this kind of technologyto filter down to affordable clothing ranges anytime soon! Medical methods…
That gives a neat link into another prime user of flexible substrates, though not necessarily electronics-based. Manufacturers of biosensors – those small, invariably disposable products that provide instantaneous data on everything from pregnancy to blood sugar levels – employflex substrates (not flex circuitry) as a base to deposit layers of live enzymes at nominal thicknesses of 25-50 microns.Some are porous substrates (paper or cardboard forinstance) and therefore need to be handled at high volumesand at very high speeds with accuracies similar to finepitch SMT assembly. Here again, fixturing of individualsubstrates is complex and cost prohibitive, even in large,multi up arrays. So how do you fixture flexibles?
Substrate fixturing (also known as ‘tooling’) is fundamental to accurate and repeatable printing. Three key planes need to remain parallel in the printing process: the print head, the stencil and the substrate surface. So support of the underside of the substrate is imperative and impossible to compromise without impacting negatively on the imaging results, meaning that secure fixturing is vital. From a process perspective, the requirements are the same for printing a flexible, continuous substrate as for that of a conventional rigid PCB. Factors like image resolution, deposit thickness control, and screen separation must be addressed, but doing so demands some different materialhandling design considerations. Typical continuous substrates are presented in a strip from a roll, with step and repeated images created and plotted side-by-side along the entire length of the roll, which may be up to 150 meters long. The reel is indexed under the print position, where screen and image fiducials are identified and aligned. The substrate is raised into contact with the stencil, printed, and then brought back down and transported away ready for the sequence to be repeated – much in the same way that individual boards would be processed. However, a continuous flexible stripposes unique problems that must be addressed. These include considerations relating to: substrate thickness, where the flexible roll needs to be handled sensitively without twisting or stretching; and substrate tension control, as a function of holding the flex flat in the print area during the print stroke. Control of possible smearing as the flex is indexed forward after the print stroke, especially as the distance between step and repeated images is invariably minimized to make the best use of the substrate material – which can mean that a wet printed portion of the continuous roll is not fully clear of the stencilwhen the subsequent image is indexed into the print area. Image to image offset also needs consideration as substrate manufacturing tolerances may cause the location of subsequent images to vary from the nominal indexcenterline. Pull down, index and back off
An optimum solution that addresses all these considerations comprises a feeder system incorporating two vacuum beds that supply an even pull-down forceacross the width of the substrate. When the moving edge-edgeclamps(see figure 1) pull the substrate through duringindexing, the even vacuum force ensures no twist or stretchon the outer edges of the roll. Once in position, automaticcentre clamps then hold the stationery substrate firmly inplace for printing. 
The feeder motion velocity and acceleration are adjustable to suit any type of substrate. Critically, the system also incorporates a ‘back off’ motion sequence which reverses the substrate back towards the machine after the centre clamps are activated, allowing some slack in the output side of the continuous roll (see figure 2) and ensuring that the table is free to move upwards without tugging or stretching the substrate. By using the intrinsic tension created by pulling the substrate through the print area (and against the vacuum) combined with precise timing of the clamping system and vacuum plates, the sophisticated transport mechanism (see figure 3) succeedsin keeping the substrate fl at during printing. 
But what about smearing?
Naturally, the potential for smearing still exists even with this sophisticated substrate transport mechanism, particularly with the necessary back-off phase. However, the risk is virtually eliminated through the use of dedicated vacuum plates on the print table section, along with overhanging substrate clamps and well-designed stepetched stencils. Such a configuration permits the roll to hang over the edge of the vacuum plate with a forcing guide from the substrate clamps. The stepped etch stencil serves to increase the distance from the trailing edge of thelast image printed. When manufacturing a continuous roll substrate, one image is created then step-repeat-plotted side-by-side over the entire length of the reel. In practice, alignment between images can vary as much as 0.2mm, which effectively prevents the printing of more than one image at a time (see figure 4). The only solution is a highly accurate and repeatable alignment system – probably a fundamental requirement of any reel-to-reel process. However, unlike the printing of singlulated (one at a time) boards, adjusting the position of the substrate relative to a stationary stencil image is not possible without inducing twisting, or stretching, and most likely smearing too. Instead, the system design must accommodate an alignment technology that movesthe stencil relative to a stationery image position. 
An optimum situation would be to locate the largest gap in the images and print between that gaps from image to image. Unfortunately this is not possible due to the offset between images. Instead, given that the print function has to be effected from image to image, there needs to some gap available. Quite simply: the larger the gap, the more robust the process. A very large gap would be excellent, but it would be at the expense of substrate ‘real estate’, which is likely to have an impact on the number of images that could fit on a roll, which in turn would have a cost and/or productivity implication. So there’s always a compromise or balance between the number of images and the gap between them. However, a gap of just 1.5 mm has beenshown to offer a reliable process window. Round up
Flexible substrates offer a number of distinct benefits. Flex is a good option for reducing weight and space, saving on assembly costs, and even managing high temperatures. Reel-to-Reel technology is a handling solution that permits precision processing onto almost any type of continuous substrates, including flexible circuits and advancedsynthetic materials increasingly popular in industry today. It’s true that the Reel-to-Reel process demands some changes to the conventional methods used for precision alignment and clamping with rigid substrates. And yes, there’s a little more equipment sophistication needed to deal with the unique demands of large, flexible substrates, such as vacuum hold-down and back-off indexing procedures. But by deploying accurate indexing and tension management mechanisms, Reel-to-Reel processing can successfully automate printing onto substrates typically over 150 meters in length and up to 508mm wide. Alignment accuracies of 1.6 Cpk at 25 microns are typical and printing proceeds atspeeds up to 500mm per minute. | 
