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STEP 4: Printing Flexible Substrates
December 31, 1969 |Estimated reading time: 6 minutes
By Michael Brown, DEK
In fast-paced manufacturing, continuous flexible substrates help meet complex challenges of sub-miniature assemblies. Reel-to-reel processing offers automation compatible with flexible PCBs, organic LEDs (OLEDs), non-conductive materials, fuel cell membranes, and diverse other substrates. It requires significant changes to standard rigid methodologies.
Continuous web, or reel-to-reel, processing of continuous flexible substrates solves a range of end-product challenges in mobile telephony and laptop computers — even rope lights, automotive instruments, and RFID tags. It requires some significant changes to the standard methods of precision alignment, clamping, and conventional rigid-substrate hold-down to deal with different demands of large, flexible substrates.
Continuous flexible substrates play an important role in meeting complex challenges of sub-miniature assemblies. Use of flexible substrates, whether as circuits or not, will continue to grow for numerous reasons. Fixturing of non-rigid substrates in individual formats is both complex and expensive — often cost prohibitive. Large-area, multi-up processing is one approach, but it does not eliminate fixturing and material handling issues.
Flexible Solutions
Unlike the cyclical performance of rigid PCB markets, flexible PCB demand has grown rather continuously, particularly over the past several years. Though miniaturization has been a long-standing design driver for electronics, in recent years, portability has emerged as a significant trend in the industry. Along with typically cited advantages of flexible circuits — weight and space savings, higher circuit density, and the removal of bulky connectors and associated wiring — design and styling increasingly are critical product and brand differentiators, pointing to flexible circuitry’s ability to break traditional limitations and achieve the design elegance crucial to competing and market survival.
Automotive electronics represent an increasingly large user segment for flexible circuitry. Bulk and weight reductions are critical to nearly every aspect of automotive engineering. As electronics become a larger part of new auto design, shrinking the wire harnesses and reducing connectors and interconnections between subsystems grow increasingly important. Sensors can be integrated readily into the wiring harness itself. As more sensor data need to be transmitted around the automobile to support new features such as collision avoidance, automatic parking, and lane-change warnings, flex circuits are being designed into steering wheel and control levers for lights and windshield wipers.
LED technology continues revolutionizing virtually all areas of lighting, including exterior automobile lighting. Here, combined with flexible circuitry, LED technology enables dramatically new design styling. Sports and children’s apparel designs incorporate flexible circuitry and LED packages to provide safety lighting superior to simple reflective strips.
The manufacture of biosensors — small, often disposable products that provide instantaneous data on everything from pregnancy to blood sugar levels — employs flexible substrates, not flex circuitry, upon which are deposited layers of live enzymes at nominal thicknesses of 25?50 μm. The porous substrates need high-volume/high-speed handling with accuracies similar to fine-pitch SMT assembly. Once again, fixturing of individual substrates is complex and costs prohibitive, even in large, multi-up arrays.
These are a sampling of an increasingly popular manufacturing solution to meet the interwoven demands of lower cost, smaller and lighter product configurations, and high-volume processing. Namely, thinner and more flexible substrates processed in one continuous web.
Fundamental Requirements
Basic process requirements remain essentially the same for flexible substrates as for rigid ones, whether in printing, placement, reflow, inspection, or other familiar steps. For printing, which is the further focus of this analysis, issues of stencil or screen alignment, image resolution, deposit thickness control, screen separation, and so forth, must be addressed accordingly.
Substrate fixturing, or tooling, is a fundamental element of accurate and repeatable printing. Parallelism of the three key planes — print head, stencil, and substrate surface — and underside support of the substrate will affect imaging results negatively if compromised. Alignment accuracy and repeatability necessitate secure positional fixturing of the substrate. From a process point of view, the requirements are the same for flexible, continuous substrate printing as for rigid PCB printing. Achieving those requirements calls for some different material handling design considerations.
Material Handling
Substrates in reel-to-reel printing are presented in a continuous strip, typically a roll, with step-and-repeat images created and plotted side by side along the roll’s entire length, which may be up to 500 feet. Simply stated, the reel is indexed under the print position, where screen and image fiducials are identified and aligned together. The substrate is raised into contact with the stencil, printed, and then brought down and away for the sequence to be repeated, much in the same way that individual boards would be processed.
However, a continuous flexible strip poses problems. Due to the narrow substrate thickness, systems are challenged to handle the substrate without twisting or stretching. Controlled substrate tension will ensure that it is held flat in the print area. With smearing risks, the challenge is to minimize the distance between stepped and repeated images, to make maximum use of the substrate material. This can mean that a wet printed portion of the continuous web is not completely clear of the stencil when the subsequent image is brought to the print area. Finally, image-to-image offset can occur. Substrate manufacturing tolerances will cause the location of subsequent images to vary from the nominal index centerline.
Figure 1. Moving clamps pull substrates through the process without twisting or stretching.
A feeder system that incorporates ample vacuum beds offers gripping force across the substrate’s width so that, when moving clamps are pulling the substrate through, there is no twist or stretching on the outer edges of the substrate (Figure 1). Feeder motor speed and acceleration can be adjusted to suit the substrate. A back-off command to move the substrate back into the machine after the center clamps are down allows the table freedom to move upwards without pulling the substrate or stretching. Using the natural tension created by pulling the substrate through the machine, combined with the timing of the clamps and vacuum plates, helps keep the substrate flat during printing (Figure 2).
Figure 2. Natural tension of transport system pulling a substrate through print area assists with flattening.
The smearing issue can be overcome with the use of dedicated vacuum plates on the print nest table section, overhanging substrate clamps, and step-etched stencils. This configuration, combined with the back-off command, allows the substrate to hang over the edge of the vacuum plate with a forcing guide from the substrate clamps. A stepped-etch stencil increases the distance from the trailing edge of the last image printed. In reeled-substrate manufacturing, one image is created and plotted side-by-side over the entire length of the reel. Unfortunately, alignment between images can vary as much as 0.2 mm, which prevents printing more than one image at a time (Figure 3). A highly accurate and repeatable alignment system is a fundamental requirement of any reel-to-reel process. Additionally, that system must align the stencil relative to the image position. Unlike single-up board processing, it is not possible to manipulate the position of the substrate to a stationary stencil image without inducing the twisting and smearing problems referred to earlier. The ideal situation is to find the biggest gap in the images and print between that gap from image to image.
Figure 3. Step-and-repeat substrate tolerances require individual image alignment.
This unfortunately is not possible because of the offset between images. So, when having to print from image to image, an available gap is required. While the larger the gap, the more robust the process, a gap of 1.5 mm has been demonstrated to offer a reliable process window.
Conclusion
In today’s marketplace, flexible substrates offer designers and engineers an array of choices with a growing number of benefits. Flex often is the way to go for managing high temperatures, reducing weight and space, and saving on assembly costs. Reel-to-reel is a handling solution that enables precision processing onto continuous substrates, such as flexible circuits and advanced synthetic materials. It requires some significant changes to the standard methods of precision alignment, clamping, and conventional rigid-substrate hold-down to deal with different demands of large, flexible substrates. However, use of accurate positional indexing and tension management mechanisms lets reel-to-reel processing methods print onto substrates typically 500 feet in length and up to 20" wide. Alignment accuracies of 1.6 Cpk at 25 μm are typical, and at speeds of up to 500 mm/min. SMT
ACKNOWLEDGEMENTS:
Ashmore, Clive, ”Medical HVM Challenges“ Circuits Assembly Magazine, November 2007.Joselyn, Louise, ”Breaking Barriers,“ New Electronics, May 2007.
Michael Brown, SEMICON product specialist, DEK International, may be contacted at Granby Industrial Estate, Weymouth, Dorset, U.K. DT4 9TH; +44 1305 208416; mbrown@dek.com.