As more and more designs move to flexible materials to take advantage of space, weight or packaging benefits, it has been clear that flexible circuits require a different set of rules than their rigid counterparts. We spend quite a bit of time working through the design to make sure that the flex is as robust as possible. We also spend quite a bit of time on material selection, again to ensure that the flexible circuit withstands the flexing that will be required and performs properly in the end environment.
One thing we do not often talk about is what happens behind the scenes during the fabrication and assembly of the flexible circuit. What types of special handling considerations are in place throughout the manufacturing process to accommodate these thin materials? When you are auditing a potential new supplier, what things should you be asking about and looking for in their procedures?
Undoubtedly, the largest source of defects in flexible circuit manufacturing can be traced back to material handling. Drawing from my own knowledge and soliciting the expertise of several industry veterans involved in flex circuit manufacturing—David Moody with Lenthor Engineering, Anaya Vardya with American Standard Circuits, Jim Barry with Eltek and Mike Vinson with Averatek—I have put together an insider’s view of the nuances involved in manufacturing flexible circuits.
Everyone agreed that it is the handling of the thin flexible materials that is the key to the successful manufacturing of flex and rigid-flex designs. A wrinkle, ding or dent in the copper material can easily, and will most likely, cause a defect. In fact, wrinkles are typically the leading cause of defects for trace and spaces errors in process imaging. So, what do fabricators do to mitigate this damage?
Skilled technicians are at the very top of the list. Thin, flexible materials require a unique set of processing parameters and significant time and effort is put into training operators on material handling. The movement of product between process steps is as critical as the precautions that need to be taken during each process.
When moving flexible materials between process steps, transport frames, slip sheets, and trays are required to provide the extra support needed to keep these panels absolutely flat—remember a ding or fold in the material will create a defect. When picking the material up for processing, consideration needs to be given to grasping the opposing corners to keep the panel flat.
Special consideration and handling is also needed when processing. Most equipment is not specifically set up to handle thin core, flexible materials. For example, moving product through the etching process or other conveyorized equipment requires “leader boards” or some type of frame to be taped to both ends of the sheet of flex material to provide stability and prevent the sheet from being caught up in the equipment rollers. If not done carefully, the process of applying the frame or leader and the subsequent process of removing the support structure is also an operation prone to damaging the thin materials.
Prior to wet processing, the panels are in full copper sheets. Once the excess copper has been removed to form the space and trace pattern, the panels are even more susceptible to handling damage. Care is given when creating the panel artwork to leave as much excess copper as possible on the panel. This could be the outer edges of the panel, the outer edges of each array, and between individual parts. It is not uncommon that the need for extra copper to add stability takes priority over the desire to maximize panel utilization.
Dimensionally, flex is far less stable than glass-reinforced rigid boards. The added copper in the panels also helps mitigate the material movement. This material movement creates unique challenges for registration both with coverlay application and in layer-to-layer registration for both multilayer flex and rigid-flex. Each manufacturer has a preferred method for registration and how they set up their tooling pin systems to best fit their processes.
Lamination is another area with unique requirements and special equipment for flex processing, including both lamination plates and specific lamination driver materials. Specialized materials are needed to fill air gaps and provide support through lamination. In the case of very thin core (.0005” polyimide) a base support layer may be needed.
More and more fine line flex circuits, particularly medical and sensor applications, are using extremely thin polyimide substrates with densities requiring additive processing rather than subtractive etch processing. These products are primarily double-sided with one side much more densely plated than the other, using both gold and copper to form traces on 0.0005” polyimide or thinner. Because of this, any plating stress will cause the parts to curl. For routing operations, UV-sensitive tape can be added to the panel to improve stability and support and improve handling. This technology is similar to what is used in wafer processing. The parts will remain flat until the UV tape is removed. When removing the UV tape, the simple effort of being aware of the direction the material stress will cause curling, and then removing the tape by pulling against that direction, will help minimize the effect.
Whether the flexible circuit has just a couple of components and is hand assembled, or the circuits are going to be run through a surface mount process, the number one thing that needs to be taken into consideration is the need to bake the material before subjecting the flexcircuits to high temperatures. Hand assembly is especially prone to defects with flexible materials. This process requires special consideration as to temperature and time and is yet another area that operator training is critical.
Flex circuits (and thin core boards in general) require some support mechanism to run through either wave solder or a reflow process. There are several options to accomplish this. A design with many FR stiffeners may move forward by using an FR4 carrier panel designed to provide stability to the array until assembly is complete and then have the excess FR4 removed, leaving the intended stiffeners. While that is one approach, it is more common to build the flex circuit as a single, individual piece and have a carrier fixture made to transport the circuit through the assembly line. This allows the fabricator to maximize the panel real estate and provide a lower cost unit price for the flex circuit. Carrier tooling is relatively inexpensive and is generally more than off-set by the lower cost flex circuit.
Flexible circuits are certainly a growing segment of the market and require not only special design and material consideration, but special handling throughout the manufacturing process. With material handling cited as the largest cause of yield loss during manufacturing, this is an area with (and for) continuous improvement. We all agree that employee training and on-going education is the key to success. Many facilities specialize in just flex and rigid-flex processing and others have teams dedicated to this product subset, but the common theme is knowledge and specialization. Flexible circuits often have a slightly longer manufacturing lead-time than their rigid counterparts and this is often related to the special handling and processing required for flex. Whether that is extra time in tooling and process planning, extra time during wet process, extra care needed to properly register the layers prone to material movement, or extra care needed during assembly, all special handling is done to maximize yield and provide a robust product to the end user.
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For additional information, contact Tara Dunn. www.omnipcb.com