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PCB-Mar2017

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20 The PCB Magazine • March 2017 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 ma- terial handling. The movement of product be- tween process steps is as critical as the precau- tions that need to be taken during each process. When moving flexible materials between pro- cess steps, transport frames, slip sheets, and trays are required to provide the extra support needed to keep these panels completely 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, flex- ible materials. For example, moving product through the etching process or other convey- orized equipment requires "leader boards" or some type of frame to be taped to both ends of the sheet of flex material to provide stabil- ity and prevent the sheet from being caught 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 damag- ing 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 han- dling 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 uncom- mon that the need for extra copper to add sta- bility 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 in terms of coverlay application and in layer-to-layer regis- tration 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 re- quirements and special equipment for flex pro- cessing, 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 lay- er may be needed. More and more fine-line flex circuits, par- ticularly medical and sensor applications, are using extremely thin polyimide substrates with densities requiring additive processing rather than subtractive etch processing. These prod- ucts 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 plat- ing 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 in which direction the mate- rial stress will cause curling, and then removing the tape by pulling against that direction, will help minimize the effect. Assembly Whether the flexible circuit has just a cou- ple of components and is hand assembled, or the circuits are going to be run through a sur- face mount process, the number one thing that FLEX MATERIAL HANDLING: AN INSIDE PEEK " When picking the material up for processing, consideration needs to be given to grasping the opposing corners to keep the panel flat. "

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