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JANUARY 2019 I FLEX007 MAGAZINE 63 into the holes. The pressure drives the via- fill material into all the low-lying areas of the panel including the flex cavities. The material then hardens and needs to be removed. In a rigid board where the surface is very smooth, a deburr machine or sander is typically used to sand the excess via-fill mate - rial off the surface. With rigid-flex, if we go too aggressively at sanding the via-fill material off the sur- face, one can sand through the copper on the elevated parts of the outer layers at the same time. If that copper is removed, the panel must be scrapped. To accommodate this delicate process, much of the work, inspection, and re-work is done by hand. Via-filled boards typically yield 30% less than an equivalent rigid-flex board and take weeks of extra time to get them to yield adequately. An additional consideration for rigid-flex manufacturers is the copper plat- ing wrap require- ment on via- filled boards for high-reliability appli cations (IPC 6013, 3.6.2.12.1). The wrap require- ments specify how much copper must remain on the surface after planarizing the via-fill material off the surface. To accommodate this require - ment, fabricators will often plate extra copper onto the pads of the via-filled holes. This extra copper plating adds to the reli- ability of the via. At the same time, this makes it more difficult to image and plate higher den- sity trace/spacing on outer layers that are typically needed for BGA routing. Outer Layer Imaging and Copper Plating I previously mentioned that the outer layer copper surfaces on a rigid- flex will typically have some topography that needs to be overcome. Also, if you think about a production panel, there are now areas in the panel that are rigid board and areas that are flexible. The rigid areas will usually be sig- nificantly thicker than the flexible areas of the board, forming depressions or cavities in the surface of the panel. These two aspects require rigid-flex manu- facturers almost universally to use vacuum lamination to apply their photoresist and direct imaging to image the outer layers of the board. The vacuum lamination pulls the photoresist into intimate contact with the sur- face of the board, which is critical to achieving good yields in imaging and subsequent plating operations. Direct imaging is critical because if you were to use conventional phototools and printers, the phototools are distorted by the flex cavities during vacuum lami- nation, and your features will be far off of where you need them to be. This is especially true with small boards that have many flex cavities across the panel, making it virtually impos- sible to scale the phototool to match the distortion cre- ated during vacuum pull- down of the phototool. Routing and Scoring Rigid-flex Boards There are a few different techniques for provid- ing singulated rigid-flex boards and rigid-flex boards sup p o r t e d i n arrays for assembly. Singulated rigid-flex boards are usually, b u t n o t a l w a y s , cables with connec- tors on each end (Fig- ure 3). Parts that require more extensive assem- bly are usually provided in an array to hold the Figure 3: Singulated rigid-flex cable with connectors on each end.