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JANUARY 2019 I FLEX007 MAGAZINE 59 Ideally, coverlayer and bondply should only extend 0.050–0.100" into the rigid sections of the rigid-flex board using a technique called cut-back coverlayer. The reason for this is that the adhesives used on coverlayer and bondply have a higher Z-axis CTE rate than the other materials used in rigid-flex constructions. This higher Z-axis CTE rate can put undue stress on vias during assembly and other thermal excur- sions. As a rule, the adhesives also do not drill, desmear, etchback, or plate as well as flexible and glass-reinforced laminates and prepregs. There are exceptions of course, but rigid-flex manufacturers will try to use a cut-back cover- layer technique whenever they can, and espe- cially for high-reliability "never-fail" applica- tions (IPC 2223C, 5.2.2.2). The coverlayer and bondply pieces need to be cut to size so that they are only in the flex- ible sections of the board. They are usually cut with steel-rule dies, lasers, or CNC drilling and/or routing machines with the choice based on the size of the pieces and/or the dimen- sional accuracy required. Once the coverlayer and bondply are fabricated, they are kitted and held until the flexible layers are completed. No-flow or Low-flow Prepreg The no-flow or low-flow prepreg that is used to bond the final package together is fabri- cated at the same time as the coverlayer and/ or bondply. The material is somewhat similar to standard prepregs with a couple of excep- tions. No- and low-flow prepregs are designed to flow very little, so that the resin does not flow out onto the flexible areas of your design. Standard flow prepregs should not be used in rigid-flex designs. The prepregs used in rigid- flex manufacturing are thin and generally lim- ited to just two glass styles: 1080 and 106. They can be sensitive to handling damage because if they get dented or creased, it can deprive a section of the board of enough resin to encap- sulate your circuits. Once the no-flow prepreg is sheeted and kit- ted, we then fabricate the sheets to match the design of your board. Wherever there are flex- ible sections in your rigid-flex design, we will cut an opening in the no-flow prepreg accord- ingly. Designs that are very small or have lots of flex arms will have so many holes cut in them that they can look like doilies and be almost as hard to register! The no-flow prepreg is then staged until the flexible layers are completed. Flexible Laminate Layers The copper-clad flexible layers that will be the circuits in the flexible layers of the board are fabricated similarly to standard glass-rein- forced copper-clad laminates. Two exceptions are that the flex layers are usually pretty thin (0.001–0.003") and clad with a wrought, or rolled-annealed, copper foil and the surface cannot be cleaned using conventional equip- ment. Rolled-annealed copper foil is very, very soft and is easily dented during handling. Care must be taken to not dent the material either with handling or automated equipment processing. A dent can cause issues with imaging your cir - cuits faithfully, or it can pull one of the registra- tion targets out of alignment just enough that it will affect the registration of the flex layers to the rest of the package. Registration targets must be used throughout your entire package to get dissimilar materials to register to one another. Any target that is pulled out of its loca - tion ruins the rest of the package. Flexible laminates are too thin to use con- ventional scrubbing equipment to clean the surface before applying photoresist, so flex and rigid-flex manufacturers will typically use a chemical clean process line to condition the copper surfaces. Once the flex layers are fin- ished, they go through AOI and tooling for lamination similar to hardboard layers, and end up in lamination for coverlayer and/or bondply application. Assembling Coverlayer and/or Bondply The pieces of coverlayer and bondply are placed on what will become the flexible sec- tions of the board, and are tacked in place. Placement is done by hand, which can be tedious; depending on your design, this can take hours, days, or even longer to complete a job. Layup of the coverlayer and/or bondply is usually completed in a cleanroom to reduce