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52 The PCB Design Magazine • June 2015 article RIGID-FLEx PCB RIGHT THE FIRST TIME – WITHOuT PAPER DOLLS continues 4, for example). This gives rise to the term "rig- id-flex PCB," where some areas of the board are flexible and others are rigid where components are mounted. The rigid sections may include additional cores, pre-pregs and copper layers or may sim- ply have blank FR-4 or thick PI film adhered to one side as a stiffener. Figure 2 shows an illus- trated cross-section of an adhesiveless rigid-flex hybrid PCB layer stack. Author's note: Lamination of adhesiveless boards is becoming more common, in order to avoid Z-axis expansion problems with the older adhesive-based approach. Because adhesive lay- ers tend to expand greatly during solder reflow, they can adversely affect assembly production yield due to cracked via barrels. The Traditional Paper Doll Approach Design clearance issues can be mitigated us- ing paper dolls. A paper doll is a scaled (usually 1:1) printout of the 2D PCBA outline, cut into its final shape with scissors, so named after the children's activity of cutting a folded piece of paper into a doll shape and then unfolding it to form a string of connected doll shaped cutouts. Paper dolls have traditionally been used exten- sively in PCBA development, and not only for flexible electronics. Figure 3 shows an example of a typical cardboard paper doll used to model the mechanical fit of a carrier PCB with a daugh- terboard. Immediately, one can begin to see the limi- tations of this approach. While it is cheaper and faster than actually building a prototype, it is extremely difficult to accurately model the en- tire PCBA with component interferences. One method is to glue the bulky components to the cardboard or paper so a reasonable appreciation of the 3D fit can be obtained. This is tedious and wasteful. An example of a paper doll for a rigid-flex design is shown in Figure 4. A 1:1 printout of Figure 2: A typical hybrid rigid-flex layer stack cross-section. Figure 3: PCB paper doll using cardboard, mounting posts and connectors [2] .