Issue link: https://iconnect007.uberflip.com/i/1108006
APRIL 2019 I FLEX007 MAGAZINE 13 0.2-mm overlap to accommodate registra- tion tolerance of both solder mask image and stiffener location. While the solder mask formulations used by most FPC fab- ricators are referred to as "flexible," they are not in fact designed to bend, particu- larly not this sharp. Having solder mask in the bend area increased circuit stiffness precisely where the bend began Fortunately, these failures occurred during an early prototype run, so there was time to respond. The circuit was redesigned to address each of these issues; no subsequent failures were encountered. Figure 2 shows the revised design in which bend areas are relocated far from stiffened zones with SMT components. Moreover, modifications were made to the housing to relax the required bend radii. The revised design performed flawlessly. 2. Everything Is Easy Until the First Unit Is Built A flex circuit in a 3D CAD model looks pre- dictable and well-behaved (Figure 3). Every- thing nests well, there is no mechanical inter- ference, and the allotted tolerances seem gen- erous. Of course, the reality can be quite differ- ent. Mechanical fixtures may, for example, be less precise than anticipated, operator access to housing recesses may be restricted, dispens- ing equipment may be obstructed, etc. Add to this the inconsistency and variability of human operators and the flex circuit can be exercised in vastly different ways than expected. Figure 4 shows the actual circuit from Fig- ure 3 after it cracked during assembly, which required the FPC to be threaded through a cav- ity during insertion. The CAD model didn't predict this because of a complicated layer- ing of operations that wasn't apparent until the first assembly. Due to a tight bend radius, a pre-bend of the FPC was required, but the assembly process essentially meant inverting the pre-bend and then re-bending, which was apparently enough to break the circuit. Fortunately, this occurred early in develop- ment, allowing enough time to react. Modifi- cations to the housing greatly improved acces- sibility and reduced the bend requirements. Working with the FPC fabricator, we changed the material stackup, which greatly increased the flexibility of the circuit. Figure 2: Outline of the circuit from Figure 1 comparing the original design that failed (L), and the modified version (R), which did not fail. Figure 3: 3D CAD rendering, showing FPC in the installed position. The designed bends shown in this graphic are substantially different than the actual bends required during assembly. Figure 4: Yellow arrow points to a fracture in the circuit that occurred during assembly due to unanticipated bending and re-bending.