Issue link: https://iconnect007.uberflip.com/i/1348195
76 DESIGN007 MAGAZINE I MARCH 2021 this fixture, a clamp was placed to ensure that once a droplet of super glue was added, the cir- cuit bend was stabilized. is allowed a repeat- able and reliable 2:1 bend ratio. It is critical on a reduced bend ratio application that once the circuit is formed, it is immediately constrained and not allowed to relax. Once all devices were in place and a few bends were made and glued, cross-sections were taken to ensure there was no cracking or elongation of the traces, or delamination in the coverlay. Since then, hundreds of thousands of circuits have been deployed in the field with zero quality issues. When the available room and the install sce- nario were considered, the solution we came up with was a success. 3. Placing vias closer to the transition rigid-to-flex areas on a rigid-flex circuit. Oen, the reason customers go to a rigid-flex solution is because space is limited. In some cases, the space is so limited that the rules need to be broken. IPC-6013 guidelines state that minimum coverlay penetration into the rigid area is to be at least 0.635 mm (0.025") but shall be suf- ficient to provide coverage of exposed conduc- tors, and not to extend to the nearest via. My suggestion when a via needs to be even closer to this transition area is to use both drilling and punching to define the coverlay as shown in Figure 1. is allows the coverlay to stay out of the drilled via, but allows the via to be closer than recommended, ABUSS (as agreed upon between user and supplier). 4. Requiring tighter than normal tolerances. Again, IPC sets up standards for tolerances, but customers will sometimes need very tight tolerances that exceed what is outlined in IPC- 6013. Please be sure your supplier is able to perform and meet these requirements. I had a customer who required openings in the substrate with a ±10 µm (0.000393") hole location tolerance in relation to other features, and a ±12.5 µm (0.0005") tolerance on the hole size. We were able to perform this by lasering fiducials while we created the substrate open- ings. We also adjusted the hole sizes by compen- sating half the laser beam diameter. While we were able to meet the requirements, yields were reduced, which resulted in increased costs. When tolerancing your drawings, is it impor- tant to remember that these are flexible cir- cuits, not machined metal parts. Flexible cir- cuits can grow and shrink by 0.001" due to just temperature and humidity. You should have reasonable expectations of just how tightly dimensions can be controlled without affecting the manufacturer's yields. Also, any dimension that you put on your drawing will have some cost impact since the manufacturer will have to verify them. If there are features on your flex- Figure 1: Define the coverlay by using drilling and punching.