Issue link: https://iconnect007.uberflip.com/i/746712
62 The PCB Magazine • November 2016 UNIQUE IMPLEMENTATION OF A RIGID-FLEX CIRCUIT ure 12 shows a 3D X-ray image of the product with backdrill. Figure 13 shows microsection mounts of the three backdrill depths. Via hole copper plating minimum average requirements for through-holes is higher for flexible printed boards than for rigid boards. Type 3 and 4 flexible printed boards have a 25 µm (984 µin) minimum average require- ment per IPC-6013C. Rigid boards class 2 have a 20 µm (787 µin) minimum average per IPC-6012 [2] . Qualification tests included Accelerated Thermal Cycling testing of backdrilled cou- pons. No fails were seen through 600 cycles from -40°C to +90°C. The LGA interconnect for this applica- tion uses gold-over-nickel to gold-over-nickel interconnect metallurgy. For high reliability, minimum gold thickness on the rigid-flex in- terconnect pads was specified as 0.00076 mm (0.000030"). Minimum nickel thickness was specified as 0.00127 mm (0.000050"). Porosity testing, gold and nickel harnesses testing, FTIR analysis, and ESCA analysis were performed on the interconnect surfaces during qualification. Additional data collected during qualifica- tion included: • First article inspection reports from the fabricator • Itemized confirmation of compliance to all print notes • Verification data for of all print dimensions/print notes Figure 11: Impedance modeling results. Figure 12: 3D X-ray image of rigid-flex section including backdrill. Figure 13: Microsection of three backdrill depths.