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November 2016 • The PCB Magazine 63 • Process capability data for key print requirements including impedance, plating thickness and critical mechanical dimensions • Delivery of microsection pictures, measured data and physical mounts • Failure analysis of all early build fails to determine root cause • Hi-Pot test data The LGA interconnect allowed for a dense interconnect solution with good signal integrity properties. Mechanical hardware including bol- ster plates, alignment pins and screws were de- signed to provide proper alignment and normal for ce for the LGA contacts. Figure 27 shows sev- eral of these features. Use of the LGA intercon- nect eliminated thermal exposures to the raw rigid-flex due to soldering. Figure 14 shows the via and pad structure for the LGA inter connect. Conductive Anodic Filament Testing and Prepreg Rotation Low- or no-flow prepregs are often required for rigid-flex applications to control epoxy flow at the rigid flex to flex interface. With the 1 mm LGA contact pitch and 0.39 mm vias there was significant concern in early development to drive initial CAF coupon build in parallel with initial part builds and early testing. Test methods and coupons were similar to the conductive anodic filament (CAF) resistance test: X-Y axis as defined in IPC-TM-650 number 2.6.25 [3] . The coupon cross-section replicated the rigid portion of the rigid-flex. Fails occurred within the first 50 hours of testing. A signifi- cant numbers of fails occurred within the first 150 hours of testing on coupons built with both standard and spread glass pre-pregs. A team consisting of the prepreg supplier, the fabricator and the end user evaluated op- tions which included additional glass types, bet- ter CAF performing materials, additional flow testing in conjunction with additional cutting back of prepregs and alternate lamination con- ditions. Based on available data from this and other programs and engineering judgement all options had significant risk due to short pro- gram schedules. The team then considered prepreg rotation. With the LGA connector on both ends of this rigid-flex and no additional vias on the part, the original design had all vias on a 1 mm x 1 mm grid. While this is unique compared to complex flex or boards with logic, it is fairly common for flex cables used in interconnect solutions. By rotating the prepreg 26.6 degrees as shown in Figure 15, the via to via distance was increased from .75 mm to 2.0 mm. Prepregs were rotated instead of rotating parts to facili- tate manufacturing the progressive bookbinder sub-composite lengths in multiple parts across a panel. UNIQUE IMPLEMENTATION OF A RIGID-FLEX CIRCUIT Figure 14: Via and LGA pad structure. Figure 15: Via-to-via distance with rotated and non-rotated prepregs.