The factors interfere insertion loss of rigid-flex board

Abstract

Because rigid-flex board contains the advantage of pure rigid board and flex board, choosing it as base design for analysis. Dividing the rigid-flex board into four parts for the discussion, including the raw material of FCCL, the length of the flex part exposed, the design of signal reference layer, whether the back drilling and back drilling depth to investigate how each factor interferes the signal integrity in different frequency. Use vector network analyzer measurement instrument. For the effect of FCCL material, LCP has the smallest insertion loss below 10 GHz. As frequency is getting higher and over 10 GHz, the insertion loss of modified PI is lower than LCP. The effect of flex length exposed is opposite in normal PI and LCP, the insertion loss of no flex part exposed is relatively low for normal PI, but length of flex area=8” is relatively low insertion loss design for modified PI. For LCP, longer the flex length is, larger the insertion loss is. Reference plane with hatch copper causes larger insertion loss in contrast to rigid copper. Based on ±0.03 mm capability of stub length control for back drill, drilling off 80% of maximum drilling depth leads to minimum of insertion loss.