Publications Details

Insulation Resistance of Flat Flexible Circuit Boards

Cordaro, Joseph G.; Dillinger, Troy

The insulation resistance (R_I) between parallel copper traces embedded on flexible, polyimide-based circuit boards was measured as a function of temperature, substrate type, copper thickness, and feature size. The results show a strong dependency on temperature: the measured R_I decreases nearly seven-orders of magnitude upon a 90degC change. Also measured was R_I as a function of the trace and space of parallel copper circuits. This dependency is less obvious but a correlation showing narrower spacing leads to lower R_I is evident. The thickness/width of the copper traces has less of an effect on the measured R_I. The thickness of the polyimide-based substrate, or core of the circuit boards, was also varied. Measurements of the R_I versus substrate thickness were inconclusive. The height of the copper traces was changed and shows that heavier (or taller traces) result in a lower measured R_I. Lastly, it was shown that the adhesive found between the copper traces and the polyimide core on so-called "LF" Pyralux^(R) boards significantly reduces the measured R_I compared to "AP" Pyralux^(R) boards of the same dimensions. The presences of this adhesive in all LF materials, including LF coverlays should be avoided if high R_I is desired for narrowly spaced copper traces. Alternative materials like AP Pyralux^(R) or HT Pyralux^(R) available from DuPont^TM should be investigated when designing copper-based flat-flex circuit boards that require a demanding environmental specifications.