  Flexible printed cables and circuitry based on copper-polyimide materials are
widely used in experiments looking for rare events due to their unique
electrical and mechanical characteristics. However, past studies have found
copper-polyimide flexible cables to contain 400-4700 pg $^{238}$U/g, 16-3700 pg
$^{232}$Th/g, and 170-2100 ng $^{nat}$K/g, which can be a significant source of
radioactive background for many current and next-generation ultralow background
detectors. This study presents a comprehensive investigation into the
fabrication process of copper-polyimide flexible cables and the development of
custom low radioactivity cables for use in rare-event physics applications. A
methodical step-by-step approach was developed and informed by ultrasensitive
assay to determine the radiopurity in the starting materials and identify the
contaminating production steps in the cable fabrication process. Radiopure
material alternatives were identified, and cleaner production processes and
treatments were developed to significantly reduce the imparted contamination.
Through the newly developed radiopure fabrication process, fully-functioning
cables were produced with radiocontaminant concentrations of 20-31 pg
$^{238}$U/g, 12-13 pg $^{232}$Th/g, and 40-550 ng $^{nat}$K/g, which is
significantly cleaner than cables from previous work and sufficiently radiopure
for current and next-generation detectors. This approach, employing witness
samples to investigate each step of the fabrication process, can hopefully
serve as a template for investigating radiocontaminants in other material
production processes.
