The use of flexible printed circuits has become prevalent in diagnostic imaging systems; in particular, the flex shown in Figure 4 is utilized in an ultrasound probe. The circuit shown has features of 4Mm-thick copper, 25Mm-thick polyimide, conductor pitch of 50Mm, and 25Mm-diameter laser-drilled vias.

Test Element Groups (TEGs) or process monitor coupons are critical components in the fabrication of reliable, high-quality flexible interconnect. TEGs are routinely used for standard process control (SPC), certification of compliance (COC), and reliability or qualification testing. A number of standards can be used for interconnect compliance, such as IPC, JDEC, ASTM, etc. TEGs can be utilized as in-process monitors, for post-fabrication validation, and as reliability screening coupons.

In-process monitoring examples include resistivity or Kelvin test structures to insure metal thickness after plating operations. Four-point probe resistivity measurements can be performed, prior to pattern and etch operations, to measure uniformity of metal deposits. Other in-process coupon examples include lithographic resolution structures (critical dimension measurement vehicles), impedance coupons, elongation/ductility coupons, flexural endurance coupons, and via interconnect strings. Via string coupons allow the manufacturer to test an array of vias (200-500) on the device panel for a measure of via integrity during and after flex circuit manufacturing. Via integrity can be measured by passing current through the via structure to insure that sidewall metal coverage is adequate for the circuit operation environment. The via string can combine all metal layers and metal layer pairs to aid in troubleshooting process fabrication issues (via drilling, via cleaning, via metallization). This coupon allows for circuit troubleshooting and, if performed in-process, can save process time and costs by identifying issues early in a process cycle. Specific via string designs allow for accurate measurements of metal contact resistance.

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