Flexible circuit interconnects for ultrasound transducer applications are among the most difficult to fabricate and make good representative circuits for medical applications. The polyimide dielectric can be as thin as 12 micron. They often contain ultra-high density regions that are push the edge of currently available technology. Lines and spaces can be as small as 20 to 30 micron, and via diameters can be as small as 25 micron. The interconnects are typically fabricated starting with adhesiveless copper on polyimide flexible substrates manufactured using sputtering and electroplating techniques. High performance polyimides provide mechanical and thermal stability needed to metallize thin dielectric without significant wrinkling or creasing. The availability of thin copper and smooth interfaces facilitates fine-line etching and high density features. Good adhesion is critical to ensure fine-line features remain intact throughout the fabrication process. Peel strength exceeding 6 lb/in is achieved by pretreating the polyimide surface with plasma and applying a suitable tiecoat metal between copper and polyimide. Laminate dimensional stability after etching and heating exceeds the IPC specification and promotes good alignment and registration of circuit features. Fine-line features imply strict copper quality requirements. Pinholes represent an important aspect of copper quality. Systematic analyses have been done to classify pinholes and link different types to their likely causes. The classification scheme and detailed analyses serve as a basis for pinhole quality control in the manufacturing environment. The approach has been used to help reduce pinholes and serves to allow rapid containment if pinholes become an issue.
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