Advanced Multilayer Polyimide Substrate Utilizing UV Laser Microvia Technology

This paper presents ongoing work on an advanced flexible circuit interconnect substrate featuring a 24 x 24 (576 element) pad grid array pattern at 300 micron pitch. This eight metal layer circuit is being developed as a highdensity, pixel-level interconnection substrate for a two-dimensional (2D) ultrasound imaging transducer MEMS device. The flexible circuit substrate is constructed using laminate fabrication methods, whereby multiple sets of two-sided metallized and patterned 1 mil polyimide circuit pairs are laminated together using dielectric adhesive sheets to form a multilayer structure. UV laser-drilled via-in-pad blind vias are used to connect buried trace elements in the circuit stack to the array pattern on the top surface. The flex circuit has feature patterns as small as 35 μm traces and spaces. The UV laser (Electro Scientific Industries, Portland, OR) is used in several other process steps to fabricate the multilayer device including patterning precisely registered pin alignment features in the circuit pairs. Construction of the multilayer device is also possible using advanced substrate materials with improved electrical performance characteristics, including laminate materials such as liquid crystal polymers (LCPs). LCP substrates have a significantly lower dielectric constant, improved dimensional stability and lower TCE compared to polyimide. Although being developed as part of an ultrasound transducer imaging array project, the advanced substrate has wide applicability to new chip-scale packaging and MEMS electronic integration efforts.