Evaluation of commercially available, thick, photosensitive films as a stress compensation layer for wafer level packaging

Commercially available, photosensitive dielectrics for use as a bump encapsulation or “stress compensation layer” (SCL) for wafer level chip-scale packaging (WL-CSP) were evaluated. Four materials were identified as potential SCL´s. Filled and unfilled photosensitive materials were evaluated. To be successful as a SCL in WL-CSP, the commercially available material must be at least 125 μm thick, be photosensitive, have 1:1 via and saw street resolution, have a total process time of less than 4 hours, have a cure process that does not effect the under-bump metallurgy, adhere to underlying passivation, and have a radius of curvature greater than 1.25 m. One commercially available unfilled photosensitive material had a glass transition temperature of 285°C, a CTE of 55 ppm/°C, and a E of 2.5 Gpa. Since the company supplying this material had significant experience in thick, photosensitive film development and processing, they had excellent potential as a partner for evaluation. Another unfilled, photosensitive material supplied for this study had a glass transition temperature of 330°C, a CTE of 80 ppm/°C, and an elastic modulus of 1.0 GPa. A filled, photosensitive material evaluated had a glass transition temperature of 130-150°C, a CTE of 45 ppm/°C, and an elongation at break of 2.5%. Of the four materials evaluated, all types of materials showed potential