To service the fast growing market within PDA and cell phone, this smaller chip size is essential. In 2001, ASE licensed Ultra CSP® from Kulicke & Soffa's Flip Chip Division. ASE also provided several enhanced structures called "aCSP™" by polyimide, PBO, or thicker Cu RDL to meet various customer demands. aCSP™ is a wafer level CSP package that can be Direct Chip Attached to the PCB board without any interposer. Also, aCSP™ provides the shortest electrical path from Al pad to PCB board to enhance the electrical performance. It has been broadly used in portable devices in passives, analog and logic applications, and is extending to RF applications.