A method of manufacturing a cartridge assembly of a hard disk drive, including a step of forming a part of a wiring circuit into flying leads and a step of forming a coating layer on the respective opposite surfaces of the bonding surfaces of the flying leads, which will be connected to bonding terminals of a flexible printed circuit board. The method also includes a step of placing an ultrasonic tool in contact with the opposite surfaces of the flying leads via the coating layers, and applying ultrasonic vibration from the ultrasonic tool in a state where the flying leads are pressed onto the bonding terminals to ultrasonically bond the flying leads and the bonding terminals together. Additionally, the method includes a step of heating and melting the coating layers to smooth surfaces of the coating layers that have been roughened by contact with the ultrasonic tool during ultrasonic bonding.