This paper presents a method of improving the signal integrity of the universal serial bus (USB) by analyzing transmission line impedance. There are many accessories for smartphones in the marketplace due to the recent fast proliferation of smartphones. Some of these accessories have data sync functions to connect smartphones with host-like computers. Currently, most smartphones comply with USB2.0 high-speed (HS) specification for the data sync. Signals for USB2.0 HS in smartphones are extremely vulnerable to mismatched impedance when adding peripheral electronic parts such as switches, capacitors, transmission lines, etc. However, because the accessories supporting data sync are made of printed circuit boards (PCB), flexible printed circuit boards (FPCB), or both, if the transmission line impedance is designed carelessly, USB communication will not operate. This paper presents a transmission line impedance design for USB2.0 HS signals in Power Pack that is a portable charging device accessory for the Samsung Galaxy S. This device was assembled using connected PCB and FPCB because of its sleek design and hence was more vulnerable to impedance mismatch. This paper simulated PCB signal integrity and impedance design with Si8000, developed by Polar Instruments, and was verified with USB-HS eye-diagrams in the time domain.