When sourcing PCB sheets, you must prioritize the Base Material (Substrate), such as FR-4 (Flame Retardant), which is the industry standard for its excellent strength-to-weight ratio. For high-frequency applications, look for PTFE (Teflon) or Ceramic-filled materials. Key metrics include Dielectric Constant (Dk), which affects signal integrity, and Glass Transition Temperature (Tg); high-reliability electronics typically require a Tg of 170°C or higher to withstand thermal stress during soldering.