As a PCB expert, I can provide insights into the processes a PCB circuit board undergoes, including potential issues like residue formation leading to decreased surface insulation resistance. To ensure electrical system reliability, evaluation of different fluxes and consistent use of the same flux on the same PCB is crucial. Detectable defects like thin intermetallic compounds and cracks can lead to failure, while hidden defects require reliability tests and analyses for detection. Lead-free product reliability can be uncertain due to hidden defects, necessitating careful consideration of lead-free materials, design, and processes from the start of product design.