What is a factor that can affect the quality of solder connections?

The presence of excessive impurities in the solder is indeed a significant factor that can affect the quality of solder connections. When impurities are introduced to the solder, they can lead to poor wetting of the solder to the surfaces being joined. This lack of effective wetting results in weak mechanical bonds and can create areas where solder does not properly adhere, leading to potential failure points in the electrical connection. Additionally, impurities can interfere with the electrical properties of the solder joint, increasing the risk of electrical issues such as shorts or increased resistance.

In contrast, the other factors listed positively contribute to the quality of solder connections. For example, using high-quality solder ensures that the alloy composition is suitable for providing effective joints. Thorough cleaning of the PCB is crucial as it removes contaminants that can inhibit soldering effectiveness, while proper temperature control ensures that the solder melts and flows correctly, leading to optimal bonding between components and pads.