What characteristic of solder allows for effective joining at low temperatures?

The characteristic of solder that allows for effective joining at low temperatures is low viscosity when molten. When solder is heated and becomes molten, its low viscosity means that it can flow easily and fill gaps between the surfaces being joined. This property ensures that the solder can create a strong bond without requiring extremely high temperatures, which could potentially damage the materials being joined.

In contrast, high melting point solder would require more heat to achieve a liquid state, making it unsuitable for projects where low-temperature working is essential. Low thermal conductivity is not directly related to the ability of the solder to flow and fill gaps efficiently; instead, it influences how heat is transmitted through the solder joint. High density does not contribute to the smooth flow or spreading characteristics of solder when molten; rather, it pertains to the weight of the solder material itself, which is not relevant to its effectiveness at low temperatures. Thus, the flow properties imparted by low viscosity are crucial in achieving strong, reliable joints with minimal heat exposure.