Solder paste consists of solder alloy and a flux system. A typical solder paste composition by volume is split between about 50% alloy and 50% flux. The metal load mass (solder alloy powder) is around 90%, with the remaining 10% mass a flux system. The primary purpose of the flux system is to remove the contaminants from the solder joints during the soldering process. The capability of removing contaminants is determined by the activation level of the type of solder paste. The preferred solder paste metal alloy has a lead-free composition (SnAgCu where Ag is 3–4% and Cu is 0.5–1%). A no-clean solder paste is preferred to eliminate any risk of improper cleaning that could leave active residue beneath the device and other BTC components. The paste must be suitable for printing the solder stencil aperture dimensions. Type 4 paste is recommended for better paste release performance. When using a solder paste, you must adhere to the handling recommendations of the paste manufacturer.