Solder paste consists of solder alloy and a flux system. A typical solder paste composition uses about 50% alloy and 50% flux by volume. By mass, solder alloy powder makes up about 90%, and the flux system makes up about 10%.
The flux system removes contaminants from the solder joints during the soldering. The solder paste activation level determines how effective the flux system is at removing contaminants. The preferred solder paste metal alloy has a lead-free composition (SnAgCu where Ag is 3–4% and Cu is 0.5–1%). Use a no-clean solder paste to reduce the risk of cleaning errors that leave active residue under the device and other BTC components. Select a paste suitable for printing the solder stencil aperture dimensions. Use Type-4 paste to improve paste release performance. Follow the paste manufacturer’s handling recommendations when you use solder paste.