Issue link: https://iconnect007.uberflip.com/i/1526114
18 SMT007 MAGAZINE I SEPTEMBER 2024 and the intended use. e second factor is the solder powder size, based upon the appli- cation and the PCBA requirements: print- ing, dispensing, jetting, etc. e third fac- tor is the type of flux desired: water-solu- ble or no-clean, activity level, halide/hal- ogen content, and special properties. e first two factors influence the flux choice, as the flux is formulated to work specifically with certain solder alloys and solder powder sizes in the intended application. How does the composition of solder paste affect its performance and reliability? e two major components of solder paste are the flux and the solder powder. Fluxes are formulated to optimize the formation of sol- der joints with that solder powder. Fluxes are tuned for the application (print, dispense, jet) and reflow conditions (high- or low-tempera- ture soldering, nitrogen atmosphere, laser sol- dering, etc.) e solder alloy has a major influence on the reliability of the solder joint. e structural integrity of the solder joint depends upon the solder alloy as well as the other materials of the PCBA. e alloy also determines the reflow temperature and oxidation potential that the flux must accommodate. What is the difference between a "standard" solder paste and a "high reliability" solder paste? Standard solder pastes are typically made with common fluxes and alloys like SAC305 and Sn63/Pb37. ey perform well in Class 1 and 2 electronics with short to moderate expected life and work in "office" type environments. High-reliability solder pastes usually are named so due to the solder alloy. High reliabil- ity solder alloys are formulated to withstand harsh environments and challenging usage conditions. Automotive under-hood electron- ics, aerospace electronics, telecommunica- tions equipment, etc., are all used in challeng- ing conditions, and the solder alloy must stand up to these conditions. "High reliability" may also refer to no- clean fluxes formulated to withstand chal- lenging electrochemical and environmental conditions. Some fluxes enhance solder joint strength. Other fluxes survive high heat and humidity cycling to freezing conditions. Some fluxes are designed to provide increased elec- trochemical resistance for high voltage or high current applications. No-clean flux technology can be used to enhance the reliability of the PCBA for some applications. Solder pastes are available in different "types." This generally refers to the solder sphere size. What are the different types and how are these types selected? e size of the solder powder is chosen based on the size of the smallest stencil aperture for printing applications. e "5-Ball" rule (five of the largest solder balls should be able to fit across the width of the smallest rectangular stencil aperture) is oen used as a rough guide- line for solder powder size. It is common for manufacturers to change solder powder size based upon print perfor- mance. For example, if Type 4 solder paste Tony Lentz