Issue link: https://iconnect007.uberflip.com/i/1526114
20 SMT007 MAGAZINE I SEPTEMBER 2024 In harsh environment applications, no-clean fluxes are typically encapsulated in conformal coating or potting materials, and the flux must be compatible with those materials. Some harsh environment applications require flux removal, and the flux must be able to be removed whether no-clean or water-solu- ble. Some fluxes can be challenging to remove. When no-clean flux was first introduced, it was uncommon to clean circuit assem- blies after reflow. Today, more assemblers are choosing to clean their assemblies after reflow, even when reflowed with no-clean solder pastes. Did this require a change in the flux formulation to allow it to be more easily cleaned? No-clean fluxes are always evolving to improve performance, including cleanability. Older technology no-clean fluxes used resins and other ingredients that formed a hard shell or a sticky/gummy residue. Dissolving a hard res- inous shell or a sticky/gummy residue can be difficult. Today's fluxes are formulated with soer, more pliable flux residues in part to aid in cleanability, but also to facilitate electrical probe testing through the flux residue. Addi- tives such as surfactants are commonly used for dissolution or suspension of the flux ingre- dients, but surfactants can also provide better cleanability. There has been quite a bit of discussion about low-temperature solder pastes. What are the benefits and drawbacks of utilizing low-temperature solder pastes? Low-temperature Pb-free solder pastes are used to minimize temperature-related defects on the PCBA. BGA components are notori- ous for warpage which leads to head-on-pil- low and non-wet-open defects. ese defects can be challenging to troubleshoot as the BGA can have intermittent electrical connections. Lower reflow temperatures minimize warp- age and can reduce the rate of warpage related defects. Low-temperature solder pastes can also reduce temperature-related damage to the PCB and component materials. Flex circuits, CEM (paper based) laminates, and some com- ponents can be damaged by standard SAC305 reflow temperatures. Low-temperature solder pastes are typically made with bismuth/tin or indium/tin alloys. Both bismuth and indium reduce the melting point of tin significantly. High indium alloys can be costly as compared to standard SnAgCu alloys. A drawback of using bismuth-based low tem- perature solder paste is the high air reactivity of bismuth. Bismuth/tin alloys oxidize read- ily in air and need additional protection from the flux to prevent this. e flux formulations are typically more aggressive for bismuth/tin alloys which may lead to shorter shelf and sten- cil life. High bismuth alloys can also be brittle which leads to solder joint fracturing when drop shock is a risk. How does the storage and handling of solder paste affect its quality and performance on the production line? Honestly, I get this type of question more than any other, and I could write a book on this topic. I will try to give a short answer.