Introduction of SMT lead-free
SMT SMT technology lead-free flux should adapt to the high temperature and poor wettability of lead-free alloy. Measures should be taken to improve the activation temperature (high temperature resistance) and activity. The temperature curve should be set correctly according to the melting point of solder joint alloy and the activation temperature of flux. If the control is not proper, it will affect the weldability, and will cause too many welding defects in the SMT chip processing process, thus affecting the reliability.
There is chemical reaction between the lead-free flux and the alloy surface, so different flux should be selected for different alloy composition. Due to the poor wettability of lead-free alloy, high flux activity is required, and the wettability of lead-free alloy is poor, and the amount of flux needs to be increased. Therefore, it is required to have less residue after welding and no corrosivity to meet the ICT probe capacity and electromigration Gold has a high melting point, so it is necessary to increase the activation temperature of flux to adapt to the high temperature of lead-free welding.
Lead free flux is a water-based solvent flux. If water is not volatilized during welding, it will cause solder spatter, porosity and cavity. Therefore, the preheating time is required to be increased, and the welding time of manual welding is longer than that with lead. The key to printability and solderability is flux. After the lead-free alloy is determined, the key is flux.
Lead free flux must be specially formulated. Clean free Sn Pb solder paste has been used for many years and is a mature technology. In the early days, the lead-free solder paste was simply mixed with Pb Sn solder, no cleaning flux and lead-free alloy, and the results were very poor. The chemical reaction between the flux and the solder alloy in the solder paste affects the rheological properties of the solder paste (which is very important to the printing performance). Therefore, lead-free flux must be specially prepared.
The development of new flux with stronger activity and better wettability should match the preheating temperature and welding temperature, and meet the requirements of environmental protection. Similarly, VOC free flux in wave soldering also needs special preparation. Water soluble fluxes for lead-free pastes and wave soldering are also required for some products.