Xiang Wei, Ph.D., Kyle Loomis, Jennifer Allen, Bruno Tolla, Ph.D.
Itasca, IL, USA
Surface insulation resistance (SIR) and electrochemical migration (ECM) are two of the primary reliability drivers in consideration of the performance of electronic assemblies. In recent years, the concern of "partially-activated" flux residue and their influence on reliability have been significantly raised due to the miniaturization along with high density design trend, selective soldering process adoption, and the expanded use of pallets in wave soldering process.
When flux residue becomes trapped under low stand-off devices, pallets or unsoldered areas (e.g. selective process), it may contain unevaporated solvent, "live" activators and metal complex intermediates with different chemical composition and concentration levels depending on the thermal profiles. These partially-activated residues can directly impact the corrosion, surface insulation and electrochemical migration of the final assembly.
In this study, a few application tests were developed internally to understand this issue. Two traditional liquid flux and two newly developed products were selected to build up the basic models. The preliminary results also provide a scientific approach to design highly reliable products with the goal to minimize the reliability risk for the complex PCB designs and assembly processes.
This research establishes the basic approach and test methods for new PCB assembly materials development, specifically for the selective soldering process which is drawing more attention in electronic assembly industry. In order to obtain higher SIR reliability for through-hole assembly, two recommendations are offered based on this study:
1. Optimize the process settings and choose the right soldering method to limit amount of "partially-activated" flux residue on the board surface:
- Design an appropriate soldering profile by considering the thermal mass from the board and components. For example, thermocouples should be attached to several locations on the board and under any large thermal mass components during profiling.
- Adjust the nozzle or jet settings to obtain accurate deposit location, spread area and appropriate flux volume.
2. Choose the right formulation for the different soldering methods. Particularly for selective soldering, the formula should:
- Do not clog the drop-jet system head while providing low cleaning frequency requirement.
- Have higher surface tension to control the spread area.
- In addition have a good SIR/ECM reliability under different temperature exposure conditions,
For these reasons, fluxes like Product C and D are not appropriate formulas for the wave pallet and selective soldering application in the high reliability field; while Product A and B are suitable products for these applications regarding the reliability aspect. Product A is the best selection for selective soldering due to its higher surface tension property for controlled spread area.
Initially Published in the SMTA Proceedings