The Effect of Reflow Profiling on the Electrical Reliability of No-Clean Solder Paste Flux Residues

An estimated 80% of all SMT assembly in the world is performed with a no-clean soldering process, largely due to the predominance of consumer-type electronics. The continuing trend of increasing miniaturization that dominates modern electronics devices requires no-clean flux residues to be as benign and electrically resistive as possible. Solder pastes with a IPC J-STD-004 [1] classification of ROL0 or ROL1 rely heavily on two basic mechanisms to render the flux residue as “no-clean”: (1) the encapsulating properties that the rosin provides and (2) the heat activation/decomposition of the chemicals in the flux, commonly known as “activators.” The latter is generally known in the industry, but is rarely taken into consideration for reflow profiling in SMT assembly. Optimization of a reflow profile often focuses on mitigating defects such as voiding, tombstoning, graping, slumping/bridging, etc. However, little thought is given to the reflow profile’s effect on the electrical reliability of the no-clean flux residue. Because of the wide variation in size and thermal density of SMT components and PCBs, achieving a reflow profile that equally heats the entire assembly can be challenging and often impossible. The temperature under a large component, such as a BGA, is often markedly cooler than a smaller component, such as a passive resistor or capacitor. This paper will discuss an experiment that studied the effect of reflow profiling on the electrical reliability of no-clean flux residues that can be measured using IPC J-STD-004[1] surface insulation resistance (SIR) testing. Both a halogen-free (ROL0) and a halogen-containing (ROL1) Pb-free no-clean solder paste, exposed to various reflow profiles, were used in this study.