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Effect of Reflow Profiling on Solder Paste Flux Residues
Effect of Reflow Profiling on Solder Paste Flux Residues
Paper studied the effect of reflow profiling on the electrical reliability of no-clean flux residues using surface insulation resistance testing.
Production Floor

Authored By:
Eric Bastow
Indium Corporation
Clinton, New York USA
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Summary
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 classification of ROL0 or ROL1 rely heavily on two basic mechanisms to render the flux residue as "noclean": (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 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.

Prior work had exposed the impact on SIR values of entrapping a solder paste flux residue under a component body or RF shield. What was unclear in that work, is the impact of the reflow profile. Invariably, flux underneath a device does not get exposed to the same heat that an exposed flux does. So, performing an experiment that focused solely on the effect of heating seemed pertinent.
Conclusions
There are many variables that affect SIR performance. And with the specific knowledge of the impact of proper heating on a flux's SIR performance, such an experiment as this seemed appropriate. What was surprising is that even with a very short "soak" the SIR performance of a residue can be "improved" more than by using a higher peak temperature.

Such knowledge could be useful in such problematic situations as temperature sensitive assemblies and flux residues trapped under component bodies and RF shields. The latter situations can produce unusual visual anomalies and gooey flux residues, as was discovered in a prior work, with less than optimum SIR performance. As the acumen of knowledge increases relative to the parameters which affect SIR performance of flux residues, no-clean processes can be honed to provide reliable products.

For those processes involving cleaning/removal of no-clean residues, especially with the ever decreasing standoff of SMT components, more work should be done to understand the impact of partial or incomplete removal of such residues.
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