Authored By:
Mike Bixenman, DBA, David Lober, Anna Ailworth - Kyzen Corporation
Bruno Tolla, Ph.D., Jennifer Allen, Denis Jean, Kyle Loomis - Kester Corporation
Summary
Reliability Expectations of Highly Dense Electronic Assemblies is commonly validated using Ion Chromatography and Surface Insulation Resistance. Surface Insulation Resistance tests resistance drops on both cleaned and non-cleaned circuit assemblies. It is well documented in the literature that SIR detects ionic residue and the potential of this residue to cause leakage currents in the presence of humidity and bias. Residues under leadless components are hard to inspect for and to ensure flux residue is totally removed. The question many assemblers consider is the risk of residues that may still be present under the body of components.
A recent research study10 of both flux activator systems and cleaning under bottom terminated components found that different no-clean flux packages have chemical properties that induce failure at different rates. The study also found that residues that were not fully cleaned under leadless components could be a reliability risk. Electrochemical methods (EIS) provide insight into the corrosion potential of a residue, in this case, flux residue. Electrochemical methods have not been commonly used for assessing corrosion potential on electronic devices. The purpose of this research study is to run Electrochemical Methods on the four flux systems used in the SIR study to determine if EIS data findings have commonality on the SIR data findings.
Conclusions
Flux is an aid used within the electronic assembly industry to join metals needed to create a metallurgical bond. Reliability of the flux residue left after the soldering process depends on the reactivity of the post reflow residues when contacted with moisture and electrical bias. The activity of the flux residue on the surface and under component terminations is an important factor in controlling the reliability of an electronic assembly.
To determine the corrosion potential of flux residue, IPC J-STD-004 and J-STD-005 specify several test methods. One of those test methods, IPC-TM-650 2.6.3.7 measures the surface insulation resistance to quantify the deleterious effects of fabrication, process or handling residues in the presence of moisture and bias11. The test method measures the resistance between the cathode and anode on the component tested. The test method requires specifically designed test boards, instrumentation, humidity, bias and time. The length of the test is 168 hours.
EIS test methods are commonly used by many industries to test corrosion resistance and corrosion rates.
Electrochemical corrosion measurements can be completed in minutes to hours since corrosion is accelerated by polarizing the sample. Electrochemical methods are not commonly used to measure the corrosion resistance and corrosion rates of flux residue left on the assembly post soldering. The objective of this research was to evaluate the activity of the four fluxes used in the SIR study and to determine if EIS test methods find commonality of the corrosion rate for each of these four activator systems. We wanted to determine if the electrochemical corrosion measurements were similar to what was found from the SIR study.
The SIR study found a wide divergence based on the activator system used in the solder paste, the reflow conditions used to solder the assembly and the level of cleanliness. For each of the components represented on the test board, the SIR study provided current leakage potential from the activity of the residue that was left under the body of the component tested. There was a clear delineation between the activity of the flux post soldering, reflow conditions and cleanliness levels.
The EIS methods used on these four flux activators also found a wide divergence based on the activator system used within the solder paste based on the remaining flux residue. The EIS analysis only evaluated the flux residue on a copper coupon using the Ramp-to-Spike reflow profile. No cleaning was performed for this EIS study. The electrical leakage from the SIR values, on the four activator systems tested, showed a similar trend to the EIS values found in this study.
Electronic reliability data finds that corrosion takes place when a reactive metal comes in contact with active flux that is mobilized with moisture under bias. The electrolytic solution initiates electrochemical reactions at the surface of the metal. The data from this study found that reactions are characteristic of the metal - electrolyte interface. The corrosion rate and corrosion potential of the four flux activators was determined using EIS methods. The EIS data findings compared with the SIR data findings. More work is needed to better understand, validate and optimize EIS test methods. The data findings in this paper are a starting point.
Initially Published in the IPC Proceedings
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