Effect of flux systems on electrochemical migration of lead-free assembly

This study shows that both physical and chemicalacceleration factors for ECM failure have to be consideredwhen the engineer chooses or designs the SIR/ECMreliability test method. Each factor is not independent, butimpacts each other and results in different test outcomes.From four products A-D in this study, here is a summaryand some recommendation for the reliability test:For no-clean solder paste, IPC SIR 2.6.3.3 is morechallenging than 2.6.3.7 due to the highertemperature and voltage conditions. The highertemperature accelerates the chemical reaction andion mobility, but also softens the rosin layer thusexposing the active ingredients. Even though thehigh bias voltage may blow out the dendrites, itsignificantly accelerates the ion migration in thisshort 7 days test. IPC 2.6.3.3 method can beimproved by increasing the reading frequency toprovide more detailed resistance data. For no-clean chemical flux especially those in theVOC-free category, IPC SIR 2.6.3.7 is morechallenging than 2.6.3.3, contrary to the no-cleansolder paste product because of the formulationchemistry differences explained above. The 40Cset point in this newer method preserves theresidue activity, and the low voltage slows downthe ion depletion and the resistance recovery rate.The dewing test can further stress assemblymaterials, as performed by the customized method#1 & #2. These procedures purposely create watercondensation and alter the residue physicalappearance, either cracks or softened rosin residue. For long term ECM tests, a lower voltage willfavor dendrites growth without blowing them out.The higher reading frequency also helps catchingany possible ECM failure. B25 board used inECM test can provide higher voltag