Issue link: https://iconnect007.uberflip.com/i/1498238
32 SMT007 MAGAZINE I MAY 2023 What is ECM? ECM is the dissolution and movement of metal ions in the presence of electric poten- tial, which results in the growth of conduc- tive dendritic structures between cathodes and anodes. is results in electrical shorts. e process is most observed in printed circuit boards where it may significantly decrease the insulation between conductors. How does moisture serve as a catalyst for ECM? ere are three basic "ingredients" of ECM: electrical bias, conductive residues, and moisture. The main factor facilitating ECM is humidity (moisture). Usually the process involves several stages: water adsorption, anode metal dissolution, ion accumulation, ion migration to the cathode, and dendritic growth. While ECM is not a new phenomenon, it's occurrence is at historically high levels. What is driving the historically high levels of ECM? While there are several factors influencing the rising levels of ECM, the two primary factors are widespread adoption of no-clean fluxes and miniaturization. When no-clean fluxes were first introduced in the early 1990s, cir- cuit assemblies were vastly different than they are today. Modern circuit assemblies and components are considerably smaller than any time in history. Previously, through- hole components were larger and placed far- ther apart. Larger spacings between cathodes and anodes provide a higher degree of residue tolerance. Modern circuit assembly designs have all but eliminated the spacing between conductors, reducing an assembly's tolerance for residues. e combination of a higher density of resi- dues combined with smaller spacing between conductors has increased the propensity for ECM. Isn't it true that no-clean fluxes are residue-free? All fluxes, including no-clean, leave behind residues. It is accurate to describe the resi- dues from no-clean fluxes as both invisible and relatively benign. e primary issue at hand is not the residues le on an assembly from the no-clean flux, rather it is the total- ity of all process residues le behind on the assembly aer reflow. Formerly, before the introduction of no-clean fluxes, virtually all circuit assemblies were cleaned aer the sol- dering process. Not only did the cleaning pro- cess remove the flux residues, it also removed all process residues. What other residues besides flux are left on the assembly? Residues from board fabrication and compo- nent fabrication, as well as process and human residues, combine with flux residues to create a problematic totality of ECM-causing residues. One should consider that when the indus- try stopped removing flux residues via a post- reflow cleaning process, it also stopped remov- ing all residues. I frequently remind assemblers Mike Konrad