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In today's electronics manufacturing environment, assemblers continue to overlook areas of localized contamination that are capable of causing product failures. By neglecting to examine especially sensitive, critical, or tightly packed areas on an assembly, opportunities for electrochemical failures loom.
The following case study involves a customer seeing visible, white residue and dendritic growth in a connector area. Fortunately, the residue was visible, which alerted them to investigate the source and implications of the residue. Often, residues are invisible and can go unnoticed until field problems occur.
One assembly sample was sent to our laboratory for analysis to help troubleshoot this problem. In this project, we utilized FTIR, SEM/EDX and ion chromatography (IC) analyses. FTIR only identified flux residues. SEM/EDX revealed concentrations of tin, lead, oxygen and copper.
We concluded, due to the high concentrations of tin and lead, that electromigration was occurring in this area, creating the visible white residue and dendritic growth. Based on this finding, we analyzed the white residue area with IC using C3 (Critical Cleanliness Control) in order to identify the contaminants present and to better understand the impact of the manufacturer's processes.
The C3 enabled us to extract residue samples from 0.1 in2 areas on both the connector and its casing, plus reference areas near both sites. The C3 quickly showed "Dirty" test results for the connector and casing areas with the white residue. Tests on the connector and casing reference areas (no visible white residue) with the C3 read "Clean". IC analysis on these four C3 localized extractions indicated high levels of chloride, sulfate and weak organic acid (WOA) flux residues on the two areas with white residue; low, acceptable levels of ionic residue species on the two reference area samples.
We suspected that an external fluid contaminant had been introduced to the assembly at some point, causing the white residue and dendrites. Visual inspection of the board revealed that the white residue was in a line, suggesting a drip—possibly caused by tap water, which tends to be high in chloride and sulfate.
We recommended to this customer to monitor their processes, especially looking for the source of an external fluid contaminant—the cause of this reliability issue.
This is just one of numerous cases we have seen in our lab that demonstrate the importance of evaluating the cleanliness of specific, local areas of the board and/or assembly. Since the tested reference areas were acceptably clean and the vast majority of the assembly was free of the visible residue, standard industry test methods (bulk extraction), such as ROSE testing, would not identify specific, potential problem areas – the test result is an average cleanliness level for the entire assembly.
With passing (ROSE) test results, the presence of the white residue could be interpreted as not threatening product reliability – a benign situation. Actually, the residue in this case was highly corrosive and already causing product problems that would only intensify in use. Localized extraction methods can lead to identification of problem areas in an assembly, even before residues become visible or dendrites appear.
The C3 automatically, as part of a programmed cycle, extracts a sample from a selected, small (0.1 in2), product location of interest. Localized extraction is a significant advancement in electronics manufacturing quality control.
Terry Munson is the founder and president of Foresite Inc. Terry has extensive experience with how cleanliness issues are affecting product reliability for electronic hardware. He is the primary developer of ion chromatography analysis for use in the electronics manufacturing industry and is an active member in the IPC.