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Residues on Probing PCBAS-Consistent Connections Across No-Clean Fluxes
Analysis Lab |
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Authored By:Paul Groome, Ehab Guirguis Digitaltest, Inc. Concord, CA, USA Bruno Tolla, Denis Jean, Kyle Loomis Kester SummaryNo-clean fluxes have been in use for many years, but there is a need for knowledge that allows the industry to better understand the robustness of probing methodologies across different wave and reflow processes. Can different fluxes be used reliably? What pressures and probes are required? This paper discusses the variety of challenges presented by probing solder pads, including the use of different types of solder, fluxes and land designs. In this study, concerns involving the reliability of probing different solder/flux compounds, the land configurations, the contact pressures and probe styles will be discussed in detail. ConclusionsOverall, land patterns appear more probable than patterns configured with a via. It seems that the presence of vias result in the occurrence of a residue more difficult to penetrate on top of the solder joint. This effect is apparent at all speeds and for all compounds, except for the wave flux. The vias induce a segregation phenomenon during the process, where the high-molecular weight / poorly probable components of the flux tend to concentrate at the surface, while the lighter/low viscosity components find an escape route in these channels. The wave flux formulations are too low on high-molecular weight components for this effect to be prevalent. Looking at the results collected on the patterns with vias (e.g. the most discriminative ones), there are clear distinctions in the probability of the components tested in this study. The pastes are generally harder to probe than the selective flux, while the wave soldering formula is the most process-friendly compound. These differences can come from the process (wave soldering vs conventional reflow), but also the formulation strategies adopted for these different classes of compounds. Strong differences in heating profiles, especially in temperature ranges where the organic compounds start to decompose, and atmospheres, combined with the residue-washing effect of the wave can alone induce these significant distinctions in probability. Based on these fundamental processing differences, noclean pastes will yield larger amount of residues on top of the solder joint. Also, Paste fluxes are generally much more active and prone to leaving hard-to-penetrate residues than wave soldering fluxes. In addition to being responsible for the cleaning of the soldered surfaces, these fluxes also have to interact with reactive (e.g. high-surface surface area) solder powders until consumed (e.g. during their shelf-life), which generally requires the use of larger amounts of activators. Out of the two tested solder pastes, the High-reliability Paste seems more process-friendly than the General market paste. Significant formulation differences resulted in these distinct behaviors. The key aspect is the formulation of the polymer - rosin components, as well as the presence of high-boiling point solvents which present a plasticizing effect. The end-residue will therefore have different mechanical properties, as highlighted by these probing tests. In addition, it should be noted that the reflow process should have a very significant impact on probing results: both the reflow profile and the reflow atmosphere will affect the physicochemical characteristics of the residue, hence its probability. Therefore, a probing study should be integrated as an extra response to process optimization work. Looking at the other class of compounds, the relatively better probability of wave soldering fluxes compared to selective soldering fluxes can also be explained from a process and a formulation perspective. First, the wave soldering fluxes require relatively larger amounts of fluxes in the soldering area. Also, the silver free alloy used in combination with the wave soldering fluxes resulted in significantly higher bath temperatures than the SAC305 alloy employed for the selective process. Finally, from a formulation perspective, the selective flux was designed to be more heat resistant, to cope with the longer preheating time associated with the selective process, but also to the use of thicker boards. To achieve these performance improvements, the amount of heat-resistant formulation components was significantly increased, resulting in larger amounts of residues. These residues were thoroughly tested for their electrochemical reliability (Electrochemical Migration, SIR, Corrosion, according to IPC-TM-650 standards) in a very large process window envelope (from room temperature to reflow). It is interesting to note that in all situations (pad configurations, class of compounds, process, formula specificities) there is always a robust probing technique available as long as one is willing to spend the time to execute studies described above. Initially Published in the SMTA Proceedings |
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