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58 SMT Magazine • April 2014 Adhesion testing/thermal shock testing was originally conducted on Practical Component SABER test assemblies; however, after multiple tests it was determined that the required data could be collected using standard B-24 test cou- pons. In addition to a considerable cost sav- ings, it eliminated variables that could have clouded the results including the presence of ionics, mold release agents and coating thick- ness variability. The findings of the adhesion testing yielded both favorable and unexpected results. The bal- ance of this work focuses on solder paste. We did not test wire solder residues and all liquid fluxes where the conformal coating wet and ad- hered to the substrate at the time of coating/ curing passed all subsequent tests. Initial testing of thermal shock at -60°C +125°C showed gross delamination. Initially, it was thought the failure was due to movement of the flux residue having softened at 125°C. Further examination revealed that there was a cohesive failure of the flux residue, wherein the flux remained firmly adhered to the PCB sub- strate and to the coating, but failed internally (Figure 10). This phenomenon was present on all coatings in varying degrees (other than sili- cone). In general, UV materials performed the worst, with solvent-based acrylics better and silicones the best, with no delamination. A failure was considered any evidence of delami- nation. It was not determined if delaminated coating that remained contiguous was still ef- fective in protecting the underlying substrate. Ultimately, we found the modulus of the CONFOrMal COaTiNG OvEr NO-ClEaN FlUX continues fEATurE coating is directly correlated to cold tempera- ture failure. The CTE mismatch of the residue and a high modulus coating were enough to fracture the cold hardened flux residue. Flux medium used in solder paste is typically a resin- based material and after reflow, the residue is hard. The colder the environment is, the harder the residue. To test this theory, we varied the residue and the coatings using harder and softer materials. UV curable silicone having the low- est modulus of the materials tested and UV cure urethane the highest. We also tested a paste that is not resin based with residue that is waxy rather than hard. As depicted below, reducing the modulus of either the coating or the resi- due eliminated the delamination failure. We also noted that solvent-based acrylic coatings outperformed UV cured urethane ma- terials although it was product specific. It is believed that the solvent would facilitate a more intimate bond between the residue and the coat- ing lessening the adverse effect of the CTE. We went a step further to determine what the lowest temperature a resin-based no-clean paste and acrylic or acrylate/urethane coating can withstand before suffering delamination. The results of these tests were scattered, but none of the material sets were capable of withstanding more than -35°C for more than 10 cycles. With this information, it would seem the simple solution to this problem would be to in- corporate a softer residue solder paste to remedy the delamination issue. Unfortunately, there is a significant impact to the SIR characteristics as detailed below in Figure 1. figure 1: Moisture absorption after conformal coat. figure 2: results: 1 to 5 (worst to best). Uv Curable Urethane paste paste paste hybrids TG a B C Comment a 40 1 1 1 complete delamination, combination with paste c was the worst b 25 3 1 4 Delamination but not global c 3 4 3 4 Wetting issues, slight delamination D -60 5 5 5 perfect, no delamination