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78 The PCB Magazine • March 2017 (an inert high boiling inert heat transfer fluid) at peak temperatures of 240°C. Vapour phase soldering typically induces a more uniform dis- tribution of heat to the board by the process of latent heat of condensation, enabling a more uniform solder quality of the different compo- nents. It also provides reduced operational costs and risk from fires, due to the more safe heating method, and so for these reasons is often used in manufacturing [6] . Highlighted in Figure 4 is a plot of the aver- age force of the wetted solder weight after two seconds immersion for the different finishes and pre-treatments. The ionic liquid–ENIPIG results showed that the amount of solder wet- ted varied little between the different process- ing conditions and that the temperature ageing of the board induced only small performance changes. The finish performed on par with ex- isting industry finishes such as ENIG. Figure 5 shows a plot of the average solder wetting time taken to reach 2/3 of the final wetting force. This shows how fast the solder wetted to the surface, where a short time is de- sired. The ionic liquid–ENIPIG showed a range of behaviours, which differed depending on the pre-treatment conditions. Its performance dropped—shown by the increase in time to wet—for the application of convection reflow. Convection reflow is processed at higher peak temperatures than vapour reflow and so the im- pact of thermal expansion on topography, and thus solder-ability, would have been greater. Re- gardless, the performance drop was not signifi- cant and the time to wet was less than the ma- jority of the other surface finishes. This showed that the surface topography of the deposit was of a sufficient quality to enable wetting to the board and displayed a high surface energy like the other surface finishes, despite heating dam- aging its surface. The final evaluation of the solderability was the wetting dot test. This evaluated the ability for solder paste to flow across test pads plated with a surface finish [7] . The test was performed using the test coupon highlighted in Figure 1 again. Twenty-two solder paste dots were stencil printed onto the six tracks above the rectangular tracks, with an example shown in Figure 6. The board was then subjected to reflow heating MACFEST: BENCHMARKING A NEW SOLDERABLE PCB FINISH Figure 4: Average solder wetted weight after two seconds immersion. Test performed on five different finishes with each pre-processed under three different conditions.