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80 The PCB Magazine • March 2017 which melted the solder paste. Upon melting, the paste spread across the surface of the pad and coalesced when it came into contact with surrounding spots. After a heating cycle the number of spots remaining on the board surface was counted, two or more coalesced spots were counted as one. A surface finish with high wettability allows for increased coalescence and a smaller number of counted solder spots on the surface after re- flow heating. The results for the spot test are displayed in Figure 7, showing the number of spots counted for HASL, ENIG, ionic liquid– ENIPIG, immersion tin and OSP. The test was performed with different pre-treatment condi- tions, which were convection, vapour phase, no pre-processing, and the application of nitrogen, which is used to aid soldering and to prevent surface modification [8] . When populating both sides of a PCB with components and applying two heating opera- tions, an unintentional pause may be intro- duced in the manufacturing process between heating cycles, due to the work load of the as- semblers. During this pause in processing oxi- dation occurs on the metal surface, negatively influencing the performance of the board for soldering [9] . For this reason, an additional test was performed whereby convection and vapour phase reflow operation were performed individ- ually on test boards, and a hold period intro- duced—to simulate degradation which may oc- cur when leaving the boards during component assembly—before spot wetting testing. The results showed that the most effective finish for coalescence of spots was the HASL, and the least effective was the OSP, which were unsurprising results. The performance of the finishes dropped with the introduction of a hold cycle and for processing at higher temper- atures—such as processing under convection reflow—which was also as expected. The ionic liquid–ENIPIG performed well relative to the other finishes, where coalescence was high on its surface regardless of the pre-treatment con- ditions. The finish specifically performed better than the ENIG, which had a similar material composition and topography. MACFEST: BENCHMARKING A NEW SOLDERABLE PCB FINISH Figure 5: Average solder wetting time taken to reach 2/3 of the final wetting force. Test performed on five different finishes with each pre-processed under three different conditions.