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March 2017 • The PCB Magazine 77 soldering was evaluated from the total increase in weight of solder adhered onto the board and its wetting speed [5] . Solder wetting was performed on the test coupon, as highlighted in Figure 1, where the sample was lowered into a bath and the rect- angular pads took up solder. The process is out- lined in Figure 3, showing a plot of the weight of solder, measured as force against time, which is officially known as a wetting curve. The tests were benchmarked against a variety of other widely used surface finishes including organic solderability preservative (OSP), electro- less nickel/immersion gold (ENIG), immersion tin and hot-air solder levelled (HASL). The dif- ferent finishes are applied in PCB manufacture due to the differing costs of materials and the conditions they will be expected to perform un- der, for example, use in the defence, aerospace, or space industries, etc. Two test boards were pre-treated before sol- dering. This was to mimic the conditions which the boards would undergo if they were pro- cessed in component assembly. A board may require several solder cycles dependent on the complexity of the component assembly, such as when components are added to both sides of the PCB. This will occur in two separate pro- cess heating operations. One such process could involve: the pre-treatment of the surface by a flux solution, the application of solder paste to the conductive pads, the application of the components to the paste covered pads and, fi- nally, heating of the board to a temperature be- yond the melting point of the solder, to enable the solder paste to reflow onto the pads, whilst aligning the components into the correct posi- tions on the pads. If the board requires further component assembly then the pads on the un- processed side of the board will have degraded in surface quality, due to thermal expansions caused by the first heating operation. There- fore, it was useful to test the condition of the test boards after a heating operation to evaluate changes in quality. Different commonly applied heating meth- ods are convection reflow, which in these tri- als applied 200 mg, Sn-Ag-Cu (SAC) lead free solder alloy at peak temperatures of 260°C and vapour phase reflow, which applied Galden® Figure 2: X-ray fluorescence measurements obtained at Merlin Circuit Technology, of the ionic liquid–ENIPIG finish plated at the University of Leicester. Results show average thickness for different metals on rectangular and circular pads of sizes 10.5 mm2 and 1.8 mm2, respectively. Figure 3: Solder wetting balance test with plot of adhered solder weight force against time on a wetting curve. MACFEST: BENCHMARKING A NEW SOLDERABLE PCB FINISH