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April 2016 • SMT Magazine 77 consisTEnT conTrol ovEr ThE sElEcTivE soldErinG ProducTion ProcEss square inch) makes it complicated to effect a pro- cess control on this critical process parameter. The surface tension of a flux (influenced by surfac- tants in flux) isn't even defined in the datasheet. The surface energy of the solder mask can be mea- sured using simple ink pencils with different inks. The line of ink should remain unchanged for two seconds without turning into droplets; this indicates that the surface energy of the board is the same or higher than the surface tension of the ink. A typical value for selective soldering is 35 mN/m. This parameter will not change from board to board, but requires verification when a differ- ent batch of bare boards are used. In a previous study on a selective soldering application with respect to flux spreading, the data is converted into a Pareto chart: When monitoring these two parameters (sur- face energy of solder mask and flux amount), 99% of the influence on flux spreading is covered. Some selective soldering machines utilize a laser to measure flux flow. However, a laser sys- tem can only measure the time that a flux flows, and then uses an algorithm to convert this into a flux amount more accurate method is to use a thermal micro sensor. This sensor technology measures these very small amounts of flux. The short response time and high precision allows accurate monitoring of high dynamic fluxing processes. Sample times of 10 ms are achiev- able. The flow of the flux is very small 0.25–1.25 g/min (wet flux). For machine capability analysis, the flux amount was also measured manually by spray- ing on a coupon and weighting the amount on a balance. Process engineers use sensitive paper to verify flux spreading. What type of paper used depends on the flux. A water-based flux requires a differ- ent type of paper than an alcohol-based one. For reliability reasons a selective soldering flux will be alcohol-based (rosins don't dissolve in water). Dropjet calibration is required to achieve a machine-to-machine consistency. Some appli- cations may have a dual dropjet to reduce the flux cycle. In these circumstances it's required that both machines dropjet spray the same Figure 1: Higher surface energy of solder mask makes flux flow. Figure 2: Ink pencils to help define surface energy of a PCB. Table 1: Recommended methods to control flux spreading.