Process parameter optimization of selective soldering

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Dr. Illyefalvi-Vitéz Zsolt
Department of Electronics Technology

The subject of my thesis was the selective soldering. I made experiments with the Pillarhouse Jade S200-type selective wave soldering machine to determine the optimal values of the soldering parameters.

Designing the experiments I have chosen a multilayered, FR4-type, mixed technology printed circuit board, and a plug connector, with 112 pins, arrayed in five rows.

I started the optimization of the process parameters with defining the preheat settings. For this, I used the lower preheat unit - realized by the solder fountain - and the upper, infrared preheat unit of the machine. To adjust the intensity of preheating, we can use the speed and the distance in the case of the first one, and the number of the used lamps, the power and the duration of heating in the second one. I took measurements with thermal profiler to adjust the values of these parameters, to meet the requirements as it was given by the datasheet of the flux. For the soldering experiments I used the settings of the adequate thermal profiles, that gave the less amount of temperature difference between the lower and the upper side of the board. These settings were: 15 mm/s speed of fountain, 20 mm distance from the board, preheating the double rows in one step, along their central line, and the upper unit using the first two lamps with 78 % power for 60 s.

I continued the optimization of the process with the soldering experiments. The parameters of this were the temperature, the distance and the speed of soldering. In the course of the parameter changeovers, I modified the parameters one by one for each case, to find the relationship between these parameters and the quality of the joints. I analyzed the joints also with optical and X-ray microscope, primarily how well the barrels are filled, their uniformity, the extent of the voids, and also the formation of solder balls. With some of these joints I made cross-sectional analysis too, to get more exact information about the filling of the barrels and the process of the wetting. Along the inspections I found out that the soldering distance is the most determining parameter, which affects the other parameters and the soldering quality. Increasing the distance, at a given soldering temperature, the soldering speed, which results in equal quality, decreases significantly. Primarily I looked for the optimal settings with low soldering temperature and high soldering distance. With these conditions the recommended settings according to my experiments were: 290 °C soldering temperature, 3,5 mm soldering distance and 5 mm/s soldering speed.

The most common defects during my experiments were the short-circuits, the voids and the solder balls. With the proper settings of preheat I achieved better quality, with much lower amount of these defects. Applying the machine's wave-decreasing function at the end of the lines as a new parameter, I managed to eliminate the short-circuits.


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