You have been asked to determine the feasibility of using

Chapter , Problem 7.106

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You have been asked to determine the feasibility of using an impinging jet in a soldering operation for electronic assemblies. The schematic illustrates the use of a single, round nozzle to direct high-velocity, hot air to a location where a surface mount joint is to be formed. For your study, consider a round nozzle with a diameter of 1 mm located a distance of 2 mm from the region of the surface mount, which has a diameter of 2.5 mm. (a) For an air jet velocity of 70 m/s and a temperature of 500 C, estimate the average convection coeffi- cient over the area of the surface mount. (b) Assume that the surface mount region on the printed circuit board (PCB) can be modeled as a semi-infinite medium, which is initially at a uniform temperature of 25 C and suddenly experiences convective heating by the jet. Estimate the time required for the surface to reach 183 C. The thermophysical properties of a typical solder are 8333 kg/m3 , cp 188 J/kg K, and k 51 W/m K. (c) For each of three air jet temperatures of 500, 600, and 700 C, calculate and plot the surface temperature as a function of time for 0 t 150 s. On this plot, identify important temperature limits for the soldering process: the lower limit corresponding to the solders eutectic temperature, Tsol 183 C, and the upper limit corresponding to the glass transition temperature, Tgl 250 C, at which the PCB becomes plastic. Comment on the outcome of your study, the appropriateness of the assumptions, and the feasibility of using the jet for a soldering application.