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 coefficient 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 thermo physical properties of a typical solder are p = 8333 kg/m³, c p = 188 J/kg ∙ K, and k = 5] W/m ∙ K.

(c) For 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 solder's eutectic temperature, T_sol = 183°C and the upper limit corresponding to the glass transition temperature, T_gt = 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.

Transcribed Image Text:

1 mm -T. V. 2 mm Surface-mount region (solder joirt) Printed-circuit board (PCB) - 2.5 mm -