Reflow Soldering

By Fred Dimock
Senior Process Engineer
BTU International

How does the convection rate affect a solder reflow profile?

The heating and cooling of a printed circuit board is all about heat transfer. This is accomplished with temperature differentials (delta T between the board and gas) and time. A reflow oven uses a controlled belt speed as well as zone temperatures to produce a repeatable profile, but some ovens allow the fan speed to be controlled. Varying the fan speed changes the pressure in the zone and, thus, the speed of the gas impinging upon the board (AKA Convection Rate). This increase (or decrease) alters the amount of energy transferred to the board for heating and cooling and, thus, changes the profile. Controlling the convection rate in the zone can vary the peak temperature of a board by as much as 10°C.  Manufacturers are finding that controlling the convection rate is important for lead-free solder profiles with the reduced process window.

This Q&A was previously published in our Volume 7 web edition of leadfreemagazine.com