The present invention relates to a process for the heat treatment of a series of objects in an enclosure, situated in a surrounding gaseous fluid, particularly air, and defining an internal treatment zone, the said enclosure comprising at least one entrance passage and at least one exit passage, the said passages being provided between the treatment zone and the exterior of the enclosure; the process being of the type that comprises the following steps:    (a) the treatment zone is heated;    (b) a gaseous treatment fluid is injected into the treatment zone;    (c) the said objects are transported through the said treatment zone; and    (d) a gaseous isolating fluid is injected in the vicinity of at least one of the said passages.
This process applies to the assembling of electronic boards by soldering.
In a process of the abovementioned type (WO 91/11284), components to be assembled are placed on a board in areas covered with a solder cream. The board is then introduced into a oven where it undergoes a thermal cycle which carries out the soldering.
To improve the quality of this soldering and in particular avoid defects on boards of complex structure caused by the small size of the components, a stream of inerting nitrogen is injected into the oven. The oxygen concentration in the oven is thus brought down below a limit value of a few hundred ppm (parts per million). This operation, which can be described as “inerting”, under a nitrogen atmosphere, prevents in particular oxidation of the parts to be soldered during the heating.
Such processes are not completely satisfactory. Specifically, in order to keep the oxygen content below a few hundred ppm in the oven, and thereby maintain the quality of the soldering, a high nitrogen flow rate is required (some 20 to 40 m3/h)