The present invention relates to a method for the closing, by laser, of packages of electronic circuits, notably hybrid circuit packages, said method minimizing mechanical stresses. It can be applied, more particularly, to the closing, by laser, of large-sized hybrid circuit packages, called macro-hybrid packages, where a metal lid is closed on a frame which is itself soldered to a ceramic substrate bearing screen-printed tracks and electronic components.
More generally, the invention can be applied to any hybrid circuit packages for which it is necessary to minimize the mechanical stresses related to closing by laser. These would be, for example, large-sized, all-metal solid packages.
The method of closing a metal package by laser is known to those skilled in the art. This method provides for tightly sealed soldering through the melting of the metal constituting the lid of the package. The parameters of the laser are determined as a function of the nature of the metals and the thickness of the lid. In the case of macro-hybrid packages for example, the substrate of which is constituted by a ceramic wafer supporting the hybrid electronic circuits, the frame is soldered to this ceramic substrate and not to a metal substrate. Experience has shown that any stress contributed by a solder made by means of a laser may prompt a fault at the solder joint between the frame and the ceramic. These stresses are actually related to the heating of the metals that get soldered to each other. The deformation thus created in the unit makes it impossible to obtain tight sealing in accordance with prevailing standards. Known methods consist, for example, in the soldering, transparently, of a lid made of an iron-nickel alloy to a frame made of an iron-nickel alloy or an iron cobalt-nickel alloy for example so that the laser beam, starting from a point of the perimeter of the macro-hybrid, travels along this perimeter and returns to the starting point. However, methods of this type lead to the above-mentioned deformations, prompted by the mechanical stresses exerted by the laser soldering. Indeed, before closing, the ceramic substrate of a macro-hybrid supporting the soldered frame generally displays a convexity of the order of 100 to 200 .mu.m due to the soldering with the frame whereas the macro-hybrid package, which has a surface area of the order of one dm.sup.2 once it is closed, displays a concavity of the order of 100 to 200 .mu.m corresponding to the stresses applied to the frame by the laser soldering. This results in defects of impervious sealing between the frame and the ceramic substrate, preventing any encapsulation in accordance with certain standards as laid down. Furthermore, when the laser soldering is done in the form of a continuous soldering, as described here above, the imperviousness between the lid and the frame is not acquired in a single pass: it is then necessary to double and even triple the seam of solder in the unsealed zones. This contributes greatly to increasing the stresses exerted by the soldering and increases manufacturing costs.