Vented cavity, hermetic solder seal

#Soldering is an effective method for joining metallic components and can even join many non-metallic components provided that the faying surfaces are suitably metallized. Accordingly, many types of solders, soldering processes, and solder joint designs are known. Of the many types of solder joints, one of the most difficult to make is a large-area joint that provides a hermetic seal and is free of internal voids. In this context, a large-area solder joint is a solder joint covering an area having a smallest dimension more than 2 mm long.

#Hermetic seals are difficult to form with large-area joints because the most reliable method of guaranteeing joint hermeticity is to ensure the formation of a continuous edge fillet around the entire perimeter of the joint. Meeting this requirement becomes progressively more difficult for joints having larger areas and perimeters.

#Voids are a problem for a large-area solder joint because at dimensions greater than 2 mm, gas bubbles that are trapped between components or evolve internally on heating to the soldering temperature cannot overcome the hydrostatic pressure of the molten solder and escape via the edges of the joint. The bubbles thus remain trapped in the solder and form voids when the solder solidifies. FIG. 1 illustrates the dependence of the percentage of voids in a solder joint on the minimum joint dimension for some conventional solders. As can be readily seen from FIG. 1, the problem of voids in solder joints increases with the dimensions of the joints. These voids generally impair the electrical, thermal, and mechanical properties of a solder joint.

#FIG. 1 is a bar graph illustrating the effect that the size of a conventional solder joint has on the percentage volume of voids in the solder joint.

#FIG. 2A and 2B are cross-sectional views of solder joints including vented cavities in accordance with embodiments of the invention.

#FIGS. 3A and 3B are cross-sectional views of assemblies including multiple solder joints in accordance with embodiments of the invention.

#In accordance with an aspect of the invention, one or more vented cavities can be provided within the area of a large-area solder joint to improve the release of gas bubbles and reduce the volume of voids in the solder joint. The pressure variation process for solder can further reduce the volume of voids in the solder joint without disturbing the seal that the solder forms. Use of the pressure variation process in this manner is counter-intuitive in that the solder joint deliberately includes cavities, the very feature that normally precludes application of the pressure variation process.

#In accordance with another aspect of the invention, a cavity to be hermetically sealed by a solder joint is provided with a vent to permit a pressure variation process that reduces voids in the solder joint. After the solder joint is complete, the vent can be sealed to create a closed cavity.

#In accordance with yet another aspect of the invention, providing a cavity at the location of aligned vent holes in components being soldered substantially improves yield of hermetic joints through modification of the peripheral fillet, which is now also at an interior perimeter of the cavity in addition to the exterior perimeter of the joint.