Method and apparatus for applying external coating to grid array packages for increased reliability and performance

Abstract

A method and apparatus are disclosed for selective removal of a conformal coating from the solder balls of grid array packages such that the benefits of the coating are realized. An ancillary benefit of the invention is improved process-ability of the grid array package by improving the mechanical containment of the solder during the reflow process and improved electrical isolation between the individual solder attachment points. For example, a method for coating a ball grid array is provided, which includes coating the ball grid array with a thin layer of parylene. Next, the solder ball side of the part is butter smeared or squeegeed with a water soluble coating and assembled wet. A mask having holes in the same pattern as the balls in the grid, and a thickness that is about 80% of the height of the balls, is applied to the solder ball side of the part. This side of the part is then butter smeared again with the water soluble coating, and the entire assembly is allowed to dry. At this point, about 20% of each parylene-coated solder ball protrudes higher than the surface of the mask. The solder ball side of the part is then grit blasted with an abrasive material. The extent that the abrasive material removes the parylene coating from the solder balls is limited by the mask and the layer of water soluble coating. Therefore, the grit blasting removes the parylene coating from only the protruding areas (e.g., about top 20%) of the solder balls. Water is then used to remove the water soluble coating, and the parylene coated part is baked to remove moisture. Thus, a parylene coated ball grid array (or column grid array) is provided that is highly impervious to moisture, has a very high dielectric strength, and thereby improves the electrical performance and reliability of the surface mounted part.