Aluminum decal for transferring solder spheres during electronic package assembly

Ball grid array (BGA) and flip chip package designs use solder spheres or bumps for electrically connecting the integrated circuit to another electronic component such as a printed circuit board. This paper introduces a novel method for transferring solder spheres to BGA substrates or flip chip wafers that uses a decal made from aluminum foil. This decal design differs from most other decals in that preformed solder spheres, and not plated or evaporated solder bumps, are utilized. Aluminum decals offer a solder source that can be applied at all stages of assembly including initial sphere attach. For BGA applications, aluminum decals should reduce equipment costs and save valuable floor space by decreasing the complexity and physical size of the sphere loading equipment. For flip chip applications, expensive plating equipment and sometimes difficult to control chemical processes are avoided. For a BGA rework application, the aluminum decal can be viewed as the solder sphere equivalent to the solder grid preforms commonly used for reworking BGA packages. The paper focuses on the design, fabrication, and experimental testing of an aluminum decal for a 256 I/O BGA organic substrate. Benefits and disadvantages of such an approach will be discussed. Reflow of the solder spheres to the BGA substrate and the resultant shear data are reviewed.