(1) Field of the Invention
This invention relates to integrated circuit (IC) packaging.
(2) Brief Description of Related Art
In the IC packaging art, the IC chip size tends to be smaller as time progresses. In cutting such small dimensions, chemical etching now replaces the mechanical sawing, because the damage caused by sawing propagates toward the IC itself.
FIG. 1 shows a prior art substrate 10 for an array of IC packages. Each package or block, as shown within the dot-dash line boundary, has two metal plates 11 and 12 serving as terminals for an IC chip 15, which is mounted on the metal plate 11. The bottom electrode of the IC chip 15 is coupled to the metal plate 11, and the top electrode of the IC chip 15 is wire-bonded by wire 16 to the metal plate 12. All the metal terminals are originally from a common metal substrate, but are etched through by chemical etching along the orthogonal grooves 14. The grooves are separated by narrow necks 13 to hold the metal in one piece. The necks 13 are later broken off to yield individual blocks.
The width W of the etched groove 14 should be as narrow as possible to increase the packing density of the packages per IC wafer. However, during chemical etching of the grooves 14, the minimum width W is proportional to the thickness of the metal substrate as shown in FIG. 2. In the upper section (A), the metal substrate 11, 12 is thin with thickness of T1 and the minimum separation of the groove is W1, because the etching solution etches both horizontally and vertically. If the metal substrate 11, 12 is thicker with a thickness of T2 as shown in section (B), then the minimum width of the groove W2 must necessarily be wider than W1. Thus, the prior art narrow groove is not compatible with thick metal substrate.
Hence, if the substrate on which the IC chip is mounted is too thin, the substrate is too fragile and tends to break. If the substrate is thick, the chemical etching widens the etched separation between adjacent blocks, as well as the separation W between leads on the same block, and lowers the packing density.