1. Field of the Invention
The present invention generally relates to chip packaging, and more particularly to a method of forming metallic bumps on the I/O pads of a semiconductor device
2. The Prior Arts
Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate (or lead frame) of the package. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad.
An UBM generally contains an adhesion layer usually made of aluminum or chromium, a barrier layer usually made of copper, lead, or platinum, and a wetting layer usually made of gold, arranged in this order on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps and gold bumps.
To form solder bumps on the UBMs, usually either electroplating or printing method is used. For electroplating, patterned resists are first formed on the UBMs and then solders are plated. For printing, solders are first printed on the UBMs and the solders are thermally cured into the bumps. The two methods, however, all suffer disadvantages such as excessive bubbles, easy to peel off, etc and the production yield is therefore limited.
Electroplating similar to that used for solder bumps is also usually used to form gold bumps on UBMs so as to facilitate the subsequent Tape Carrier Package (TCP) or Chip On Glass (COG) process. Another method is to use a specialized wire bonding apparatus capable of melting gold wire into ball shape to perform bonding on the UBMs several times until the gold bumps reach a required height. Gold bumping, however, is not widely popular due to its high cost.