Page 357 - A Practical Guide from Design Planning to Manufacturing

Microprocessor Packaging 327

the insulation to the metal pads, which will ultimately connect the bumps
to the circuitry of the die (step #1). Sputtering is then used to coat the
die in layers of metal called the under-bump metallization (UBM) and
a new layer of photoresist is applied (step #2).
The lead and tin of the solder bumps are not very compatible with the
metal layers and insulation of the die. The material of the solder bumps
does not cling well to the die and diffuses into the interconnects and
insulation. The UBM is deposited to provide a better interface between
the die metallization and the solder bumps. There is not a single good
choice of material that clings well to the die metal pad, blocks diffusion of
the solder, and clings well to the solder. As a result, UBM usually comprises
three or more layers of metal. An adhesion layer provides a good mechan-
ical connection to the die. Adiffusion barrier prevents contamination of the
die, and a solder “wettable” layer provides a surface to which molten
(“wet”) solder will cling. Common materials for these layers are alloys of
titanium, chromium, and nickel.
The new layer of resist is exposed and developed to open holes over
the pads for formation of the bumps (step #3). One method of deposit-
ing the material of the solder bumps is electroplating. The wafer is
placed in a solution containing a probe coated in the material to be
deposited. A voltage is attached across the probe and the UBM coating
the wafer. Ions of metal from the probe are electrically attracted to the
exposed regions of UBM. As they attach to wafer, they become electri-
cally connected to the UBM and attract still more ions from the solution.
The bump material is rapidly built up past the level of the photoresist
layer (step #4).
After electroplating the resist is stripped away and a wet etch removes
the UBM wherever it is not covered by solder. The result is a mushroom-
shaped structure of solder material left behind (step #5). Applying heat
allows the solder material to flow into its natural spherical shape (step #6).
The die are now ready to be packaged, but first they must be separated
from one another.
Die separation is the process of cutting the wafer into individual die.
Before separation, each die on the wafer goes through preliminary test-
ing. Contact probes touch the C4 bumps and allow at least basic func-
tionality to be tested. Defective die will be discarded and not packaged.
For die separation, the wafer is mounted on a circle of adhesive plastic
that will hold the individual die in place as the wafer is cut. A protec-
tive coating is sprayed over the die, and a saw cuts through the wafer
but not through the entire thickness of plastic. After repeated cuts have
divided the entire wafer, the cutting debris and protective coating are
washed off. Die that tested as functional are picked off the plastic sheet
to be packaged and defective die are left behind. The remaining steps
in packaging the processor are shown in Fig. 10-13.