Semiconductor Manufacturing: Assembly
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In Assembly, the wafers are cut, and the die are removed, packaged, and prepared
for final testing and shipping.
Backgrind
Using grindstone wheels, the thickness of the wafer is reduced to provide both
a clean, uniform surface and a specified product thickness. Backgrind is capable
of grinding each wafer down to approximately 0.3 millimeters—as thin as an average
piece of paper.
Wafer
Mount / Saw
This process attaches a wafer and a film frame to ultraviolet-sensitive adhesive
tape. This tape holds the wafer and die in place during subsequent processing.
A diamond-edged saw blade, approximately the thickness of a human hair, cuts
the wafer into individual die. The blade spins at 45,000 revolutions per minute
and cuts at a speed of 8.9 centimeters (3.5 inches) per second. During the cutting
process, water sprays on both the blade and the wafer to keep the temperature
low and remove the debris. After the wafer is cut, a final high-pressure rinse
washes it.
Die Attach
Using the wafer map created in Probe, the good die are identified, removed
from the wafer, and placed with adhesive on a leadframe or "interposer"
at a speed of up to 4,000 die per hour. To remove the die from the tape, needles
push up from beneath the tape as a vacuum tip lifts the die from the top. The
unqualified die are left on the adhesive as illustrated below.
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The good die are then adhered to the interposer--or
lead frame--and batches of interposers are cured in an oven to set the
adhesive/epoxy.
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Wire
Bond
Thin gold wire—99.9999% pure and thinner than a human hair—is attached to the
die and the interposer. This wire provides the communication path (circuitry
connection) between the die and the computer. Ultrasonic gold-ball bonding,
a technique that combines ultrasonic energy, heat, and force, is capable of
interconnecting the die bond pads to the interposer/leadframe bond pads.
Encapsulation
During Encapsulation, the die and a small portion of the interposer/leadframe
are covered with a hard plastic compound to protect the die. The equipment
encapsulates the die by moving the interposers into a mold area, using force
to inject heated compound into the mold cavities, and curing the compound. The
mold is opened, and the interposers are pressed out and cleaned
Lead
Finish
Product must now go through either an Electroplating or Solder-ball
Attach process. In electroplating, the exposed metal on a leadframe
is covered with a conductive metal coating. While submerged in a
tin and lead solution, leadframes are charged to attract the tin and lead ions.
This results in a uniform plated coating that increases conductivity, keeps
the leads from rusting, and provides a clean, even surface.
In solder-ball attach, solder balls and flux are placed on gold-plated pads
located on the substrate. When heat is applied to the part, the solder
balls adhere to the pads. Leads or solder balls provide the final interconnect
between the component and the board application in the end-use product.
Trim and Form
In Trim & Form, leadframes are loaded into trim-and-form machines
where the leadfingers are formed step by step until finally the chips are severed
from the frames. An opens-and-shorts test is performed on each device, and the
devices are sorted into good or reject trays or tubes.
The various positions and shapes of the leads and the package size and shape
depend on the final application and the customer's packaging requirements.