CHAPTER 3GENERAL PROCESS FLOW FOR CHIP ASSEMBLY

Generally, the chip assembly line in Unisem is divided into 2 parts, which are

Front of Line (FOL) and End of Line (EOL). Referring to Figure 2.4 FOL is in blue

color and EOL is in yellow color.

Figure 1.4: The Process Flow

3.1 DESCRIPTION OF FRONT OF LINE (FOL)

The process of Front of Line department II is illustrated in Figure 1.5.

Figure 3.5: The FOL Process Flow

3.1.2Wafer Mount

Wafer mounting is a process of providing support to the wafer to facilitate the

processing of the wafer from wafer saw through die attach. During wafer mounting,

the wafer and a wafer frame are simultaneously attached on a wafer or dicing tape.

The wafer frame may be made of plastic or metal, but it should be resistant to

warping, bending, corrosion, and heat. The dicing tape is just a PVC sheet with

synthetic adhesive on one side to hold both the wafer frame and the wafer.

Figure 3.6: The Wafer Mount Process

3.6.1Wafer Saw

Wafer saw is the step that actually cutting process to disintegrate the wafer into

individual die for assembly in IC packages. The wafer saw process consists of the

following steps. Firstly, the frame mounted wafer is automatically aligned into

position for cutting. Then, the wafer is cut thru its thickness according to the

programmed die dimensions using a resin-bonded diamond wheel rotating at a very

high rpm. Finally, the wafer goes through a cleaning process using high pressure DI

water sprayed on the rotating work piece and then dried by air-blowing.

Figure 3.7: The Wafer Saw Process

3.2.1.1 Second Optical Gate

The optical inspection process is to ensure that the wafers are sawed accordingly to

specifications and the die is without sawing defect, glassivation, scratches on die,

metallization voids, metallization corrosion, metallization adherence and other

defects. Any damaged die will be rejected and disposed. A 100% screening will be

performed.

Figure 1.8: Wafer view by microscope

3.2.2 Die Attach

Die Attach, also known as Die Mount or Die Bond, is the process of attaching the

silicon chip to the die pad or die cavity of the support structure of the semiconductor

package. The complete bonding process is repeated continuously and automatically.

Only good dies are picking up from the wafer and accurately bonded onto the die

pads using vacuum, form the Top Stack Loader to the Programmable Indexer. The

Indexer transports the lead frame to the dispense position where it is accurately

placed using a laser sensor. Epoxy (liked glue) is placed onto the die pad using the

Syringe Dispenser. The lead frame is then moved to the bond position where it is

positioned by a further laser sensor before a chip is placed onto the die pad. After

bonding, the lead frames are placed in a magazine. The magazines are taken to the

oven to cure the epoxy so that the epoxy is hardened and the dies are bonded tightly

to the die pad. A die shear test monitor is performed to ensure the strength of the

bonding conforms to the specifications of the customer.

Figure 1.9: Die Attach Process

3.2.3 Wire Bonding

Wire bonding is the process of providing electrical connection between the silicon

chip and the external leads of the semiconductor device using very fine bonding

wires. At Unisem, the wire used in wire bonding is gold (Au) wire. The bonding

method used on the Wire Bonder 3006 (machine used in Unisem) is a thermo sonic

ball bonding process, the predominant method in semiconductor assembly industry.

The wire bonding process (ball and wedge bond) is a precisely controlled

combination of heat, force, time and ultrasonic energy. It involves ultrafine gold wire

with a typical diameter of approximately 1 mil and high purity of 99.99% gold. In

the region where force is applied, diffusion and plastically deformation occurs

between the gold wire and the metallization surface to produce a metallurgical

bond. The device to be bonded is placed in contact with a heat source on the work

holder to heat the lead frame to the required temperature of 150°C to 200°C.Three

reliability tests, the wire pull test, the ball shear test and the wire peel test will

be performed on samples from the magazines.

Figure 1.10: Wire Bond Process

3.2.4 Third Optical Gate

The high powered scope inspection is aimed to check for partial attached die, foreign

matter, faint ink, die mask different from Build sheet, crack or chipped die, tilted die,

lifted die, epoxy coverage (non-conformance to specifications), wrong bonding, metal

scratches or smears, wrong die orientation, broken or extraneous wires, bonding on

epoxy, cratering and other visual defects on die after bonding. The 3rd optical QA gate

inspection is the last inspection in Front Of Line (FOL) assembly before units are sent

for molding.

Figure 1.11: Inspection of chip using microscope