Tape Automated Bonding (TAB)

Tape Automated Bonding, or simply TAB, is the process of mounting a die on a flexible tape made of polymer material, such as polyimide. The mounting is done such that the bonding sites of the die, usually in the form of bumps or balls made of gold or solder, are connected to fine conductors on the tape, which provide the means of connecting the die to the package or directly to external circuits. Sometimes the tape on which the die is bonded already contains the actual application circuit of the die.

The TAB bonds connecting the die and the tape are known as inner lead bonds (ILB), while those that connect the tape to the package or to external circuits are known as outer lead bonds (OLB).

The tape used in Tape Automated Bonding is usually single-sided, although two-metal tapes are also available. Copper, a commonly-used metal in tapes, can be electrodeposited on the tape or simply attached to the tape using adhesives. The metal patterns of the circuit are imaged onto the tape by photolithography.

Standard sizes for polyimide tapes include widths of 35 mm, 45 mm, and 70 mm and thicknesses between 50 to 100 microns. Since the tape is in the form of a roll, the length of the circuit is measured in terms of sprocket pitches, with each sprocket pitch measuring about 4.75 mm. Thus, a circuit size of 16 pitches is about 76 mm long.

Fig. 1. Example of TAB devices

To facilitate the connection of the die bumps or balls to their corresponding leads on the TAB circuit, holes are punched on the tape where the die bumps will be positioned. The conductor traces of the tape are then cantilevered over the punched holes to meet the bumps of the die.

There are two common methods of achieving a bond between the gold bump of the die and the lead of a TAB circuit: 1) single-point thermosonic bonding; and 2) gang or thermocompression bonding.

Single-point bonding, as the name implies, connects each of the die's bond site individually to its corresponding lead on the tape. Heat, time, force, and ultrasonic energy are applied to the TAB lead, which is positioned directly over the gold bump, forming intermetallic connections between them in the process. Single-point bonding is a more time-consuming process than gang bonding.

Gang bonding employs a specially designed bonding tool to apply force, temperature, and time to create diffusion bonds between the leads and bumps, all at the same time. Without the use of ultrasonic energy, this type of bonding is simply referred to as 'thermocompression' bonding. Gang bonding offers a high throughput rate, and is therefore preferred to single-point bonding.

After gang bonding, the die, bonds, tape leads, and part of the tape are covered with an encapsulant, which provides mechanical and chemical protection to the circuit after its curing. The die is then electrically tested, after which the useable part of the tape is punched from the frame for assembly into the final application.

Fig. 2. Examples of Tape Automated Bonding Equipment

Tape Automated Bonding offers the following advantages: 1) it allows the use of smaller bond pads and finer bonding pitch; 2) it allows the use of bond pads all over the die, not just on the die periphery, and therefore increases the possible I/O count of a given die size; 3) it reduces the quantity of gold needed for bonding; 4) it limits variations in bonding geometry; 5) it has a shorter production cycle time; 6) it results in better electrical performance (reduced noise and higher frequency); 7) it allows the circuit to be physically flexible; and 8) it facilitates multi-chip module manufacturing.

Tape Automated Bonding, on the other hand, has the following disadvantages: 1) time and cost of fabricating the tape; 2) the need to 'tailor-fit' the tape pattern after each die; and 3) capital expense for TAB equipment since TAB manufacturing requires a set of machines different from those used by conventional processes.

Thus, Tape Automated Bonding is a better alternative to conventional wirebonding if very fine bond pitch, reduced die size, and higher chip density are desired. It is also the technique of choice when dealing with circuits that need to be flexible, such as those that experience motion while in operation, e.g., printers, automotive applications, folding gadgets, etc. Tape Automated Bonding is generally more cost-effective for use in high-volume production, since returns on the time and cost of developing the tape will be maximized under this situation.

Front-End Assembly Links: Wafer Backgrind; Die Preparation; Die Attach; Wirebonding; Die Overcoat

Back-End Assembly Links: Molding; Sealing; Marking; DTFS; Leadfinish

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