Chip-on-glass (COG) Packaging

COG is a typical flip-chip packaging process. In 1983, Citizen Co. in Japan first an-nounced it was using COG packaging in the palmtop television LCD.^[7] After that, many manufacturers have developed various COG packaging methods. COG has the advantages of high density, low cost, high efficiency, and small size. Especially in recent years, with the developments in the density and capacity of flat panel displays, COG packaging has become dominant in display driver chip packaging.

Like other flip-chips, COG needs bumps to be fabricated on the active surface of chips as interconnection pins. The bumps used for COG packaging are mostly “upright wall”

Au bumps fabricated with the photolithography technique. The electrodes on glass sub-strate usually use indium tin oxide (ITO) transparent conductive material. Because of the restriction of size and layout of flat panel displays, COG basically uses slender chips, the bumps of which align along two sides in parallel or interlace and mostly have rectangular cross-sections. Usually the distance between the centers of adjacent bumps is called pitch, and the gap between adjacent bumps is called gap, which are two main parameters for the density of COG packaging, as shown in Figure 7.9.

Upward bumps Single row bumps

P G

Figure 7.9 A schematic structure showing the pitch and gap

Flip-chips are divided into two main types according to the method of interconnecting between bumps and electrodes on the substrate, namely, metallurgy contact and mechanical bumps and corresponding electrodes reacting with solder joints. Under these conditions, bumps of chips are usually fabricated in solder material. In the soldering process, after an IC is mounted on a substrate, the reflow process will connect the IC to the substrate through the formation of solder joints. After reflow, underfill is usually applied to seal and protect the interconnections and the chip. Mechanical contact, by definition, is electrical conduction between the bumps and corresponding electrodes through mechanical contact where the connections are maintained by additional adhesive. Usually, mechanical contact may be direct contact between bumps and electrodes or through other conductive materi-als, such as anisotropic conductive film (ACF). Owing to the limitations of temperature, cleanness, efficiency, and cost of fabricating flat panel displays, with the exception of some reports on the development of COG eutectic bonding by some researchers and institutes, all COG processes are mechanical contact film (ACF), isotropic conductive adhesive (ICA), or nonconductive film (NCF).

7.3 Packaging of Flat Panel Display Modules 135

1. ACF Bonding

ACF is an anisotropic conductive thin adhesive film, which completes interconnection and adhesive curing at the same time in the packaging process. ACF is currently the most widely used packaging material in COG. ACF is fabricated by disseminating uniformly 0.5%–5%

conductive particles in a polymer matrix. In the COG process, ACF is first placed onto the glass substrate, covering all of the ITO electrodes. A driver IC is then mounted on the glass substrate through alignment, prebonding under certain temperature and pressure conditions. At last the main bonding is completed under higher temperature and pressure conditions, which needs typically 5–10 s to fully cure the adhesive. Therefore, the main bonding is the critical step of COG packaging in terms of efficiency. Dispersed conductive particles in ACF usually consist of an elastic polymer core coated by Ni/Au metal layers.

In the thermocompression process, some conductive particles are captured by bumps and corresponding ITO electrodes and deformed to form the conductive path between bumps and electrodes. In the other direction, because of low distribution, conductive particles cannot form conductive paths, so the anisotropic interconnection is established, as shown in Figure 7.10. A typical conductive particle is resin ball plated with Ni-Au, as shown in Figure 7.11. The balance of stress is the main consideration.^[8]In an ACF bonded component, the compression stress provided by the adhesive and the elastic stress of the conductive particle are balanced, which is good for reliability. The polymer in ACF is commonly thermoset, which cures by heat in the bonding process. More than 80% curing is typically required for good mechanical and electrical properties of COG packaging. The main function of the adhesive is to attach the die and seal the COG. In addition, since the glass substrate is transparent, some ACF in COG is photodefinable material.^[9] This kind of ACF cures in ultraviolet light during packaging. The COG process is thus performed at room temperature, which is the most outstanding advantage. This type of material is particularly suitable to the low-temperature COG process.

To sum up, the advantages of ACF are as follows:

(1) Fine pitch flip-chip interconnection.

(2) No underfilling.

(3) Low process temperature.

(4) Simple, flexible, and low cost.

Figure 7.10 A schematic view of an anisotropic interconnection

Au Ni

Resin core

Figure 7.11 Conductive particles in ACF

136 Chapter 7 Module Assembly and Optoelectronic Packaging

2. ICA Bonding

Isotropic conductive adhesive is a packaging material that has the same conductivity in all directions, a typical composition of which is polymer with Ag filler.^[10]The ICA bonding process is similar to the soldering process. First the ICA is printed on the electrode pads on the substrate. Then the chip is aligned and mounted onto the substrate, the polymer in the ICA cures through thermal compression to bond the chip onto the substrate. At the same time, conductive filler in the ICA forms a conducting path between the chip and substrate.

At last the underfill is applied between the chip and substrate for sealing and protection.

Unlike soldering, ICA packaging doesn’t need to reflow.

ICA is one of the first packaging materials developed in the COG process. Citizen Co.

in Japan first used ICA in the COG packaging of palmtop liquid crystal displays.^[10]In the process, bumps of IC chips were Au plated Cu mushroom bumps, with a pitch of 200μm.

In addition, they used ICA in the production of a small-sized LCDs with a bump pitch of 150μm.^[11] The ICA was transferred and attached to the bumps, instead of printing on the ITO electrodes of the glass substrate. The ICA-COG process developed by Panasonic Co.

in Japan used stud bump technology.^[12]The fabrication of stud bumps is similar to the wire bonding process. Metal wire fuses to a ball on the chip pad and is drawn to break on top by bonding force to form nail head bumps. Then all bumps are pressed to the same height.

Compared to plating bumps, this kind of fabrication of bumps is simple and cheap. The diameter of metal wire used in Panasonic Co. fabrication was 20μm, so that the size of the square pad could be reduced to less than 70μm, which is the pitch of bumps (Fig. 7.12).

Compared to ACF packaging, the ICA bonding process is relatively more complicated with lower bonding strength. Especially because the contact area between ICA and the chip and ICA and the substrate is small, many chips are detached before underfilling. In addition, high mechanical residual stress remains inside devices after underfilling, which will cause product failure in reliability tests. Therefore, up to now ICA has been limited in application to packaging small size and large bump pitch products.^[13]

IC

Glass substrate Electrode ICA Insulating bonder Nail head type bump

Figure 7.12 A schematic of stud bump bonding

3. NCF Bonding

The function of NCF (Nonconductive film) is similar to ACF, which is to establish elec-trical conducting paths between different components for interconnection.^[10] Unlike ACF, which introduces new conductive particles as a conducting medium, in NCF, all conductive particles are removed and the electrical connection is established by direct contact between chip bumps and ITO on substrate through compression. After the thermal compression process, thermoset nonconductive film can provide mechanical fixation for the direct con-tact to ensure the stability of the electrical connection. Because NCF is used to ensure the connection of an IC and substrate both mechanically and electrically, the curing condition is very important for NCF.

In an NCF-bonded device, stress is an important factor for electrical and mechanical interconnection reliability. Thermal stress, binding force, and internal shrinkage provide

7.3 Packaging of Flat Panel Display Modules 137 the bonding force of the assembly and the compression force between bumps and the ITO.

Because of the low elastic deformation of glass, the elastic deformation of the bumps is extraordinarily important for releasing the internal stress of the device. Experimental results showed that the bumps with better elastic properties performed much better than those bumps with low elasticity in reliability tests.^[14]One of the key requirements of NCF bonding is that every bump must be compressive to ensure the electrical conductivity of the interface after bonding and in service conditions.

NCF bonding avoids possible electrical shortages between bumps, as in the case of ACF bonding. Therefore, it could be an interconnection technique for ultra–fine pitch IC bonding.

However, the current bumping technology cannot ensure the planarity of the bumps, which is the requirement for bumps to be under compression after bonding and during service conditions. Therefore, the NCF technique has not been used in mass production.