The back-end process: Step 4 - Wire bonding step by step
04/01/2001
BY STEPHAN RüEGG AND DOMENICO TRUNCELLITO
In the semiconductor and overall electronics industry, the move continues unabated toward further miniaturizing of packages, components and modules while also increasing their functionality. This trend challenges the back-end assembly process in that high-density packages push the limits of equipment and process in terms of accuracy and speed, especially for the wire bond process. This article gives an overview of the process of connecting the chip to the external contacts of the package.
Wire Bonder Processes
Different types of wires are used for the wire bond process: gold (Au), aluminum (Al) and copper (Cu). Gold and aluminum are both highly conductive and ductile enough to withstand deformation during the bonding process while remaining strong and reliable. Each material has its advantages and disadvantages and is bonded by a different method.
 Figure 1. Wire bonding is a highly sophisticated assembly operation. |
Economical considerations, as well as various electrical benefits (good thermal and electrical conductivity, lower intermetallic growth and higher stiffness), are the drivers to use copper wire for enhanced applications. Chip manufacturing companies, together with equipment suppliers, are starting to produce high volumes with the copper bonding process.
Copper Wire Bonding
The key for a reliable ultra fine-pitch copper wire process is the formation of a round, reproducible free-air ball. To prevent the free-air ball from oxidizing, an inert atmosphere around the tail-electronic flame-off (EFO) blade area during the flame-off phase is created. Considering the harder copper-wire, which causes more stress on the die pad and increased surface oxidation on the substrate, special bond processes must be taken into consideration. Programmable dual stage bondforce and ultrasonic profiles to set in the bond program on the wire bonder are required to enhance both ball- and wedge-bond contacts. Together with the bond process, the capillary design should also be taken into consideration to achieve a high bond quality.
Gold Ball Wire Bonding
Gold ball wire bonding equipment makes up the largest portion of capital investment for the package interconnection segment. Gold wire has many advantages over other bonding materials. Gold is the best available room-temperature conductor and is excellent for transferring heat. Its properties greatly limit oxidation and corrosion, ensuring a reliable wire bond process under cleanroom conditions without requiring a protective atmosphere.
The thermosonic wire bonder process requires heat, ultrasonic power and force. Gold ball and aluminum on the chip pad are forced together under heat while ultrasonic power is applied. The result is an intermetallic connection (weld). The typical bond temperature for copper leadframe-based devices is approximately 180 to 250°C. Organic substrates (ball grid arrays) require low temperatures of approximately 100 to 150°C. Higher ultrasonic power at the same frequency compensates for lower bond temperatures to ensure a solid and high shear value (force required to shift away the bonded ball from the aluminum pad). The most commonly used gold wire in production today is 25 microns (1 mil).
 Figure 2. Bond recipe transfer. |
Wire bonding is a sophisticated assembly operation. Several hundred wires must be perfectly positioned on the chip and welded to the environment. The distance between the chip pads becomes incrementally smaller because of higher integration. A single misplacement of the bonded ball to the pad results in a faulty connection and dramatically reduces the production yield. Gold ball wire bonding's flexibility and reliability have made it the most widely used technology (Figure 1).
Step by Step Wire Bonding
After the silicon chip is attached to the leadframe or substrate, the interconnect process begins. The wire bonder welds a gold ball onto the chip, loops the fine gold wire to the lead and joins it.
The device with the attached chip is first placed and held down (mechanically or by vacuum) in the process zone of the wire bonder to heat up. A camera integrated in the bond head moves over the device on the process zone looking for known patterns on substrate and chip. After the patterns (eye points) are found, they are aligned. Simultaneously, the finger finder is used to locate the centers of the leads to ensure an accurate wedge placement.
After the alignment is performed, all coordinates of the ball and wedge positions are accurately defined and centered. The wire bonder is now ready to start the bond process, connecting the bondpad on the chip with the leadfinger on the substrate.
Production Flexibility
The flexibility of today's fully automatic wire bonder equipment allows a fast and secure conversion from one product material to another. To change the production load, the new recipe on the wire bonder, if necessary, changes the gold wire and capillary and starts the new production. Today's equipment is able to handle small lots that require frequent changes to different products, as well as high-volume production, without any difficulties.
 Figure 3. Ultra fine-pitch bonding. |
Programming a new bond recipe is becoming easier because of available PC offline programming tools. In a short timeframe, these tools allow an operator to create a wire bond recipe on a PC outside a production site (such as in an office). Production can thus be continued because no wire bonder has to be taken out of production just for programming the parameters of the new material. With the new offline programming tools, manufacturers increase their responsiveness as the new bond recipe can be taught before the new material has arrived at the facility. In addition, bond recipes can easily be distributed and transferred to other machines (Figure 2).
Thanks to a comprehensive calibration concept, the same production result is achieved on all machines. Important bond process parameters, like bondforce and ultrasonic energy, are being calibrated with external devices ensuring identical static and dynamic behavior.
The productivity of the equipment has increased significantly in the last few years. Today, the wire bonders in production have essentially the same performance. The cost of ownership of the gold wire bonder process compared to other interconnect processes is much lower.
Equipment Requirements
Ultra fine pitch bonding: The driving forces behind today's wire bonder advancements are productivity and ultra fine-pitch bonding (Figure 3). What was nearly impossible a few years ago is standard today. The 50 µm distance from pad center to pad center on the chip is now possible. The industry asks equipment suppliers to provide equipment that is able to bond pad pitches of 35 µm (Figure 4). This requires a smaller ball size and more precise placement so the wires don't touch. The bonded ball must stay completely inside the pad (the smaller the pad, the better). Because only a limited number of bond pads can fit along the perimeter of a chip, some devices have staggered bond pad layouts.
Successful ultra fine-pitch bonding requires many conditions, including consistent free-air ball formation, closed-loop bond process control, accurate ball placement below 4 µm, high productivity, high frequency, accurate and repeatable wire looping, and flexible material handling.
Consistent free-air ball formation: A precisely controlled current/voltage of the electronic flame-off unit (EFO) has a direct impact on the formation of the free-air ball (melted ball before actual deformation during the bond process). The EFO generates the spark that melts the end of the gold wire to form the tiny ball.
Closed-loop bond process control: Constant ball and wedge dimensions are important for the connection quality. Slight variations in the deformed ball size cause yield loss. During the bond process, the bondforce, as well as the ultrasonic power, is closed-loop controlled (i.e., the applied bondforce is checked and controlled according to its predefined values). This process control system for bondforce and ultasonic power accurately forms each ball and wedge the same way; it guarantees the best connection for ball and wedge bond on the chip pad and lead finger.
 Figure 4. Pad pitch of 35 µm. |
Accurate ball placement below 4 µm (3 sigma): Considering the ultra fine-pitch needs of the industry, ball placement is a crucial factor. Imagine a chip pad opening of 45 µm and a bonded ball size of 35 µm placed to it: The placement range to position the ball is only ± 5 µm. The most accurate bond placement can be achieved by using air-bearing technology. Air bearings have no slip-stick effect (unforeseen bondhead stop when static friction suddenly hits) like linear bearings. The air bearing bondhead stops exactly at the right spot, no matter what speed. A high-resolution direct X/Y/Z position measurement and an accurate pad recognition system are additional premises for accurate ball placement. In addition, a thermal compensation system on the equipment finally levels out the thermal expansion of bondhead parts to prevent placement drifts of a few microns. A closed-loop placement correction implemented in the machine can be used to correct the last microns when ultra fine pitch is bonded.
Productivity: Cycle speeds to lay a wire can run below 75 ms. However, there is more to consider than just unit per hour numbers. Uptime, set-up time and maintenance can impact the overall productivity of the equipment. Optional visual bond inspection, which checks online wire straightness, loop height and placement accuracy, decreases productivity. However, it monitors the quality of the assembled devices and may justify a lower productivity when dealing with higher priced devices.
High frequency: The continued trend in ball grid array packages creates a need for lower bond temperatures. An increase in the ultrasonic frequency applied during the bond process can compensate for the temperature drop. 125kHz is an ideal bond frequency.
Looping: To meet the ultra fine-pitch requirements in current production, precise and repeatable wire looping is needed. It is essential that the capillary on the bond head complete an exact trajectory between the ball and the wedge. Any winded and sagging wire can create a short between its neighbor wire. Because of the shrinking of ICs to reduce the silicon real estate, longer wires automatically result to compensate for the longer distance to the wedge site. This is a challenge considering the tiny wire and high speed at which the wire is placed. On the other hand, for chip scale packages (CSPs), very short wire loops are required, ensuring a small package size. For special applications, low wire loops with a maximum height of less than 80 µm are required to achieve very thin packages.
Material handling: On automatic wire bond equipment, the magazines will be picked up by the magazine handler and brought up to the indexer. The substrate will then be pushed out between the rails of the indexer. The indexer will transport the leadframe to the fully programmable work holder, which will hold the frame in position during bonding. After bonding, it will then be shifted into an output stack magazine. The current market demands flexible material handling on wire bonders that provide customized solutions for singulated BGA (boat applications), optoelectronic material handling and ceramics handling.
Outlook
One factor that is still driving progress in wire bonder technology is miniaturization. 35 µm ultra fine-pitch capability has successfully been tested in the lab. This new benchmark is a challenge for the suppliers of wire, capillary and equipment. The miniaturization of the bonded ball size is the crucial factor.
As wire bonding pushes its physical limits, the challenge is to maintain process reliability and process control at a standard while increasing equipment productivity. Therefore, for the near future, effort will be put into optimizing the bond process and providing better materials to get more performance out of the machine. In addition, the variety of applications to be bonded is increasing (SBGA, PBGA, QFN, MCM, etc.) and the market asks for different production solutions on the same platform. Finally, although copper bonding is already running production at some major manufacturers, it still presents a great challenge for the near future.
AP
STEPHAN RüEGG, marketing manager, can be contacted at ESEC, Hinterbergstrasse 32, 6330 Cham, Switzerland; +41 41 749 51 11; E-mail: [email protected]. DOMENICO TRUNCELLITO, senior communications manager, can be contacted at ESEC (USA) Inc., 9830 South 51st Street, Suite B-111, Phoenix, AZ 85044; 480-893-6990; Fax: 480-893-6793; E-mail: [email protected].