Semiconductor packaging has tremendous contribution to the advancement of electronics manufacturing. It starts with the fabrication of a semiconductor chip and ends with the final encapsulation of the product. The semiconductor package is referred to as the first level of interconnection while the printed circuit board and the final enclosure as the second and third levels respectively. A semiconductor chip should be protected from environmental stresses and electrostatic discharge damage.
One of the major processes in semiconductor packaging is the wire bonding process. In this article, we will dwell on the discussion of the wire bonding process, the different techniques, and the challenges that are commonly encountered.
Components and Materials Used in the Wire Bonding Process
The wire bonding process is the process of bonding conductive metal wires to form electrical connections. It has greatly improved from manual wire bond machine operation to highly automated machines that have staggering throughput. Let us break down the different components and materials involved in the wire bonding process.
- Bonding Wire: The wire materials used in wire bonding processes are typically copper, gold, and aluminum. Each material type has specific advantages and disadvantages in terms of material properties, cost, and applicability. For example, gold is known to have excellent corrosion resistance, and good thermal conductivity however, it is expensive in comparison to other types of materials. The purity of the bonding wire is selected based on the needed mechanical properties, manufacturability, and cost.
- Capillary: The capillary is the ceramic tool used in the wire bonding process to bond the wire to the bonding pad. The capillary is clamped in the transducer of the wire bonding machine. Each capillary type has tool life which must be monitored. It must also be cleaned at regular intervals, defined based on the number of touchdowns of the capillary. Most advanced wire bond machines also have an auto-inhibit system to pre-alert users if the tool is near its life. Bonding quality is highly affected by the cleanliness and dimensions of the capillary.
- Bonding Pad: The bonding pad is the conductive opening on a chip or die to establish a connection between the chip and substrate or lead frame. The semiconductor die undergoes a passivation process as a final manufacturing step, however, the bonding pads must be kept uncovered to keep them accessible for the wire bonding process. The frontside metallization of the bonding pad can be aluminum or aluminum-copper layer or other metals. The bonding pad must be kept clean to attain good bonding of wire and pad.
- Wirebond Machine: The wire bond machines that are used now are automatic with advanced features, especially for automotive applications with tight requirements. These advanced features include pre and post-bond inspection, a robust pattern recognition system, and better placement accuracy. A wire bond machine consists of a work holder that puts the product in place, a bond head that contains the capillary assembly and camera system, and a magazine or reel holder.
What are the Different Wire Bonding Process Types?
Ultrasonic: In the ultrasonic wire bonding process, low or high-frequency ultrasonic power is used to scrub the wire on the pad surface to form the bond. Aluminum is the typical wire material used for ultrasonic bonding which usually comes in the form of a wedge bond. The wire is clamped behind the wedge capillary. The wire is welded into the bond pad and the clamp opens to release the wire and form a loop, then lands to the second bond location. Once the second bond is completed, the wire clamp closes and forms a tail.
- Thermocompression: Themo-compression bonding utilizes both heat and pressure to create a perfect wire bonding process. The bonding temperature can range from 300 to 400 degrees Celsius for gold wire. A hydrogen torch is needed to form a ball at the tip of the wire. The bonding tool presses the ball down and with heat and pressure, the ball is deformed to establish the bond. Gold is the material used for this wire bond technique due to the oxidation resistance of golf at very high temperatures.
- Thermosonic: Thermosonic bonding uses a combination of ultrasonic and thermocompression technology to form reliable bonds. This type of bonding is usually applicable to gold wires. During the first bonding, an electric flame-off is produced to create a parameter-controlled Free Air Ball (FAB). The ball is then pressed into the bonding pad where a scrubbing motion is applied with the application of ultrasonic energy. The main parameters influencing the bond quality include bond temperature, bond force, ultrasonic power, and bonding time. After first bonding, the wire lifts off to form a loop and proceeds to second bonding on the substrate pad or leadframe post.
What are the Challenges in the Wire Bonding Process?
Even if wire bonds have been used for a long time in the industry, challenges are still constantly met during manufacturing. Due to the complexity of the design of semiconductor packages and the decreasing size of wires, defects, and risks need to be addressed as early as product design and development. Below are the common challenges that are encountered during the wire bonding process:
- Bond Placement Accuracy: The goal of the wire bonding process is to have a centered, correctly sized wedge or ball bond within the bonding pad. Both the size and placement accuracy are measured during product set-up to ensure the quality of the wire bonding process. For fine pitch devices, capillary needs to have smaller holes and diameters to ensure bond placement accuracy.
- Wire Sweeping: Smaller wires are more susceptible to wire sweeping which may be aggravated by the encapsulation process when the mold flows and triggers movement of soft wires. Wire sweeping or swaying is more prominent in gold wires due to their soft material properties. The response during the mold encapsulation process should also be checked by doing engineering evaluations.
- Contamination: In achieving a reliable bonded wire, it is important to have a clean, halogen-free surface. Oxidation and corrosion on the pad surface interfere with the bonding process and may induce nonsticking of the wire to the pad. Contaminants such as chlorine and fluorine may lead to early failure during application due to its sensitivity to corrosion. Plasma cleaning prior to the wire bonding process is sometimes done to improve the adhesion of the wire to the pad metallization.
- Pad Cracking or Cratering: Copper wires pose some advantages in terms of electrical conductivity, cost, and reliability; however, copper also has higher hardness compared to other materials. Copper wires are more prone to pad cracking and cratering if insufficient optimization studies are performed. The bond impression should be checked using the cratering test.
Quality Verification Steps During Wire Bonding Process
To know if the wire bonding process was successfully performed, the package must go through several quality verification tests that are based on acknowledged standards in the industry. Below are the accepted tests for checking the quality of the wire bond:
- Pull Test: The pull test is a popular technique for determining the strength of the bond. Pull Tests can be done on the wire span and stitch following a standard procedure. This type of test is typically done using semi-automative pull test equipment which has a force-controllable hook. The pull strength of the wire bond must fall within the acceptable limits for it to pass. The “break mode” which is defined as the portion or location by which the wire fails also has specific criteria of acceptability.
- Shear Test: Another method to know the strength and quality of the bond is by means of a shear test. In shear testing, a tool is allowed to shear the bond in the direction parallel to the bonding surface. The shear strength must also reach the allowable range of the wire.
- Cratering Test: A cratering test is required to check if there is a pad crack present after the wire bonding process. Pad cracking is critical as it can disturb the underlying metallization and cause electrical issues. The chip is immersed in a solution to dissolve the wire and check the presence of a crack on the pad.
- Visual Inspection: There are two ways by which visual inspection can be done: manual and automatic. In manual inspection, the units may undergo sampling or 100% inspection to know if the wires are bonded according to visual criteria. With the recent advancements in manufacturing, automated optical inspection (AOI) is also being performed to increase accuracy and avoid handling-related defects. Some popular defects during visual inspection are wire shorting, non-sticking, and lifted wires.
- X-ray Inspection: Wire sweeping is a defect that will sometimes manifest only after the mold encapsulation process. A non-destructive way to know if wire sweeping occurred after molding is through X-ray inspection. Using an X-ray, the image of the wires can be retrieved and assessed if it follows quality requirements. Other wire defects that can be detected through X-ray testing are lifted and broken stitches, wire shorting, and missing wires.
