Die attach is a back-end semiconductor process performed after wafer dicing. It fixes the die to a substrate or leadframe. Wire bonding is the following packaging step, providing the electrical connection between the die and the package substrate. Among the available interconnection methods, wire bonding is one of the most widely used.
Gold Bonding Wire and Wire Bonding
Gold bonding wire is used for die interconnection in semiconductor packaging. During wire bonding, the adhesive between the die and the substrate is processed to provide suitable bonding performance. High-purity gold wire is then used to connect the die pads to the corresponding substrate or leadframe pads.
The wire is primarily gold, with a stated purity of 99.999%. Silver, palladium, magnesium, iron, copper, silicon, and other elements may be added as dopants. Different alloying elements can change the wire's hardness, stiffness, ductility, electrical conductivity, and other properties. The selected wire composition therefore affects both bonding behavior and the mechanical characteristics of the finished interconnect.
Purpose and Scope of the Bonding Standard
The purpose of a wire-bonding standard is to establish basic process requirements and define the criteria used to judge products as acceptable or unacceptable during production.
The criteria described here apply specifically to gold ball bonding. They are not intended as universal requirements for every wire-bonding method or every package structure.
Basic Conditions for Gold Ball Bonding
Gold wire bonding uses thermosonic bonding, which combines heat, pressure, and ultrasonic energy. The required temperature, bonding force, ultrasonic power, and bonding time can vary significantly with the bonding machine, die material, substrate or leadframe material, and package configuration. These parameters must therefore be established according to the equipment and material characteristics rather than copied directly from an unrelated process.
Quality Criteria for Gold Wire Bonds
1. Ball Geometry
The free-air ball and bonded ball must satisfy the specified dimensional and shape requirements. In the following dimensions, φ refers to the diameter of the gold bonding wire.
- Ball diameter: The standard range is 2.5 φ to 3.5 φ. A diameter below 2.5 φ is classified as an undersized ball, while a diameter above 3.5 φ is classified as an oversized ball.
- Ball thickness: The standard range is 0.5 φ to 1.5 φ. A thickness below 0.5 φ is classified as a flattened ball, while a thickness above 1.5 φ is classified as an excessively thick ball.
- Ball deformation: If the wire deviates from the center of the bonded ball by more than 1/2 φ, the ball is considered deformed.
2. Wire-Loop Geometry
The wire loop must maintain a controlled shape and clearance from the die and bonding surfaces. Excessive lateral movement, an irregular S-shaped profile, wire damage, insufficient clearance, or contact with the bonding surface can all indicate a defective wire profile.
- Lateral wire movement: The wire swing must be no greater than 3 φ. For an S-shaped loop, the limit is 2 φ. A value exceeding either limit is classified as a wire-profile defect.
- Wire damage: The allowable damaged portion is no more than 1/4 φ. Damage exceeding 1/4 φ is unacceptable.
- Loop clearance: The vertical distance between the die edge and the bonding wire must be at least 1.5 φ. A value below 1.5 φ is classified as a low wire. The maximum height of the wire profile above the die surface must not exceed 200 μm.
A collapsed wire is another unacceptable condition. In this defect, the portion of gold wire identified in the inspection area lies against the bonding surface. An acceptable loop should maintain a defined angle relative to the bonding surface instead of lying flat on it.
3. Bond Footprint
The bond footprint must have the specified dimensions, and the capillary impression must remain complete.
- Bond length: The required length is 0.8 φ to 1.5 φ.
- Bond width: The required width is 1.5 φ to 2.5 φ.
- Capillary impression: The impression left by the capillary must be complete. A bond outside the dimensional range or with an incomplete impression is unacceptable.
The wire tail must be no longer than 1 φ. A tail exceeding 1 φ is classified as an excessive wire tail and is not acceptable.
4. Nonbonding and Lifted-Bond Conditions
A false bond occurs when the ball bond contacts the die surface and the second bond contacts the frame surface, but the measured pull force is zero. A lifted or detached bond occurs when either the ball bond or the second bond fails to contact its intended bonding surface.
These conditions are critical because visual appearance alone may not confirm electrical or mechanical integrity. The bond must make actual contact with the intended pad or frame, and the resulting interconnection must withstand the applicable mechanical test.
5. Bond Location
Bond placement must match the designed pad locations on both the die and the package-side connection area.
- Ball placement: The ball bond must remain within the IC pad or be positioned exactly at the pad edge. A ball extending beyond the pad is classified as a ball-placement defect.
- Second-bond placement: The second bond must be located within the PCB gold finger. The bond edge must remain at least 1 φ from the edge of the gold finger. A bond extending outside the gold finger is classified as a second-bond placement defect.
- Missing wire: A missing wire occurs when no wire is present at a location where a bond is required.
- Incorrect pad connection: If the gold wire is bonded to a pad other than the designated pad, the result is classified as an incorrect bonding location.
Pull-Test and Ball-Shear Criteria
Pull-Test Method
During a wire pull test, the pull tool is positioned near the highest point of the wire loop, close to the ball bond. The test location should be controlled consistently so that the measured result can be compared with the applicable process requirement.
Pull-force control limits are defined according to the wire diameter and package or product configuration. For SOT-54 and SOT-23 products, wire bonding may use two different lead directions. The required pull force can differ with the lead direction, and the letter “J” is used to distinguish the corresponding control condition. For example, 23 and 23J refer to gold wires with the same diameter but different lead-direction requirements. Other product designations follow the same principle.
Ball-Shear Test
Ball shear is evaluated using a ball-shear tester. The required shear force is at least 16 g, and at least 60% of the gold wire must remain on the pad after the test. If either the shear-force requirement or the remaining-wire requirement is not satisfied, the result is classified as a ball-shear defect.
Process Control Considerations
Wire-bond quality depends on the interaction of the bonding wire, die pad, substrate or frame surface, capillary, and bonding parameters. Temperature, bonding force, ultrasonic power, and bonding time must be controlled together because changing one parameter can affect ball geometry, bond footprint, loop shape, and mechanical strength.
Inspection should therefore cover both appearance and mechanical performance. Ball size, ball deformation, wire position, loop clearance, bond dimensions, wire damage, missing wires, and incorrect pad connections can be checked visually or microscopically. Pull testing and ball-shear testing provide additional evidence that the bonds are mechanically connected to the intended surfaces.
The dimensional limits above are process-control criteria for the specified gold ball-bonding application. Wire diameter, package design, bonding equipment, material stack-up, and lead direction must be considered when applying them. A criterion defined for one product or wire configuration should not be transferred to every semiconductor package without technical validation.
