Research on the bonding quality of different wires on aluminum pads of chips

Comparison and reliability study of chip aluminum pad bonding wire performance

Introduction: Technical choice and industrial value of bonding materials

In the interconnect system of semiconductor packaging, metal bonding wires are like "conductive bridges" in the microscopic world, and their performance directly determines the stability and durability of the connection between the chip and the external circuit. Despite the emergence of advanced interconnect technologies such as flip chips and through-silicon vias, wire bonding is still the mainstream solution in the current packaging industry with an 80% market share. This metallurgical combination formed by the synergy of heat, pressure and ultrasonic energy requires the bonding wire to meet the electrical performance while taking into account process compatibility and long-term reliability - this triple requirement drives the technological evolution of bonding materials from traditional gold wire to multi-alloy wire.

At present, the industry has formed four mainstream material systems represented by gold wire, palladium copper wire, gold palladium copper wire, and silver wire, each of which shows significant differences in performance, cost and applicable scenarios. Production data from a packaging and testing company showed that the choice of different wires could cause product yields to fluctuate by 5-8 percentage points, with a 3-5 times difference in life and material costs of up to 100 times. Through system testing, from basic characteristics, process performance to reliability verification, this paper comprehensively analyzes the bonding quality of these four types of metal wires on chip aluminum pads, providing a quantitative basis for packaging process optimization and material selection.

1. Analysis of the basic characteristics of metal wire

The material properties of bonding wires are the fundamental factors that determine their process adaptability and reliability, and the differences in electrical properties, mechanical properties and chemical properties directly create the application boundaries of different materials.

1.1 Electrical and thermal properties: the core indicators of energy transfer

In the dual dimensions of current carrying and heat conduction, the four types of metal wires are clearly differentiated: silver wire ranks first with a conductivity of 62MS/m, which is 38% higher than gold wire (45MS/m), and silver wire has a current-carrying capacity of 1.8A at the same diameter (25μm), which is 40% higher than that of gold wire. This advantage makes silver wire unique in high-density bond designs – the wire diameter can be reduced to 20 μm at the same current, reducing pad spacing from 50 μm to 40 μm, and increasing interconnect density by 25%.

Silver wire also performs best (429W/m·K), which is 35% higher than gold wire (317W/m・K), and in high-power devices such as power MOSFETs, silver wire bonding can reduce chip junction temperature by 8-12°C, significantly extending the operating life. Experimental data show that for power devices above 10W, the thermal dissipation advantage of silver wirebonding can increase the thermal cycle life by more than 30%, while in low-power scenarios below 5W, this difference has an impact of less than 5% on overall performance.

1.2 Mechanical Properties: Key constraints of the bonding process

The mechanical properties of metal wire directly affect the bonding quality and pad protection, which are mainly reflected in three aspects: gold wire has the lowest Vickers hardness (HV 80-90), palladium copper wire has the highest (HV 140-160), and gold palladium copper wire (HV 120-130) and silver wire (HV 100-110) are in between. This difference is particularly critical in thin aluminum pad (<0.8μm) bonding – the high hardness of palladium and gold palladium copper wires causes "craters" of 8-12% compared to only 1-2% for gold wires. Failure analysis of an RF chip showed that pad damage caused by palladium-copper wire bonding accounted for 23% of the total failure when the thickness of the aluminum pad was 0.6μm. The gold wire has the lowest CTE (14.2×10⁻⁶K⁻¹), and the matchability with silicon chips (2.6×10⁻⁶K⁻¹) is better than other materials, and the interfacial stress of gold wire bonding is 40% lower than that of palladium copper wire in the temperature cycle of -55°C~150°C, significantly reducing the risk of delamination. The minimal yield strength of gold wire (about 120MPa) means that the pressure required for bonding is only 60-70% of that of palladium-copper wire, and the consistency of arc forming (CPK value of 1.6-1.8) is also better than that of other materials (1.2-1.4). These characteristics determine the basic guidelines for material selection: gold or silver wire is preferred for thin aluminum pad devices, and palladium-copper alloy wire can be considered for thick aluminum pads (>1μm) to reduce costs.

1.3 Chemical properties: the core guarantee of environmental adaptability

In complex packaging environments, the chemical stability of metal wires is directly related to long-term reliability: gold wires exhibit absolute advantages and do not have any oxidation when burned in air, while palladium copper wires, gold palladium copper wires and silver wires must be protected by nitrogen (oxygen content < 50ppm), otherwise an oxide layer of 10-50nm (CuO/Cu₂O) will form on the surface or AgO), resulting in a 30-50% decrease in bond strength. An experiment showed that the yield rate of palladium copper wire in air environment was only 68%, while under nitrogen protection it could reach 99.5%. The corrosion rate of gold wire in a humid and hot environment (85°C/85% RH) containing chloride ions (100ppm) < 0.01μm/day, while that of silver wire and 0.03μm/day for palladium-copper wire. This difference makes gold wire irreplaceable in harsh environment applications such as automotive electronics. The difference in chemical properties is directly reflected in the process cost: the nitrogen protection system of the palladium-copper alloy wire increases the cost of a single line by about 150,000 yuan, which can be eliminated by the gold wire.

2. System comparison of ball welding process performance

The actual performance of the bonding process is the result of the synergy between material properties and equipment parameters, and the process window and quality characteristics of different metal wires can be clearly identified through control variable experiments.

Pelletizing performance and parameter optimization

Pelleting tests on the K&S Iconn bonding machine showed that all four types of wires can form homogeneous free air balls (FABs) with a diameter of 50±3μm under optimized parameters: gold wire: no shielding gas, electron flame (EFO) energy 25-30mJ, burn time 150-200μs, FAB roundness error < 5%; Palladium copper wire / gold palladium copper wire: nitrogen flow rate 0.5L/min, EFO energy 35-40mJ, burning time 200-250μs, oxide layer thickness < 5nm; Nitrogen flow rate 0.5L/min, EFO energy 30-35mJ, burn time 180-220μs, surface roughness Ra<0.1μm; Scanning electron microscopy confirmed that the FAB of all metal wires had no porosity, cracks and other defects, and met the basic requirements of bonding, but the spherical parameter window (energy fluctuation allowed ±5mJ) of palladium-copper-based alloy wire was narrower than that of gold wire (±8mJ), which required higher equipment accuracy.

2.2 Morphology and strength characteristics of the first solder joint

The first solder joint formed with optimized bonding parameters (Table 1) showed significant differences in dimensional consistency and mechanical strength:

Morphological characteristics: The arc height (15-20 μm) and ball height (25-30 μm) of all solder joints are less different, but the incidence of edge burrs (5%) of palladium copper wire solder joints is higher than that of gold wire (1%), which is directly related to their higher bonding pressure.

Strength performance: Palladium copper wire had the highest thrust (35g) and gold wire had the lowest thrust (28g), but the thrust consistency (CV value of 6.8%) of gold wire was significantly better than that of other materials, indicating better process stability. The tensile test shows a similar trend - the tensile CV value of gold wire (7.2%) is nearly half lower than that of palladium copper wire (13.5%), which is more suitable for automated mass production. It is worth noting that the aluminum pads showed no signs of crushing damage after the ball was removed, indicating that all four types of wires can safely bond 4μm thick aluminum pads under optimized parameters.

2.3 Growth kinetics of intermetallic compounds

The high-temperature storage test (HTST) revealed the growth pattern of IMC at the interface between different metal wires and aluminum pads: the gold-aluminum interface IMC grew the fastest, with a thickness of 3.2μm after 175°C/500h; silver-aluminum is followed by (2.8 μm), gold-palladium-copper-aluminum is the slowest (1.5 μm), and palladium-copper-aluminum is slightly higher (1.8 μm). This suggests that the addition of palladium reduces IMC growth rate by 40-50%. The IMC of all systems showed rapid growth from 0 to 500 h (gold-aluminum average rate of 6.4 nm/h), and after 500 hours it entered the hysteresis stage (the rate decreased to < 1 nm/h), and this nonlinear feature was directly related to the formation of the diffusion barrier layer. The gold-aluminum interface mainly forms AuAl₂ (η phase) and Au₂Al (θ phase), the silver-aluminum interface is dominated by Ag₂Al, and the palladium-copper alloy wire interface detects the complex phase of CuAl₂ and PdAl₃, which is less brittle than pure copper-aluminum IMC。 Although the growth of IMC causes the bond strength to decrease over time, the thrust retention rate of all samples is still above 80% after 500h aging, meeting the basic reliability requirements.

3. Comprehensive evaluation of bonding reliability

Through multi-dimensional reliability testing, the performance differences of different metal wires in extreme environments can be revealed, providing a key basis for application scenario selection.

Commercial-grade reliability test results

Commercial-grade tests conducted according to the JESD22 standard show that all four types of wire bonded devices have passed all assessments: high-temperature storage (HTST 1000h/150°C): all devices are functioning normally, and the thickness of the wire IMC is up to 4.5μm, but there is no failure; Hot and cold cycling (TCT 1000 cycle/-40°C~125°C): no functional failure, the lead fatigue degree (5% increase in resistance) of silver wire bonding is slightly higher than that of other materials; High-pressure cooking (PCT 168h/121°C/100% RH): All qualified, the lead corrosion degree (weight loss of 0.02mg) of gold-palladium copper wire is the lightest. Of particular note is that although gold wire IMC is the thickest, it does not cause functional failure, suggesting that IMC thickness is not the only determinant of reliability, and its morphology is as critical as its distribution – the continuity of the gold wire IMC layer (more than 90%) is better than that of silver wire (82%), which explains its ability to maintain reliability in the thick IMC state.

3.2 Limit performance of military-grade reliability testing

After increasing the test rigor to the MIL-STD-883E military standard, differences between materials began to appear: Extremely long hot and cold cycles (6000 times): No failure of all materials, indicating that the bonded structure can withstand extreme temperature stresses; Long-term high-temperature storage (2000h/175°C): gold wire fails at the earliest time (1500h), with a failure rate of 10%, and gold-palladium copper wire performs best (no failure in 2000h); Extended PCT (504h): The gold-palladium copper wire failed for the first time at 168h, and the palladium copper wire performed best (no failure at 504h) Unbiased high acceleration stress (uHAST 336h): The silver wire failed at 240h, and the gold wire and palladium copper wire persisted until the end

Failure mode analysis showed that all failures stemmed from "ball detachment" – the interface separation between the ball and the aluminum pad, and the microscopic mechanisms include: The Kirkendall void density of the gold wire IMC layer is 15 / μm², which is much higher than that of palladium copper wire (5 / μm²)

； In the PCT environment, Cl⁻ ions (concentration 50ppm) released from the plastic encapsulation react with silver wires to form AgCl, resulting in interfacial peeling. The high CTE of silver wire leads to cyclic stress concentration and accelerates interfacial fatigue These results reveal the environmental sensitivity of material selection - gold palladium copper wire is preferred in high-temperature environments, palladium copper wire is preferred in humid environments, and gold wire is moderate in comprehensive extreme environments.

4. Material selection strategy and technical outlook

Based on comprehensive performance evaluation, a scientific bonding wire selection framework can be established and future technology development directions can be predicted.

4.1 Differentiated selection guidelines for application scenarios

The selection of optimal materials in different fields is clearly differentiated:

Consumer electronics: gold wire (cost-sensitive) or silver wire (high-performance type), balancing yield (>99%) and cost (material proportion < 10%); Automotive electronics: palladium copper wire (power system) or gold palladium copper wire (vehicle radar), meeting the requirements of -40°C~150°C wide temperature; Aerospace: Gold wire is the first choice, with a lifespan of more than 15 years guaranteed to override cost considerations (material proportion can be tolerated up to 30%); Industrial control: Gold palladium copper wire is the best value for money, with a total cost of ownership that is 20% lower than gold wire at 1000 power cycles

According to the calculations of a new energy vehicle manufacturer, changing the bonding wire of the IGBT module from gold wire to palladium copper wire reduces the cost of a single vehicle by $15, while the reliability still meets the 150,000 kilometers / 8 years requirement.

Three major innovation directions of technological development

The evolution of bonding wire technology shows a clear trend: the development of palladium-gold-copper ternary gradient alloys to reduce the IMC growth rate by another 30% while maintaining good bond plasticity; 5-10nm diamond-like coating (DLC) is deposited on the surface of the silver wire to increase the corrosion resistance to 80% of the gold wire level;  The real-time parameter adjustment system based on machine learning reduces the thrust CV value of palladium copper wire from 14.3% to less than 8%; These innovations will drive bonding technology towards "higher reliability, lower cost, and wider compatibility" to provide stronger interconnect support for advanced packaging.

conclusion

The wire bonding quality on the aluminum pad of the chip is the result of the combination of material properties, process parameters, and application environment. This study confirmed through system testing that gold wire, palladium copper wire, gold palladium copper wire and silver wire all meet the basic requirements of semiconductor packaging, but each has its own focus on performance and applicable scenarios:

Gold wire is still the first choice for high-demand areas due to its excellent process stability and comprehensive reliability, but its high cost limits its popularity

； Palladium copper wire has outstanding performance in the balance of strength and cost, suitable for mid-to-high-end industrial applications, but the process window is narrow; Gold-palladium copper wire has the strongest IMC suppression ability and the best high-temperature reliability, making it an ideal choice for power devices. The electrical and thermal conductivity advantages of silver wire are significant, suitable for high-density and high-power consumption scenarios, but the corrosion resistance needs to be improved. Reliability failure analysis showed that IMC thickness was not a determining factor, but its growth morphology and interface integrity were more critical, and all failures were manifested by the stripping of the solder ball and the aluminum pad, which provided a clear direction for process optimization. In the future, the development of bonding technology will focus on alloy composition innovation and process intelligence, continue to reduce costs under the premise of ensuring reliability, and promote the evolution of semiconductor packaging to higher density and higher performance.