Preeti S. Chauhan

Copper Wire Bonding Concerns and Best Practices

Copper wire bonding of microelectronic parts has developed as a means to cut the costs of using the more mature technology of gold wire bonding. However, with this new technology, changes in the bonding processes as well as bonding metallurgy can affect product reliability. This paper discusses the challenges associated with copper wire bonding and the solutions that the industry has been implementing. The paper also provides information to enable customers to conduct qualification and reliability tests on microelectronic packages to facilitate adoption in their target applications.

Critical Review of the Engelmaier Model for Solder Joint Creep Fatigue Reliability

A solder interconnect fatigue life model was developed by Werner Engelmaier in the early 1980s as an improvement upon the inelastic strain range-based Coffin-Manson model. As developed, the model provides a first-order estimate of cycles to failure for SnPb solder interconnects under power and thermal cycles. While the model has been widely adopted for SnPb solder joint reliability prediction, many issues that arise from simplifications in formulating input model parameters as well as from the complex physics of solder degradation challenge the models ability to accurately estimate cycles to failure. Deficiencies with the model have been reported by a number of researchers. This paper reviews and summarizes the major issues with the Engelmaier model in its applicability to predict solder joint thermal fatigue life.

In Situ Interconnect Failure Prediction Using Canaries

A physics-of-failure-based canary approach for early identification of solder interconnect failures has been developed. The canary is composed of a resistance path formed by a near-zero ohm ceramic chip resistor soldered to pads designed to produce failure earlier than standard pad resistors, which are the target structures. The time to failure of the canary can be adjusted by adjusting the printed wiring board pad dimensions and, hence, the solder interconnect area. The developed canary approach is demonstrated through temperature cycling of the resistors. The pad width of a standard resistor is reduced by 80%, thereby reducing the interconnect life. The results from the temperature cycling experiment prove that the developed canary approach provides advanced warning of failures of the standard pad resistors. The FEA results suggest that there is a 78% increase in the strain range in the canary resistor, as compared with the standard resistor. The Engelmaier model, a physics-of-failure-based model for solder interconnect life estimation under thermal cycling, is modified to take the solder interconnect area into account. The model provides time to failure estimates for the canary and target structures. A comparison of the results from the Engelmaier model and temperature cycling experiment shows that the model provides a good estimate of time to failure of standard resistors and a conservative estimate of time to failure of the canary resistors.

EFFECT OF ISOTHERMAL AGING ON MICROSTRUCTURE AND CREEP PROPERTIES OF SAC305 SOLDER - A MICROMECHANICS APPROACH

SnAgCu (SAC) solders undergo continuous microstructural coarsening during both storage and service. In this study, we use cross-sectioning and image processing techniques to periodically quantify the effect of isothermal aging quantitatively on phase coarsening and evolution, in SAC305 (Sn3.0Ag0.5Cu) solder. SAC305 alloy is aged for (24–1000) hours at 100C (~ 0.7–0.8Tmelt). The characteristic parameters monitored during isothermal aging include size, volume fraction, and inter-particle spacing of both nanoscale Ag3Sn intermetallic compounds (IMCs) and micronscale Cu6Sn5 IMCs, as well as the volume fraction of pure tin dendrites in SAC305 solder. Effects of above microstructural evolution on secondary creep constitutive response of SAC305 interconnects were modeled using a mechanistic multiscale creep model. The mechanistic phenomena modeled include: (1) dispersion strengthening by coarsened nanoscale Ag3Sn IMCs and reinforcement strengthening by micronscale Cu6Sn5 IMCs, respectively; and (2) load sharing between pure Sn dendrites and the surrounding eutectic Sn-Ag phase. The coarse-grained polycrystalline Sn microstructure in SAC305 solder was not captured in the above model because isothermal aging did not appear to cause any significant change in the initial grain morphology of SAC305 solder joints. The above model is shown to predict the drop in creep resistance due to the influence of isothermal aging on SAC305 solder joints.

Impact of Thermal Aging on the Growth of Cu-Sn Intermetallic Compounds in Pb-Free Solder Joints in 2512 Resistors

Interfacial intermetallic compounds (IMCs) in solder joints are formed during soldering and continue to grow after assembly. Excessive interfacial IMC growth may impact the reliability of solder interconnections due to changes in material behavior. The impact of thermal aging on IMC growth can be determined by subjecting assemblies to elevated temperatures and determining the interfacial IMC growth. This paper discusses the interfacial IMCs formed in the solder-Cu interface for SAC305, SAC105, and Sn-0.7Cu-0.05Ni+Ge (SN100C) assemblies. Test assemblies were produced using tin-finished 2512 resistors soldered onto OSP-finished copper lands. The assemblies were subjected to aging conditions of 100°C for 24 h and 600 h; and 150°C for 24 h and the impact of high temperature aging on the thickness of IMCs formed at solder-Cu interface was studied. Samples stored at room temperature for 600 h were the reference specimen for the experiment. The IMC growth observed in the lead-free solders was compared with that in eutectic SnPb. Interfacial IMCs formed in room temperature conditioned specimens were scallop shaped and non-uniform. The IMC structure evolved with aging temperature and duration resulting in smoother and more uniform IMCs in 100°C/600 h aged samples. A comparison of IMC thickness in the solders at given aging conditions revealed that SAC305 exhibited highest IMC thickness, followed by SAC105 and SN100C. SnPb showed the least IMC thickness at all aging conditions except at 150°C for 24 h. At this condition, SnPb showed IMC thickness comparable to SAC305 solder and was higher than other solders.Copyright

Impact of thermal aging on the thermal fatigue durability of Pb-free solder joints

It is known that isothermal aging of lead-free solder materials at elevated temperatures leads to changes in microstructure. A growth of the intermetallic interface between bulk solder and substrate material can be observed as well as the growth of intermetallic particles in the bulk solder itself. Isothermal aging is used as preconditioning for reliability and mechanical tests. The idea is to bring the solder microstructure to a common baseline condition for comparison purposes and relax the stress in the solder joint coming from solidification. However, limited studies on the impact of thermal aging on the reliability of the solders are available. For solder interconnections, thermal aging will impact both the interconnect interfaces and the bulk material. Thermal aging has been shown to increase the interfacial intermetallic compounds (IMC) thickness and changes IMC composition which can result in reduced interconnect reliability. However, the thermal aging can also change the distribution of IMC particles in the bulk material that will also influence interconnect reliability. The focus of the present study is the evaluation of the temperature cycling fatigue reliability of solder joint interconnects after a exposure to extended isothermal aging at 100°C. This paper discusses the impact of isothermal aging on the thermal fatigue durability of SAC305, SAC105, and SN100C and SnPb assemblies. The test structures consist of chip resistors (CR2512 with Sn-finish) soldered to Cu/OSP-finish pads on an FR4 substrate. The test structures were subjected to isothermal aging of 100°C for 24, 100, 500 and 1000 hours. Subsequent, the aged structures were exposed to temperature cycling test conditions (−55°C to 125°C, 15 min dwell, ramp rate: 10°C/min) while monitoring electrical continuity of conductive paths formed with the specified solders. Failure identified as an electrical discontinuity was measured during the temperature cycle exposure and the impact of high temperature aging was documented. Specimens aged at 100°C for 24 hours acted as the controls for the experiment. The thermal cycling reliability of the lead-free solders was compared with that of eutectic SnPb.

Concerns and Solutions

This chapter provides an overview of the concerns with Cu wire bonding and the industry’s solutions to these concerns. Although Cu wire bonding is gaining widespread acceptance in the industry, there are a few challenges associated with it that need to be overcome. Cu wire bonding poses concerns related to Cu’s hardness, propensity to oxidize, and sensitivity to corrosion, as well as the wire bonding process, bonding in specialized packages, and low yield. The industry solutions to these problems, such as the use of thicker Al pads than are used in Au wire bonding, Ni-based pad finishes, specialized capillaries, palladium-coated Cu wires, and bonding in an inert gas atmosphere, are also discussed.

Wire Bond Pads

This chapter examines the pad materials and finishes for wire bonding. Cu wire bonding on Al and Cu pads is discussed. The common pad finishes, including NiAu, NiPdAu, PdAu, electroless nickel immersion gold, electroless nickel/electroless palladium/immersion gold, and electroplated silver, are considered. The effect of the thickness of surface finish layers on bond strength is also explained. The chapter also discusses the effects of surface treatment on the reliability of wire bonds. The sources of contamination on bond pads, including fluorine, chlorine, carbon, oxygen, silicon, and titanium, are examined, along with their influence on wire bond reliability. The effect of lead surface contamination and pad surface roughness on wire bond strength is considered. The surface treatments, including organic coating to prevent pad oxidation and plasma cleaning to remove surface contaminants, are also explained.

Wire Bond Evaluation

This chapter discusses the evaluation of wire bonding performance. The criteria for good bonds are described, along with pre- and post-bonding inspection techniques. Wire bond functionality tests, such as bond accuracy tests, electrical resistance measurements, and material characterization of wire bonds, are covered. Destructive and nondestructive mechanical tests, shear tests, and pull tests to evaluate the wire bond strength are discussed. The industry standards and best practices for wire bonding quality assurance and testing methods, and the common reliability tests for wire bonds, are also explained.

Thermal Reliability Tests

This chapter covers the thermal reliability tests conducted on Cu wire bonds. High-temperature storage (HTS) tests on Cu and PdCu wires on Al-, Au-, and Ni-based pads are discussed, and reliability test data are provided. Comparisons are made between the HTS strengths of Cu and PdCu wires. The effect of HTS on Pd distribution, as well as its effect on wire bond strength, is discussed. Cu wire bond reliability under thermal cycling and thermal shock testing is also presented.