Influence of shear strength on long term biased humidity reliability of Cu ball bonds

This paper compares and discusses the influence of shear strength (in terms of shear-per mils-square) of Au and Cu ball bonds on the biased humidity reliability performance in SOIC 8LD leaded package. Biased (HAST) highly accelerated temperature and humidity stress test, 130 °C, 85 % RH) has been carried out to estimate the long term reliability of Au and Cu ball bonds. Lognormal reliability plots have been plotted for the three legs (control, leg 1 and leg 2) whereby Shear-per-mil-square of 6.68 is identified to have better mean-time-to-failure (t₅₀) compared to other two legs. Open failure from biased HAST 96, 192 h are subjected for secondary electron microscopy cross-sectioning and found typical interfacial CuAl intermetallic compound corrosion microcracking. HAST failure rates have been analyzed and the Cu ball bond lifetime has been established by using Peck model. The obtained Cu ball bond lifetime, of SPMS of 6.68 is >25 years and belongs to wearout reliability data point. This proves significant influence of SPMS on biased HAST failure rate. The higher the ball bond shear strength the lower the failure rate of biased HAST test. Hence, we should implement control on the average SPMS of ≥7.50 g/mil².     

镀Au键合Ag线性能对键合质量的影响研究

利用扫描电镜、能谱仪、拉力-剪切力测试仪等研究了不同镀Au厚度的镀Au键合Ag线Free Air Ball(FAB)特性和不同力学性能的镀Au键合Ag线对键合强度及其可靠性的影响规律，研究结果表明：镀Au键合Ag线镀层厚度过小会造成Electronic-Flame-Off(EFO)过程中的FAB偏球及球焊点形状不稳定，镀层厚度过大会导致FAB变尖；高强度、低伸长率会造成焊点颈部产生裂纹而造成焊点的拉力偏低并在颈部断裂，低强度、高伸长率引起颈部晶粒粗大进而降低颈部连接强度；镀Au键合Ag线颈部应力集中或内部组织结构不均匀，在冷热冲击周期性形变作用下，球焊点颈部产生裂纹并引起电阻增加，进而导致器件失效.     

Fundamentals of thermo-sonic copper wire bonding in microelectronics packaging

Fine copper wire bonding is capable of making reliable electrical interconnections in microelectronic packages. Copper wires of 0.8–6 mil diameter have been successfully bonded to different bond pad metallized and plated substrate materials such as Al, Cu, Ag, Au and Pd. The three metallurgical related factors; solid-solubility and diffusion of dissimilar contact metals, oxide film breakage and plastic deformation of asperities play a critical role in the bonding. Plastic deformation of an asperity is the most significant factor one has to consider to attain good bonding. Soft aluminum metal (30–40 VHN), with a lower % asperity threshold deformation is easier to wire bond than harder metallic surfaces (Ni, W, Mo, Cr, Co, Ta) of 150–500 VHN. Good adhesion of wire bonding is achieved for the surface roughness (Ra) of 0.01–0.15 μm and 0.02–0.6 μm of bare and plated surfaces respectively. It is rationalized that the application of ultrasonic energy principally breaks the oxide film and deform the asperities, while a compressive force increases the proximity of asperities. Hence wire welds to bond pad surface by molecular attraction and inter diffusion. Storage of copper ball bonds at 175 °C for 100–1,000 h forms copper aluminide at the interface. EDAX and Auger analysis reveal 22 at% Al + 78 at% Cu composition of the aluminides and Cu₃Al₂ empirical formula is calculated, which, does not match with any of the reported copper aluminides. Hardness of the copper ball bonds and stitch bonds are higher than wire exhibiting work hardening of the bonds on processing.     

Electromigration at gold–aluminium interfaces and in thin aluminium tracks

The in situ method to study accelerated ageing of electronic interconnections under current stress has been applied to (a) Au ball bonds on A11% Si metallization, and (b) thin aluminium tracks. For the Au ball bonds, it has been shown that direct current stresses above 10⁸ Am^?2 cause electromigration effects at the Au-Al interface, and that the ageing behaviour depends on the direction of the applied direct current stress. It was also shown that the addition of 1 per cent Cu to the metallization strongly retards void formation, and hence open contact failure. For the thin aluminium tracks, it has been shown that the early stage of electromigration, which can be studied accurately with the in situ technique, is closely related to the stress relaxation within the track. It is therefore concluded that the kinetics of electromigration can only be described if the kinetics of stress relaxation are well understood.     

Comparative performance of gold wire bonding on rigid and flexible substrates

This paper reports comparative performance of wire bondability of electrolytically plated Au/Ni/Cu bond pads on rigid FR-4 and bismaleimide trazine (BT) PCBs, as well as flexible polyimide (PI) substrate. The metallization surfaces were treated with plasma to study the effect of bond pad surface cleanliness on wire bondability. Process windows were constructed as a function of bonding temperature and bond power for the individual substrate materials. Significant improvements of wire pull strength and process window were noted after plasma treatment with a substantial reduction in minimum bonding temperature from 120°C to 60°C for both the rigid and flexible substrates. The minimum bond power required to produce successful bonds decreased with increasing bonding temperature. At a bonding temperature of 120°C, the process window for the flexible substrate was wider than the rigid substrates. The wire bondability and wire pull strength of rigid substrates decreased with increasing bonding temperature above 120°C due to softening of the substrate which adversely affected the effective bond force and the transmission of ultrasonic energy. In contrast, the wirebonding performance of the flexible substrate remained stable at 120°C or above because the thermo-mechanical properties of flexible PI substrate were rather insensitive to temperature. The process windows of flexible substrates with and without stiffener showed similar bondability.     

Investigation on high temperature mechanical fatigue failure behavior of SnAgCu/Cu solder joint

In this paper, high temperature mechanical fatigue tests on SnAgCu/Cu solder joints were carried out under three test temperatures (100, 125, 150 °C). Failure mechanism was analyzed through observation of micro-crack evolution and fracture morphology. The results show that the deformation curve of solder joint under high temperature mechanical fatigue tests can be divided into three stages: strain hardening stage, stable deformation stage and accelerated failure stage, which is similar to the curve under creep test condition. In addition, the cyclic life decreases rapidly with increasing temperature. Deformation field in the solder joint is non-uniform and shear strain concentration occurs in solder close to the intermetallic compound (IMC) layer. Micro-crack initiates at the corner of the solder joint and then tend to propagate along interface between Cu substrate and solder. The fracture morphology under three temperatures all exhibits ductile fracture mode and the failure path transforms from cutting through the top of Cu₆Sn₅ to propagation in solder matrix close to IMC layer with increasing temperature.     

Deformation behaviors of flip chip plastic ball grid array packages subjected to temperature change using moiré interferometry

When the temperature of the package assembly is changed, non-uniform deformation and local stress may occur owing to the different thermal expansion coefficients of the constituent materials, while concentrated thermal stress may cause significant failure. In this study, we carry out an experiment and analysis of the thermal deformation of a flip chip plastic ball grid array package with respect to temperature change. Interference fringe patterns representing the displacement distribution of each temperature stage are obtained using a Moiré interferometry and the bending deformation behavior and the strain of the solder ball are analyzed thereafter. We compare the deformation behavior of a single-sided package assembly and a double-sided package assembly. The effective strain, which significantly affects the failure of the solder ball, is found to be the largest in the solder ball just outside the edge of the chip in the case of the single-sided package assembly. In the case of the double-sided package assembly, the maximum effective strain is found in the solder ball just inside the edge of the chip, with a value approximately 50 % larger than in the case of the single-sided package assembly.     

A review of interconnect materials used in emerging memory device packaging: first- and second-level interconnect materials

The main motivation of this review is to study the evolution of first and second level of interconnect materials used in memory device semiconductor packaging. Evolutions of bonding wires from gold (Au) to silver (Ag) or copper (Cu) have been reported and studied in previous literatures for low-cost solution, but Au wire still gives highest rating in terms of the performance of temperature humidity test, high temperature storage, and bond-ability, etc. However, a new bonding wire material, Au-coated Ag, is recently developed to be an alternative solution which gives comparable performance, but lower cost compared to Au wire. In the first section of the article, the influence of a variety of factors were reviewed, which includes reliability performance and interfacial reaction that determines the performance of Au-coated Ag to reach for developing high reliability of bonded devices. With respect to second-level interconnects, SAC305 and SAC302 solder alloys give a balance performance between temperature cycling testing and drop testing, which are widely used in many field applications, such as mobile, consumer and computer. SAC405 and LF35 are developed for specific requirements such as SAC405 owns better temperature cycling performance, whereas LF35 gives excellent drop performance compared to SAC305 or SAC302. However, with market demands on automotive electronics get strong in recent years, solder joint reliability is being reviewed and discussed, especially in temperature cycling performance. Typical solder alloys on Ni/Au surface finish were not designed for automotive application to fulfill the requirement of board level reliability. Hence, newly developed solder alloys with Sn/Ag/Cu/Bi/Ni elements and Cu-OSP substrate surface finishes will be reviewed in the second section of the article.

     

Bond pad cratering study by reliability tests

Silicon cratering is one of the major obstacles to turn thermosonic copper wire bonding technology into a mass-production mode. The effects of reliability tests, i.e. aging test, temperature cycle test, and autoclave test on silicon craters of copper and gold wire-bonded Si substrate are discussed in this paper. Prior to reliability tests, wire-bonded specimens were examined by initial bond etch test. Results showed that there was no crack or silicon cratering observed in the selected samples. It was found that reliability tests, unlike improper bonding parameters, did not induce silicon craters at the Si substrate. However, reliability tests degraded the Si substrate and bonded interfaces, which were exposed later during destructive tests, i.e. wire pull test and ball shear test.     

Copper pillar bump design optimization for lead free flip-chip packaging

The interfacial delamination between the under bump metallurgy and the aluminum (Al) pad was observed in a copper (Cu) pillar bump evaluation in the work. The finite element analysis was employed to investigate the failure mechanism and the cu pillar bump geometry designed optimization. The finite element simulation result shows that the Cu pillar bump sustains the largest tensile stress at the temperature cycling test. The larger diameter Cu pillar bump can reduce the tensile stress significantly at the high temperature stress environment. Besides, an experiment confirms the finite element simulation result. Furthermore, an underfill with high glass transition temperature (T _g) and high modulus also improve the flip-chip ball grid array packaging reliability.     

Interfacial reactions of Sn-3.5% Ag and Sn-3.5% Ag-0.5% Cu solder with electroless Ni/Au metallization during multiple reflow cycles

The increasing industry awareness of lead-free activities has prompted original equipment manufacturers and suppliers to investigate lead-free solder systems in detail. The reliability of lead-free solders has been studied a lot recently, but the knowledge of it is still incomplete and many issues related to them are under heavy debate. In this study, the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu (wt.%) solders with Cu/Ni(P)/Au ball grid array (BGA) pad metallization were systematically investigated after multiple reflows. The peak reflow temperature was fixed at 260°C. It was found that relatively high consumption of Ni(P) was observed in the case of Sn-3.5%Ag solder alloys during multiple reflow cycles. A white layer of P rich Ni-Sn compound was observed above the dark Ni₃P layer for Sn-3.5%Ag solder after several reflows. It was noticed that the mean thickness of the intermetallics and the dark P-rich Ni layer at the interface was decreased just by adding 0.5% Cu in Sn-3.5%Ag solder alloy with less overall interfacial reaction at the solder joint.     

Thermal and Thermo‐mechanical Analysis for Design Evaluation of an Automotive Radar Module

The influence of the substrate technology, assembly method, and housing material on the thermal, thermo‐mechanical and cost performance of a radar module for automotive applications has been studied to address the product reliability aspects during the design phase. Flip chip and wire bonding have been evaluated for Multi‐Chip Module—Laminate/Deposition (MCM‐L/D) and Multi‐Chip Module—Deposition (MCM‐D) substrate technologies used for electronic packaging solutions in a harsh environment. Solder ball and direct attachment have been investigated as second‐level assembly. As a result of thermal and thermo‐mechanical simulations and cost analysis, radar module designs combining MCM‐D and MCM‐L/D with wire bonding have been revealed, which are preferable for use in different temperature environments with respect to two performance criteria, the maximum junction temperature and the manufacturing cost. Simulation‐based guidelines have been developed for designing radar modules used in automotive applications while satisfying temperature and stress constraints provided for the module. Copyright © 2004 John Wiley & Sons, Ltd.     

Oxidation of Au4Al in gold ballbonds

Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al-1 wt.% Si-0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au₄Al oxidation. With wire Type A the dominant compound underneath the ball was Au₈Al₃. A thin layer of Au₄Al (≈ 1 μm thick) was observed between the Au₈Al₃ and the gold ball. Ball lift failures occurred in the Au₄Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au₈Al₃. With wire Type B, a double layer Au₄Al was dominant after long term ageing and Au₈Al₃ was confined to the ball periphery. Consequently, because of the much greater volume of Au₄Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au₈Al₃ compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.     

Effects of package geometry on thermal fatigue life and microstructure of Sn–Ag–Cu solder joint

Abstract

Soldering experiments of chip scale package devices were carried out by means of diode laser soldering system with Sn–Ag–Cu solders. In addition, pull tests and a scanning electron microscope were used to analyse the effect of processing parameters on mechanical strength of solder joints. Viscoplastic finite element simulation was utilised to predict solder joint reliability for different package geometry under accelerated temperature cycling conditions. The results indicate that under the conditions of laser continuous scanning mode as well as the fixed soldering time, an optimal power and package geometry exists, while the optimal mechanical properties of microjoints are gained.     

Further characterization of Pd-coated copper wire on wirebonding process: from a nanoscale perspective

The objective of this research is to investigate the nanomechanical properties of ultra-thin Pd-coated copper (Cu) wire (ψ = 0.6 mil) and nanotribology along the interfacial between free air ball (FAB) and aluminum (Al) bond pad during wirebonding process. Two major analyses are conducted in the present paper. In the first, the characteristic of heat affected zone and FAB for Cu wire has been carefully experimental measured. Nanoscale interfacial tribology behavior between Cu FAB and Al pad is examined by atomic force microscopy. Secondary, the dynamic response on Al bond pad and beneath the pad during wirebonding process has been successfully predicted by finite element analysis. Micro-tensile mechanical properties of Cu wire before and after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Experimental obtained hardening constant in Hall–Petch equation has significantly influence on the localize stressed area on Al pad. This would result in Al pad squeezing (large plastic deformation) around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB is also carefully investigated by nanoindentation instruments. A real-time secondary EFO scheme has been conducted to reduce the strength of Cu wire and increase the bondability. All the measured data serve as material inputs for the finite element model based on explicit software ANSYS/LS-DYNA. In addition, nanoscale bondability on Cu-Al intermetallic compound is simulated by molecular dynamics. A series of comprehensive parametric studies were conducted in this research.     

Intermetallic and void formation in gold wirebonds to aluminum films

Gold ball bonds attached to either pure Al films or Al films with Cu and Si additions were annealed at temperatures in the range 77–277°C for periods of up to 3,000 h and then shear tested. The resulting fracture surfaces were observed to change progressively with time and temperature of annealing. The intermetallic phases and voids present after each annealing treatment were characterized by microscopic examination of the bond cross sections. Shear test results are discussed in terms of the observed microstructures.     

Stress/strain characteristics of Cu-alloy sheath MgB2 superconducting wires

The mechanical properties of Cu and Cu-alloy (Cu-Zr, Cu-Be and Cu-Cr) sheath in situ PIT-processed MgB₂ superconducting wires were studied at room temperature (RT) and 4.2 K. The effects of stress/strain on the critical current (I_c) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. Alloying the Cu sheath significantly increased the yield stress of the wires. The 0.5% flow stresses of the Cu-alloy sheath wires were 147-237 MPa, whereas that of Cu was 55 MPa. At RT, the serration in the stress-strain curves corresponding to the multiple cracking was observed around a strain of 0.4% and the curve almost saturated beyond that point. The strain dependence of I_c prior to the critical strain (ε_irr) was different depending on the magnetic field; being almost constant at 2 T and increased with strain at 5 T. The I_c decreased beyond ε_irr, which was much larger for Cu-alloy sheath wires as compared with Cu sheath wire. The magnitude of ε_irr is due to the difference in the thermal compressive strain in the MgB₂ core, which was relaxed by yielding in the sheath materials. The transverse compression tests revealed that the I_c of the Cu-alloy sheath wire did not degrade up to about 95 MPa, which is also higher than that of Cu sheath wire.     

Effects of Aging on Interfacial Microstructure and Reliability Between SnAgCu Solder and FeNi/Cu UBM

Effects of thermal aging on the interfacial microstructure and reliability of the SnAgCu/FeNi‐Cu joint are investigated. It is found that aging effects depends strongly on the temperature. Aging at low temperature, e.g., at 125 °C, a submicron meter thick FeSn₂ IMC layer formed at the SnAgCu/FeNi‐Cu interface during reflowing grows at a rate twenty times slower than the growth rate of the IMC at the SnAgCu/Cu interface. At high temperature, e.g., at 180 °C, the Cu element is found to diffuse through FeNi layer to produce the (Cu, Ni)₆Sn₅ IMC and this IMC layer grows even faster than the IMC at the SnAgCu/Cu interface. Solder ball shear test results show that the SnAgCu/FeNi‐Cu joint has a comparable strength to the SnAgCu/Cu joint after reflowing, and the strength drop after aging at 125 °C is less than that of the SnAgCu/Cu joint. However, after aging at 180 °C, the strength of the SnAgCu/FeNi‐Cu joint is degraded to a low value, along with a shift in failure mode from the solder fracture to the brittle intermetallics fracture.     

Overstressed interatomic bonds in stressed polymers

Infrared (IR) spectroscopy is used to find how the applied mechanical stress imposed upon a sample is distributed among the interatomic bonds. The distribution is highly heterogeneous: 80–95% of the bond population experience stresses close to the applied stress, the stress on the rest of the bond population varies over a wide range and reaches 1000–2000 kg/mm². The overstressed interatomic bonds lie in the amorphous regions of the polymer and are oriented in the direction of the mechanical force. The maximum stress on interatomic bonds is determined by the magnitude of the breaking stress on them. The breaking stress is shown to be a function of the applied stress, time, and temperature. This dependence is due to scission of stressed bonds induced by thermal fluctuations.     

Strain characteristics of Nb3Sn multifilamentary wires with CuNb reinforcing stabilizer

Bronze processed multifilamentary Nb₃Sn superconducting wires, with a CuNb reinforcing stabilizer instead of the conventional Cu stabilizer, were fabricated. The mechanical properties and the strain dependence of the critical current I_c were evaluated at 4.2 K and a magnetic field of 15 T. A remarkable increase in the yield stress (70%) and the plastic flow stress as compared to the values for the wire with Cu stabilizer was observed. The strain for the peak I_c was also increased by 0.2%. I_c on unloading was reversible within the strain range of 1.5%. The strain sensitivity of I_c in the CuNb/Nb₃Sn wire was almost the same as that of the Cu/Nb₃Sn wire. A decrease in the wire diameter from 0.8 to 0.5 mm resulted in a slight increase in the yield stress of the CuNb/Nb₃Sn wire, but no change in the strain dependence of I_c. An increase in the heat treatment temperature from 700 to 750°C resulted in a decrease in the flow stress of 15%, but no change in the strain dependence of I_c. A marked change in the morphology of the Nb filament in the CuNb reinforcing stabilizer was evidenced during heat treatment.