Wearout Reliability and Intermetallic Compound Diﬀusion Kinetics of Au and PdCu Wires Used in Nanoscale Device Packaging

Wearout reliability and diﬀusion kinetics of Au and Pd-coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. is paper discusses the HAST (with bias) and UHAST (unbiased HAST) wearout reliability performance of Au and PdCu wires used in �ne pitch �GA packages. In-depth failure analysis has been carried out to identify the failure mechanism under various wearout conditions. Intermetallic compound (IMC) diﬀusion constants and apparent activation energies ( 𝐸𝐸 aa ) of both wire types were investigated aer high temperature storage life test (HTSL). Au bonds were identi�ed to have faster IMC formation, compared to slower IMC growth of PdCu. PdCu wire was found to exhibit e�uivalent or better wearout reliability margin compared to conventional Au wire bonds. Failure mechanisms of Au, Cu ball bonds post-HAST and UHAST tests are been proposed, and both Au and PdCu IMC diﬀusion kinetics and their characteristics are discussed in this paper.


Introduction
In recent years, Cu wire bonding has been widely adopted in recent nanoelectronic packaging due to its conductivity, material properties, and cost effectiveness. However, there are few key technical barriers to be seriously considered in order to achieve fully transition from Au to Cu ball bonds in semiconductor packages. Gan et al. [1][2][3] reported the key challenges of Cu wire bonding deployment in nanoelectronic packaging while Tan et al. [4] and Uno [5], Zhong [6], Chen et al. [7] investigated the technical barriers and engineering solutions of bare Cu and Pd-coated Cu wire bonding in semiconductor packaging. Harman [8] reported the challenges and moisture reliability of Cu wire bonding in early years. Hang et al. [9] investigated post isothermal aging of CuAl ball bonds are mainly attributed to CuAl IMC interface corrosion and induce interface microcracking. However, there are limited studies on the wearout reliability of palladium-coated Cu wire, bare Cu wire, or Au wire bonds in nanoelectronic device packaging. It is crucial to conduct knowledge-based reliability studies and understand the wearout reliability models [10] and its associated failure mechanism with Cu wire bonding in nanoelectronic device packaging which will ensure successful Cu wire bonding deployment in high pin count and nanoscale devices. McPherson [11] laid out the time to failure reliability modeling in semiconductor physics and reliability stressing. Gan et al. [12] characterized the wearout reliability on Au and Pd-coated Cu ball bonds used in �neline �GA �ash memory packages. Some researchers have investigated and compared the IMC diffusion kinetics and calculated the apparent activation energy for Cu and Au ball bonds IMCs aer high temperature aging [13][14][15][16][17][18][19][20][21] and predict aa for isothermal Cu wire interfacial fracture [22]. In the �rst part of this study, Au and Pd-coated Cu bonds reliability are investigated under biased HAST and UHAST; resulting wearout reliability plots are generated, and

Materials and Preparation.
e key materials used include 0.8 mil Pd-coated Cu wire and 4 N (99.99% purity) Au wire and 110 nm �ash devices packaged into forti�ed �ne-pitch BGA packages, with green (<15 ppm Chloride in content) molding compound and substrate. In this Cu wire development study, there are a total of 6 legs comprising of Pd-coated Cu wire and 4 N Au wire bonded on Fine pitch 64-ball BGA packages on a 2 L substrate. Sample size used is 80 units for each stresses. e corresponding stress tests and its conditions are tabulated in Table 1. Aer electrical test, good samples were then subjected for preconditioning and 3 times re�ow at 260 ∘ C as described in JEDEC IPC-STD 020 standard, followed by unbiased HAST stress testing per JESD22-A118 at 130 ∘ C/85% RH [23], biased HAST stress per JESD22-A110 at 130 ∘ C/85% RH, and 3.60 V and temperature cycling per JESD22-A104 at −40 ∘ C to 150 ∘ C. Electrical testing was conducted aer several readpoints of stress as well as to check Au and PdCu ball bonds integrity in terms of their moisture and thermomechanical reliability.
Another set of materials were used to characterize IMC (intermetallic compound) growth rate, diffusion kinetics, and its apparent activation energies ( aa ). e key materials used include 0.8 mil Pd-coated Cu wire and 4 N (99.99% purity) Au wire, �ne pitch BGA package, 110 nm device that to be packed in forti�ed �neline BGA package, green (<20 ppm Chloride in content) in molding compound and substrate. All direct material used in this evaluation study for the 110 nm �ash device (with top Al metallization bondpad) are for packaging purpose. A total of 2 legs of 45 units of Au and Pd-coated Cu wires bonded on Fine pitch 64-ball BGA packages are subjected for 150 ∘ C, 175 ∘ C, and 200 ∘ C aging temperatures. Electrical testing was conducted aer each hour and cycle of stress to check Au and PdCu ball bonds integrity in terms of its high temperature ball bonds reliability with various aging conditions. IMC thickness measurements were carried out for each aging hours. and apparent activation energies ( aa ) of AuAl and CuAl IMC diffusion kinetics were analyzed as tabulated in Table 2. e package construction of our evaluation vehicle is shown in Figure 1 with 110 nm device encapsulated with green molding compound and bonded with PdCu or Au ball bonds.

Wearout Reliability of Au and PdCu Ball Bonds in HAST
and UHAST. Wearout reliability of Au and PdCu ball bonds were �tted to the Weibull distribution. e characteristic values of Weibull plots for both Au and PdCu ball bonds are tabulated in Table 1, covering HAST and UHAST. For HAST testing, it clearly indicates that the PdCu ball bond exhibits higher wearout reliability margins with a higher time to �rst failure ( �rst ) as well as a higher mean time to failure ( 50 ). In UHAST, PdCu ball bonds demonstrate a lower time to �rst failure compared with Au bonds the mean time to failure is similar between the two. e respective characteristic life or scale parameter ( 63.2 , ), and shape parameter ( of each weibull distribution are calculated and shown in Table 1. All reliability plots belong to wear out reliability region in conventional reliability bathtub curve since the shape parameters ( are more than 1.0 for biased HAST, UHAST, and TC stresses. Samples loaded for temperature cycling (TC) stress (−40 ∘ C to 150 ∘ C). It also reveals higher cycles-to-failure for PdCu ball bonds compared to Au ball bonds.

Weibull Plots Analysis and Characterization.
Cu wire is well known to be less corrosion resistant compared to Au wire in nanoelectronic packaging. Moisture reliability of bare Cu wire is identi�ed as a key technical barrier of Cu wire deployment in nanoelectronic packaging. Cu ball bond interfacial fracture is one of the bonding failures attributed to interface CuAl IMC corrosion [1-5, 7, 8, 12, 24, 25]. Figure 2 indicates the wearout reliability Weibull plot comparing PdCu wire and Au wire. In our evaluation, the PdCu Weibull plot actually shows a higher reliability margin than Au in HAST stress. is is most likely due to that Pd-coated Cu ball bond exhibits higher moisture reliability margin as Pd in Cu ball bond is more resistant towards moisture corrosion under biased HAST conditions. In our study, we use alternative method for performing Weibull plotting through the use of Weibits. e conversion of cumulative fraction failed, into  Weibits, that is given by Weibits is equivalent to ln[− ln(1− [11]. However, PdCu wire exhibits slightly lower UHAST stress compared to conventional Au wire as shown in Figure 3. is could be attributed to the variation of PdCu ball bond integrity in semiconductor assembly. However, both PdCu and Au wire legs in UHAST stress still far exceeded the industrial JEDEC standard of 96-hour surviving hours. e �rst hour rate to failure of PdCu and Au ball bonds are at 3000 hour and 4000 hour, respectively. is indicates that the bare Cu wire coated with palladium plays an important role in improving its biased HAST wearout reliability compared to Au ball bond in FBGA package with longer surviving hours. ermomechanical degree of Au and PdCu ball bonds was investigated through Temperature Cycling, (−40 ∘ C to 150 ∘ C condition TC). Figure 4 reveals wearout reliability plots  �tted to �eibull distribution of both wire types. PdCu ball bonds shows better and larger cycles to failure compared to Au ball bonds on 110 nm device FBGA 64 package. PdCu ball bonds still exhibit longer cycle-to-failure compared to Au ball bonds in terms of thermomechanical stress, and PdCu is believed ball bond with better �rst ball bond integrity and able to withstand longer cold and hot cycling test with material contraction and expansion.
AuAl IMC microcracking is also observed in post UHAST-2000 hour. e failure mode is similar to post biased HAST 1553 hr for Au ball bonds on Al bondpad (as depicted in Figure 6). oxide which is a resistive layer, and ionic Cl − is usually found at the corroded ball bond [1][2][3]. Equation (1) Cracking of the Al 2 O 3 interface of Cu to the Cu IMC might be due to outgassing of H 2 during hydrolysis (as shown in (2) and (3)) in between Cu IMC to Cu ball bonds. Cracking usually starts at Cu ball bond periphery and will propagate towards center of Cu ball bond [1][2][3]. EDX analysis conducted on failed CuAl interface reveals presence of Cl, O, and Al peaks. is indicates that aluminium(III) oxide is formed a as results of corrosion reactions [3]. A representative PdCu ball bond cross-section SEM image con�rms CuAl interface �neline cracking as shown in Figure 7. e �neline cracking would probably cause electrical opens in HAST stress for PdCu. e failure mechanism of Au ball bond is found similar to PdCu ball bond with similar �neline microcracking along the AuAl IMC region and caused electrical opens. Similar edge Cu ball bond microcracking is observed in post-UHAST 1000 hour opens of PdCu ball bond on Al bondpad (as shown in Figure 8).
Representative PdCu ball bond cross-section SEM image con�rms CuAl interface �neline cracking as shown in Figure 9. e �neline cracking would probably cause

Diffusion Kinetics of Au and PdCu Ball Bonds
In this section, we discuss the thermal aging test that was used to accelerate the intermetallic thickness growth of CuAl and AuAl IMCs. e IMC thicknesses, diffusion coefficient ( ) and required apparent activation energies ( aa ) of interdiffusion of Cu and Au atoms in Al were reported. Fick's �rst law of diffusion considering the concentration gradient does not change with time. It is given by (4) where is the diffusion coefficient (cm 2 /s). e term in the square bracket is negative of the concentration gradient, , so that the equation can be rewritten as in (5): .
e diffusion coefficient, , contains the temperature dependence of the jump frequency as well as the information about the interplanar distances, which depend on crystal structure through constant in (6). Graph ln versus ( ) can be plotted by using (7) exp aa , ln aa + ln , where self-diffusion coefficient, , is a constant with unit m 2 /s, aa is apparent activation energy in eV for the diffusion process, is molar gas constant in Jmol K , and is temperature in unit K. Table 2 tabulates the results of aa and of Au and PdCu IMCs formation of previous engineering studies [13][14][15][16][17][18][19][20][21] and compares them to this work. e values obtained for aa (in eV) of AuAl IMC are similar to the values investigated by Zulrich et al. [21] while aa of CuAl IMC interdiffusion is close to the value reported by Kim et al. [19,20]. e value of for CuAl IMC obtained in this study is smaller than the value of Xu et al. [13][14][15] while the value of AuAl IMC is smaller than those previous literature values. It has been reported that intermetallic growth in AuAl and CuAl IMCs follows a parabolic law during thermal aging as in (8) [13][14][15][16][17][18][19][20][21] where is the IMC thickness at time and is the diffusion coefficient. In our study, value of AuAl is 1 magnitude smaller than obtained for CuAl IMC. Hence, it can be concluded that the Au atom diffuse much faster than PdCu atoms in Al metallization and produce much thicker AuAl IMC layer than CuAl IMC.
Calculation of (diffusion coefficient) can be achieved by using (9) below. e linear relationship is consistent with IMC growth being diffusion controlled, and this kinetic relationship can be described by an empirical power law, where is the average thickness at annealing time , is the initial IMC thickness at 0, and is the diffusion coefficient (in Table 3). , 2 , and 3 are the diffusion coefficients obtained aer cumulative 500 hours, 1000 hours,  Figure 10(c)).  of ball bond shear and wire pull strength are analyzed and compared to time zero before stress testing.

Postextended Reliability Stresses Ball
It is observed that the ball shear strength degraded gradually over time. However, they far exceed the minimum shear value of 14 g (as shown in Figure 11(a)). Figure 11(b) indicates similar degradation trend for wire pull strength when comparing Au and PdCu ball bonds. PdCu ball bonds exhibit faster degradation trend compared to Au ball bonds.

Conclusion
In Au and PdCu wire bonding evaluations on 110 nm devices, we have successfully characterized the wearout reliability margins for HAST and UHAST and determined the diffusion kinetics of both wire types used in nanoscale semiconductor packaging.
e technical �ndings are summarized as the following.
(1) PdCu ball bond exhibits a higher time to failure (Weibull �tted distribution) compared to Au ball bonds in HAST wearout reliability plots. PdCu ball bonds showed a slightly lower UHAST wearout hour to failure but still far exceeding the JEDEC standard of minimum 96-hour surviving hours rate.
(2) Wearout failure mechanism of HAST and UHAST stress testing belong to CuAl and AuAl IMCs interface corrosion and microcracking which induced electrical ball bond opens.
(3) e values obtained for aa (in eV) of AuAl IMC formation (1.04 eV) is similar to the values investigated by Zulrich et al. [21], while aa of CuAl IMC interdiffusion (1.18 eV) is close to the value reported by Kim et al. [19,20]. e value of for CuAl IMC obtained in this study is smaller than the value of Xu et al. [13][14][15], while of AuAl IMC is a bit smaller than those previous literature values. In our study, value of AuAl is 1 magnitude smaller than obtained for CuAl IMC. Hence, it can be concluded that the Au atom diffuse much faster than PdCu atoms in Al metallization [8,16,21,27].
(4) It clearly indicates that Au atoms diffuse at least 5 times faster than PdCu atoms in Al metallization of the 110 nm �ash device tested. is could be easily estimated from the comparison of IMC thickness developed over time comparing CuAl and AuAl IMCs.
(5) Ball shear strength degraded gradually over time for both PdCu and Au ball bonds. However, it is still far exceeding the minimum shear value of 14 g and wire pull value of 2.5 gf. A similar degradation trend for wire pull strength comparing Au and PdCu ball bonds was also observed. Additionally, PdCu ball bonds exhibit faster degradation trend compare to Au ball bonds.