Effects of Wire Type and Mold Compound on Wearout Reliability of Semiconductor Flash Fineline BGA Package

C.L. Gan,1&2, Senior Member, IEEE, C. Francis1, B.L. Chan1, E.K. Ng1, U. Hashim2

1Spansion (Penang) Sdn BhdPhase II Free Industrial Zone, 11900 Bayan Lepas, Penang, Malaysia

2Institute of Nano Electronic Engineering (INEE)Universiti Malaysia Perlis (UniMAP), 01000 Kangar Perlis, Malaysia

Email: clgan_pgg@yahoo.com

ABSTRACT

This paper discusses the influence of wire type (Cu and Au) and the effect of mold compound on package reliability after exposure to component reliability stress tests. Failure analysis is conducted to identify associated failure mechanisms, and wearout reliability of both wire types investigated after biased HAST (HAST) and Temperature Cycling (TC). Weibull plots are plotted for each reliability stress. Cu (Copper) ball bonds are found to have better reliability margin in biased HAST stress compared to Au (Gold) ball bonds for both mold compounds. Cu wire also exhibits a better reliability margin compared to Au ball bonds in the TC stress test. Failure mechanisms of HAST and TC are proposed for both wire alloys in this paper.

Key wordsWearout reliability, weibull plot, HAST, TC, wire type

INTRODUCTION

The primary considerations of finding an alternative to replace gold wirebonding in microelectronic packaging are driven by new technologies coming to market, in line with ever decreasing cost expectations [1]. The currently common Au-Al intermetallic compound (IMC) growth has been widely characterized and analyzed [4-6]. Work is still ongoing with Au wire, as Zulkifli MN et al. [7] suggested a new approach of examining the effect of the individual phases and surroundings on the strengthening produced by the Au–Al intermetallic compound; combining FEA based on friction and wire bonding parameters and correlating TEM results with results obtained from other techniques should enable a more detailed understanding of the bondability and strength of thermosonic gold wire bonds. Copper wirebonding, however,appears to be the best candidate to meet cost and market considerations, and various engineering studies on copper wire deployment have been previously reported [2, 3]. Key technical barriers such as Intermetal dielectric (IMD) cracking due to excessive bonding, copper ball bond corrosion under moist environments and extended reliability of copper ball bonds have been identified accordingly [8-10]. In previous works, Copper ball bondshave been shown to be more susceptible to moisture corrosion compared to gold ball bonds, and undergo different corrosion mechanisms in microelectronic packaging [11-12]. Our previous studies indicate that Pd-coated copper ball bonds actually outperform gold ball bonds in biased HAST wearout reliability [13]. Extended reliability of High temperature storage life (HTSL) of copper ball bonds in TSOP package is found with apparent activation energy (Eaa) of ~ 0.70 eV compared to gold ball bonds [14]. Blish et al. [15] investigated Eaa of typical Au-Al IMC of 1.0 eV ~ 1.5eV which is dependent on Al thickness as well as dopants. A similar range may be appropriate for Cu as more reliability papers are published.

Hence, extended reliability is crucial to determine the lifetime of gold and copper ball bonds in microelectronics packaging. Cu-Al and comparable Au-Al IMC growth kinetics have been studied and have shown Cu-Al growth to be at least 5X slower than Au-Al IMC [16]. However, copper wirebonding still has some reliability challenges and complex failure mechanisms to understand which are primary barriers to the complete replacement of gold with copper in wirebonded packages [17]. To help address this, in this study, we have prepared 64 ball BGA (Ball Grid Array) packages assembled with Gold and Pd-coated copper wire and submitted for biased HAST (HAST) and Temperature Cycling (TC) tests. The test vehicle construction is indicated in Figure 1.

Figure 1: FBGA Package Interconnect Construction

EXPERIMENTAL RESULTS AND DISCUSSION

Materials used include 0.8 mil Pd-coated Cu wire (Cu) and 4N (99.99% purity) gold (Au) wire, and 110 nm flash devices packaged into fortified Fine-Pitch BGA packages with green (< 20 ppm Chloride in content and pH values of 5.30 to 5.80) molding compound and substrate. In this study, there are two variables, namely mold compound type (A or B) and Wire type (Pd-coated Cu wire and 4N Au wire) yielding four legs, all bonded on Fine pitch 64-ball BGA packages on a 2L substrate. The sample size used was77 units for HAST and TC stresses, and the corresponding stress tests conditions are tabulated in Table 1. After electrical test, good samples were then subjected for Level 3 Preconditioning and 3 times reflow at 260ºC as described in JEDEC IPC-STD 020 standard.Each leg then went through HAST stress testing per JESD22-A110 at 130ºC/85%RH, 3.6V as well as TC per JESD22-A104 at -40ºC to 150 ºC. Electrical testing was conducted after several readpoints of stress as well as to check Au and Cu ball bond integrity in terms of its moisture and thermo-mechanical reliability.

Table 1: Summary of experimental matrix (for Au and Cu wires)

Compound Stress Test Test Conditions SSA HAST 85%RH, 130 ºC, 3.60V 77

A TC -40ºC to 150 ºC 77

B HAST 85%RH, 130 ºC, 3.60V 77

B TC -40ºC to 150 ºC 77

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All reliability data belong to the wearout portion of the reliability curve (i.e. the bathtub curve) since the Weibull ß (shape parameter) is greater than 1.0 for all legs. Mold compound B reveals a greater time to failure in HAST and higher cycle to-failure (tfirst , t50 and t63.2) in the TC test compared to mold compound A, even thoughthey have similar specifications (See Table 2). Possibly, the slightly lower pH of EMC A might have played a role in the reduced reliability as observed in other works [19]. Additionally, Cu ball bonds show superior extended HAST reliability compared with Au ball bonds for both mold compounds A and B (See Figure 2).

y = 4.9585ln(x) - 39.652R² = 0.8804

y = 5.0585ln(x) - 41.954R² = 0.9477

y = 9.9595ln(x) - 80.853R² = 0.996

y = 8.676ln(x) - 72.19R² = 0.9419

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

500 5000 50000

Wei

bits

Hours

Weibull Plot (HAST Wearout Reliability)

Au_EMC ACu_EMC AAu_EMC BCu_EMC B

Figure 2: Biased HAST Weibull reliability plots of Au and Cu ball bonds with different EMC (Epoxy Molding Compounds)

The wire type does not appear to be a key factor affecting the TC reliability performance, although we did observe Cu ball bonds to have slightly better TC extended reliability performance (higher t first ,t50 and t63.2) compared to Au ball bonds (Figure 3). The coefficient of thermal expansion (CTE) of mold compounds A and B are similar,but perhaps the CTE difference between the two with respect to silicon or the pH values (as mentioned earlier) might play a role.EMC B (with α2 of 3.0 x 10-5/°C) is closer to silicon die CTE of 3.0 ppm/°C compared with EMC A(α2 of 0.7 x 10-5/°C) shows higher TC reliability margin and longer cycles-to-failure for Au and Cu wires. Increased internal stresses due to CTE mismatch might result in faster dislocation movement and IMC formation. The key material properties of EMC A and B are tabulated in Table 2.

y = 8.4311ln(x) - 81.641R² = 0.9304

y = 7.2471ln(x) - 71.315R² = 0.8618

y = 11.956ln(x) - 115.45R² = 0.9754

y = 8.347ln(x) - 82.541R² = 0.9129

-5.00

-4.00

-3.00

-2.00

-1.00

2000 20000

Weib

its

Cycles

Weibull Plot (TC Wearout Reliability)

Au_EMC ACu_EMC AAu_EMC BCu_EMC B

Figure 3: Extended TC Weibull reliability plots of Au and Cu ball bonds with different mold compounds.

FAILURE ANALYSIS AND MECHANISMS

Typical failure mechanisms of both biased HAST (HAST) of gold (Au) and copper (Cu) ball bonds indicate IMC microcracking along the AuAl and CuAl interfaces with the ball bonds. Figure 4 shows arepresentative image of Cu ball bond microcracking and open after 3000 hours in HAST. We observed crack initiates from the edge of Cu ball bonds and it appears to propagate towards the center of ball bond, typically due to the ball bond force by the capillary, and consistent with literature [9, 10, 13].

Figure 4: Representative SEM image shows Cu ball bond microcracking along CuAl interface after HAST 3000 hours

Figure 5 shows the proposed Cu ball bond corrosion mechanism in HAST test. The proposed corrosion mechanism as described in Eq. 1 to 3 below.

Cu9Al4 + 6H2O 2 (Al2O3) + 6H2 + 9Cu (1)CuAl2 + 3H2O Al2O3 + Cu + 3H2 (2)2AlCl3 + 3H2O Al2O3 + 6 HCl (acidic) (3)

Cu ball bond corrosion is most probably attributed to Cl- attacking the edge of Cu ball bond region. Hydrolysis of IMC and AlCl3 (an intermediate product) in a moist environment forms Aluminum (III) oxide which is a resistive layer and ionic Cl- is usually found at the corroded ball bond [1, 10]. Equation 1 indicates the hydrolysis of Cu9Al4 into Al2O3 and out gassing. Cracking of the Al2O3 interface of Cu to the Cu IMC might be due to out gassing of H2 during hydrolysis (as shown in Eq. 2 and 3) in between Cu IMC to Cu ball bonds. Cracking usually starts at the Cu ball bond periphery and will propagate towards center of Cu ball bond [9, 10, 13]. There is a possibility that under moist conditions, internal oxidation of intermetallic phases can result in oxidation of aluminium, precipitation of the noble metal (Au or Cu), and generation ofhydrogen. Possible reactions resulting in formation of aluminium oxide that might occur with gold and copper ball bonds are given in Eq. 4 and Eq. 5 [11, 12].

2Au4Al + 3H2O Al2O3 + 8Au + 6H (4)Cu9Al4 + 6H2O 2(Al2O3) + 9Cu + 12H (5)

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Figure 5: Proposed HAST failure mechanism of Cu ball bond

Au ball bonds also can exhibit Au oxidation and corrosion during HAST stress. Figure 6 reveals the representative SEM image of Au ball bonds and microcracking after HAST 2000 hours stress. Au is commonly thought to have high corrosion resistance compared to Cu ball bonds and has been deployed in microelectronics packaging for more than 25 years. However we observe higher HAST reliability margin of Cu ball bond compared to Au ball bond (Figure 2). The AuAl HAST corrosion failure mechanism is illustrated in Figure 7.

Figure 6: Representative SEM image shows Au ball bond (microcracking along AuAl interface after HAST 2000 hours)

Lifted ball bond

Al Bondpad

4

ball bondMoisture penetrate fromEdge of Auball bond

AuAl IMC will be react with oxygenFrom moisture>oxidised AuAlOlayer 1

2

Al Bondpad

ball bond

3 Formation of AuAl IMC micro- cracking

Al Bondpad

Figure 7: Proposed Failure mechanism of AuAl HAST oxidation failure.

Temperature cycling was conducted on the 4 legs of Au and Cu ball bonds to examine its thermo-mechanical reliability performance assembled with different epoxy mold compounds (EMC). It should be noted that there is a mismatch in CTE (Coefficient of Thermal expansion) between both Cu (17 ppm/°C) and Au ball bond (14 ppm/°C) compared to the silicon die (3.0 ppm/°C). This, combined with the CTE mismatch between Au and Cu ball bonds with Al bondpad of silicon die, will likely result in differential thermal stresses during hot cycles (150°C) and cold cycles (-40°C). IMC formation initiated at the edge of the ball bond (again, due to the ball bond force of the capillary) and micro cracking was observed after long cycles of thermal cycling effects. The micro cracking occurredin between the ball bond and the IMC (as shown in Figure 8).

Figure 8: Representative SEM image of Cu ball bond CuAl interfacial microcracking after TC 9500 cycles

Figure 9 indicates the representative AuAl IMC microcracking after 9000 cycles of TC (Temperature Cycling) test in our wearout reliability study.

Figure 9: Representative SEM image of Au ball bond AuAl interfacial microcracking after TC 9000 cycles

EFFECT OF MOLDING COMPOUND ON RELIABILITY

The epoxy mold compound (EMC) used in this evaluation are from suppliers A and B. The key material characteristics of EMC A and B are shown in Table 2. In general, the material specifications are very similar between the two compounds. EMC B exhibits a longer time-to-first-failure in HAST extended reliability (Figure 2) and a higher cycles-to-first-failure TC extended reliability tests (Figure 3) compared to EMC A. EMC B also shows higher t50 and t63.2

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compared to EMC A in HAST and TC stresses. These results are summarized in Figure 10. It should be noted that both EMCs show promising extended reliability results which far exceed the typical requirements of 96 hours of HAST and 1000 cycles of TC according to the JEDEC JESD47 standard.

Figure 10: Plot of TC (cycles-to-failure) and HAST (hours-to-failure) against EMC and wire types

Table 2: Key material characteristics of Epoxy Mold Compound (EMC) A and B

EFFECT OF WIRE TYPE ON RELIABILITY

Many previous studies reported Au wirebonds to have higher reliability margins compared to Cu wirebonds. However, there isvery little published data on extended reliability of Au and Cu ball bonds. Table 3 tabulates the major material properties comparing Au and Cu metals. In our study, Cu ball bonds show significant highert50 (mean-time-to failure) and t63.2 (characteristic life) in HAST and TC reliability compared to Au ball bond in FBGA package (as indicated in Figure 10). Although Cu is commonly oxidized under moist environments like the HAST test, an optimized package bill-of-material (such as that being used in our flash memory packages)show excellent results and exceed Au performance with either EMC.

Similarly, Cu ball bonds exhibit a higher number of cycles-to-failure in TC test compared to Au ball bonds (Figure 3) regardless of

EMC types. This is an interesting finding since the mismatch in CTE (Coefficient of Thermal expansion) between Cu (17 ppm/°C) is greater than that of Au (14 ppm/°C). Normally speaking, reducing CTE mismatch would tend to result in an improvement in reliability, but there are clearly other factors at work here. Perhaps the fast IMC growth of the AuAl interface plays a role in this case.

Table 3: Material properties of Au and Cu

Material Properties Units Au Cu

Thermal Conductivity W/mK 320 400

Electrical Resistivity 10-8 Ωm 2.2 1.72

Young’s Modulus GPa 78 130

Vickers hardness MPa 216 369

CONCLUSION AND SUMMARY

From the study, we analyzed the effects of wire alloy on extended reliability of HAST and TC stresses. Cu ball bonds show significant higher HAST reliability compared to Au ball bonds in theFBGA package. In addition, Cu ball bonds are also superior to Au ball bonds in an extended TC stress test. Overall, EMC B exhibits higher TC and HAST reliability margins compared to EMC A with either wire type.. However, both EMCs far exceed the minimum requirement of 96 hours of HAST and 1000 cycles of TC according to the JEDEC JESD47 standard.

ACKNOWLEDGMENT

The authors would like to render appreciation to Gene Daszko andTony Reyes for their encouragement on the publication of this technical paper.

REFERENCES

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Material characteristics

Units EMC A EMC B

Linear coefficient of thermal expansion 1

(CTE, α1)

10-5/°C 1.1 +/- 0.3 0.7+/-0.3

Linear coefficient of thermal expansion 2

(CTE, α2)

10-5/°C 4.5 +/- 1.0 3.0+/-1.0

Glass transition temperature (Tg)

°C 125 +/- 15 125 +/- 15

Cl- content ppm < 10 ppm < 10 ppm

Na content ppm < 10 ppm < 10 ppm

pH n/a 5.30 5.80

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