Thermal Aging Effects on the Thermal Cycling Reliability ... - IEEE Xplore

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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 3, NO. 8, AUGUST 2013

Thermal Aging Effects on the Thermal Cycling Reliability of Lead-Free Fine Pitch Packages Jiawei Zhang, Zhou Hai, Sivasubramanian Thirugnanasambandam, John L. Evans, Michael J. Bozack, Yifei Zhang, and Jeffrey C. Suhling, Member, IEEE

Abstract— The microstructure, mechanical response, and failure behavior of lead-free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn–1.0Ag–0.5Cu (SAC105), Sn–3.0Ag– 0.5Cu (SAC305), and Sn–37Pb solder ball interconnects. Package sizes range from 19 mm with 0.8-mm pitch BGAs to 5 mm with 0.4-mm pitch BGAs with three different board finishes (ImSn, ImAg, and SnPb) previously studied. This paper presents the latest results from an on-going investigation on the aging temperatures were 25 °C, 55 °C, 85 °C, and 125 °C, applied for a period of 12 months. Subsequently, the specimens were thermally cycled from −40 °C to 125 °C with 15 min dwell times at the high temperature. Weibull analysis of failures versus cycle number show a ∼57% reduction in package lifetimes when aged at 125 °C compared to no aging for 19 mm BGAs, for MLFs the degradation is even worse, more than 58% reduction in experiment result. In contrast, the reliability performance of Sn–37Pb is much more stable over long time up to 12 months and temperature. We also study the evolution during isothermal aging, which is one of the major failure modes of solder joints due to its high homologous temperature. The degradation is observed for both SAC alloys on all tested package sizes and board finishes. For the 19 mm SAC105 case, e.g., there was a 53% (57%) reduction of characteristic lifetime at 125 °C for 6 months (12 months) compared to room temperature aging. The trends are in the expected directions; namely, the reliability is reduced when using higher aging temperatures, smaller solder balls, and SAC105. The dominant failure mode can be associated with the growth of Cu6 Sn5 intermetallic compounds during the aging, particularly on the pad side. Index Terms— Board finishes, isothermal aging, lead-free, microstructure, reliability, SnAgCu, solder.

PBGA CSP QFN ENIG SMT JEDEC

N OMENCLATURE Plastic ball grid array. Chip scale package. Quad-flat no-leads package. Electroless nickel immersion gold. Surface mount technology. Joint electron device engineering council.

Manuscript received September 26, 2012; revised February 26, 2013; accepted March 5, 2013. Date of publication March 28, 2013; date of current version July 31, 2013. Recommended for publication by Associate Editor C. Basaran upon evaluation of reviewers’ comments. The authors are with the Center for Advanced Vehicle and Electronics, Auburn University, Auburn, AL 36849 USA (e-mail: zhangjiawei19831010@ gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2013.2251932

PCB RoHS SEM NSMD WEEE CTE

Printed circuit board. Restriction of hazardous substances. Scanning electron microscopy. Nonsolder mask defined. Waste from electrical and electronic equipment. Coefficient of thermal expansion. S YMBOLS

Ag Cu Im Pb Sn β η ρ

Silver. Copper. Immersion. Lead. Tin. G REEK S YMBOLS Slope. Characteristic life. Probability plot. S UBSCRIPTS

Tg

Glass transition temperature. I. I NTRODUCTION

D

UE TO implementation of WEEE/ROHS legislation, the reliability of electronic assembly processes using leadfree solders has attracted increased scientific interest. Concurrently, electronic components have become smaller with higher densities, which means that the fine-pitch package contains more solder joint interconnects within a given area, with smaller solder balls and smaller distances between the solder balls. Further, most lead-free alloys require higher reflow temperatures, which increase the probability of failure due to a multitude of physical processes, which depend on temperature. From the perspective of manufacturability, cost, availability, and reliability, Sn–Ag–Cu (SAC) series solder alloys have become the standard for lead-free applications. However, the long-term reliability of lead-free solders is not well understood [1]. Both the package and board level reliability are primary concerns. Smaller solder joints are generally impacted more by the thickness and morphology of intermetallic compounds (IMCs) and the composition of the bulk solder alloy than larger solder joints [2]. The microstructure, mechanical response, and failure behavior of lead-free solder joints continually evolve during isothermal aging and/or thermal cycling environments [3]–[17]. Prior investigations reported microstructure coarsening and intermetallic layer growth. Ma et al. [3] explored the effects of

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ZHANG et al.: THERMAL AGING EFFECTS ON THE THERMAL CYCLING RELIABILITY

room temperature aging on the mechanical properties of SAC alloys and found large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 40%) during the first 6 months after reflow solidification. Zhang et al. [9] showed that the creep performance of lead-free solders significantly degrade after aging at elevated temperature. They also showed that isothermal aging effects on the mechanical properties for eutectic Sn–37Pb are relatively small. The creep behavior of lead-free and Sn–37Pb solders have a “cross-over point,” where lead-free solders begin to creep at higher rate than Sn–37Pb. They further found that microstructure evolution during aging is the underlying reason for the creep behavior of solder alloys. The evolving, coarsened second phases in the alloys are less effective in blocking dislocation movements, with a consequent loss of strength. These findings highlight the potential risks of lead-free solders in high temperature and harsh environments. Lee et al. [2] investigated the interaction between isothermal aging and the long-term reliability of fine-pitch ball grid arrays (BGAs) with Sn–3.0Ag–0.5Cu solder interconnects. Two different surface finishes with 0.4 mm fine-pitch packages and 300 mm diameter SAC solder balls were used. During thermal cycles from 0 °C to 100 °C with 10 min dwell time, they found package lifetime reduced by ∼44% during aging at 150 °C. Aging at 100 °C had less effect on package lifetimes. In this paper, we explore the effects of elevated temperature isothermal aging on the long-term thermal reliability of SAC105 and SAC305 assemblies in board level packages. Correlating tests using 63Sn–Pb eutectic solder were carried along in the experimental design for comparison purposes. Smetana et al. [4] have performed an extensive study on the effects of prior isothermal preconditioning (aging) on the thermal cycling lifetime for a variety of components. Similar to the investigations discussed above, it was observed that prior aging reduced the thermal cycling characteristic life of SAC BGA assemblies subjected to 0 °C to 100 °C cycling. It was also found that changes occurred in the Weibull slope, suggesting other failure modes were created by aging. They also found that prior aging increased the thermal cycling reliability of certain components (e.g., 2512 chip resistors and certain QFNs). Similar results of improved reliability with aging were found for components subjected to a smaller thermal cycling range 20 °C to 80 °C. This led them to conclude that aging does not universally reduce solder joint fatigue life. Previous literatures [1]–[10] show that the microstructure and mechanical behavior of Sn–Ag–Cu soldering alloys can change a lot over time when exposed to isothermal aging. A product from the manufacturing factory to consumers, needs long cycle, which needs months or even years. So, the aging effect on the hand-held consumer products is also important. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball over long-term isothermal aging. Besides this, current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the

Fig. 1.

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Assembled test vehicle.

calculations with the new lead-free SAC alloys that illustrate dramatic aging phenomena. II. E XPERIMENT A. Test Vehicle Description Daisy chain components allowed for continuous sampling of component reliability during the accelerated life tests. The fine-pitch PBGA packages measured 5×5, 10×10, 15×15, and 19 × 19 mm and contained SAC105, SAC305, and Sn–37Pb solder joints for each size fine-pitch PBGA, we also extended our test with 7 × 7 mm CSP and 5 × 5 mm MLF. The PBGA components used NSMD pads for better analysis of the aging effect on the reliability. The test component matrix is shown in Table I, which is covering some popular device models in current industry. The test vehicle, dubbed TV7, is shown in Fig. 1. The test vehicle used FR-406 glass epoxy laminated with a glass transition temperature Tg of 170 °C. The dimensions of the board design were 100.076 × 67.056 mm with a thickness of 1.574 mm (measured laminate to laminate). There are four circuit layers with reasonable copper distribution to provide optimum copper balance and CTE for thermal cycle testing. B. Accelerated Temperature Cycling and Aging Storage After assembly and aging, the boards were oriented vertically in the cycling furnace and wired to provide a daisy chain current diagnostic to determine when an open circuit occurred, indicating joint failure. The various daisy chain networks were monitored during cycling using a high-accuracy digital multimeter coupled to a high-performance switching system controlled by LabView software. Based on the IPC9701 standard, the practical definition of solder joint failure is the interruption of electrical continuity > 1000 ohms. In this paper, we defined “failure” to occur when the daisy chain resistance reached > 300 ohms for five sequential resistance measurements. The failure data are reported using a twoparameter Weibull analysis. The matrix testing plan is shown in Table II, which is based on our previous studies [3], [8], aged for various durations (0–12 months) at room temperature (25 °C), and several elevated temperatures (55 °C, 85 °C, and 125 °C). The first column refers to (control) test boards, which were cycled but not aged. This set contained a total of 24 boards with 8 containing ImSn finishes, 8 with ImAg,

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TABLE I T EST C OMPONENT M ATRIX Type

Body Size (mm)

Die Size (mm)

Ball/Lead Count

Pitch (mm)

Ball Alignment

BGA BGA BGA BGA CSP MLF

19 × 19 15 × 15 10 × 10 5×5 7×7 5×5

12.0 × 12.0 12.7 × 12.7 5.0 × 5.0 3.2 × 3.2 5.9 × 5.9 4.5 × 4.5

288 208 360 97 84 20

0.8 0.8 0.4 0.4 0.5 0.25

Perimeter Perimeter Perimeter Full array Perimeter

TABLE II

The Weibull plots show several interesting trends:

T HERMAL T ESTING M ATRIX

No Aging

Thermal cycle 40 °C∓ 125 °C

6/12 Months Aging

ImSn

ImAg

SnPb

ImSn

ImAg

SnPb

8

8

8

8 (55 °C)

5 (25 °C)

4 (25 °C)

8 (85 °C)

5 (55 °C)

4 (55 °C)

5 (85 °C)

4 (85 °C)

5 (125 °C)

4 (125 °C)

and 8 with SnPb finishes. The second column refers to boards, which were isothermally aged for 6 months (at the specified temperature) and then cycled over 40 °C–125 °C. All the ImSn vehicles were one side reflowed due to yield problems when employing multireflow processes. III. R ESULTS AND D ISCUSSION A. Accelerated Temperature Cycling Fig. 2 shows Weibull plots for the thermal cycling results on isothermally aged SnPb, ImAg, and ImSn 19 mm BGA samples both no aging and 12 months aging. A continuous degradation in reliability is observed for both SAC alloys on ImAg and ImSn at elevated aging temperatures. Contrastingly, the SnPb were still minimally affected by aging over 12 months over the temperature tested. In Fig. 2(b), the characteristic lifetime for SAC105 was reduced to 1123 cycles after aging at 125 °C for 12 months, and compared to 2559 cycles for no aging. Compared in Fig. 8, the characteristic lifetime was reduced to 1214 cycles after aging at 125 °C for 6 months. The degradation rate becomes slower from 6-month aging to 12-month aging. But the degradation is continuous. This is a 57% reduction in the characteristic lifetime. For 85 °C aging, there is a 50% reduction of lifetime, which means after 12-month aging, the 85 °C aging degradation is close to 125 °C aging effect. Fig. 2(c) shows the case for aged SAC305 on ImAg. After 125 °C for 12 months, there was a 50% reduction of characteristic lifetime, nearing to 57% with SAC105 on ImAg. For the same package size (19 mm) and aging conditions (125 °C/12 months), the characteristic lifetime of SAC alloys was much larger than Sn–37Pb solder. There is little change in reliability as a function of board finish.

1) higher Ag content improves the reliability of SAC alloys; 2) aging has a larger negative impact on reliability for finepitch packages; 3) aging has little impact on the characteristic lifetime for SnPb; 4) higher aging temperatures result in higher rates of degradation for the SAC alloy solders tested; 5) only small changes in reliability were observed versus board finish. Figs. 3 and 4 show the characteristic lifetime for 19 mm SAC105 (SAC305) on ImAg and ImSn compared to control Sn-37Pb. “As assembled” SAC alloy solders outperform Sn37Pb but show dramatic decreases in lifetime with higher isothermal annealing temperatures. After 6 and 12 months of aging at 85 °C, the characteristic lifetimes of SAC on both ImAg and ImSn are lower than Sn–63Pb. While “as assembled” Sn–37Pb has a slightly worse characteristic lifetime than SAC alloys, it is much more stable over time and temperature. Table III compare the characteristic lifetime versus annealing precondition for 15 mm SAC105 and SAC305 on ImAg and ImSn. All SAC solder alloys showed reduced characteristic cycle lifetimes with elevated annealing temperatures. The degradation rate becomes slower at 125 °C aging compared with 55 °C and 85 °C. There seems to have isothermal aging degradation boundary of lead-free solder joints. Weibull distributions for 0.4 pitch 10-mm BGAs with SAC105 and SAC305 on ImAg are studied. The relatively small volume of the solder balls was more sensitive to elevated temperatures at 55 °C and 85 °C than the cases reported above having larger solder balls. After 12 months of aging at 55 °C (85 °C), the cycle lifetime of SAC105 undergoes a ∼34% (∼41%) reduced lifetime compared with “no aging” specimens. Comparing with 6-month aging, the degradation is not stopped, but has a slower degradation rate. There is only 1% degradation from 6-month aging at 125 °C to 12 months 125 °C. The thermal reliability test results (Table III) show that both SAC105 and SAC305 have better thermal resistance than Sn-37Pb at the “as-assembled” stage. After elevated temperature aging, however, the thermal performance degrades significantly. Increasing the Ag content in SAC alloys offers some resistance for aging. For Sn–37Pb, the effect of elevated isothermal aging is insignificant. In MLF packages, isothermal aging caused a serious degradation in solder fatigue life, shown in Fig. 5. There is up to


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(a)

(c)

(b)

(d)

(e) Fig. 2. Weibull plots versus thermal cycle for isothermally aged 19-mm BGA samples. (a) SnPb on SnPb. (b) SAC105 on ImAg. (c) SAC305 on ImAg. (d) SAC105 on ImSn. (e) SAC305 on ImSn.

58% degradation for 5 mm MLF after 12 months 125 °C aging (Fig. 5). B. Failure Analysis and Microstructural Characterization The microstructure, mechanical response, and failure behavior of lead-free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments [8], [13], [15]–[17]. The observed material behavior variation during thermal

aging/cycling is universally detrimental to reliability and failures arise as solder joints relax thermal stresses through creep and fatigue mechanisms in response to thermal cycling. The different thermo-mechanical response of the various parts of the assembly to temperature cycles results in low-cycle fatigue and eventual failure. Creep dominates this strain accommodation (relaxation) in solder joints because of the high homologous temperature in operation (typically 0.48 < Th < 0.81) for the −40 °C to 125 °C temperature range [3].

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5000

4000

3000

SA AC105 2000

1000

0

(a)

No Aging

12 Months

6 Months

(a) 5000

S SAC305 4000

3000

(b) Fig. 4. Eutectic Pb–Sn Solder has the best aging resistance among all various combination of solder and board finish.

2000

1000

0

No Aging

6 Months

12 Month s

(b) Fig. 3. Weibull characteristic lifetime versus isothermal annealing conditions for 19 mm SAC105 BGAs and 19 mm SAC305 on ImAg.

In previous studies, we found that aging induced changes in the creep strain rates of SAC alloys are much larger than analogous changes observed for conventional eutectic Sn–37Pb solder [8], [17]. The measured creep rate evolution curves for Sn–37Pb solder samples at various aging temperatures. It is observed that the creep rates for Sn–37Pb are restricted to a narrow range of values between ε˙ = 2 × 10−6 and ε˙ = 1.5 × 10−5 , roughly one order of magnitude over the entire range of aging temperatures. In contrast, under the same conditions, Figs. 6 and 7 shows that SAC105 and SAC305 begin with creep rates lower than Sn–37Pb solder immediately after reflow (zero aging) but the creep rate changes dramatically with elevated temperature aging relative to Sn–37Pb. “Cross-over points” occur during the first few days of aging where SAC alloys begin to creep faster than Sn–37Pb. In these plots, the shaded regions represent the extent of the creep rate variations for Sn–37Pb (brown shading) and SAC alloys (blue shading). The fastest crossover point occurs for SAC105, but eventually the creep rate of SAC305 also begins to exceed Sn–37Pb. In general, the aging time required to obtain the creep rate cross-over depends in a complicated manner on the SAC alloy under

Fig. 5. Weibull plots versus thermal cycle for isothermally aged 7 × 7 MLF.

consideration, the reflow profiles utilized, the applied stress level, and the aging temperature. The measured creep strain rate variations indicate that the effects of aging are big issues for both SAC105 and SAC305 alloys. The creep strain rate increases were calculated by taking the ratios of the maximum creep strain rates to the corresponding creep strain rates for the non-aged materials. The changes for SAC105 was large at 9700× and SAC305 was at 361×, respectively (Table IV). The degradation in Weibull characteristic cycle lifetime is related to the microstructure evolution of solder joints. Fig. 8 shows backscattered electron images of the alloy microstructures before aging for SAC105, SAC305, compared to the

ZHANG et al.: THERMAL AGING EFFECTS ON THE THERMAL CYCLING RELIABILITY 10 -1

SAC105, RF T = 25 oC

10 -2

Cross-Over Points

SAC105 Creep Range

= 15 MPa

-1

Strain Rate (sec )

10 -3 10 -4

Tin-Lead Creep Range

10 -5 10 -6 10 -7 Aging Aging Aging Aging Aging

10 -8 10 -9

at at at at at

RT 50 oC 75 oC 100 oC o 125 C

10 -10 0

1

2

3

4

5

6

Aging Time (months)

Fig. 6.

Creep strain rate comparison for SAC105 and Sn–37Pb.

10-1 SAC305, RF T = 25 oC

10-2

=15 MPa

Strain Rate (sec-1)

10-3 10-4

Tin-Lead Creep Range

10-5 10-6 10-7

SAC305 Creep Range

Aging at RT Aging at 50 oC Aging at 75 oC Aging at 100 oC Aging at 125 oC

10-8 10-9 10-10 0

1

2

3

4

5

6

Aging Time (months)

Fig. 7.

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TABLE III (a) N O A GING C HARACTERISTIC L IFETIME η

β

19 × 19 BGA SnPb 2370 15 × 15 BGA SnPb 3413 10 × 10 BGA SnPb 7 × 7 CSP SnPb 4376 5 × 5 QFN SnPb 7258 Board finish: ImSn

5.151 3.238

19 × 19 15 × 15 5×5 19 × 19 15 × 15 5×5

5.602 4.714 5.732 4.365 4.109 4.337

Package

Alloy

Board finish: SnPb

BGA SAC105 2421 BGA SAC105 2257 BGA SAC105 4079 BGA SAC305 3644 BGA SAC305 3422 BGA SAC305 5055 Board finish: ImAg

19 × 19 BGA 15 × 15 BGA 10 × 10 BGA 5 × 5 BGA 19 × 19 BGA 15 × 15 BGA 10 × 10 BGA 5 × 5 BGA 7 × 7 CSP 5 × 5 QFN

SAC105 SAC105 SAC105 SAC105 SAC305 SAC305 SAC305 SAC305 SAC305 SAC305

2559 2926 2419 4841 4719 3743 3329 5171 7184 8054

4.565 4.798

7.827 4.686 3.515 4.798 3.245 3.516 4.364 5.143 8.439 4.45

Creep strain rate comparison for SAC305 and Sn–37Pb.

Ag3Sn 10µm

Ag3Sn 10µm

Ag3Sn

10µm

(a)

Ag A 3Sn

Ag3Sn S 10µm

10µm

Ag3Sn

10µm

(b) Fig. 8. Backscattered electron images of (a) SAC105 and (b) SAC305; view of the alloy before aging (a); view after 6 months/125 °C (b); view after 12 months/125 °C (b).

microstructural appearance after aging at 125 °C/6(12) months. The characteristic microstructure of SAC alloys contains a Sn matrix with imbedded Ag3 Sn second phase material. SAC alloys with greater Ag content have higher Ag3 Sn particle densities. After aging at elevated temperatures, it is clear that the Ag3 Sn particles undergo coarsening caused by solid state diffusion [3]. We surmise that, since diffusion processes are highly dependent on temperature, it is likely that the diminished reliability of SAC alloys under elevated temperature

annealing is connected to the lowered ability of larger sized coarsened particles of Ag3 Sn to block dislocation movement and reduce grain boundaries sliding. Contrastingly, eutectic Sn–37Pb solders lack Ag3 Sn particles. Quantitative analysis of IMCs in SAC105 and SAC305 is shown in Table V. Specifically, the coarsening of alloy grain and phase structure is governed by the self-diffusivity of atoms, interstitials, and vacancies, given by an Arrhenius relationship [18] Q (1) D = D0 exp − kT where D is diffusivity, D0 is a constant, k is the Boltzmann constant, Q is the activation energy, and T is the absolute temperature. It is interesting to note that IMC thickness for TC aging is higher that isothermal aging [1], shown in Fig. 9. And it may be due to higher stresses generated from TC aging condition. And a thick IMC layer can result in weaker solder joint interface strength. The most common failure mode during our tests was solder cracking at the corners of the BGA package. For the “no aged” 0.8 pitch 19 and 15 mm BGAs, the failures were located at the package side along the Cu6 Sn5 interface. The cracks initiated at the corner of the package and propagated along the IMC as shown in Fig. 9(a). Similarly like 6 months aging, after 12 months aging at 85 °C and 125 °C, cracks on the board side were also observed. The cracks began at the lower corner at the board side and propagated along the interface of Cu6 Sn5 . And from our experiment, there was no observed difference between room temperature aging and no aging failure mode.

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TABLE III (c) 12-M ONTH A GING C HARACTERISTIC L IFETIME

TABLE III (b) 6-M ONTH A GING C HARACTERISTIC L IFETIME Package

Alloy

19 × 19 BGA

SnPb

Aging (°C)

η

β

Lost %

Board finish: SnPb

15 × 15 BGA

5 × 5 BGA

Package 19 × 19 BGA

55 85

2078 2072

4.76 4.239

12.36 12.57

55 85

3066 2920

3.416 3.797

10.17 14.43

55 85

2934 2975

4.442 3.551

4.24 2.87

2295 2060 1757 1214 2489 2269 1656 1429 4137 3088 3016 2388 4243 3786 3260 2989 3328 2789 2241 2082 4581 3991 3699 3209

7.993 6.58 5.842 5.355 3.856 3.246 3.559 3.057 4.798 6.514 6.452 5.837 3.542 3.713 3.376 2.803 3.311 3.424 4.818 3.716 5.269 4.2 4.517 4.027

10.32 19.5 31.34 52.56 14.94 22.45 43.4 56.16 14.54 36.21 37.7 50.67 10.1 19.8 30.81 36.65 11.1 25.5 40.2 45.37 11.4 22.81 28.74 37.95

15 × 15 BGA

SnPb

SnPb

5 × 5 BGA

Board finish: ImAg 19 × 19 BGA

SAC105

15 × 15 BGA

SAC105

5 × 5 BGA

SAC105

19 × 19 BGA

15 × 15 BGA

5 × 5 BGA

SAC305

SAC305

SAC305

25 55 85 125 25 55 85 125 25 55 85 125 25 55 85 125 25 55 85 125 25 55 85 125

Board finish: ImSn 19 × 19 BGA

15 × 15 BGA

5 × 5 BGA

19 × 19 BGA

15 × 15 BGA

5 × 5 BGA

19 × 19 BGA

15 × 15 BGA

10 × 10 BGA

5 × 5 BGA

19 × 19 BGA

15 × 15 BGA

10 × 10 BGA

5 × 5 BGA

SAC105 55 85

2117 1765

4.551 3.097

12.56 27.1

55 85

1678 1382

3.368 5.556

25.65 38.77

55 85

2866 2433

4.36 4.676

29.74 40.35

55 85

3084 2697

4.303 2.761

15.37 25.99

55 85

2569 1997

3.019 2.449

24.93 41.64

SAC105

SAC105

19 × 19 BGA 15 × 15 BGA 5 × 5 BGA

SAC305

SAC305

SAC305 55 85

3650 3447

4.696 4.226

27.79 31.81

19 × 19 BGA 15 × 15 BGA 5 × 5 BGA

Alloy Aging (°C) η Board finish: SnPb SnPb 25 2069 55 1882 85 1752 125 1518 SnPb 25 3071 55 2764 85 2594 125 2358 SnPb 25 2887 55 2516 85 2264 125 2144 Board finish: ImAg SAC105 25 1976 55 1438 85 1277 125 1124 SAC105 25 2377 55 1906 85 1569 125 1325 SAC105 25 1954 55 1598 85 1439 125 1079 SAC105 25 3849 55 2964 85 2814 125 2269 SAC305 25 3906 55 2872 85 2688 125 2216 SAC305 25 3096 55 2246 85 2107 125 1884 SAC305 25 2938 55 2155 85 1874 125 1551 SAC305 25 4266 55 3709 85 3456 125 3012 Board finish: ImSn SAC105 55 1854 85 1524 SAC105 55 1484 85 1240 SAC105 55 2631 85 2414 SAC305 55 2774 85 2577 SAC305 55 2306 85 1856 SAC305 55 3451 85 3178

β

Lost %

3.857 4.05 3.953 4.733 3.376 3.444 3.737 3.269 4.584 4.527 4.167 3.966

12.7 20.59 26.08 35.95 10.02 19.02 24 30.91 5.75 17.86 26.09 30

4.885 7.278 6.876 5.877 4.014 3.941 4.241 3.981 4.053 4.284 4.662 4.079 4.258 4.997 4.875 4.599 4.086 4.149 3.348 3.53 3.83 3.742 3.927 3.701 4.243 3.617 3.666 3.729 6.1 5.571 5.909 3.835

22.78 43.8 50.1 56.08 18.76 34.86 46.38 54.72 19.22 33.94 40.51 55.39 20.49 38.77 41.87 53.13 17.23 39.14 43.04 53.04 17.29 39.99 43.71 49.67 11.75 35.27 43.71 53.41 17.5 28.27 33.17 41.75

4.894 4.562

25.39 38.67

3.73 4.487

34.25 45.06

4.379 4.093

35.5 40.82

4.5 3.597

23.87 29.28

3.363 3.314

32.61 45.76

4.325 4.345

31.73 37.13


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TABLE IV I NCREASING VALUES IN C REEP S TRAIN R ATE W ITH A GING (6 M ONTHS AT

100µm

100µ µm

125 °C)

Alloy

Strain Rate (Non-Aged)

Strain Rate (After 6 Months Aging at 125 °C)

Increase

SAC105 SAC305

10.0 × 10−8 3.6 × 10−8

9.7 × 10−4 0.130 × 10−4

9700× 361×

(a)

TABLE V

(b)

Q UANTITATIVE A NALYSIS OF IMC S IN SAC105 AND SAC305 Number of Particles

Averaged Particle Size (μm)

100µm

100µm

SAC105 As reflowed

271

2.5

25 °C (6/12 months) aging 125 °C (6/12 months) aging

265/231

2.6/2.8

128/118

4.1/4.2

SAC305 As reflowed 25 °C (6/12 months) aging 125 °C (6/12 months) aging

382 295 /271

1.7 2.4 /2.6

196 /174

3.7 /3.9

(a)

(b)

(c)

(d)

Fig. 9. Failure modes for 0.4-mm pitch BGA. (a) As reflowed and thermal cycling (no aging). (b) After 85 °C aging/12 months and thermal cycling. (c) After 125 °C aging/12 months and thermal cycling. (d) After 25 °C aging/12 months and thermal cycling.

The images in Fig. 10 illustrate crack propagation in the strain localized region of 0.7-mm pitch CSPs. Similar to BGAs, the “no aging” solder joints had cracks through the upper corners of the solder joint. After aging and cycling, some solder joints exhibited crack paths at an angle down through the solder bulk and, with higher aging temperatures, the angle became larger. This is difficult to explain but is

(c)

(d)

Fig. 10. Failure modes for 0.7 pitch CSP. (a) As reflowed and thermal cycling (no aging). (b) After 55 °C aging/12 months and thermal cycling. (c) After 85 °C aging/12 months and thermal cycling. (d) After 125 °C aging/12 months and thermal cycling.

(a)

(b)

(c)

(d)

Fig. 11. Failure regions at high magnification for MLFs. (a) As reflowed and thermal cycling (no aging). (b) After 25 °C aging/12 months and thermal cycling. (c) After 85 °C aging/12 months and thermal cycling. (d) After 125 °C aging/12 months and thermal cycling.

probably related to the degradation of mechanical properties of SAC solders during aging, which is caused by microstructural evolution [1]–[3] and dramatic coarsening. As shown in Fig. 11, there is a high density of small Ag3 Sn second phase

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particles, which can block the movement of dislocations and alter the grain boundary evolution. The solder microstructure appearance in the vicinity of crack regions is shown in Fig. 11. With increased aging time and temperature, the IMCs formation consumes Cu near the interface region and accelerates the coarsening rate. Cracks initiate and propagate easily in these regions. IV. C ONCLUSION The relationship between elevated temperature isothermal aging and the long-term thermal reliability of fine-pitch packages with Sn–1.0Ag–0.5Cu, Sn–3.0Ag–0.5Cu, and 63Sn–Pb solder ball interconnects was investigated. Significant cycle lifetime degradation was observed for both SAC105 and SAC305 in 19, 15, 10, and 5 mm PBGA, CSP, and QFN packages. Similar to 6-month aging, there were dramatic degradation after 12-month aging. It is interesting to note that compare with 7- to 12-month aging, big degradation happened in 1- to 6-month aging on 125 °C. Smaller solder balls were more sensitive to aging reductions in lifetime. Failure analysis showed significant bulk Ag3 Sn coarsening and intermetallic Cu6 Sn5 growth at the solder joint interfaces. After 85 °C and 125 °C aging, the cracks appeared at the lower corners at the board side interface and propagated along the Cu6 Sn5 . The reduced Weibull lifetimes occur coincidently with increasing Cu6 Sn5 layer growth at board and package sides of the solder joint. In related work in our laboratory, Cai et al. [16] has studied doped SAC–X alloys (X = 0.1% Bi) and finds similar joint degradation with aging for all aging temperatures (25 °C, 50 °C, 75 °C, 100 °C, and 125 °C) in the experimental matrix. We are currently exploring the manufacturability of new doped alloys and board level reliability in our ongoing work.

[9] T. C. Chiu, K. Zeng, R. Stierman, D. Edwards, and K. Ano, “Effect of thermal aging on board level drop reliability for Pb-free BGA packages,” in Proc. 54th Electron. Compon. Technol. Conf., 2004, pp. 1256–1262. [10] Y. Ding, C. Wang, M. Li, and H. S. Bang, “Aging effects on fracture behavior of 63Sn37Pb eutectic solder during tensile tests under the SEM,” Mater. Sci. Eng., A, vol. 384, no. 1, pp. 314–323, 2004. [11] J. C. Madeni, S. Liu, and T. Siewert, “Casting of lead-free solder bulk specimens with various solidification rates,” in Proc. ASM Int. Conf., Indianapolis, IN, USA, 2002, pp. 1–6. [12] Q. Zhang, A. Dasgupta, D. Nelson, and H. Pallavicini, “Systematic study on thermo-mechanical durability of Pb-free assemblies: Experiments and FE analysis,” J. Electron. Packag., vol. 127, no. 4, pp. 415–429, 2005. [13] J. H. L. Pang, T. H. Low, B. S. Xiong, L. Xu, and C. C. Neo, “Thermal cycling aging effects on Sn–Ag–Cu solder joint microstructure, IMC and strength,” Thin Solid Films, vols. 462–463, pp. 370–375, May 2004. [14] C. M. Chuang, T. S. Liu, and L. H. Chen, “Effect of aluminum addition on tensile properties of naturally aged Sn-9Zn eutectic solder,” J. Mater. Sci., vol. 37, no. 1, pp. 191–195, 2002. [15] Q. Xiao, L. Nguyen, and W. D. Armstrong, “Aging and creep behavior of Sn3.9Ag0.6Cu solder alloy,” in Proc. 54th Electron. Compon. Technol. Conf., 2004, pp. 1325–1332. [16] Z. Cai, Y. Zhang, J. C. Suhling, P. Lall, R. W. Johnson, and M. J. Bozack, “Reduction of lead free solder aging effects using doped SAC alloys,” in Proc. 60th Electron. Compon. Technol. Conf., 2010, pp. 1493–1511. [17] J. Zhang, Z. Hai, S. Thirugnanasambandam, J. L. Evans, M. J. Bozack, R. Sesek, Y. Zhang, and J. C. Suhling, “Correlation of aging effects on creep rate and reliability in lead free solder joints,” J. SMTA, vol. 25, no. 3, pp. 19–28, 2012. [18] S. Thirugnanasambandam, Z. Jiawei J. Evans, F. X. Fei Xie, M. Perry, B. Lewis, D. Baldwin, K. Stahn, and M. Roy, “Component level reliability on different dimensions of lead free wafer level chip scale packages subjected to extreme temperatures,” in Proc. 13th IEEE Intersoc. Conf. Thermal Thermomech. Phenomena Electron. Syst., 2013, pp. 612–618. [19] J. Zhang, “Isothermal aging effects on the thermal reliability performance of lead-free solder joints,” in Proc. 45th Int. Symp. Microelectron., 2012, pp. 801–808. [20] M. Motalab, Z. Cai, J. C. Suhling, Z. Jiawei J. L. Evans, M. J. Bozack, and P. Lall, “Improved predictions of lead free solder joint reliability that include aging effects,” in Proc. IEEE 62nd Electron. Compon. Technol. Conf., May–Jun. 2012, pp. 513–531. [21] P. A. Thornton and V. J. Colangelo, Fundamentals of Engineering Materials. Englewood Cliffs, NJ, USA: Prentice-Hall, 1985, pp. 227–229.

R EFERENCES [1] J. Zhang, S. Thirugnanasambandam, J. L. Evans, M. J. M. J. Bozack, and R. Sesek, “Impact of isothermal aging on the long-term reliability of fine-pitch ball grid array packages with different Sn-Ag-Cu solder joints,” IEEE Trans. Compon., Packag. Manuf. Technol., vol. 2, no. 8, pp. 1317–1328, Aug. 2012. [2] T.-K. Lee, H. Ma, K.-C. Liu, and J. Xue, “Impact of isothermal aging on long-term reliability of fine-pitch ball grid array packages with SnAg-Cu solder interconnects: Surface finish effects,” J. Electron. Mater., vol. 39, no. 12, pp. 2564–2573, 2010. [3] H. Ma and J. Suhling, “A review of mechanical properties of leadfree solders for electronic packaging,” J. Mater. Sci., vol. 44, no. 5, pp. 1141–1158, 2009. [4] J. Smetana, R. R. P. Coyle, R. Popowich, D. Fleming, and T. Sak, “Variations in thermal cycling response of Pb-free solder due to isothermal preconditioning,” in Proc. SMTAI Int. Conf., Fort Worth, TX, USA, Oct. 2011, pp. 641–654. [5] S. W. Lee, Y. K. Tsui, X. Huang, and C. C. Yan, “Effects of room temperature storage time on the shear strength of PBGA solder balls,” in Proc. ASME Int. Mech. Eng. Congr. Exposit., 2002, no. IMECE2002-39514, pp. 1–4. [6] Y. K. Tsui, S. W. Lee, and X. Huang, “Experimental investigation on the degradation of BGA solder ball shear strength due to room temperature aging,” in Proc. 4th Int. Symp. Electron. Mater. Packag., 2002, pp. 478–481. [7] J. H. L. Pang, B. S. Xiong, and T. H. Low, “Low cycle fatigue models for lead-free solders,” Thin Solid Films vols. 462–463, pp. 408–412, Sep. 2004. [8] Y. Zhang, Z. Cai, J. C. Suhling, P. Lall, and M. J. Bozack, “The effects of aging temperature on SAC solder joint material behavior and reliability,” in Proc. 58th IEEE Electron. Compon. Technol. Conf., Orlando, FL, USA, May 2008, pp. 99–112.

Jiawei Zhang received the B.S. degree in electrical engineering from the Beijing University of Aeronautics and Astronautics, Beijing, China, in 2006, and the M.S. degree in electrical engineering and the Ph.D. degree in industrial and systems engineering from Auburn University, Auburn, AL, USA, in 2009 and 2012, respectively. He is currently with Broadcom Corporation, Irvine, CA, USA, as a Staff II Packaging Engineer, working on packaging technology research and high speed signal packaging design. His current research interests include lead-free technology, fine pitch packaging, wafer level packaging, flip chip processes, and electronics reliability in harsh environments.

Zhou Hai received the B.S. degree in electronic engineering from Nanjing University, Nanjing, China, in 2009. He is currently pursuing the Ph.D. degree in industrial and system engineering from Auburn University, Auburn, AL, USA. He is currently a Research Assistant with the Center for Advanced Vehicle and Extreme Environment Electronics, Auburn University. His current research interests include lead-free solder alloy technology, SMT packaging manufacturing, and aging effect on the reliability of Lead-free solder joint.


Sivasubramanian Thirugnanasambandam received the B.Eng. degree in mechanical engineering from Anna University, Chennai, India, in 2009. He is currently pursuing the Doctoral degree in industrial and systems engineering with Auburn University, Auburn, AL, USA. He is currently a Research Assistant with Cave3, Auburn University. His main responsibilities include FE simulation of microelectronic packages, solder joint fatigue modeling, analytical, and experimental investigations of flip chip interconnect in microelectronic packages or assemblies. His current research interests include design-for-reliability in electronics packaging, nonlinear FEA, lead-free and lead bearing solder joint reliability, and aging mechanism of solder materials and structures. John L. Evans received the M.S. degree in electrical engineering from Auburn University, Auburn, AL, USA, and the Ph.D. degree in engineering management and manufacturing systems from the University of Alabama, Huntsville, AL, USA. He is currently a Thomas Walter Technology Management Professor of industrial and systems engineering and an Associate Director with the National Science Foundation’s Center for Advanced Vehicle Electronics, Auburn University. Before joining Auburn University in 2001, he spent 17 years at DaimlerChrysler Corporation, Huntsville. During his tenure at DaimlerChrysler, he worked as a Design Engineer, Lead Engineer, Financial Specialist, Electronics Packaging Supervisor, and Technology Manager. The last five years he served as a Manager of Strategic Business and Advanced Technology for DaimlerChrysler Huntsville Electronics. He also served as an Adjunct Assistant Professor with the Department of Industrial and Systems Engineering and as a Lecturer with the Administrative Science Department, University of Alabama, from 1991 to 2001. Michael J. Bozack received the B.S. and M.S. degrees in physics from Michigan State University, East Lansing, MI, USA, and the Ph.D. degree in surface physics from Oregon Health and Science University, Portland, OR, USA. He is currently a Professor of physics with Auburn University, Auburn, AL, USA, and the Director of the AU Surface Science Laboratory. He was the Chief Surface Scientist with Intel Corporation, Santa Clara, CA, USA. He has authored over 200 publications in areas ranging from semiconductor surface physics, epitaxial growth,gas- and metal-surface interactions, liquid metalion sources, semiconductor contacts, thin-film growth, wetting and adhesion, Pbfree solder alloy technology, and tin whiskering phenomena.

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Yifei Zhang received the Bachelors degree in materials science and engineering from the Beijing University of Aeronautics and Astronautics, Beijing, China, and the Masters degree in high temperature structural metallic materials from the Beijing Institute of Aeronautical Materials, Beijing, in 1998 and 2001, respectively, and the Ph.D. and M.B.A. degrees in mechanical engineering from Auburn University, Auburn, AL, USA, in 2010. He was with the National Key Laboratory of Advanced Structural Materials, Beijing, for three and a half years as a Project Manager in charge of several state-funded projects. From 2004 to 2010, he was with the Center for Advanced Vehicle Electronics, Auburn University, as a Research Assistant and specialized in developing new generation lead-free and mixed formulation soldering technologies as well as evaluating electronic package reliability under various design and environmental conditions. He was a Project Manager wotj Commercial Aircraft Corporation of China Ltd., in 2010, leading an engineering team dedicated to the localized production of materials and components for China’s first ever large passenger aircraft C919 Program. Then, he joined AVIC Commercial Aircraft Engine Co., Ltd. and started to take responsibility for the AVIC/CFM Joint Venture Project for Providing CFM LEAP-1C Engine to C919 Aircraft Program as well as other international cooperation affairs. He has authored about 20 technical papers in several journals and conference proceedings, especially under the subject of lead-free surface mount technology. Mr. Zhang was the recipient of the Best Paper Award of ECTC Conference 2010.

Jeffrey C. Suhling (M’07) received the Ph.D. degree in engineering mechanics from the University of Wisconsin, Madison, WI, USA. He joined the Department of Mechanical Engineering, Auburn University, Auburn, AL, USA, in 1985, where he is currently a Quina Distinguished Professor. At Auburn, he serves as the Director of the NSF Center for Advanced Vehicle Electronics, which is an Industry University Cooperative Research Center with 22 member companies. He has authored or coauthored over 225 technical publications, including four papers selected as the Best of Conference. He has advised over 50 graduate students at Auburn University. His current research interests include the application of analytical, numerical, and experimental methods of solid mechanics to problems in electronic packaging and paper mechanics. Dr. Suhling is a member of ASME, IMAPS, SEM, SMTA, and TAPPI.