Vibration Fatigue of pBGA Solder Joint

P.L. Tu, Y.C. Chan, Senior Member IEEE, C.W. Tang, and J.K.L. La? Department of Electronic Engineering, and

‘Department of Physics & Materials Science, City University of Hong Kong

Abstract This paper studies the vibration fatigue failure of pBGA

solder-joints reflowed with different temperature profiles, and ageing at 120°C for 1, 4,9, 16,25, 36 days. The effect of the thickness of the Ni3Sn4 and Cu-Sn intermetallic compound (IMC) on the fatigue lifetime is also reported. During the vibration fatigue test, in order to identify the failure of pBGA solder joint, electrical interruption was monitored continuously through the daisy-chain network. Our results show that the fatigue lifetime of the solder joint firstly increases and then decreases with increasing heating factor (Q,,), which is defined as the integral of the measured temperature over the dwell time above liquidus (1 83°C) in the reflow profile. The greatest lifetime occurs when Q, is near 500 SOC. Moreover, the lifetime of the solder joint decreases linearly with the increasing fourth root of the ageing time. The SEMEDX inspection shows that only Ni3Sn4 IMC and Cu6Sn5/Cu3Sn IMCs are formed between the solder and the nickel-plated PCB pad, and the solder/component- metallization interface respectively. For non-aged samples reflowed with different profiles, the fatigue crack generally initiates at valleys in the rough surface of the interface of the Ni3Sn4 IMC with the bulk solder. Then it propagates mostly near the Nilsolder, and occasionally in the IMC layer or along the Nilsolder interface. For aged samples, the fatigue crack mostly initiates and propagates in the Cu6Sn5-phase/bulk- solder interface or the Cu3Sn/Cu6Sn5 interface on component- metallization. Evidently, the intermetallic compounds contribute mainly to the fatigue failure of pBGA solder joints. The thicker the IMC layer, the shorter the fatigue lifetime of solder joint. The initial formation of the IMCs at the interface during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick IMC layer influences the solder joint strength, which results in mechanical failure due to volume shrinkage during the transformation from solid phase to the intermetallic compound. Introduction

The micro ball grid array (pBGA) package has been successfully applied in many electronic products, i.e. computer processor, flash memory and mobile phone. Current issues include availability, cost, testability, assembly- inspection, rework methods, etc. Solder joint reliability is one of the most critical issues in the development of these technologies [l]. The pBGA structure includes a low stress die-attach elastomer between the silicon die and the solder bump array, which dissipates thermally induced stress caused by mismatches between the silicon and the substrate, allowing good solder joint reliability under thermal shock and temperature cycling [2]. However, in normal use, locomotor and portable products are always faced with various kinds of mechanical and environmental stresses, such as vibration and

bending of a PCB with pBGAs soldered to its surface [l]. Moreover, vibration load due to the operation of vehicle may also cause fatigue failure of solder joints too [3-41. Thus, it is essent‘ial to evaluate the ability of these devices to withstand vibration stresses. The aim of this paper is to discuss the vibration fatigue of pBGA solder joint reflowed in N2 with different temperature profiles, and aged at 120°C for 1, 4, 9, 16,25,36 days.

During the soldering process, the formation of intermetallic compound (IMC) between tin-based solder and substrate metallization is inevitable [2,5-8, 121. During our investigation, we noticed that Ni3Sn4 IMC and Cu6Sn5/Cu3Sn IMCs are formed at bulk-soldedplated-nickel and bulk soldedcopper metallization on component interface respectively. The growth of intermetallic compounds during reflowing and high temperature storage can strongly affect the subsequent solderability and the strength of solder joints, which result in mechanical failure of the joint [7]. It is important to investigate in order to gain some insight of the potential reliability issue that may result from the growth of this compound, because little is known about the IMCs’ effect on pBGA solder joint reliability under mechanical vibration. Therefore, this paper also focuses on the effect of growth/thickness of the Ni3Sn4 and Cu-Sn IMCs on mechanical fatigue. The temperature profile would be engineered to optimize soldering performance based on the observed relationship of the fatigue lifetime with Q,,, and in conjunction with our previous research works [2]. Experimental Procedure

Soldering The pBGA CSP46T.75-DC24 with SdPb-eutectic solder

ball was placed and soldered on FR-4 printed circuit boards (PCB) using CAS10 YCM-5500V and BTU VIP-70N oven in NZ. The PCB pads are of copper of 105pm thickness, plated with 15pm of nickel and a less than lpm gold flash. The pBGA packages comprised a dummy die with metallization forming a daisy chain in conjunction with the substrate metallization designed to permit monitoring of critical solder joint regions by electrical continuity. The assemblies were then reflowed with six different temperature profiles. The time-resolved temperature during reflow between the component and the PCB was measured using a wireless profiler (Super M.O.L.E, E31-900-45/10) and plotted in Fig. 1. The length of time spent and the temperature above the 183°C liquidus are important parameters for the formation of a good solder joint. In this study, the integral of the measured temperature T(t)“C in the liquidus temperature, w:th respect to time is used to approximate the term t x AT. This integral is given the name “heating factor” Q,, and is considered to be characteristic of the reflow profile [2]. The heating factor, peak temperature, and time above the liquidus corresponding to each reflowing profile are summarized in Table 1.

0-7803-5908-9/00/$10.00 02000 IEEE 1369 2000 Electronic Components and Technology Conference

Table 1. Reflow parameters corresponding to various profiles in Fig. 1

Tupl- 1 T~pl-2 Tup13

No. 186 17 33 0.091 0.773 191 36 205 0.136 0.833 197 31 307 0.159 0.894

Time above liquidus (Sec.)

Tupl-4 TUP]-5 Tupl-6

203 49 682 0.242 1.015 207 5.1 864 0.333 1.106 212 100 2004 0.606 1.56

Q,=682 32.7 50.3 65.5 78.5 92.1 105.2

1 18.25 129.1 143.3

180.6

Table 2. Vibration test results of samples reflowed with different Q,

Q,=864 1.1 3.8 9.3 17.1 28.4 45.2 70

101.9 148.8

45.2

Failure samples No.

1 2 3' 4 5 6 7 8 9 10

50% failure Total samples

number

Q,=205 0.75 1.8 5.1 10.5 19.3 30.1 47.4 70.2 100.5 141.5 100.5

18

Failure samples No.

1 2 3 4 5 6 7 8 9 10 11

Q,=307 16.1 30.5 43.2 60.1 72.3 90.5 109.9 123.3 146.5

146.5

18

50% failure Total samples

number

6.2 10.4 13.8

20.75 28.7 38.3 49.1 63.3 80.2 98.9 124.3

Vibration time of the samples cycled to failure (hours)

3.1 7.3 9.5 15.3 20.2 25.2 34.5 40.5 52.7 67.5 108.9 159.8

Q,=33 0.5 1.8 4.7 9.5

17.75 29.8 50.5 84.1 147.9

23.2

1.8 4.8 9.1 15.1 22.5

45.3 62.2 87.5 124.3

32.5

11

1.75 4.8 5.75 8.8 15.1

33.1 47.5 63.5 82.8 130

23.8

24 I 12

Table 3. Vibration test results of aged samples Vibration time of the samples c j

Aged-4 1 Aged-9 No aged 32.7 50.3 65.5 78.5 92.1 105.2

1 18.25 129.1 143.3

Aged- 1 11.75 20.25 26.6 39.5 49.3 64.2 84.1 100

135.9

24 I 16

63.3 1 40.5

16 1 16

Q,=2004 0.15 0.3 2

4.25 9.2 17.1 30.2 53.3 85.8 158.9 30.2

14

12 1 12 I

~

Aged-36 0.5 1.2 4.1 9.4

16.25 25.5 41.2 60.3 99.75

16.25

10

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The effect of IMC growth on fatigue failure is studied by ageing the solder joints. Samples reflowed with Q, = 682s"C were aged isothermally in oven at 120°C for 1, 2, 4, 6, 9, 12, 16,25, and 36 days. Then, they were reflowed again at 230°C for 20 s to remove grain coarsening in the solder joint. The mean thickness of the intermetallic layer was measured by uskg a powerful image processing system, OPTIMAS, together with a Nikon optical microscope. The micro- structural details of the IMC layer were observed by using a SEM (Philips XL40G).

250 -7 2 225

5 200

,g IS0

6 175 P

I

' 125

100

75

50

25

O l

Reflowtime (Sec.)

0 100 200 300 400 500

Fig. 1 The temperature profiles for reflowing pBGA assemblies

0 Reliability Test Two set of samples, one is prepared by reflowing in six

different profiles and the other by ageing at different time, were subject to vibration fatigue test. A vibration simulator system (King Design 9363) is used to test the vibration fatigue lifetime of the solder joints. PCB with pBGAs soldered to its surface was fixed to an electrodynamic shaker with four bolts positioned at each comer. A steel vibration stud of 56g weight was bonded to the top of every pBGA package. The shaker was performed with a sinusoidal excitation with an acceleration of RMS (root-mean-square) 9.lg, and a frequency 30 Hz, such that the peak to peak displacement of the pBGA PCB is about 5.05 mm. The vibration cycling continues until failure occurs. Thus, RSM of sinusoidal load applied on solder joints under one package is calculated by P = m (package self-weight + stud weight) x a = 5 (N).

During the test, a computer monitoring system equipped with AD/DA cards was used to monitor any electrical interruptions in the current through a "daisy chain" network. The interruption is caused by a complete through crack, which creates an open circuit greater than 25 microseconds. Testing was concluded when a significant number of failures had occurred. The number of cycles at solder joint failure is recorded, and is defined as the fatigue lifetime of the sample.

Finally, failed joints are cross-sectioned and analyzed using a Philips XL4OG Scanning electron microscope. Test results and discussion

0 Reflowing temperature profile The results of the vibration cycling test of the first set of

samples reflowed with six different temperature profiles are

listed in Table 2. The failure distribution of pBGA solder joints is plotted in Fig. 2 by data fitting with "Weibull'

I w 0.9 e

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

.-

1 0 15 30 45 60 75 90 105 120 135 150 Vibration cyclmg t k (hours)

I Fig. 2 Failure distribution of solder-joints for various

heating factor function 121. When Q, = 683, and 307 s°C, the distributed lifetime of the solder joints is the greatest amongst the six different reflow profiles, and hence the probability of failure during the early vibration cycling is very small. The lifetime distribution of solder joints can be characterized by the N W ~ , which is the vibration time to failure when 50% of samples have failed. The NSO% as a function of heating factor is shown in Fig. 3. The fatigue lifetime first increases and then

m

e 160 g 180

g 140 2 120 2 100

$ 6 0 .- Eo40

.a 80

Y

E 2 0 5 0 .o

0 250 500 750 lo00 1250~1500 1750 m m Heahng factor (degree sec.)

Fig. 3 Relationship between the statistical fatigue lifetime and the heating factor

decreases with increasing heating factor. The greatest lifetime occurs when Q, is near 500 SOC. The optimal range of heating factor should be between 200 and 700 SOC. Evidently, solder joints can't be formed successfully if the heating factor is less than 200 s"C because of the time requirement for fluxing reaction [9]. However, too large a heating factor results in thickening of the IMC layer, which increases the probability of the failure of the solder joint [l 11. The SEM and EDX inspection reveals that the Ni3Sn4 IMC and the Cu6Sn5/Cu3Sn IMCs are formed between the solder/nickel- plated PCB pad and solder/component-metallization interface respectively, as shown in Fig. 4 and 5. When Q, =500- 2004, the Ni3Sn4 layer and Cu-Sn IMCs layer are thickened linearly from 0.2pm to 0.6pm, and 0.955pm to 1.56pm respectively.

0 Ageing The vibration tests results of aged samples are summarized

in Table 3. The percentage fatigue failure against vibrating

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Fig. 4 SEM micrographs of solder joints showing the Ni-Sn IMC layers, reflowed under 4,,=33,682 and 2004s"C.

0 1 2 3 4 5 6 7 Ageing time(square/fourth root day)

Fig. 5 SEM micrographs of solder joints showing the Cu-Sn IMC layers, reflowed under Q,=33,682 and 2004s"C.

time is plotted in Fig. 6 by data fitting with 'Weibull' function [2]. As shown in the figure, the percentage failure curves of aged solder joints shift up very much, that is, the fatigue lifetime becomes shorter than that of the non-aged joints. The longer the ageing time, the larger the failure potentiality. The NSPh vibration fatigue lifetime is plotted against the ageing time as shown in Fig. 7. It can be seen that the fatigue lifetime of solder joints decreases quickly, then slowly with the increasing ageing time. It decreases linearly with the increasing fourth root of the ageing time.

1 0.9 0.8

E 8 0.7 0.6

$ 0.5 0.4 0.3 0.2 0. I 0

0 15 30 45 60 75 90 105 1u) 135 150

Mt" cyclins tim(h0Urs)

Fig. 6 Failure distribution of solder joints aged at 120°C for 0-36days.

I .̂ - 5 200 Q 175 .s 150 3 125

100

.E * 50

.3 z 75

*NSO% vs square

+ N50% vs fourth

lifetime and the ageing time __-____

2 , 10 - 1.8 5 1.6 2 1.4

0 1 5 0.8 2 0.6 6 0.4

1

g 1.2 Y

c

g 0.2

Fig. 8 IMC thickness as a function of ageing time. The sample is reflowed with Q,,=682s°C and aged at 120°C

The lifetime variation can be attributed to the intermetallic compounds (IMCs) growth. The IMCs thickening influences both the lifetimes of solder joints whether during reflowing or storing (ageing) [12]. During aging, Ni3Sn4 and Cu-Sn IMCs are thickened linearly with the increasing root of ageing time, as shown in Fig. 8. Since the re-reflowing after ageing removes grain coarsening in the solder joint, so the effect of the ageing time on solder lifetime is mainly created by the IMCs thickening. Therefore, we can describe that the fatigue

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lifetime of solder joints decreases linearly with the increasing square root of the IMCs thickness.

0 Failure analysis During the vibration test, fractures mostly failed in the

outermost solder ball (farthest from the neutral point of the package), same for twisting and thermal shock tests [13]. The fiactograph of the solder joint presents typical fatigue mode.

Fig. 9 Fractograph of break solder joint showing fatigue striation

Fig. 10 Cross-section of the failed solder joints showing crack initiation and propagation, (A) at PCB side, (B) in Ni3Sn4 layer/ at PCB side, (C) at PCB side and component side, (D) at Cu6Sn5 surface near bulk soldedthe component side, (E) between Cu6Sn5 and Cu3Sn IMC /at component side.

The typical fatigue striation resulted by crack propagating is shown in Fig. 9, which the sample has undergone 129 hours vibration cycling. The figure reveals that the rate of crack propagation is approximately 0.5pm per cycle.

The failed samples were cross-sectioned and examined by scanning electron microscope to look for signs of crack initiation and propagation, as shown in Fig. 10 (A)-(E). The IMC growth not only influences the fatigue lifetime of solder joint, and also dominates where the failure crack initiates. For the non-aged solder joints, the fatigue failure occurs mostly at the inside edge of the joints at the PCB side, shown as Fig. 10(A). The crack initiates at valleys in the rough surface of the interface of the Ni3Sn4 with the bulk-solder, then thrills through IMC layer toward IMCMi interface [ 141. And then it propagates near the Ni/solder, and occasionally in the Ni3Sn4 IMC layer or along the Ni/IMC interface, see Fig. 10(B). Along with IMCs growth, the site that the crack occurs tends towards the Cu-Sn IMC (Cu-IMC) layer of the component side from the Ni-Sn IMC (Ni-IMC) layer of the PCB side. When reflowing with Q, = 2004 s°C, and 1 day ageing after reflowed with 682 s°C, a little of fatigue failure occurs at inside edge of the joints at the component side, as shown in Fig. lO(C). The fatigue crack can either occur at Ni-IMC layer, or the Cu-IMC layer. Both of the Cu-IMC and Ni-IMC are responsible for the failure of solder joint [15]. Furthermore, for the solder joint aged more than 1 day, the fracture is caused mainly by the Cu-IMC on component metallization (copper pad). The fatigue crack initiates and propagates on the surface of the interface between Cu-IMC and bulk-solder, as shown in Fig. 10(D), or it initiates and propagates between Cu6Sn5 and Cu3Sn IMC layer, as shown in Fig. 10(E). The effect of the Cu-IMC on fatigue failure of

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solder joints is much greater than that of the Ni-IMC after ageing.

0 Discussion The IMC effect on solder joint failure can been well

interpreted by its brittleness and volume shrinkage. Thick IMC layer poses potential reliability issues due to a 10.7%, 8.5% and 5% volume shrinkage during the transformation from solid phase to the intermetallic compound Ni3Sn4, Cu3Sn, and Cu&5. The internal strain, pile-up of dislocation, and intercrystalline defects are created and increase gradually in severity at the IMCs interface with increasing IMC thickness. The site becomes the cradle of the crack initiation. In addition, the brittle IMC layer possesses low dynamic ductility. In mechanical fatigue, strain accumulation around the IMC can wake the crack initiating and propagating.

Fig. 11 SEM micrographs showing IMC growth by ageing, (A) Ni3S%/Q,,=682s"C, (B) Ni3Sn4/ Ageing 36 days, (C) Cu-Sn IMC/Q,=682s°C, and (D) Cu-Sn IMC/Ageing 36 days

-

There exists a difference of the influence level of Ni-IMC and Cu-IMC on the crack initiation and propagation. When the Ni3Sn4 thickness SNi<0.455-0.606pm, the corresponding Cu-JMC thickness 6c,<l.56-1 .99pm, the Ni3Sh is responsible for fatigue failure of solder joints. But when 6cU>l.56-1.99pm or &,>0.455-0.606pm, Cu-IMC is main contributor to solder failure, see Table 1, Fig. 8 and Fig. 10. The reason can be comprehended as follows. Under normal reflowing (Q,,<IOOOs"C), CdSn IMC layer possesses smooth surface, which the Ni3Sn4 layer is peaky and spiky, and its crystal-whisker comes into solder along Sn/Pb interface, see Fig. 1 1(A) and (C). At Ni-IMC valley or aiguille, vibration fatigue strength is weaker than that in solder and at Cu-IMC due to strain accumulation by vibration, which result in failure occurs mainly in that region [14]. When IMCs grow during reflowing or ageing, the surface shapes of Ni-IMC and Cu- IMC become similar, as show in Fig. 1 1(B) and (D). As a result, the volume shrinkage during the formation of IMCs becomes a main factor of the crack initiation and propagation, especially the IMCs growth during ageing. Although the rate of the volume shrinkage of the Ni-IMC (10.7%) is larger than that of the Cu-IMC (5-8.5%), the thickness of the Cu-IMC increases faster than the Ni-Sn IMC. With 36days ageing, the Cu-IMC thickness increases by 6.36pm, but the Ni-IMC thickness only by 1.5pm. This volume-shrinkage would create a large internal strain, so the fatigue crack initiates and propagates in Cu6Sn5 interface. Conclusions

The optimal heating factor for reflowing the pBGA assemblies in Nz should be between 200 and 700 SOC. If the heating factor is less than 200 s°C, solder joints can't be formed successfully because of time requirement for fluxing reaction. After the optimal Q,,, the fatigue-lifetime of solder- joint decreases with the increasing heating factor, due to growth of intermetaIlic compound (IMC).

The fatigue lifetime of solder joints decreases quickly, then slowly with the increasing ageing time. It decreases linearly with the increasing fourth root of the ageing time (i.e., the square root of IMCs thickness).

For non-aged solder joints, most crack initiates at valleys in the rough surface of the interface between the Ni3Sn, and the bulk solder at the PCB side. Then it propagates mostly near the Nilsolder, and occasionally in the IMC layer or along the NVIMC interface.

For aged solder joints, fracture mainly occurs at surface of the Cu-IMC at component side. The fatigue crack initiates and propagates on the surface of the interface between Cu- IMC and bulk-solder, or the interface between Cu6SnS and Cu3Sn IMC layers.

The mean thickness of the IMC layer linearly increases with the increasing heating factor and also with the increasing square root of ageing time. Under normal reflow, the Ni3Sn4 layer is peaky and spiky, unlike the smooth Cu-IMC. Moreover, the rate of volume shrinkage of the Ni3Sn4 is larger than that of the Cu-IMC. Therefore, Ni-IMC is responsible for the solder joint failures. AAer ageing or long term storage, Cu-IMC is the main contributor of solder joint failure due to a thicker Cu-IMC layer with larger volume shrinkage.

i

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