Micro Impact Characterisation of Solder Joint for Drop ... - IEEE Xplore

Micro Impact Characterisation of Solder Joint for Drop Impact Application E.H. Wong1,2, Y-W Mai2, R.Rajoo1, K.T. Tsai3, F. Liu3, S.K.W. Seah1, C-L Yeh4 1

Institute of Microelectronics, [email protected] University of Sydney, Centre for Advanced Materials Technology 3 Instron Singapore Pte Ltd 4 Advanced Semiconductor Engineering Inc, Stress-Reliability Lab

2

The IC package and the board level interconnects in the portable electronic product are expected to experience a wide range of operating conditions. The effects of these conditions on board level solder joints are investigated in this paper.

Abstract Good correlation has been established between high speed shearing of solder joint at component level and board level drop tests, endorsing high speed shearing as a viable quality assurance test for manufacturing and incoming inspection. The high speed shear characteristics of solder joints under different test conditions (shear speed, shear angle, and temperature) and aging conditions (multiple reflow, temperature humidity, and salt spray) have been evaluated. Preliminary S-N characteristic for SnPb_OSP and SnAg_OSP solder joints have been generated using high speed cyclic bends test. These could be devolved into a life prediction model for board level solder joints in product drop impact.

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2.1 Instrument Instron Micro Impactor (Fig. 1) achieved shear speed from 0.2 m/s to 1 m/s using a patented flexure based drive system. A load transducer and Linear Variable Displacement Transducer (LVDT) attached to the striker provide the force and displacement. The direct attachment of the load cell to the striker minimizes noise from the machine response. The force, time and displacement history of the striker could be filtered digitally to remove any undesirable noise. Peak load, total shear deformation, total fracture energy, and fracture energyto-peak load are provided. Two cameras provide magnified views of the solder joint in 2 perpendicular directions for display on a monitor. This is useful for set-up of shear tool, as well as for optical inspection of the fractured surface.

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Introduction Compared to SnPb solder, ternary Pb-free solder alloys have been found to be particular susceptible to brittle fracture in the intermetallic compounds (IMC) [1]. This has hightened the need for more stringent quality control for manufacturing process, mainly the ball attachment process, and raw materials, such as pad finishing on the substrate and PCB. It is not economically viable to use board level drop tests (BLDT) such as described in JEDEC STD JESD22-B111 [2] for quality control. Rather, a low cost component level test is preferred. The basic requirement for a valid component level test is to be able to reproduce the failure mode observed in BLDT. However, the industry-practice component level shearing of solder joints at up to 1mm/s could only induce bulk failure in all solder joints, including those with Pb-free solders [3,4], especially in the case of unaged samples [5]. On the other hand, brittle IMC fracture has been successfully reproduced in the impact shearing of Pb-free solder joints at component level [3,4]. IMC failure has also been observed in pulling of solder joints at lower speeds [3, 5]; but clampinginduced damage, especially in the case of fine solder joints, remains a serious concern. High speed shearing of solder joints have been reported using the split Hopkinson bar technique (3 m/s) [1, 6], miniature Charpy tester (1 m/s) [7], motorised shear tester [3], and Micro-Impactor (0.6 m/s) [4]. The impact shear strength and impact toughness of a number of solder alloys and pad finishes have been investigated [4] and it has been found that while the impact shear strength of some Pb-free solder joints could be higher than that of eutectic SnPb solder joints, the impact toughness of all Pb-free solder joints investigated are inferior to that of eutectic SnPb solder joints. However, it was unclear what characteristics of impact shear, if any, could be correlated with the BLDT. This is to be investigated in this paper.

1-4244-0152-6/06/$20.00 ©2006 IEEE

Experiment

Striker

Specimen

Fig. 1 Instron Micro Impactor

2.2 Test Sample and Test Conditions The design of the test sample for BLDT is depicted in Fig. 2. The IC package was simulated with organic substrate; the PCB and IC package were fabricated from the same panel with identical pad designs and finishing. The four corner joints were designed to experience significantly higher magnitude of stress/strain than the inner joints, ensuring controlled failure at the four corner joints. The symmetrical design ensures identical loading on the four corner joints. Two adjacent corner joints were separated by adequate inner joints to ensure independency; i.e. the failure of one corner joint would not alter the loads bearing on the adjacent corner joints. Thus, the design allows collection of four independent data from a single test specimen. The four corner joints were wired individually using 4point measurement technique which allows monitoring of minute change in electrical resistance in each joint. The inner pads are connected into a daisy chain for monitoring of

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2006 Electronic Components and Technology Conference

characteristics: Peak load (FP); total energy (ET); Energy to peak load (EP); and ductility (ζ) which is defined here as the ratio of the total deformation to the diameter of solder joint on the pad (∅0.3 mm).

electrical connectivity, which should remain intact throughout the test. This is to assure that the corner joints remain well supported by the inner joints throughout the test. The number of drops to first electrical failure in individual corner joint was recorded. The same IC package design of Fig. 2 was used for impact shear test. This minimises experimental error due to design and process variation. The solder alloys, pad finishes, pad and solder mask design, and test conditions are described in Table 1.

∅4.5

7 6

e

Force (N)

5 4

h

3 2 1

5

0

IC Package

g

f

0

PCB

0.04

0.12

0.16

(b) Force

Thickness= 1 mm All dimensions in mm

Peak Load (FP)

Total Energy (ET) Energy to Peak Load (EP)

5x1

8

Total deformation 1 1 1 1

∅0.3

Fig 2 PCB and IC component dimensions and layout for BLDT

3.1 As Reflow (Pristine) Six solder_pad combinations each containing 6 data points were evaluated. The results are summarised in Table 2 together with ranking. Observations: • The contrasting difference in performance between the SP_ENIG joint and the SP_OSP joint hightens the importance of pad finishing. • NSMD design could be better for pads with ENIG finish. • There is general agreement between impact toughness and ductility but not with impact shear strength. • The SP_OSP joint has the best impact toughness and ductility while the SA_OSP joint has the best impact shear strength. Question: which property is a better indication of board level drop impact robustness?

Table 1 Description of experiment

Pad finishes Pad & Mask design

Test Conditions

Displacement

Fig 3 (a) Typical force-displacement from impact shear (b) terminologies

1.5

Design Solder alloys

0.2

Displacement (mm)

10 12

c

0.08

40

10

c SP_ENIG d SP_OSP e SA_OSP f SAC_ENIG g SAC_OSP h SP_NSMD d

Energy

230

(a)

Description Sn37Pb (SP) Sn3.5Ag (SA) Sn3.8Ag0.7Cu (SAC) Electroless Ni-P/Au (ENIG) Organic surface preservative (OSP)

Solder mask defined with φ500µm pad and φ300µm pad opening (SMD) Non-solder mask defined with φ300µm pad and φ500µm mask opening (NSMD) BLDT Half-sine acceleration on support: Amplitude 1000G and duration 0.5 ms Impact Shear Baseline: 0.4 m/s, 200C, 00 shear angle As reflowed (Pristine) Shear Speed (0.2 m/s, 0.4 m/s, 0.45 m/s) angle Temperature (200C,500C, 800C) Shear angle (00, 150, 300) Multiple-reflow (2,3,4) Moisture conditioning (25hr, 50hr, 75hr) Salt spray (240hrs)

Table 2 Summary of impact shear test on pristine specimen Alloy

Fp (N)

Rank

SP_ENIG SP_OSP SA_OSP SAC_ENIG SAC_OSP SP_NSMD

3.46 4.38 5.64 3.57 4.86 3.94

6 3 1 5 2 4

Ep Rank (mJ)

Et Rank (mJ)

0.09 0.27 0.18 0.04 0.10 0.19

0.32 0.70 0.63 0.10 0.37 0.43

5 1 3 6 4 2

5 1 2 6 4 3

ζ

Rank

0.50 0.70 0.66 0.18 0.49 0.57

4 1 2 6 5 3

Notes: (i) Solder joints with SMD pad design are not explicitly indicated; (ii) SP_NSMD implies SP_ENIG_NSMD

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Results & Analysis The typical force-displacement of the solder joints obtained from impact shear test are depicted in Fig. 3(a); Fig. 3(b) illustrates the terminologies that describes the

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3.2 Correlation with BLDT Four solder_pad combinations each containing 2 specimen of 8 data points were evaluated in BLDT. Failure in BLDT typically occurred on the PCB side of the solder joints as predicted by FE analysis [8] (recalled that the PCB and the IC package has identical pad dimension and finishing) . The average number of drops to failure (NDf) is summarised in Table 3. Table 3 also displays the normalised data for BLDT and impact shear test (extracted from Table 2). The normalised impact shear characteristics are plotted against normalised BLDT in Fig. 4. The gradients are displayed in Table 3 which provides indication of correlation. Observations: • The impact toughness and ductility from impact shear correlates well with BLDT reliability - High impact toughenss and ductility correlates to better BLDT reliability. EP displayed the strongest correlation, followed by ET and ζ. • There is virtually no correlation between impact shear strength and BLDT reliability. Fig. 5 compares the fractographs of SP_OSP, SA_OSP, and SAC_OSP joints from impact shear and BLDT. Note that impact shear test is capable of duplicating the ductile fracture mode in the SP_OSP joint and the brittle fracture mode in the lead-free solder joints. Detailed scrutiny revealed signatures of impact shear: (i) ductile failure exhibits shear mark on bulk solder; this contrast to ridge-line feature in drop impact; (ii) brittle failure tends to leave behind a small patch of remaining solder or collapsed solder mask at the exit edge of the shear tool; this is in contrasts to small patche of solder at times found on the fracture of BLDT sample that points to the site of initiation.

(a)

(b)

Solder

(c) Collapsed Solder mask

Fig. 5 Fractographs of (a) SP_OSP, (b) SA_OSP and (c) SAC_OSP subjected to impact shear and BLDT

3.3 Impact Shear Speed Five solder_pad combinations were evaluated at three speeds. The impact shear characteristics, averaged over 12 data points, are tabulated in Table 4. Table 5 gives the normalised impact shear characteristics averaged over all solder_pad combinations. Observations: • Impact shear strength increases while impact toughness and ductility decreases with increased shear speed, as expected [9]. Impact toughness and ductility exhibited much more sensitivity than shear strength. • Different solder joints appeared to exhibit different levels of sensitivity to shear speed. The impact toughness and ductility of the SP_ENIG joint increased significantly with reduced shear speed, but the SAC_ENIG joint remains brittle even at shear speed of 0.2 m/s. Fig. 6 compares the fractographs of the solder joints at two shear speeds. For SMD pad design, the size of the patch of remaining solder appeared to have increased with reduced shear speed for most solder joints. The fracture mode of the NSMD pad appeared to be tearing of laminate, and there is no distinguishable difference for the fracture mode at the two shear speeds.

Table 3 Summary and correlation of impact shearing with BLDT Alloy SAC_OSP SP_ENIG SA_OSP SP_OSP

NDf (No.)

NDf

FP

2.7

0.09

0.86

0.36

0.53

0.70

ζ

7.7

0.26

0.61

0.32

0.46

0.72

11.8

0.39

1.00

0.67

0.90

0.95

30

1.00

0.78

1.00

1.00

1.00

-0.02

0.75

0.56

0.33

Gradient

Normalised impact shear characteristics

Normalised EP ET

1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3

BLDT

Impact Shear

Table 4 Summary of impact shear data at 3 speeds Speed (m/s) Fp S Ep Et { ζ 0

0.2

0.4

0.6

0.8

SP_ENIG SA_OSP SAC_ENIG SAC_OSP SP_NSMD

1

Normalised BLDT characteristic Fig. 4 Impact shear – BLDT correlation

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FP (N)

EP (mJ)

ET (mJ)

ζ

0.2 0.4 0.45 0.2 0.4 0.45 0.2 0.4 0.45 0.2 0.4 0.45 2.6 4.1 3.0 3.9 3.1

2.8 4.5 3.0 4.3 3.5

3.0 4.8 2.7 4.3 3.5

0.1 0.2 0.1 0.1 0.2

0.1 0.2 0.0 0.1 0.2

0.1 0.1 0.0 0.1 0.1

0.3 0.5 0.1 0.3 0.4

0.2 0.5 0.1 0.2 0.4

0.1 0.3 0.1 0.2 0.4

1.6 1.7 0.3 1.3 1.5

0.5 0.7 0.1 0.3 0.6

0.3 0.3 0.1 0.2 0.5


• The impact toughness and ductility of the SP_ENIG joint increases significantly with increased temperature, but the SAC_ENIG joint remains brittle even at 800C. The fractographs of the solder joints at 800C are depicted in Fig. 7. Compares with Fig. 6, there is noticeable increased in the bulk solder region associated with higher temperature. Small patch of pad peel-off was observed for the SA_OSP joint specimen; and the cratering fracture mode of the NSMD pad is replaced with simple pad peel-off at higher temperature, suggesting rapid reduction in padlaminate adhesion with increased temperature.

Table 5 Normalised impact shear data at 3 shear speeds Shear Speed

FP

EP

ET

ζ

0.2/0.45 0.4/0.45 1

0.91 0.99 1.00

1.00 0.97 0.78

1.00 0.87 0.70

1.00 0.34 0.22

Gradient

0.16

-0.30

-0.47

-1.43

(a) Solder

Table 6 Summary of impact shear data at 3 temperatures

Solder

(b)

Solder

(c)

FP (N)

EP (mJ)

ET (mJ)

ζ

Temp (oC)

20

50

80

20

50

80

20

50

80

20

50

80


2.8 4.5 3.0 4.3 3.5

3.5 4.6 3.5 4.4 3.7

3.3 4.6 3.4 4.5 3.7

0.1 0.2 0.0 0.1 0.2

0.1 0.3 0.1 0.1 0.3

0.2 0.3 0.1 0.1 0.3

0.2 0.5 0.1 0.2 0.4

0.5 1.1 0.1 0.4 0.6

0.7 1.1 0.1 0.6 0.7

0.5 0.7 0.1 0.3 0.6

1.0 1.5 0.3 0.8 1.3

1.5 1.5 0.2 0.9 1.4

Table 7 Normalised impact shear data at 3 shear temperatures

(d)

Temperature

FP

EP

ET

ζ

20/80 50/80 1

0.91 1.00 0.99

0.57 0.90 1.00

0.39 0.82 1.00

0.39 0.93 1.00

Gradient

0.11

0.57

0.81

0.82

(b)

(a) Solder Solder

Solder

(e)

Pad peeled

(d)

(c)

0.2 m/s

Solder

0.45 m/s

Fig. 6 Fractographs of (a) SP_ENIG, (b) SA_OSP, (c) SAC_ENIG, (d) SAC_OSP, (e) SP_NSMD at 2 shear speeds (e)

3.4 Shearing Temperature Five solder_pad combinations were evaluated at three temperatures. The impact shear characteristics, averaged over 12 data points, and the normalised impact shear characteristics, averaged over all solder_pad combinations, are tabulated in Table 6 & 7, respectively. Observations: • Impact shear strength, impact toughness, and ductility all increases with increased shear temperature. Total fracture energy and ductility exhibited very high sensitivity towards shear temperature.

Fig. 7 Fractographs of (a) SP_ENIG, (b) SA_OSP, (c) SAC_ENIG, (d) SAC_OSP, (e) SP_NSMD at 800C

3.5 Impact Shear Angle Five solder_pad combinations were evaluated using shear tool of three shear angles at 200C, 0.4 m/s test conditions. The

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impact shear characteristics, averaged over 12 data points, and the normalised impact shear characteristics, averaged over all solder_pad combinations are tabulated in Table 8 & 9, respectively. Note that the angle is measured from the vertical plane; i.e. 00 gives a standard striker with vertical plane. Observations: Increasing the shear angle gave rise to a mild reduction in shear force and a significant increase in deformation (and impact toughness, which is attributable to the increased deformation). Increased shear angle is expected to increase the ratio of peal-to-shear action on the solder joints. This appeared to be the case for the SP_NSMD joints which transites from “tearing” to “scooping” (Fig. 8). However, the scenario appeared to be more complex for the case of the SA_OSP joint whose fracture mode transited from brittle failure at 00 shear angle to pad peeling at 150 shear angle and to shear failure in bulk solder at 300 shear angle. The unexpected shear failure at shear angle of 300 is believed to be attributed to the sharp shear tip of the shear tool and could be eliminated through the use of a shear tool with more rounded shear tip.

3.6 Multiple Reflows Five solder_pad combinations were evaluated after multiple reflows at 200C, 0.4 m/s test conditions. The impact shear characteristics, averaged over 6 data points, and the normalised impact shear characteristics average over all solder_pad combinations are tabulated in Table 10 & 11, respectively. Observations: • Multiple reflow gave mildly positive effects to the impact characteristics of the solder joints with total fracture energy showing the strongest sign. • There is no distinguishable difference in the fractographs of the solder joints subjected to different times of reflow. Table 10 Summary of impact shear data at 3 multiple reflows

Table 8 Summary of impact shear data at 3 shear angles FP (N)

EP (mJ)

ET (mJ)

FP (N)

EP (mJ)

ET (mJ)

ζ

Reflow No.

2

3

4

2

3

4

2

3

4

2

3

4


4.1 4.7 3.0 3.7 4.1

4.2 5.3 3.4 4.5 4.2

3.4 5.0 3.5 4.1 4.2

0.1 0.1 0.0 0.0 0.2

0.1 0.1 0.0 0.1 0.2

0.1 0.2 0.1 0.2 0.2

0.3 0.4 0.1 0.2 0.5

0.2 0.4 0.1 0.4 0.4

0.2 0.4 0.2 0.4 0.4

0.5 0.5 0.1 0.3 0.7

0.4 0.5 0.1 0.5 0.6

0.4 0.6 0.4 0.6 0.5

ζ

Angles (o)

0

15

30

0

15

30

0

15

30

0

15

30


2.8 4.5 3.0 4.3 3.5

2.8 4.8 3.5 4.7 4.8

3.3 3.4 2.7 3.8 3.9

0.1 0.2 0.0 0.1 0.2

0.1 0.4 0.1 0.3 0.3

0.2 0.2 0.1 0.3 0.4

0.2 0.5 0.1 0.2 0.4

0.3 0.8 0.2 0.5 0.5

0.5 0.5 0.2 0.5 0.7

0.5 0.7 0.1 0.3 0.6

0.6 1.1 0.4 0.8 0.6

0.9 0.9 0.5 0.7 1.2

Table 11 Normalised impact shear data at 4 multiple reflows Reflow No.

FP

EP

ET

ζ

1/4 2/4 3/4 1

0.83 0.91 1.00 0.93

0.85 0.69 0.65 1.00

0.76 0.81 0.88 1.00

0.85 0.83 0.87 1.00

Gradient

0.17

0.16

0.32

0.20

Table 9 Normalised impact shear data at 3 shear angles Shear Angle

FP

EP

ET

ζ

0/30 15/30 1

1.00 0.99 0.96

0.47 0.80 1.00

0.54 0.84 1.00

0.50 0.69 1.00

Gradient

-0.04

0.53

0.46

0.50

(a)

(b)

(c)

(d)

3.7 Moisture Conditioned SP_ENIG_NSMD pad on conventional halogenated and phosphorus based halogen-free boards were evaluated at 200C, 0.4 m/s after moisture conditioning at 859C/85%RH over 3 durations. The average and normalised impact shear characteristics, over 15 data points, are tabulated in Table 12 & 13, respectively. Observations: • The impact toughness and ductility of both halogenated and halogen-free boards decreases with increased moisture exposure. • Halogen-free board exhibited significantly higher sensitivity to moisture exposure, which could be attributed to its higher moisture absorption property [10]. The fracture mode for the halogenated and the halogenfree board are distinctly different as shown Fig. 9. However, there was no distinguishable difference in failure mode with increased moisture absorption for both boards.

Solder Pad peeled

Table 12 Summary of impact shear data after moisture conditioned 0

0

Fig. 8 Fractographs of SP_NSMD at (a) 15 shear (b) 30 shear, of SA_OSP at (c) 150 shear (d) 300 shear

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850C/85%RH (hrs) 25 50 75

FP (N)

Halogented EP ET (mJ) (mJ)

3.67 3.93 3.84

0.46 0.47 0.43

0.22 0.24 0.19

ζ 0.69 0.66 0.59

FP (N) 3.26 3.19 3.28

Halogen-free EP ET (mJ) (mJ) 0.25 0.21 0.18

0.12 0.08 0.09

ζ 0.38 0.34 0.29


Table 13 Normalised impact shear data after moisture conditioned Halogented

0

85 C/85%RH (hrs)

FP

EP

ET

25/75 50/75 1

0.96 1.02 1.00

0.99 1.00 0.92

0.92 1.00 0.78

Gradient

experienced by the PCB in the JEDEC test is very different from that in the drop-impact of a product such as mobile phone, even when the phone was in the horizontal orientation at impact [12], not to mention the PCB assembly in a portable electronic product is expected to experience drop-impacts from different heights and in different orientations in its lifetime. The difference in load spectrum between the JEDEC test and the portable product makes a quantitative correlation very challenging. The gap between a board level drop test and a product drop-impact might be bridged using a failure model based on physics of failure. As a start, the fatigue life of the solder joints could be conveniently described with an S-N model, where S may be either stress range or plastic strain range and N is the number of cycle before failure. BLDT, as described in JESD22-B111 does not render itself easily to the construction of an S-N model for the solder joints for the simple reason that the flexing amplitude could not be kept constant. The flexing of the PCB assembly with the resultant differential flexing between the PCB and the IC package has been established as the dominant failure driver for BLDT with inertia being negligible [8, 12, 13]. This implies that the fatigue characteristic of the solder joints could be evaluated simply by flexing the board at the frequency range experienced by PCB assembly in the drop-impact of a typical portable product. This could be accomplished with a high speed cyclic bend tester (HSCBT) as shown in Fig. 10. The tester is capable of delivering 0.003 fiber strain in the PCB at cyclic frequency from less than 1 Hz to 500 Hz, either continuously or for just a single flexing cycle. In another words, flexing spectrum of any desired profile could be generated.

Halogen-free ζ 1.00 0.96 0.85

FP

EP

ET

0.99 0.97 1.00

1.00 0.83 0.74

1.00 0.67 0.76

ζ 1.00 0.88 0.75

0.07 -0.10 -0.20 -0.22 0.01 -0.40 -0.35 -0.38

Halogenated

Halogen-free

Fig. 9 Fractographs of halogenated and halogen-free boards

3.8 Salt Spray Conditioned Five solder_pad combinations were evaluated at 200C, 0.4 m/s test conditions after 240hrs of salt spray exposure. The average impact shear characteristics, averaged over 12 data points, are tabulated in Table 14. Salt spray did not seem to have significant effects on the impact shear characteristics of solder joints. There is also no distinguishable difference in failure mode; in another words, no embirttlement of solder joints was observed after salt spray conditioned. Table 14 Summary of impact shear data after 240hrs salt spray

Duration (hr) SP_ENIG SA_ENIG SAC_ENIG SAC_OSP SP_NSMD

4

FP (N) 0 240

EP (mJ)

ET (mJ)

ζ

0

240

0

240

0

240

2.8

3.2

0.1

0.1

0.2

0.3

0.5

0.7

4.5

5.1

0.2

0.2

0.5

0.5

0.7

0.7

3.0

3.8

0.0

0.1

0.1

0.1

0.1

0.2

4.3

4.5

0.1

0.2

0.2

0.3

0.3

0.4

3.5

3.4

0.2

0.1

0.4

0.1

0.6

0.7

(a)

(b)

Discussions

4.1 Quality Assurance The impact toughness and ductility of the solder joints obtained from the component level impact shear test has been shown to correlate very well with the board level drop test (BLDT). Similar conclusion has also been reached [11]. Thus, impact shear has the potential to be used as a quality assurance test in the manufacturing processes such as solder ball-attachment for IC packages or surface finishing for organic laminates. It could also serve as a quality control test for IC assembly manufacturers in the incoming inspection of raw materials such as solder balls and organic substrate.

Cam with sinusoidal profile

Holder and anvils PCB assembly

(b)

Fig 10 High speed cyclic bend tester (a) Pictorial view – without specimen holder attachment; (b, c) pictorial view and front view of specimen and holder

4.2 Qualification & Life Prediction JEDEC board level test standard JESD22-B111 is not intended to be a qualification test as clearly stated in the standard. For the JEDEC test to be a qualification test requires a quantitative correlation between the JEDEC test and the product level drop-impact. Unfortunately, the load spectrum

Fig. 11 [14] demonstrates a typical fiber strain on PCB for two sinusoidal flexing cycles at 140 Hz using the board design of Fig. 2. It shows that flexing of PCB assembly has been

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that accumulative damage, and not overstress, is the mechanism for low cycle fatigue failure of solder joints. • The cyclic life of solder joints is highly sensitivity to the cyclic frequency. While the cyclic life of both the SP_OSP and the the SA_OSP joints decreases with increased cyclic frequency, they have exhibited different level of sensitivity towards cyclic frequency. This has two important implications: (i) The natural frequencies of the board in a board level test ought to match, as close as possible, the natural frequencies of the board in typical portable electronic products. (ii) The high frequency components in the flexural response ought to be treated seriously and be given more weightage. High frequency components are more pronounced in the initial timeresponse in the JEDEC STD drop test, but are especially pronounced in the presence of knocking.

effectively arrested after each cycle with negligible residual flexing.

PCB Fiber Stain (x10-3)

2 1.5 1

Time (ms)

0.5 0 -0.5 0

50

100

150

200

250

300

350

-1 -1.5 -2

Fig 11 PCB strain waveforms generated by HSCBT

The fractographs of solder joints from HSCBT and BLDT are presented in Fig. 12, which shows identical fracture modes.

Table 15 Elastic-plastic properties of solder Elastic modulus (GPa) 25

(a)

0.14

Plastic strain on solder joint

(b)

(c)

Strain 0.0012 0.01 0.2

Stress (MPa) 30 (yield stress) 37 42

ε=5N-0.7 -0.5 SP_OSP, 140 Hz, ε=0.8N y SA_OSP, 40 Hz, ε=0.2N-0.25 S SP_OSP, 40 Hz,

0.12 0.1

SA_OSP, 140 Hz, ε=0.15N-0.3

0.08 0.06 0.04 0.02 0

HSCBT

200

400

600

800

No of cycle to failure, N

BLDT

Fig 13 S (plastic strain) – N characteristic of solder joints

Fig. 12 Fractographs of (a) SP_OSP, (b) SA_OSP and (c) SAC_OSP subjected to HSCBT and BLDT

The S-N characteristics of the solder joints subjected to non-constant load amplitude and the effects of the sequence of amplitude loading would be investigated. With these, a comprehensive failure model could be constructed; and a quantitative relation between a board level drop test and a product level drop-impact could be established.

Using the board design of Fig. 2, The S (PCB fiber strain) – N characteristics of two solder alloys at two cyclic frequencies for PCB fiber strain range between 0.0018 and 0.003 have been established [14]. Using FEA and the solder properties defined in Table 15, the S (equivalent plastic strain) – N characteristics of the the solder joints have been established and is shown in Fig. 13. In order to circumvent the difficulty of plastic singularity, the solder joints have been modeled using beam element. The equivalent plastic strain were obtained for the corner solder joints at the end joining the PCB. The S-N characteristics of the solder joints provides interesting insights: • The S-N characteristic could be approximated by a Power Law relation. A threshold was not found down to the lowest experimental PCB strain at 0.0018. This suggests

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Conclusions The impact toughness and ductility, but not shear strength, obtained from component level high speed shearing of the solder joints has been found to correlate well with board level drop test, endorsing high speed shearing as a viable quality assurance test for manufacturing and incoming inspection. The high speed shear characteristics of the solder joints under different test conditions (shear speed, shear angle, and temperature) and aging conditions (multiple reflow, temperature humidity, and salt spray) have been evaluated and the following conclusions have been found:

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OSP finish consistently outperformed ENIG finish. The fractographs of OSP finish typically revealed a small region of bulk solder amix brittle fracture of IMC while ENIG finish typically gave 100% IMC fracture. • Impact shear strength increases while impact toughness and ductility decreases with increased shear speed. Fractographs revealed reduced region of bulk solder for OSP finish with increased shear speed. • Impact shear strength, impact toughness, and ductility increases with increased shear temperature. Fractographs revealed increased region of bulk solder for OSP finish with increased shear temperature. Increased pad-laminate delamination was observed with increased temperature. • Increasing the shear angle gave rise to a mild reduction in shear force and a significant increased in impact toughness and ductility. The expected increased in peeling fracture with increased shear angle was suppressed due to sharp shear tip. • Multiple reflow generally gave mildly positive effects with total fracture energy showing the most enhancements. • The impact toughness and ductility of both halogenated and halogen-free boards decreases with increased moisture exposure. Halogen-free board exhibited significantly higher sensitivity to moisture exposure. • Salt spray did not seem to have significant effects on the impact shear characteristics of the solder joints. Finally, high speed cyclic bend testing has been shown to be a valuable tool for generating the S-N characteristics of the solder joints and the following conclusions could be made: • Accumulative damage, and not overstress, is the mechanism for the failure of the board level solder joints in the event of multiple drop-impact. • The cyclic life of the solder joints decreases significantly with increased cyclic frequency.

References [1] D.M. Williamson et al., “Spall, Quasi-Static and High Strain Rate Shear Strength Data for Electronic Solder Materials”, Internal Report for IME-NUS-Cambridge Project, Cavendish Laboratory No. SP 1113, Oct 2002. [2] JEDEC Standard JESD22-B111, “Board Level Drop Test Method of Components for Handheld Electronic Products”. [3] K. Newman, “BGA Brittle Fracture – Alternative Solder Joint Integrity Test Methods”, Proc. 55th Electronic Components and Technology Conf, 1194-1201, 2005. [4] E.H. Wong, et al., “Drop impact: Fundamentals and Impact Characterisation of Solder Joints”, Proc. 55th Electronic Components and Technology Conf, pp. 12021209, 2005. [5] T.C. Chiu, et al., “Effects of Thermal Aging on Board Level Drop Reliability for Pb-Free BGA Packages”, Proc. 54th Electronic Components and Technology Conf, pp. 1256-1262, 2004. [6] Siviour, C. R., et al., "Dynamic Properties of Solders and Solder Joints," J. Phys. IV, France 110, pp. 477-482, 2003. [7] M. Date, et al., “Impact Reliability of Solder Joints”, Proc. 54th Electronic Components and Technology Conf, pp. 668-674, 2004. [8] E. H. Wong, et al., “Board Level Drop Impact Fundamental and Parametric Analysis,” ASME TransJournal of Electronic Packaging, Vol. 127, Issue 4, pp. 496-502, 2005 [9] M.A. Meyers, “Dynamic Behavior of Materials”, John Willey, 1994. [10] R. Rajoo, et al., “Trends and Challenges of Environmentally Friendly Laminates”, PC Fab, Cover story, pp. 26-31, Mar 2003. [11] C-L Yeh, et al., “Correlation between Package-level Ball Impact Test and Board-level Drop Test”, Microelectronics Reliability, In Press. [12] E.H. Wong, et al., “Drop Impact Reliability – A Comprehensive Summary”, Proc. HDP’05, Shanghai, pp 207-217, 2005. [13] Seah S.K.W., et al., “Understanding and Testing for Drop Impact Failure,” Proc. InterPACK ’05, IPACK2005-73047, pp 1-6, 2005. [14] S.K.W. Seah, et al., “Test Methodology and Failure Criteria for Drop Impact Reliability”, to be presented at 56th Electronic Components and Technology Conf.

Acknowledgments The authors would like to acknowledge Ahmad Abdillah of Ngee Ann Polytechnic for the fractographs, as well as YiShao Lai of ASE and Keith Newman of SunMicrosystem for the technical discussions.

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