High-speed-solder-ball-shear-and-pull-tests.pdf - 第9页

Cu 6 Sn 5 IMC and Cu layers. The brittle failu re of the OSP specimens after thermal ag i ng occurred between the Cu 6 Sn 5 and Cu 3 Sn IMC phases. From the com parison of Figures 15 t o 17 (cross-section) and 18 to 20 (…

package substrate surface finish showed a more rapid

degradation with thermal aging than those with an ENIG

finish [12-16].

(a) Brittle fracture surface of

solder joint

(b) Brittle fracture surface of

the matching pad of (a)

indicated by rectangle in (a)

(d) Close-up view at location

indicated by rectangle in (b)

Figure 18. Brittle fracture surface after drop test

(SAC405 + OSP, 500 hours aging)

(a) Brittle fracture surface of

sheared ball

(b) Brittle fracture surface of

the matching pad of (a)

indicated by rectangle in (a)

(d) Close-up view at location

indicated by rectangle in (b)

Figure 19. Brittle fracture surface after high-speed ball

shear test (SAC405 + OSP, 500 hours aging, 500 mm/s)

3.5 Correlations between BLDT and High Speed Ball

Shear/Pull Tests

Previous evaluations of high-speed solder ball shear and

pull testing have observed brittle fractures that appeared

similar to the brittle fracture mode observed in BLDT

assemblies, but little definitive cross-sectional evidence has

been provided. Partly this has been due to the difficulty of

such studies, both in terms of retrieval of individual sheared

or pulled balls and matching them to their corresponding pad,

and the subsequent cross-sectional work.

(a) Brittle fracture surface of

pulled ball

(b) Brittle fracture surface of

the matching pad of (a)

indicated by rectangle in (a)

(d) Close-up view at location

indicated by rectangle in (b)

Figure 20. Brittle fracture surface after high-speed ball pull

test (SAC405 + OSP, 500 hours aging, 50 mm/s)

(a) Ball shear (ENIG) (b) Ball shear (OSP)

Figure 21. Correlation of drops-to-failure and brittle failure

percentage of ball shear and pull tests at different test

speeds.

Notes: (i) All failures in drop test are brittle fractures.

(ii) The curves disappear if the brittle failure percentage

(ball shear and pull) at different aging time rises to 100%,

e.g. 3000 mm/s for ball shear and 500 mm/s for ball pull.

In this paper, painstaking effort has resulted in the

definitive images shown in Figures 15 to 20. From the failure

analysis of drop test specimens as shown in Figures 15 and

16, the brittle failure on the ENIG was induced between the

IMC and the Ni layers. For the OSP specimens without aging

(with two times reflow), the brittle failure was found between

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IMC and Cu layers. The brittle failure of the OSP

specimens after thermal aging occurred between the Cu

and Cu

Sn IMC phases.

From the comparison of Figures 15 to 17 (cross-section)

and 18 to 20 (fracture surface), it is clear that brittle fracture

interfaces from drop testing show a striking similarity with

those from high-speed ball shear and pull tests. These figures

provide clear evidence of a close resemblance between the

brittle fracture modes of BLDT and high-speed solder ball

shear and pull.

(a) Ball shear (ENIG) (b) Ball shear (OSP)

Figure 22. Correlation of drops-to-failure and normalized

ball shear and pull force with the optimal testing speed

(a) Ball shear (ENIG) (b) Ball shear (OSP)

Figure 23. Correlation of drops-to-failure and normalized

ball shear and pull energy with the optimal testing speed

Due to their complexity and requisite length of explanation,

it is not possible in this brief paper to elaborate on the various

mathematical correlations relating the solder ball shear/pull

and drop test results. Nonetheless, an innovative approach is

graphically summarized in Figures 21-23. Figure 21 relates

the brittle fracture percentages from shear and pull solder ball

testing to the drops-to-failure for the specific packages and

drop test conditions used in this study. Briefly, this plot is

achieved by plotting the drops-to-failure number for each

time point against the equivalent data from shear or pull

testing, followed by power law curve fitting. It should be

noted that each curve in Figure 21 corresponds to one solder

ball shear or pull test speed. These curves can be employed to

estimate the “drops-to-failure” number from the brittle

fracture percentage obtained in either the ball shear or pull

test with a specific test speed. For demonstration purpose, the

5 mm/s ball pull test curve in Figure 21(d) is used as an

example. If 60% of brittle fracture percentage is observed

during this kind of test, then people can draw a vertical line at

60% on the horizontal axis and going upwards. When the

vertical line hits the designated curve, people can turn left and

find its corresponding vertical coordinate (or just plug 60%

into the fitting function), which is 39. This value is the

estimated “drops-to-failure” number for the same type of

specimens subject to the mechanical drop test with the

conditions used in the present study. In other words, the

curves in Figure 21 may provide people with certain

“prediction” capability to estimate the drop test results from

the high-speed ball shear or pull test data.

Figure 22 details the exponential relationship between

normalized solder ball shear/pull force and drops-to-failure

for the brittle failure mode data points only. The shear and

pull data come from the optimal speeds for each time point.

Figure 23 shows similar graphical summaries for the solder

ball shear/pull energy. These graphs highlight that a moderate

shift in brittle fracture rate, fracture energy or force for high-

speed solder ball shear/pull testing can have a significant

impact on the predicted drop testing lifetime. It is suggested

that a manufacturer might establish solder ball shear/pull

failure mode, fracture energy or force acceptance criteria for a

particular product based upon a similar analysis.

Future papers will further evaluate the various relationships

between drops-to-failure, failure mode, force and fracture

energy, drawing upon the much larger database generated in

the full study (only a subset of the results are included in the

Figures 21-23 correlations due to limitations on paper length).

4. Conclusions

1) Solder ball shear and ball pull tests produced a high

incidence of brittle fracture with increasing test speed,

independent of pad finish or aging time.

2) The IMC on packages with an OSP pad grew faster than

on devices with ENIG plating, and samples with OSP

pads generated more brittle solder joint failures in ball

shear/pull tests after thermal aging.

3) Solder ball pull testing (compared with shear) generated a

higher percentage of brittle failures at all test speeds.

4) Compared to the specimens with ENIG pad finish, the

ball shear/pull strength and fracture energy of specimens

with OSP decreased more rapidly with aging time. This

phenomenon most likely relates to the thicker Cu-Sn

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phase IMC formed in OSP specimens compared to those

with ENIG.

5) Thermal aging showed a significant effect on board level

drop reliability in this study for both ENIG and OSP

package substrate pad finishes. SAC + ENIG exhibits

longer drop impact life after thermal aging than that of

SAC + OSP. From SEM inspection, all drop test failures

were brittle fracture at an IMC interface, regardless of

pad finish or aging time.

6) From the failure analysis of drop test specimens, the

brittle failure on the ENIG was induced between the IMC

and the Ni layers. For the OSP specimens without aging

(with two times reflow), the brittle failure was found

between Cu

IMC and Cu layers. The brittle failure of

the OSP specimens after thermal aging occurred between

the Cu

and Cu

Sn IMC phases.

7) Detailed comparison of brittle fracture interfaces from

drop testing with those from high-speed shear ball shear

and pull tests showed a striking similarity. Therefore,

based on these microstructural observations, high-speed

ball shear and pull tests can be used as an indicator of

board level drop test performance.

8) Power law relationships can be obtained at different

shear/pull testing speeds using the combined results of

drops-to-failure and brittle failure percentages of ball

shear/pull tests with thermal aging time. Using this

correlation, drop impact lifetime can be estimated

according to the brittle failure percentages obtained in the

high-speed ball shear and pull tests.

References

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