High-speed-solder-ball-shear-and-pull-tests.pdf - 第3页

length. Figure 3 plots the m easured IMC thickness against time, yielding the expected linear fit when plotted against the square root of the aging tim e. The experimental data foll ows classical diffusion theory (Fic k’…

hot-air convective reflow oven. The Pb-free soldering profile

had a 150 ±2

C pre-heat, with a peak temperature of 260

Thermal aging to accelerate IMC growth was conducted at

125

C in an oven for several time durations (100, 300 and

500 hours). After thermal aging, some PBGA specimens with

solder balls were molded, cross-sectioned and etched

(2%HCl+98%methanol). They were inspected and analyzed

by scanning electron microscopy (SEM). The compositions of

IMC phases grown during reflow and thermal aging were

evaluated.

Similar BGA samples were assembled on test boards and

dropped using a dual-rail guided device. Since the packages

used in the present study were relatively large, the

specifications of the test boards (142x142 mm, 8 layer Cu,

2.35 mm thick) were different from those given in JESD22-

B111 [9]. Some board level test samples were also subjected

to thermal aging, as above. All samples were equipped with

daisy chains and subjected to real time data acquisition

monitoring.

The ball shear and pull tests were performed with different

speeds on the lead-free solder balls after thermal aging using

a DAGE 4000HS bond tester (Figure 1). Each set of ball

shear or pull test data consisted of a series of 20 individual

measurements.

(a) DAGE 4000HS Bond tester

(b) Ball shear system (c) Ball pull system

Figure 1. DAGE 4000HS Bond tester system

A summary of various sample and test parameters is listed

in Table 1. High-speed video capture (2,000–20,000 frames/s)

was conducted for selected solder ball shear/pull samples.

Various kinetic effects of the test hardware and sample

fixturing were observed. Test speeds above approximately

100 mm/s proved the most uniform during solder joint

fracture.

Table 1. Description of mechanical tests for solder balls

Test Method HS Shear Test HS Pull Test

Rates

10, 100, 500, 1000

and 3000 mm/s

5, 50, 100, 250, and

500 mm/s

Shear

Height

50 µm

－

Clamping

Force

－

2.2 bar

Solder

Composition

Sn4.0%Ag0.5%Cu Sn4.0%Ag0.5%Cu

Pad Finish ENIG and OSP ENIG and OSP

Sample

Status

As-reflowed

(two times reflow)

As-reflowed

(two times reflow)

(a) ENIG, 0 hour (b) OSP, 0 hour

Figure 2. IMC growth and morphology changes subject to

thermal aging at 125

C (ENIG and OSP)

Figure 3. Correlation between IMC thickness and aging time

3. Results and Discussion

3.1 Intermetallic Growth

Representative photos of the intermetallic structures and

thickness for BGA samples (unattached to a circuit board) are

shown in Figure 2. Given the highly non-uniform topography

of the IMC layer, the average thickness was determined by

dividing the cross-sectional area of the IMC by its base

1505 2007 Electronic Components and Technology Conference

length. Figure 3 plots the measured IMC thickness against

time, yielding the expected linear fit when plotted against the

square root of the aging time. The experimental data follows

classical diffusion theory (Fick’s law), which specifies a

linear relationship between the thickness of IMC layer and the

square root of time. Figure 3 also shows that the IMC growth

rates in solders on OSP surface finish are higher than those on

ENIG. This demonstrates that the Ni layer in ENIG serves as

a good barrier to inhibit growth of Cu-Sn IMC [8].

(a) Ductile mode (100% area with solder left)

(b) Quasi-ductile mode (<50% area with exposed pad)

(d) Brittle mode (almost no solder left)

(e) Pad lift with brittle

Figure 4. High-speed ball shear and pull failure modes

(a) 10 mm/s (ENIG) (b) 10 mm/s (OSP)

(e) 500 mm/s (ENIG) (f) 500 mm/s (OSP)

(g) 1000 mm/s (ENIG) (h) 1000 mm/s (OSP)

(i) 3000 mm/s (ENIG) (j) 3000 mm/s (OSP)

Figure 5. Failure mode distribution in ball shear of

specimens with ENIG and OSP pad finishes

3.2 Solder Ball Shear/Pull Failure Modes

In this investigation, the solder ball shear and pull test

samples were evaluated both quantitatively (force and fracture

energy) and qualitatively (failure mode). In order to enhance

discrimination of the qualitative failure mode assessments,

mixed ductile/brittle modes were divided into two

classifications: quasi-ductile (<50% of the surface pad

remains exposed) and quasi-brittle (>50% of the surface pad

remains exposed). Representative photographs of the 5 failure

modes (ductile, quasi-ductile, quasi-brittle, brittle and pad lift)

are shown in Figure 4.

Shear Direction

1506 2007 Electronic Components and Technology Conference

(a) 5 mm/s (ENIG) (b) 5 mm/s (OSP)

(e) 100 mm/s (ENIG) (f) 100 mm/s (OSP)

(g) 250 mm/s (ENIG) (h) 250 mm/s (OSP)

(i) 500 mm/s (ENIG) (j) 500 mm/s (OSP)

Figure 6. Failure mode distribution in ball pull of specimens

with ENIG and OSP pad finishes

Graphical summaries of the solder ball shear and pull

failure mode results are shown in Figures 5 and 6,

respectively. Similar to results of previous studies, a clear

transition occurred between the ductile mode typical of low

shear/pull speeds, and a brittle mode at higher test speeds [1,

2, 8]. Although the failure mode responses are remarkably

similar between the shear and pull testing, the transition from

ductile to brittle occurred at lower speeds in solder ball pull

testing.

Figures 5 and 6 also demonstrate that the samples with an

OSP finish showed an increased incidence of brittle failure

with increased themal exposure. By contrast, the samples with

ENIG plating yielded only a slight increase in brittle fracture

frequency even at the highest test speeds. The different brittle

fracture response between ENIG and OSP may not be

surprising given the comparative IMC growth rates shown in

Figure 3.

Figures 7 and 8 plot the failure mode data in another

format, showing the percentage of brittle failure mode for

each test speed against aging time, excluding the non-solder

related failure mode (pad lift). This transformation allows the

rate of brittle fracture incidence as a function of time and

aging temperature to be described by simple linear curve

fitting. The curve fitting follows the relationship, y = ax + b,

where y is the brittle failure percentage of ball shear and pull

test after aging, x equals aging time, and a represents the

change in brittle mode percentage with aging, and b is the

brittle failure percentage for the zero time sample. It should

be noted that the different slopes are a measure of the

sensitivity of the test system for detecting changes in brittle

fracture with time or test speed. Observe that the line with the

highest slope in each graph represents the optimal test speed

for each sample to detect changes in brittle fracture rate with

time.

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

Figure 7. Brittle failure percentage of ball shear tests at

various testing speeds and aging time

(a) Ball pull (ENIG) (b) Ball pull (OSP)

Figure 8. Brittle failure percentage of ball pull test at

various testing speeds and aging time

Figure 9 transforms the linear curve fitting of Figures 7 & 8

into simplified brittle fracture rate response curves versus

solder ball shear and pull test speeds. These graphs provide a

technique to quantitatively define the optimal test speed

(transition point) at which a solder joint fracture is most prone

to shift from ductile to brittle failure mode. Additionally,

Figure 9 can be used to define a recommended test speed for

evaluating brittle solder joint fractures.

1507 2007 Electronic Components and Technology Conference