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hot-air convective reflow oven. The Pb-free soldering profile had a 150 ±2 o C pre-heat, with a peak temperature of 260 o C. Thermal aging to accelerate IMC growth was conducted at 125 o C in an oven for several tim e du…

High-Speed Solder Ball Shear and Pull Tests vs. Board Level Mechanical Drop Tests:

Correlation of Failure Mode and Loading Speed

Fubin Song

, S. W. Ricky Lee

, Keith Newman

, Bob Sykes

, Stephen Clark

EPACK Lab, Center for Advanced Microsystems Packaging, Hong Kong University of Science & Technology

SUN Microsystems

DAGE Holdings Limited

Abstract

This study compares high-speed bondtesting (shear and

pull) with board level drop testing (BLDT) of BGA packages

using Sn4.0%Ag0.5%Cu solder balls and either an ENIG or

OSP package substrate surface finish. High-speed shear and

pull testing were carried out at various speeds; failure modes

were recorded, together with force and fracture energy data.

In addition, detailed microscopic analysis (SEM and EDX)

was executed on both complementary surfaces (ball and pad)

of brittle fracture failures from both shear and pull test

samples. The results of these studies showed close similarity

to those from brittle fractures generated during BLDT of the

same packages. Furthermore, there was strong correlation

between various bondtesting parameters at which brittle

fractures occurred and the number of drops to failure seen in

BLDT. In summary, it is suggested that brittle fractures

obtained in high-speed bondtesting are a strong indicator of

BLDT behavior.

1. Introduction

The reliability of lead-free solder joints under mechanical

shock loading is a major concern. Brittle fractures at the

interfaces between solder balls and package substrate bond

pads are considered unacceptable. In principle, this kind of

solder joint reliability should be characterized by board level

drop testing. However, such testing has some major

drawbacks. Firstly, each drop test will consume several

packages and hundreds of solder joints, incurring

considerable expense. Secondly, the crack in the solder joint

may close after the impact, resulting in an undetectable failure

unless there is a high-speed real time data acquisition system

available for in-situ monitoring. Thirdly, analysis of the data

is very time consuming, adding significant expense.

Therefore, there is an imperative to find alternative methods

for evaluating solder joint integrity under mechanical shock

loading [1-3].

The present study was performed to compare high-speed

solder ball shear and pull tests with BLDT. Emphasis has

been placed on the correlation of failure mode and energy

absorption between the two methods. The objective was to

investigate the feasibility of using high-speed solder ball

shear and pull tests as an alternative method of evaluating

solder joint integrity under dynamic loading [4-8]. During the

course of this study, a comprehensive testing program was

conducted, which included BGA package constructions

employing various combinations of solder alloys, surface

finishes, substrate material, solder ball size and package

dimensions.

Due to space constraints, however, this paper describes

results for a single 316 PBGA (27 mm x 27 mm)

construction, using Sn4.0%Ag0.5%Cu (SAC405) solder balls,

but fabricated with different surface finish options: electroless

nickel immersion gold (ENIG), and organic solderability

preservative (OSP).

The samples were divided into groups which were

subjected to thermal aging at 125

C (0 to 500 hours) in order

to accelerate the formation of intermetallic compound (IMC)

at the package substrate/solder-joint interface. The ball shear

tests ranged from 10 mm/s to 3000 mm/s and the ball pull

tests ranged from 5 mm/s to 500 mm/s. An advanced, state-of-

the-art machine, the DAGE 4000HS, was used to perform all

of the tests. This high-speed testing machine was equipped

with the most updated control and analysis software and a

new generation of force transducers, which are now able to

evaluate the fracture energy of solder balls in both ball shear

and ball pull tests.

In the current testing program, the peak shear/pull force

and energy absorption were evaluated for each test, and the

corresponding failure mode documented. The second part of

this study was BLDT. During each drop test, records of

electrical resistance, circuit board strain, and fixture

acceleration were recorded. Detailed analyses were performed

to identify the failed solder joints and corresponding failure

modes. The failure modes and loading speeds of solder ball

shear and pull tests were cross-referenced with the

mechanical drop tests for comparison. From the test results,

various correlations between failure mode and loading speed

have been observed. Also, the energy absorption value

recorded during solder ball shear and pull tests is considered

an effective index to interpret the solder joint failure mode.

Future publications will document more thoroughly the

analytical relationships observed between BLDT, high-speed

shear/pull and solder joint fracture energy. Unfortunately, the

wide scope of the study precludes full description within an

individual paper.

2. Experimental Procedures

Two types of substrate pad finishes were investigated in

this study: OSP and ENIG. The chemical composition of

lead-free solder alloy used in this work is SAC405. The

objective of the present study is to investigate the correlation

of board level drop test and solder ball shear/pull tests. The

316 PBGA samples used standard 0.76 mm (0.030 in.) dia.

spheres. The package substrates were composed of BT

laminate, with a thickness of 0.36 mm. The solder bond pads

were solder-mask-defined with an opening of 0.635 mm in

diameter. The solder balls were attached to the substrates in a

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