Nanoindentation Characterization of Sn-Ag-Sb/Cu

Substrate IMC Layer Subject to Thermal Aging

Wedianti Shualdi1,2

, Badariah Bais2, Ibrahim Ahmad

3, Ghazali Omar

4, Aishah Isnin

1

1AMREC, SIRIM Berhad, Lot 34, Jln Hi-Tech 2/3, Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysia

2Electrical, Electronic & Systems Dept., Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia

(UKM), 436000 Bangi, Selangor, Malaysia 3Electronics & Communication Dept., College of Engineering, Universiti Tenaga Nasional (UNITEN),

43009 Kajang, Selangor, Malaysia4Infineon Technologies (Kulim) Sdn Bhd Lot 10 & 11, Jln Hi-tech 7, Industrial Zone Phase II, Kulim Hi-Tech Park,

09000 Kulim, Kedah, Malaysia

*E-mail: wedianti@sirim.my

Abstract-Intermetallic compound (IMC) plays great roles in

connecting components to PCB boards, as well as die attach

materials connecting chips to substrates. Cracks in IMC may

leads to failure in an electronic product function. Therefore it is

important to investigate the mechanical properties of the IMC to

ensure the reliability of the solder joints. In this paper, a

nanoindentation test was performed at IMCs that grow on the

interface between Sn-Ag-Sb lead-free solder alloy and its Cu

substrate. The test was done from planar IMC surface. Prior of

that, the specimens were subjected to thermal aging process until

1500 hours at 175o

C to accelerate the growth of IMC. The

nanotest was executed on specimens with completed aging time

for 100, 200, 400, 800 and 1500 hours. Nanoindentation results in

this paper show the hardness and Young modulus of IMC

composition as a whole, without interpretation of hardness

properties of Cu3Sn and Cu6Sn5 individually. The hardness of

Sn-Ag-Sb/Cu Substrate IMC Layer is decreasing from 5.583 GPa

to4.444 GPa while the Young modulus is decreasing from 106.475

GPa to 128.439 GPa.

Keywords: Intermetallic compound, nanoindentation, Young

modulus, SnAgSb lead free

I. INTRODUCTION

IMC is an important layer needed to create bonding

between substrate and chips in die attach assembly. However,

as temperature goes higher, the thickness of IMC layer will

increase. As a result, it will affect the drop-impact reliability

of the solder joints. In addition, solder joint reliability of such

electronic assemblies is under further research [1]. Therefore,

continuous study is needed to understand the behaviour and

properties of solder especially for lead-free solder alloys.

Solder technology nowadays is trying to remove the

application of Pb base solder because of its hazardous

properties to human health. Combinations of lead-free solder

alloys being introduced to industry and mostly of them are

from Sn base solder. Numerous research on lead-free solder

found that Sn-Ag-Cu (SAC) is among the most potential lead-

free solder to replace Pb base solder as its properties is similar

or quite similar to Pb base solder. The only constraint is the

melting temperature of SAC is much higher than Pb base

solder which is not so applicable during welding process.

Besides of SAC, another potential lead-free solder should

be continuously investigated. One of them is Sn-Ag-Sb. A

study conducted by AIM had shown that solder alloys with

addition of Sb has the lowest copper intermetallic growth rate

compared to other composition of solder alloy without Sb

addition [2]. In this paper, mechanical properties of Sn-Ag-Sb

on Cu substrate after thermal aging process was characterised

and analysed as many lead-free solder mechanical properties

have not been clarified and understood very well due to a short

term use in microelectronic assemblies [3]. Nanoindentation

test are perhaps the most commonly applied means of testing

the mechanical properties of materials in very small

dimensions [4].

II. EXPERIMENTAL PROCEDURE

A. Sample preparations

Five samples were picked randomly after die attach process.

Subsequently, they were placed into a furnace and undergo a

thermal aging process at temperature of 175oC. One sample

was taken out from the furnace after 50, 100, 200, 400 and 800

hours aging time, respectively. After completing thermal aging

process, samples were mounted in epoxy resin and hardened.

They were slightly ground and polished to expose the IMC

layer between Sn-Ag-Sb solder and Cu substrate.

B Nanoindentation

The nanoindentation test was executed in constant load for

each sample. The load is 5mN with the loading and unloading

rate is set at 1mN/second. Point of indentations for each

sample is 8 points, respectively. Indentation was conducted on

the flat IMC surface using NanoTest by Micro Materials Ltd.

Wrexham, United Kingdom with a Berkovich tip. This

machine is designed in horizontal loading so that it has low

thermal drift which is essential for accurate measurements and

for studying creep and viscoelastic effects of materials. The

selection of position of indentation is controlled under a high

resolution optical microscope. The IMCs, solder and Cu layer

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can be differentiated by recognizing their colours. The highest

maginification for this nanotest system is 10K. Therefore, it is

very difficult to distinguish the appearance of each IMC

composition which is Cu3Sn and Cu6Sn5. Because of this

limitation, the results in this paper are taking into account the

hardness and modulus of IMC layer as a whole. After

completing the test, software will give the readings of

hardness and reduced modulus of samples in the unit of GPa.

Young's modulus of a material can be calculated from the

reduced modulus according to [5]:

Where E and v are the Young's modulus and Poisson's ratio,

respectively. The subscript i refers to the indenter where Ei is

1140 GPa and Vi is 0.07. If the Poissoin's ratio for the material

is known, the calculation is straightforward. Unfortunately, the

Poisson's ratio for IMC composition being studied here are yet

to be determined. However, Young's modulus calculation can

still be done following assumption method as being done by

[6]. They used the data from a literature review which the

Poisson's ration for Ag3Sn, Cu3Sn, Cu6Sn5 and Ni3Sn4 were

measured experimentally and the values lie between 0.3 to

0.36 [7,8]. Using the same method, a Poisson's ratio of 0.3 was

assumed for IMC layer in this work. This assumption can be

validated as a change in the Poisson's ratio of 0.06 (the upper

limit for Poisson's ratio of the above IMC measured value)

only make a change of 1GPa in the calculated Young's

modulus, which is well within experimental error [6].

III. RESULTS

Points of indentations on IMC layer are shown in Fig. 1.

The load is controlled so that the indentation points would not

exceed the IMC layer thickness and to give results accurately

on the hardness properties of IMC layer.

IMC layer in this study consist of Cu3Sn and Cu6Sn5 which

had been determined using EDS [9]. As a preliminary work,

indentation results in this paper show the hardness and Young

modulus of IMC composition as a whole, without

interpretation of hardness properties of Cu3Sn and Cu6Sn5

individually.

Fig. 1. Indentation points on IMC layer between SnAgSb solder and Cu substrate The load-displacement curve for one of the samples which has aged

for 800 hours was shown in Fig. 2. In this nanoindentation test, with load of

5mN, penetration depth for IMC undergo thermal aging from 100 hours to 1500 hours were about 180 to 200nm.

Fig. 2. The load-displacement curve for samples with 800 hours aging time

Table I derived Hardness (GPa), Reduced modulus (GPa)

and Young modulus (GPa) of IMC for 100, 200, 400, 800, and

1500 hours aging time.

TABLE 1HARDNESS (GPA), REDUCED MODULUS (GPA) AND YOUNG MODULUS (GPA) OF

IMC FOR 100, 200, 400, 800, AND 1500 HOURS AGING TIME

Aging time (hours)

Hardness (GPa) Reduced

Modulus (GPa) Young Modulus

(GPa)

100 5.583214 117.4361 106.867

200 5.567643 118.1043 107.475

400 5.507643 119.2771 108.542

800 5.415574 125.7607 114.442

1500 4.444386 141.1421 128.439

The results were plotted in two graphs, hardness-aging time

curves and Young modulus-aging time curve as shown in Fig.

3.

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Fig. 3. Hardness and Young modulus (GPa) versus aging time (hours) for

IMC layer between SnAgSb solder and Cu substrate

From the plotted graph, the graph trend shows that the

hardness of IMC is decreased from 5.583 GPa to 4.444 GPa

with the increasing of aging time while the Young modulus is

increasing from 106.867 GPa to 128.439 GPa with the

increased time of aging. Chandra Rao reported a Young

Modulus of Cu3Sn and Cu6Sn5 ranging from 102 GPa to 140

GPa [10]. The samples in their study is SnAgSb undergo aging

from 500 to 1000 hours. This indicates SnAgSb mechanical

properties behaviour is quite similar with SnAgCu.

The change of modulus for IMC subject to thermal aging

time is dependent on the dynamic changes in the IMC growth

and diffusion process, which could change the IMC structure

[11]. In the perspective of hardness, the hardness of IMC layer

shows strong dependence on the aging temperature. The

higher temperature results in smaller hardness value, which

means the IMC tends to be softer at high temperature. With

the increase of temperature, the ions adsorb the thermal energy

and reach equilibrium state with wider vibration amplitude,

which in turn softens the materials [12]. Besides, the results

also shows that this nanotest results follow the aging curve

schematic diagram (Fig. 4) which tell us that as the aging time

increases, preprecipitation zones form and their size increases,

and the alloy becomes stronger and harder and less ductile. At

peak aged condition, a maximum strength will eventually

reached as the temperature is sufficiently high. It is usually

associated with the formation of an intermediate metastable

precipitate. If aging time is continued so that the intermediate

precipitate coalesces and coarsens, the alloy overages and

becomes weaker than in peak aged condition [13].

The plotted graph also indicate that, the peak aged of IMC

layer is about 100 hours to 200 hours aging time and from 200

hours aging time, the IMC started to overage and becomes

weaker and brittle. Once the IMC becomes brittle, the

reliability issues of solder joint may rise and will affect the

performance of semiconductor packages.

Fig. 4. Aging Curve Schematic Diagram [13]

IV. CONCLUSION

Nanoindentation measurements for IMC layer between

SnAgSb solder and Cu substrate were conducted. From the

results, it shows that with the increasing time of aging hours,

the hardness of IMC layer is decreased while the Young

modulus is increased. From the plotted graph, the IMC started

to overage at 200 aging time and becomes weaker and brittle.

Once the IMC becomes brittle, the reliability issues of solder

joint may rise and will affect the performance of

semiconductor packages.

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