Study of interaction between copper and melt of lead-free solders

Abstract

Problems of reactive diffusion at the interface solid phase - melt were studied theoretically and experimentally. A theoretical description of the kinetics of dissolution of the solid phase in the melt is presented for the case of planar dissolution. The main intention was to study experimentally the copper dissolution in melts of various solder alloys and the related reactive diffusion. We used pure Sn and Sn-Cu, Sn-Ag-Cu, Sn-Sb, Sn-Zn alloys as solder materials. Experiments aimed at the study of a Cu plate dissolving in the solder melt were carried out at various conditions. Concentration profiles of elements and thickness of layers of phases were determined by SEM and X-ray microanalyses (WDX, EDX) of specimens after their heating.

Full text

509
METALURGIJA 52 (2013) 4, 509-511
J. DRÁPALA, P. KUBÍČEK, G. KOSTIUKOVÁ, P. JOPEK
STUDY OF INTERACTION BETWEEN
COPPER AND MELT OF LEAD-FREE SOLDERS
Received – Prispjelo: 2012-09-12
Accepted – Prihvaćeno: 2012-02-25
Preliminary Note – Prethodno priopćenje
ISSN 0543-5846
METABK 52(4) 509-511 (2013)
UDC – UDK 669.95:548.526:532.73=111
J. Drápala, G. Kostiuková, P. Jopek, Technical University of Ostrava;
Faculty of Metallurgy and Materials Engineering; Ostrava-Poruba
P. Kubíček, Ostrava-Hrabůvka, Czech Republic
Problems of reactive di usion at the interface solid phase - melt were studied theoretically and experimentally. A
theoretical description of the kinetics of dissolution of the solid phase in the melt is presented for the case of planar
dissolution. The main intention was to study experimentally the copper dissolution in melts of various solder alloys
and the related reactive di usion. We used pure Sn and Sn-Cu, Sn-Ag-Cu, Sn-Sb, Sn-Zn alloys as solder materials.
Experiments aimed at the study of a Cu plate dissolving in the solder melt were carried out at various conditions.
Concentration pro les of elements and thickness of layers of phases were determined by SEM and X-ray micro-
analyses (WDX, EDX) of specimens after their heating.
Key words: lead-free solders, copper, tin, silver, intermetallic compounds, di usion
INTRODUCTION
This article deals with the problems of dissolution of
the solid phase A in the melt B [1, 2]. Diffusion of atoms
of the melt into the solid phase was accompanied by
creation of new phases and/or chemical reactions, and
fi nally by diffusion of atoms from the solid phase into
the melt. Those processes usually proceed at the pres-
ence of convection in the melt. At the same time the
process of signifi cant movement of the interface be-
tween the solid phase and the melt takes place, the rate
of which depends on the properties of atoms in A and B
and on their interaction, temperature and time of the
process in question. Furthermore, it depends on the geo-
metric arrangement, on the volume of the melt and con-
ditions enabling or restricting convection in the melt.
The relevant phase diagram provides a prediction of
newly appearing phases. During the reactive diffusion,
due to the rate of the solid phase dissolution in the melt,
conditions of the equilibrium state do not have to be
always met because of non-stationary processes [1, 2].
During the diffusion of atoms from the melt B into the
solid phase A new phases are formed and they grow, i.e.
the diffusion processes take place in the areas with mov-
ing interface boundaries. When new phases appear in
the course of reactive diffusion, surface and subsurface
layers of material of various compositions, properties
and thickness are also being formed.
THEORY
Let us consider a planar interface between the solid
phase A and the melt B at the time t = 0. During dissolu-
tion of the solid phase into the melt the interface solid -
liquid moves with the time. The rate of this interface
movement is usually considerably greater than the
growth of newly nucleating phases with B atoms in the
solid metal A. At the interface a diffusion layer will
form in the melt, the thickness of which depends on the
size of convection in the melt.
For the planar interface boundary when the interface
shift during the dissolution is perpendicular to the inter-
face, the relation was obtained [1, 2]
()
o
tK
o
o
tK
l
e
cce
t
o
o
−
−
+
−
−
=11
1
2
, (1)
where lo is the width of the melt column at the time t
= 0, ρ2, ρ1 are densities of the solid and liquid phases,
χ(t) is the shift of the interface boundary during dissolu-
tion, Ko is the rate constant of dissolution and co is the
saturated concentration of the metal A in the melt B.
The following equation [2] holds for the time depend-
ence c(t) of the metal A in the melt
() ()
()
ta
t
tc +
=
, a = ρ2 lo / ρ1 (2)
Relation (1) describes the time dependence of planar
dissolution. Neither the diffusion of atoms B from the
melt into solid phase A nor the volume changes during
dissolution were taken into account in the model. Con-
vection in the melt will infl uence the rate of dissolution
and therefore the exact value Ko will partly depend on
the geometry, in which the planar dissolution takes
place. The relation (1) may be used for an approximate
evaluation of the rate constant of dissolution Ko under
precisely defi ned experimental conditions. Figure 1 il-
lustrates the calculated dependences χ(t) for the Cu – Sn
system.

510 METALURGIJA 52 (2013) 4, 509-511
J. DRÁPALA et al.: STUDY OF INTERACTION BETWEEN COPPER AND MELT OF LEAD-FREE SOLDERS
The values of density ratios ρ2/ρ1 ≈ 0,766 for T = 400
°C, co = 0,115 (from the phase diagram Cu–Sn [3]), lo =
1 cm at Ko = (0,3 ÷ 1,2).10-5 s–1 were used in the calcula-
tions. Time dependences of the component A concentra-
tion in the melt, assuming perfect melt homogenisation,
are obtained from the relations (1) and (2)
cA (t) = co(1 – e–Kot). (3)
EXPERIMENTS
The uni-directional experiment was realized using
the “sandwich” specimens. A layer of a tin-based solder
was inserted between two Cu plates. Five compositions
of lead-free solder alloys were used: pure Sn, alloys
Sn97Cu3, Sn95,5Ag3,8Cu0,7, Sn95Sb5 and Sn91Zn9.
Two plates of high-purity copper were 0,5 mm thick,
the other dimensions were 20 x 10 mm. The Sn-based
solders inserted between the two Cu plates were rolled
down to the thickness of 0,6 mm. Copper plates were
covered by a layer of suitable fl ux. The soldered joint
was created using a heating element. The sandwiches
were held at the temperature of 255 ÷ 285 °C (above
liquid) for the time 40 ÷ 120 s. The prepared specimens
were subsequently annealed in the evacuated ampoules
at 300 °C for 18, 24, 48 and 92 h – see Figure 2.
The η-Cu6Sn5 grows during annealing at the tem-
perature 300 °C on the interface with the Cu. Copper
dissolves in liquid Sn-based solder up to the saturation
concentration of approx. 4 at.% (2,2 wt.%) Cu. As a
consequence of Sn diffusion into solid Cu, the phase η
grows with time and subsequently the phase ε (Cu3Sn,
75 at.% Cu and 25 at.% Sn) appears to be close to the
pure Cu. The specimens were cooled on the air after an-
nealing and the temperature decreased relatively quick-
ly below the eutectic temperature of 227 °C. The joint
copper / solder Sn95,5Ag3,8Cu0,7 is shown in Figure 3
after 24 hours of annealing as an example.
RESULTS
The growth of the phase ε should proceed with time
according to the parabolic law
()
tt =
where χε(t)
is the thickness of phase ε and α is the constant, which
characterizes the rate of the phase growth. The values
χε(t) can be obtained experimentally using the metallo-
graphic images from relatively large areas or from the
X-ray EDX microanalysis. The evaluated values α from
the metallography are presented in Table 1.
The tentative calculations of Sn diffusivities in cop-
per at 300 °C based on the growth of the phase ε into Cu
were performed using the Wagner relation [4-7]:
Figure 1 Graph of functions χ(t) – eq. (1) for the liquid
temperature of 400°C in Cu–Sn system and various
values of Ko = (0,3 ÷ 1,2).10-5 s–1 for planar dissolution
Figure 2 Scheme of the soldering experiment and description
of the found intermetallic phases after long-time
annealing
Figure 3 Microstructure a) and concentration pro les b) of the
joint Cu/SnAg3,8Cu0,7 annealed at the temperature
300 °C for 24 hours

511
METALURGIJA 52 (2013) 4, 509-511
J. DRÁPALA et al.: STUDY OF INTERACTION BETWEEN COPPER AND MELT OF LEAD-FREE SOLDERS
()








=tD
x
erfcAtxc
B
B2
,
, (4)
where B is concentration of Sn in (Cu) and A is con-
centration of Sn in the phase ε (Cu3Sn).
Based on the X-ray analysis of concentration curves
we searched for the size of the interval Δx = 2 μm, where
the Sn concentration in Cu decreased nearly to zero. We
can estimate the diffusivity D from the value Δx and the
known time of annealing t using the equation:
()
1,5; 1,5 0,034.
2
xerfc
Dt
∆≈=
(5)
Joint t /h T /°C Δx /μm D /cm2 s-1
Cu/Sn 48 300 2 ≈ 2,6.10-14
Cu/Sn97Cu3 24 300 2 ≈ 5,1.10-14
Cu/Sn91Zn9 92 300 2 ≈ 1,3.10-14
DISCUSSION AND CONCLUSIONS
In this paper the problems of reactive diffusion at the
contact of the solid phase with the solder melt were out-
lined briefl y. Examples of particular experimental ar-
rangements were presented. The main intention was to
calculate duration of dissolution of the solid phase for the
case of planar geometry. The calculations were performed
using the equations (1) and (3) describing the kinetics of
dissolution. The aim was to determine the rate constant of
dissolution Ko from experimental results.
When observing the growth of phases on planar inter-
face in the Cu–Sn system for the temperature of 300 °C,
very irregular growth of the phase η, especially into the
melt, was discovered and estimates of the rate of growth
of the phase ε into Cu according to the parabolic law were
performed, as well for other lead-free solders.
A fast growth of the phases in metal A after long-
term heating and detection of the thickness of layers of
these phases, i.e. χε(t), χη(t) pose considerable time de-
mands on structural and chemical micro-heterogeneity
of specimens.
The problems of reactive diffusion were studied for
dissolution of copper in melts of various solder alloys
both theoretically and experimentally. We used pure Sn
and Sn-Cu, Sn-Ag-Cu, Sn-Sb, Sn-Zn alloys as solder
materials. Experiments aimed at the study of a Cu plate
dissolution in the solder melt were carried out at various
selected temperatures and times. Problems, that have to
be solved preferably, were emphasized. Concentration
profi les of elements and thickness of layers of phases
formed in specimens were determined before and after
their diffusion heating by SEM and X-ray microanalyses
(WDX, EDX). A great set of data obtained will allow us
to derive relationships between the rate of dissolution of
the solid phase, i.e. displacement of the interphase
boundary χ(t) in the melt, and growth rates of interme-
tallic phases in metal A. The tentative calculations of Sn
diffusivities in copper at 300 °C based on the growth of
the phase ε into (Cu) were performed.
The dissolving rate constant Ko is a fundamental pa-
rameter characterizing the rate of dissolution for the
certain confi guration of experiment.
The reactive diffusion studied makes it possible to
create surface and subsurface layers of controlled thick-
ness in metallic materials.
Acknowledgements
The authors are grateful to the Ministry of Education
of the Czech Republic for the fi nancial support of the
international project of EU - COST MP0602, No.
OC08032 „Lead-free solders for high-temperature ap-
plications“ and of the Czech project of OP VaVPI “Re-
gional Materials Science and Technology Centre (RM-
STC)” No. CZ.1.05/2.1.00/01.0040.
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Table 1 Evaluation of the rate of growth of the phase ε
in „sandwich“ specimens at the temperature of
300 °C
Difusion joint t
/h α . 106
/cm s-½
mean α . 106
/cm s-½
Cu/Sn 18 5,84 5,57
24 5,10
48 4,38
92 6,95
Cu/Sn97Cu3 18 6,50 5,41
24 6,23
48 4,0
92 4,92
Cu/Sn95,5Ag3,8Cu0,7 18 6,6 4,27
24 3,63
48 2,40
92 4,46