10 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 24, NO. 1, FEBRUARY 2001
Effect of Cooling Rate on the Isothermal Fatigue
Behavior of CBGA Solder Joints in Shear
S. H. Fan, Y. C. Chan, Senior Member, IEEE, C. W. Tang, and J. K. L. Lai
Abstract—This paper investigates the distribution characteris-
tics of the isothermal fatigue lifetime of ceramic ball grid array
(CBGA) solder joints in shear. Placement direction of the board-
level assembly on the oven conveyor during reflow critically influ-
ences the fatigue lifetime of solder joints in shear: the front or outer
solder joints have a longer shear lifetime than the rear or inner
ones. The solder joints that moved diagonally during reflow have a
longer fatigue lifetime and a tighter distribution. Cracks initiated
in the eutectic solder region on the card and package side and tend
to propagated in that region, while final failure occurred mainly
on the card-said eutectic solder region. This phenomenon can be
explained that the front or outer solder bumps have a resistant ef-
fect to the gas fluid which passes through the rear or inner solder
bumps, and lower these solder joints’ cooling rate during solidifi-
cation. Fast cooling rate can cause a more fine-grained and homo-
geneous microstructure in eutectic solder alloy, which can delay
crack initiation and slow crack growth. When the board-level as-
sembly moves diagonally during reflow, the resistant effect of front
solder bumps to the gas fluid reduces markedly. So the fatigue
lifetime of solder joints and its distribution characteristic enhance
substantially. The theories of fluid dynamics and heat transmission
are used to calculate the decrease of gas fluid velocity and the cor-
responding reduction of mean coefficient of heat transfer ( ).
Index Terms—Ceramic BGA, cooling rate, fatigue lifetime dis-
tribution, shear cycle test, solder joint reliability, solidification.
I. INTRODUCTION
RECENT advances inwafer fabrication techniques have pro-duced smaller feature sizes, increasing gate count and chip
inputs/outputs (I/O),This trendhasput increasedemphasisonmi-
croelectronic packaging. Ceramic ball grid array (CBGA) tech-
nology makes possible the direct attachment of multilayer ce-
ramic(MLC)modules toindustrystandardepoxyglasscardswith
ahighnumberof interconnections[1]–[4]. Inrecentyears,CBGA
packages have been extensively evaluated by the computer in-
dustry to meet I/O and density requirements. Compared to other
fine-pitchperipheral leadeddevices,CBGAhastheadvantagesof
high assembly yields, more efficient use of printed writing board
(PWB) real estate, improved electrical performance, potential for
greater I/O density, and lower assembly cost.
MLC technology can accommodate both multichip module
(MCM) and single chip module (SCM) applications and is com-
patible with all types of chip interconnections. MLC carriers
have excellent electrical and thermal characteristics, including
Manuscript received November 30, 1999; revised June 13, 2000. This work
was supported by the Hong Kong Research Grants Council, Project 9040212.
S. H. Fan, Y. C. Chan, and C. W. Tang are with the Department of Electronic
Engineering, City University of Hong Kong, Kowloon, Hong Kong (e-mail:
shfan@ee.cityu.edu.h).
J. K. L. Lai is with the Department of Physics and Materials Science,
City University of Hong Kong, Kowloon, Hong Kong (e-mail: eeyc-
chan@cityu.edu.hk).
Publisher Item Identifier S 1521-3323(01)00559-7.
built-in reference planes and low inductive power connections
with multipower paths. Packaging applications demand ground
planes, thermal dissipation and greater than 400 interconnec-
tions, CBGA is the most cost-effective choice.
In SMT solder joint, fatigue caused by shear strain is the pri-
mary failure mechanism. For CBGA, the mismatch of the coef-
ficients of thermal expansion (CTE) between the ceramic chip
carrier ( – ppm/ C) and PCB ( – ppm/ C) re-
sult in solder joint fatigue in shear created during power on-off
cycles and ambient temperature changes [5]–[7]. Solder joint
metallurgy, geometry, and height can affect fatigue behavior.
It is reasonable to assume that the fatigue property differs be-
tween solder joints in a CBGA assembly. More pin-count, more
chance to occur this problem. Fatigue failure is most likely to
first occur at either the weakest solder joints or the most severely
deformed solder joints. These joints determine the reliability of
the package. Previous work [8], [9] reported that the comer joint
had the largest deformation when packaging assemblies were
subject to thermal cycling. In this work, the distribution of solder
joint fatigue behavior was investigated. This work is assistant to
optimize the CBGA reflow parameters and enhance the fatigue
performance of CBAG assemblies.
II. EXPERIMENTAL PROCEDURE
A. Samples Preparation Procedure
1) CBGA Component and PCB Board: Alumina ceramic
BGA component with affay were mounted to on FR-4
PCB. Solder spheres of diameter 0.75 mm with a composition
of 90 wt%Pb/10 wt%Sn, attach CBGA to the PCB used eutectic
Sn-Pb solder in both fillets. The FR-4 PCB had nickel- and
gold-plated Cu pads. The diameters of the CBGA and PCB
solder pad were 0.86 mm and 0.635 mm, respectively. The
solder pad pitch is 1.27 mm. The thickness of CBGA and PCB
are 1.7 mm and 1.2 mm, respectively.
2) Assembly Process: A 0.20-mm thick metal mask stencil
was used to print 63 wt%Sn/37 wt%Pb solder paste on the
PCB solder pad. Then mounting the CBGA’s on PCB. The
board-level assemblies were reflowed in a 5-zone oven with
compressed air at a temperature profile that only melts the
eutectic solder. The time-resolved temperature profiles near the
bulk solders at the “edge” and “center” of CBGA, as well as
near the CBGA component in the oven chamber were recorded
simultaneously by a wireless profiler. Fig. 1 shows a typical
“edge” (at point in Fig. 4) and the oven chamber
(or the free gas fluid) (point ) temperature profiles,
and Fig. 2 shows the “edge” (point ) and “center”
(point ) temperature profiles. After reflowing, the
1521–3323/01$10.00 © 2001 IEEE