Reliability of Interfaces in Microelectronic Solder Joints
As the microelectronics industry continues to
develop increasingly complex devices with extremely small feature sizes, creating reliable
interconnect systems becomes a significant challenge. A common method of attaching a
chip package to a composite printed circuit board (PCB) is with a ball grid array (BGA)
configuration. In this configuration, shown in Figure 1, the solder ball provides
both the electrical and mechanical connection between the bond pads on the package
substrate to those on the board. These bond pads consist of a stack of metal thin
films that can be processed with either electrolytic processes or purely chemical,
electroless processes. The electroless process is of increasing interest because it
provides a more uniform and controlled film thickness. However, the resulting films
appear to have poor adhesion properties. The bond pad system analyzed in this study
consists of a stack of Cu, Ni, and Au bonded to eutectic 63Sn-37Pb solder, as shown in
Figure 1. The objective of this research is to quantitatively characterize the
fracture and fatigue processes of these joints.
Figure 1: Schematic diagram of the solder
ball grid array configuration.
Solder of eutectic composition is deposited and
then heated to cause it to reflow. During reflow, the molten solder absorbs the
entire Au layer into solution, allowing Sn from the solder to react with the Ni layer,
creating a Ni-Sn intermetallic. This brittle intermetallic layer has been implicated
in the debonding of electroless joints. During the deposition of the electroless Ni,
a small amount of P, ~5 wt%, remains in the Ni. When the Ni-Sn intermetallic forms,
P is expelled to create a P-rich Ni layer adjacent to the intermetallic. Brittle
fracture appears to occur at this interface, as shown in figure 2.
Figure 2: Schematic diagram of the joint
cross-section after solder reflow. Brittle fracture has been seen at the interface
between the P-rich Ni and the Ni-Sn intermetallic.
In joints that are aged at elevated temperatures
for extended times, the Au that was in solution after reflow has time to react with Sn in
the solder and form a Au-Sn intermetallic. This intermetallic forms next to the
Ni-Sn intermetallic layer as shown in Figure 3, and brittle fracture occurs between these
Figure 3: Schematic diagram of joint
cross-section after aging. Brittle fracture has been seen at the interface between
the Au-Sn and Ni-Sn intermetallic layers.
Both the electrolytic and electroless processed
interfaces are studied using traditional linear elastic fracture mechanics. Samples
are made by sandwiching the solder system between two rigid substrates with the
electroless metallurgy on one interface and the electrolytic interface on the other.
These samples are then tested in pure Mode I tension to measure cyclic fatigue and
resistance curve (r-curve) effects.
Figure 4: Schematic diagram of samples
with solder sandwiched between rigid substrates. a.) Compact tension (CT)
configuration for samples with copper substrates, b.) Double-cantilever beam (DCB)
configuration for samples with FR-4 circuit board samples. Samples were loaded in
tension, and the crack length is denoted by a.
L.C. Wang, Z. Mei, R.H. Dauskardt,
"Reliability of Electroless Processed Thin Layered Solder Joints," Materials
Reliability in Microelectronics IX Mat. Res. Soc. Symp. Proc. Vol. 563. 1999 Materials
Research Society. 3-8.