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.

The Problem:
After Reflow
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.

After Aging
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 two layers.

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.

Testing Techniques:
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.

Relevant Publications:
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.

Department of Materials Science and Engineering, Stanford University

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