Double Reflow-Induced Brittle Interfacial Failures

Julie Silk, George Wenger, Richard Coyle, Jon Goodbread, Andrew Giamis
Agilent Technologies, Santa Rosa, CA USA
Andrew Corp. Wireless Network Solutions, Warren, NJ USA
Alcatel-Lucent, Reliability Eng., Murray Hill, NJ USA
Agilent Technologies, Colorado Springs, CO USA

Assembly defects can effectively shorten reliability lifetimes in addition to lowering manufacturing yields or creating premature service failures. This paper describes and characterizes an unusual open circuit failure mechanism in Pb-free ball grid array (BGA) solder joints. The failure occurred during Pb-free solder assembly of a 31 mm, 1.27 mm pitch, perimeter array, SAC305 (Sn3.0Ag0.5Cu) BGA. Due to design constraints, it was necessary to assemble some BGA components during the first reflow cycle.

Following the second reflow operation, some solder joint opens were detected on the BGA component which had been subjected to the atypical second reflow exposure. Metallographic cross sectional analysis indicated that the open solder joints initially were well-formed but the failure resulted from a brittle interfacial fracture at the package side of the solder joints. The failure mechanism and possible root cause is discussed in terms of the combined impact of stress induced by component and board warpage and the lower inherent strength of the solder joint near the melting and solidification temperatures.

A comprehensive failure analysis was performed on a complex electronic printed circuit board assembly (PCBA) that exhibited intermittent open circuit electrical failures following Pb-free solder assembly.

The following observations and onclusions are drawn from the results of the failure analysis.

• The root cause of the intermittent open circuit failures was brittle solder joint fractures at the interface between the Ni-Sn (nickel-tin) intermetallic compound (IMC) and the bulk solder at the package side of the solder joint. The solder joint fractures could not be detected until destructive cross sectional analysis was performed.

• The fractures developed in a Pb-Free perimeter plastic ball grid array (PBGA) following exposure to a second reflow cycle. The failure signature is characteristic of the so-called Double Reflow failure mechanism that has been reported very sparingly in the literature over the years. This may be the first reported example of this failure mechanism in a Pb-free solder assembly.

• The most likely scenario for describing the failure sequence is that the solder joints fracture just prior to solder melting during the 2nd reflow. In this scenario, the solder joint strength is at a minimum prior to melting and brittle solder joint fractures are induced by a complex combination of CTE and warpage stress on the solder joint during its exposure to the second reflow cycle. Thermal gradients across the BGA package during heating may contribute to the stress.

• The brittle fractures were eliminated by process changes that allowed the BGA-populated side of the PCBA to be
exposed to only a single reflow cycle.