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Brittle Failure in Lead-free BGA Solder Joints
October 8, 2013 |Estimated reading time: 2 minutes
The electronics industry continues to identify and uncover potential performance and reliability risks associated with the transition to Pb-free design and manufacturing. This is a particular concern for complex product designs requiring high reliability and extended service life. The continued integration of printed circuit board assemblies (PCBA) using more complex packages, smaller solder joints, diminishing pitch, and complex components mounted on both sides of the PCBA has resulted in increasing yield and reliability challenges. Among those issues is the emergence of solder defects that appear to be related to higher assembly temperatures due to the use of lead-free solders. These assembly defects can effectively shorten reliability lifetimes in addition to lowering manufacturing yields or creating premature service failures.
In the mid-1990s, there were published reports of brittle interface failures of surface mount solder joints that had been subjected to a second soldering operation. The first reported case occurred subsequent to a wave soldering operation, but there was another case reported subsequent to a second surface mount reflow operation. Because brittle interfacial solder joint failures were observed during a second reflow soldering operation they were called “double reflow” failures. There have been no additional published cases of double reflow failures since those early occurrences. However, the increasing complexity of electronic assemblies coupled with the transition to higher temperature laed-free soldering has given rise to another case of brittle interfacial failures induced by double reflow.
This article describes and characterizes electrically intermittent, brittle interfacial solder joint failures in a lead-free BGA subjected to two reflow cycles. The failed BGA is a 1.27 mm pitch, perimeter array with a body size of 31 mm and SAC305 (Sn3.0Ag0.5Cu) solder balls. Due to the presence of several surface mounted daughter card modules on the non-BGA side of the board, the BGA-side of the printed circuit assembly was soldered first. In effect, this constraint put the BGA components atypically on the bottom-side during the second reflow. The solder joint failures were characterized with metallographic cross-sectional analysis using optical microscopy and scanning electron microscopy. The failure mechanism and possible root cause are 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. The primary objective of this work is to document this new case of double reflow failure to enable a heightened awareness of the failure mode. These assembly defects are difficult to detect and shorten reliability lifetimes in addition to lowering manufacturing yields and creating premature service failures. This also emphasizes the importance of reviewing and checking the assembly quality and reliability of assemblies before shipping product.Read the full article here.Editor's Note: This article originally appeared in the September 2013 issue of SMT Magazine.