Comparative compliance of a representative surface mount leadless solder connection and commercial lead designs

Abstract

The long-term reliability of surface mount (SM) solder interconnections remains an important issue in many critical electronics packaging applications. Compliant leads are typically post-attached to leadless ceramic chip carriers (LCCCs) and multichip modules (MCMs) to enhance the SM attachment reliability margin on organic substrates. The various lead-forms are commercially available in edge-clip, soldered, and thermocompression (TC) bonded designs for component attachment. Compliance evaluation was performed for a representative corner-most solder connection on a LCCC. The effective stiffness of the solder joint and commercial post-attached lead designs were compared in order to demonstrate the SM interconnection reliability advantage provided by certain edge-clip and TC-bonded lead-forms. Commercial high-compliance edge-clip, soldered, and TC-bonded lead designs have diagonal-direction stiffness between nominally 10-40 lb/in, prior to circuit-board attachment. The compliant leads accommodate a large part of the component-substrate thermal expansion mismatch, significantly reducing the cyclic loads transmitted to the comparatively noncompliant solder connections. The diagonal stiffness results for the corner-most solder joint are specific for the particular contour and dimensions of the FE structural model. However, the current study provides fundamental understanding of the compliance advantage of post-attached leads compared to leadless SM interconnections. >