The various methods for improvement of package solder joint reliability (SJR) have centered on the broad categories of (i) reductions in the thermomechanical and mechanical stresses and strains applied to the joints, and (ii) strengthening of the solder interconnect interfaces and materials themselves. In practice, the success of the former depends first and foremost on the latter — an adequate and consistent interconnect ‘strength’ during the package development and production cycles. With the advancement of various pad-plating technologies (most notably ENIG – Electroless Nickel Immersion Gold), sphere chemistries, fluxes and processing conditions, each with their own stability issues, the interconnect strengths can easily undergo seemingly random drifts over time. The Dage™ Cold Ball Pull (CBP) technique, however, has emerged as an attractive alternative to the traditional ball-shear metrology as an interconnect strength monitor. The open issues preventing its adoption are related to identifying the best test conditions (e.g. aging time, pull speed, jaw pressure, etc...), all of which are addressed here. After identifying the best test conditions, we present a number of experimental results that highlight the powerful capability of this tool for optimizing and monitoring solder-joint strength. A full metrology characterization to demonstrate accuracy, repeatability and reproducibility has been performed. Moreover, interesting results have been obtained with respect to solder-aging, multiple-reflow, and time-above-liquidus effects on interconnect strength. Examples of direct correlation between CBP measurements and solder-joint shock performance are demonstrated. CBP is also shown to correlate well to other strength metrologies, such as three-point bend. Finally, CBP is used here to show how to strengthen interconnects by the proper selection of pad plating chemistries, sphere compositions, fluxes, reflow conditions, etc… Maintaining those strengths through development and production can be handled effectively using CBP as a monitor. Looking forward, CBP data presented here shows that certain material and processing choices can maximize lead-free solder interconnect strength and lead-free solder joint reliability.
