Solder Joint Reliability Improvement Using the Cold Ball Pull Metrology

Abstract

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.

Evaluation of new substrate surface finish: Electroless nickel/electroless palladium/immersion gold (ENEPIG)

“…Two factors were concerned [2][3][4][5]: First, the interface seems to be weaker in tension than in shear. It is because that pull test applied a pure tensile load at the solder ball pas interface, but in shear test, solder ball gained a combination force of shear and tensile loads resulting in difficulty identifying weak joint.…”

Brittle Failure Mechanism of SnAgCu and SnPb Solder Balls during High Speed Ball Shear and Cold Ball Pull Tests

“…In order to predict solder joint reliability under drop conditions, it is important to increase the testing speed of package level test methods such as high-speed solder ball shear and pull [1][2][3][4]. Traditional ball shear and pull tests are not considered suitable for predicting joint reliability under drop loading, as the applied test speeds, usually lower than 5 mm/s, are far below the impact velocities applied to the solder joint in a drop test [5][6][7][8]. Recently, high-speed shear and pull test equipment with testing speeds up to several meters per second (max.…”

Correlation between Package‐Level High‐Speed Solder Ball Shear/Pull and Board‐Level Mechanical Drop Tests with Brittle Fracture Failure Mode, Strength, and Energy

“…Currently the most popular method to evaluate the strength of the solder ball attachment is the ball shear test [6][7][8][9][10][11][12][13], which is adopted from the Joint Electron Device Engineering Council (JEDEC) standard JESD22-B117A [6]. Currently, a new method called the ball pull test has emerged as an attractive alternative to the traditional ball shear test as an interconnect monitor [11,12]. The ball pull test is a relatively new development, and there is a little information on the method.…”

“…Many researchers and engineers used the conventional ball shear test (shearing speed is less than 0.8 mm/sec) to evaluate the reliability of solder joints in the past decade [7][8][9][10][11][12][13]. Huang et al (2001 and2002) first studied the ball shear test in terms of the effects of shear speed and shear height [8,9].…”