On Measurement of Effective Silicon Backend Strength Using Bump Pull/Shear Techniques

Abstract

Integrating a low-K ILD layer within silicon is key to reducing RC delays. However, low-K ILD materials typically have low mechanical strength, making their incorporation with lead free interconnects an industry-wide challenge. It is well known that conversion to lead free first level interconnects increases die backend stresses due to the higher melting temperature and increased solder stiffness. The paper will focus on the measurement of the effective silicon backend strength after subjecting the dice to different fabrication and assembly steps. The effective strength will also be evaluated post reliability stress exposure to eventually understand the life of these films. The paper will describe how a commercially available Dage 4000 tool was modified for this application. Bump pull was carried out using a 100μm tweezers, while bump shear used 1mil (25.4μm) wide stylus. Static and dynamic calibration was first carried out to ensure repeatability and reproducibility of the results. Peak force and failure modes were used as metrics to compare the effectiveness of different experimental legs. Traditional failure analysis approach of mechanical polishing, or when needed, use of FIB for sample preparation, with subsequent SEM/EDX analysis was utilized to understand the failure mechanism. Data suggests that shear and pull lead to different failure modes. Bump shear mainly led to failure at the bump/polyimide interface and did not necessarily correspond to the weakest layer or interface in the silicon backend. Whereas bump-pull, which applies tensile force to the stack up, lead to failures in the weakest layer, typically the low-K ILD, in the silicon backend. Hence, bump pull provided the advantage over shear as it allowed evaluation of the weakest interface in the stack up. Two case studies are discussed to demonstrate on how bump pull/shear metrologies were used to understand the impact of different assembly/FAB process variables and highly accelerated steam test (HAST) reliability stress on silicon backend strength. First case study shows influence of assembly flux on silicon backend strength, while second case study describes impact of HAST on different FAB backend processes.

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].…”

Effects of Testing Conditions and Multiple Reflows on Cold Bump Pull Test of Pb-free Solder Balls

“…Overall, Sn-4.0Ag-0.5Cu solder exhibited higher shear strength than that of Sn-37Pb. An increase in shear strength of solder ball with increasing shear rate has been observed previously [4][5][6][7][8].…”