Experimental investigation on testing conditions of solder ball shear and pull tests and the correlation with board level mechanical drop test
by Fubin Song
Ph.D. Mechanical Engineering
xxiii, 229 leaves : ill. ; 30 cm
The BGA packages have many advantages over conventional modules in IC packaging industries. For BGA assemblies, the solder joints are not only the passage for electrical signals, power, and ground, but also provide mechanical support to hold the module in position on the printed circuit board (PCB). Therefore, solder joint reliability is a major concern for BGA packages. The conventional method to evaluate solder ball attachment strength is the ball shear test (following JESD22-B117). In the past few years, the ball pull test configuration has been developed....[ Read more ]
The BGA packages have many advantages over conventional modules in IC packaging industries. For BGA assemblies, the solder joints are not only the passage for electrical signals, power, and ground, but also provide mechanical support to hold the module in position on the printed circuit board (PCB). Therefore, solder joint reliability is a major concern for BGA packages. The conventional method to evaluate solder ball attachment strength is the ball shear test (following JESD22-B117). In the past few years, the ball pull test configuration has been developed.
In this thesis, experimental investigations were performed on the effects of testing conditions of conventional solder ball shear and pull tests for BGA packages. The emphasis is placed on understanding the progressive failure mechanisms during the ball shear and pull tests.
The critical reflow cooling rate of large Ag3Sn plate formation was studied using lead free SnAgCu (SAC) solder balls. The ball shear and pull characteristics were tested on SAC solder balls cooled at different rates, which assessed the effects of large Ag3Sn plates on the room-temperature mechanical properties of SAC solder joints. This study also describes the formation and growth of intermetallic compound (IMC) at the interface of Sn3.5%Ag and Sn4.0%Ag0.5%Cu reflow soldered on different types of substrate pad surface finishes. The effects of IMC growth after soldering and thermal aging at 150℃ are discussed. The attachment strength of solder balls subject to thermal aging was investigated by the conventional ball shear and pull tests. The test results indicate that the ball pull test method can reveal the brittle failure of solder joints with a higher sensitivity, especially under a fast loading speed. As a result, the effects of IMC growth can be identified more easily.
In order to predict the 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. Conventional ball shear and pull tests are not considered suitable for evaluation of joint reliability under drop loading since the applied test speeds, usually lower than 5 mm/s, are below the impact velocity applied to the solder joint in a drop test. It is necessary to study the characterization of high-speed ball shear and pull tests, including the failure mode and brittle fracture mechanism during these high speed tests.
The brittle fracture surfaces were systematically analyzed on SnAgCu and SnPb solder balls after high-speed ball shear and pull tests, which led to a fundamental understanding of the failure mechanisms of solder balls under high speed mechanical loading. This study includes comprehensive high-magnification SEM/EDX characterization of the brittle fracture surfaces after high-speed ball shear/pull tests. Detailed images of the fractured surfaces of both the solder balls and the corresponding package pads were recorded. To evaluate the sensitivity of high-speed ball shear and pull tests to brittle failure, the morphologies and element distributions of the brittle fracture surfaces induced by these two methods are also compared. High-magnification SEM/EDX analysis of the fracture interfaces indicated that the solder residue on the package pad is more prevalent for SnPb solder compared to SnAgCu solder, and for high-speed shear testing compared to pull testing.
The study was performed to compare the high speed ball shear/ball pull tests with the board level mechanical drop test. The emphasis is placed on the correlation of the failure mode and energy absorption between these two testing methods. The objective is to investigate the feasibility of using high speed ball shear/ball pull tests as an alternative method to evaluate solder joint integrity under dynamic loading. This study compared high-speed bondtesting (shear and pull) with board level drop testing (BLDT) of BGA packages using Sn4.0%Ag0.5%Cu solder balls and either an ENIG or OSP package substrate surface finish. High-speed shear and pull tests were carried out at various speeds; failure modes were recorded, together with force and fracture energy data. In addition, a detailed microscopic analysis (SEM and EDX) was executed on both complementary surfaces (ball and pad) of brittle fracture failures from both the shear and pull test samples. The results of these studies showed high similarity to those from brittle fractures generated during BLDT of the same packages. Furthermore, there was a strong correlation between the various bondtesting parameters at the point brittle fractures occurred and the number of drops to failure seen in BLDT. In summary, it is suggested that brittle fractures obtained in high-speed bondtesting are a strong indicator of BLDT behavior.