THESIS
2017
xxiii, 143 pages : illustrations (some color) ; 30 cm
Abstract
Pad cratering is a dominant failure mode for BGA-PCB assemblies with the non-solder mask defined pad opening configuration. Over the last 20 years, due to the implementation of lead-free reflow processes, a higher reflow temperature is required, which introduces a larger thermal impact on PCBs. In order to compensate this problem, phenolic cured epoxy resin system is used, which is more vulnerable to mechanical loadings. This frequently leads to the occurrence of pad cratering. As a typical brittle failure mode, pad cratering has no precursors, and it cannot be reworked. Due to these reasons, pad cratering has become a major consideration for board level reliability in electronic industries....[
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Pad cratering is a dominant failure mode for BGA-PCB assemblies with the non-solder mask defined pad opening configuration. Over the last 20 years, due to the implementation of lead-free reflow processes, a higher reflow temperature is required, which introduces a larger thermal impact on PCBs. In order to compensate this problem, phenolic cured epoxy resin system is used, which is more vulnerable to mechanical loadings. This frequently leads to the occurrence of pad cratering. As a typical brittle failure mode, pad cratering has no precursors, and it cannot be reworked. Due to these reasons, pad cratering has become a major consideration for board level reliability in electronic industries.
To prevent pad cratering failure, evaluation of pad cratering strength in BGA-PCB assemblies is desirable. The test methods for pad cratering strength evaluation can be classified into board level and package level. Board level tests are the most straightforward way to evaluate the pad cratering strength. They imitate the loading conditions during the normal operation of electronic components, including bending, thermal cycling (shock), and even drops. However, this type of tests is expensive and not flexible. Compared with board level tests, package level tests are the simplest and most efficient way to estimate the pad cratering resistance, and these are used extensively in the industry. IPC-9708 specifies three standardized package level test methods for pad cratering evaluation, namely the ball pull, ball shear, and (hot) pin pull tests. These tests apply pure tension or shear loading on the solder joints, thus the loading conditions do not fit the reality in board level testing. In consequence, the package level tests may reveal a pad cratering resistance inconsistent with the board level tests.
Correlations between the package level tests and board level tests are always desirable for the pad cratering strength evaluation. This research proposes an innovative pin shear test as a package level test method to imitate the board level loading conditions. With the assistance of finite element analysis, a strain dominant failure criterion of overloading induced pad cratering is defined. To verify the effectiveness of this failure criterion, both experimental and simulation studies for pad cratering strength are conducted, and the corresponding results show this failure criterion has the capability to predict the board level pad cratering failure. For a more comprehensive research under the generalized loading condition, the post-reflow residual strain is considered as a pre-loading of over-stress induced pad cratering. BGA packages with various material bases and terminal areas are evaluated, and the corresponding results reveal the BGA packaging material may bring different influences on the board level pad cratering strength.
The fatigue phenomenon is another important consideration in pad cratering failure. In a daily service environment, vibration and repetitive drops are the most relevant conditions for fatigue loadings. Thus the failure criterion for fatigue induced pad cratering is also sought. In this research, a fatigue failure criterion for pad cratering is proposed with a correlation between the board level cyclic bending test and the repetitive drop test. The time scaling scheme with the Willams-Landel-Ferry equation is applied, and finite element analysis for pad cratering loading matching is conducted to seek the equivalent cyclic bending strain of various G-level drop impacts. With the experimental study of equivalent cyclic bending tests and pad cratering failure detection, the strain-number of cycles curve is able to be defined by Basquin’s law. This curve can also be verified by repetitive board level drop tests, and it means this curve should be defined as a common failure criterion for fatigue induced pad cratering. Additionally, due to both the cyclic bending test and repetitive drop test sharing the same failure criteria for pad cratering, it is possible to use the cyclic bending test to predict the life of the repetitive drop test, which makes the testing simpler and less time consuming. From the S-N curve of pad cratering failure for both the cyclic bending and repetitive drop test, a universal life prediction method for fatigue induced pad cratering is built. In addition, the S-N curve of the pad cratering fatigue life prediction can also be regarded as a material property of a PCB, and it is recommended as a standard material characterization part for reliability design.
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