Nanoindentation study of the Pb-free solders in fine pitch interconnects

Mohankumar, K.; Tay, A.A.O.

doi:10.1109/eptc.2004.1396656

Cited by 2 publications

(1 citation statement)

References 15 publications

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“…Obviously the stress at the crack tip is reduced below the threshold needed for further IMC fracture crack growth. Based on [13,14] the fracture stress of IMC layers is at about 170 MPa. Since the IMC is not explicitly modeled, the solder stress at the crack tip was taken as result criteria.…”

Section: Investigation Of Imc Crack Length Effectmentioning

confidence: 99%

A detailed investigation of the failure formation of copper trace cracks during drop tests

Kraemer

Wiese

Rzepka

et al. 2010

3rd Electronics System Integration Technology Conference ESTC

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The development cycle of new products can be dramatically reduced if exact lifetime models are at hand. This requires the precise knowledge of the failure modes and the failure position under all test and service conditions. In case of dynamic mechanical loads like drops of BGA modules, broken copper traces at the PCB side are more and more often observed to be the ultimate failure effect. However, straightforward FEM simulations have shown unrealistic high stress and strain results not matching experimental observations which prove that a realistic representation of this failure is not trivial. A comprehensive physical investigation of the failure formation combined dye and pry tests, electron backscatter diffraction EBSD and 3-D X-ray tomography analyses. The results revealed the complex nature of the dynamic failure propagation. In addition to the ultimate fracture of the copper trace cracking of the IMC on top of the PCB pad was detected. Both failure modes are initiated during the drop event at different times and propagate with different speeds. Both failures are mutually interacting with each other with no single mode alone dominating the failure evolution. Hence, lifetime modeling needs to account for both failure modes in order to capture the copper trace stress realistically. Numerical simulations have been conducted investigating the experimental findings. A small flaw already reduces the stress level at the pad/solder interface substantially. The IMC crack is found to be initiated first reaching a stable length at the half pad diameter. In this condition, the copper trace is stressed at a lower level accumulating plastic strain enough to ultimately create the trace fracture in a realistic number of cycles. A very realistic 2nd level interconnection model has been created based on the combined failure mechanism found by comprehensive physical failure analysis, and the numerical simulations. It now allows lifetime prediction of BGA assemblies under drop test conditions with high accuracy and reliably. Hence, it will be able to replace expensive experiments by fast numerical assessments in the design optimization phase of product and technology development

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Section: Investigation Of Imc Crack Length Effectmentioning

confidence: 99%

A detailed investigation of the failure formation of copper trace cracks during drop tests

Kraemer

Wiese

Rzepka

et al. 2010

3rd Electronics System Integration Technology Conference ESTC

View full text Add to dashboard Cite

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Influence of Intermetallic Properties on Reliability of Lead-Free Flip-Chip Solder Joints

Limaye

Vandevelde

Labie

et al. 2008

IEEE Trans. Adv. Packag.

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Electroplated pure tin bumping as a lead-free alternative for ultra fine pitch applications is a relatively easy process and has provided us with comparable results to eutectic Sn/Pb for thermal cycling reliability. Experimentally, it has been reported that a significantly higher ( 40%) thermal cycle fatigue life is seen with the use of cobalt under bump metallization (UBM) instead of copper UBM for a flip-chip device assembled on an alumina substrate. In the current approaches used to estimate fatigue life of solder joints, the solder joint is treated as a homogenous material and modeled as such. However, the smaller joint sizes and higher reactivity of Sn implies that a larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behavior of these intermetallic layers within the solder joint. In order to investigate if a simplified engineering approach can provide some insight into this issue, we have attempted to explicitly model the intermetallics as a continuous but separate part of the solder joint. The main damage parameter investigated is the accumulated inelastic strain in a single thermal cycle. From the results, it is clear that the Young's modulus of the intermetallic layer plays an important role, more so when the ratio of intermetallic thickness to the solder joint standoff increases. Thickness of the intermetallic layer also influences the overall strain accumulation in the same manner. The CTE of the intermetallic layer has a relatively lesser influence on the strain accumulation. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20%-30%.Index Terms-Finite-element thermomechanical analysis, intermetallics modeling, lead-free solders, solder fatigue, under bump metallization (UBM).

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Nanoindentation study of the Pb-free solders in fine pitch interconnects

Cited by 2 publications

References 15 publications

A detailed investigation of the failure formation of copper trace cracks during drop tests

A detailed investigation of the failure formation of copper trace cracks during drop tests

Influence of Intermetallic Properties on Reliability of Lead-Free Flip-Chip Solder Joints

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