Conductive Anodic Filament Failures

“Conductive Anodic Filament Failures in Fine-Pitch Through-Via Interconnections in Organic Package Substrates”

Authors: Koushik Ramachandran, Fuhan Liu, P. Markondeya Raj, Venky Sundaram and Rao Tummala; This email address is being protected from spambots. You need JavaScript enabled to view it..

Abstract: In this study, an accelerated test condition [130°C, 85% RH, and 100V DC] was used to investigate conductive anodic filament (CAF) in two organic package substrates: 1) cyclo-olefin polymer–glass fiber composite (XR3) and 2) epoxy-glass fiber composite (FR-4). Test coupons with through-via spacing of 100µm and 200µm were investigated. CAF failures were not observed in either substrate type with spacing of 200µm. With spacing of 100µm, insulation failures were observed in FR-4, while XR3 exhibited stable insulation resistance during the test. The substrates were characterized using gravimetric measurement, and XR3 was found to exhibit significantly lower moisture absorption compared with FR-4. CAF failures in FR-4 were characterized using scanning electron microscopy and energy-dispersive x-ray spectroscopy. Results suggest a strong effect of moisture sorption of organic resins on CAF failure at smaller through-via spacing in package substrates. (Journal of Electronics Materials, November 2012)

PCB Substrates

“Data Mining for Creep Corrosion on Desktop Computers”

Author: David Cook; This email address is being protected from spambots. You need JavaScript enabled to view it..

Abstract: When the PC industry switched from traditional leaded (SnPb) solders to lead-free (SnAgCu) solders, the industry considered new board surface finishes as well. The corrosion resistance of the leading Pb-free coatings, immersion silver (ImAg) and organic solder protectant (OSP) were questioned. With several years of field experience, organizations can quantify the efficacy of ImAg and OSP with actual field data. For the past few years, Intel’s warranty return centers performed a visual inspection for corrosion on all motherboards returned. A statistical analysis was performed on all returned products to determine the general extent of corrosion and to look for subpopulations with statistically significantly higher and lower return rates. It was found that excessive flux application dramatically affected the corrosion incident rate for motherboard products with ImAg finishes. No correlation existed between the percentage of sales to particular countries or geographies and the relative corrosion incident rates for the desktop motherboard part numbers in the study. Newer motherboard part numbers with OSP coatings experienced significantly reduced incidents of corrosion compared to earlier motherboards with ImAg coatings. Components on desktop motherboards showed no obvious signs of creep corrosion in the warranty returns population studied. (SMTA International, October 2012)

Solder Joint Reliability

“Characterization of Stress–Strain Response of Lead-Free Solder Joints Using a Digital Image Correlation Technique and Finite-Element Modeling”

Authors: G. Khatibi, M. Lederer, E. Byrne, A. Betzwar Kotas, B. Weiss and H. Ipser

Abstract: The stress-strain response of miniaturized Sn-Ag-Cu (SAC) lead-free solder joints in the thickness range of 80µm to about 1.1mm was studied. A high-resolution 3D digital image correlation system was used for in situ measurement of displacement and strain fields in the solder joints during tensile testing. These measurements showed that the localization of plastic strain and stress buildup occurs mainly at the interface of the solder. With increasing solder gap thickness, the size of the plastically deformed zone in the solder increases, resulting in transformation of a brittle interfacial fracture to a ductile fracture within the bulk of the solder. The experimental deformation plots of solder joints and strain-rate-dependent tensile tests on bulk solder material were used to establish a new constitutive material model for the solder. This strain-rate- and pressure-dependent material model was implemented in ABAQUS through the user subroutine CREEP. In agreement with the experiments, the finite-element method simulation revealed a pronounced thickness effect leading to higher tensile strength of thinner solder joints. (Journal of Electronics Materials, November 2012)

“Reliability of Passive and Active Components with Differences According to Lead-Free Paste Characterization”

Authors: Jörg Trodler, Heinz Wohlrabe, Ph.D., and Rüdiger Knofe; This email address is being protected from spambots. You need JavaScript enabled to view it..

Abstract: Components were analyzed according to shear stress and solder joint cracking. The study compared different flux and solder paste formulations in terms of wetting properties, void formation and reliability results for active and passive components. For passive components, pure nickel and tin finishes were compared. Shear stress was analyzed under automotive requirements. Test conditions used in the testing procedure were temperature cycling with different parameters in terms of deltaT: -40°/+125°C (30'/10"/30'). Data are available for 2000 cycles. Active components were also tested with this procedure. These components had different finish terminations, such as tin and NiPdAu/Ag for LQFPs, and eutectic SnAg and SAC balls for BGAs. Analysis was performed by cross-section and crack length after cycling measured. (SMTA International, October 2012)

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