Olli Salmela

Reliability Analysis of Lead-Free Solder Castellations

In this paper, the reliability of lead-free solder castellations is considered. The newly developed stress-dependent Engelmaiers solder fatigue model is utilized in this task. Based on this model, it is possible to interpret the thermal cycling test results. A very good agreement between the test results and the lifetime predictions is obtained. Using the lifetime prediction model, optimal solder castellation shape is investigated. Based on the findings, the fatigue life can be improved by up to 30% simply by solder pad length optimization. Further increment in lifetime length can be expected if the solder joint shape is optimized with the help of modeling tools presented here. Understanding how the crack propagates in solder material is vital if optimal lifetime behavior is expected.

Ceramic packaging technologies for microwave applications

Laminated ceramic packaging technologies, such as high temperature cofired ceramics (HTCC) and low temperature cofired ceramics (LTCC), are a low-cost alternative for metallic packages in high reliability applications in the microwave frequency region. As in metallic packages the main concern-from an electrical performance point of view-is the resonances in cavities; in ceramic packages isolation also has to be considered. Typically good isolation has been accomplished with the use of vias in strategic regions, such as around chip cavities and between neighbouring signal lines. In this paper, the key features of laminated ceramic packaging technologies are presented with some 3-D electromagnetic simulation results concerning isolation.

BGA Interconnection Reliability in Mirrored Module Configurations

Interconnection dimensions are becoming more important due to electrical signal timing requirements and stray effects, such as unwanted inductance, leading to increasingly denser packaging. One way to shorten the signal path is to use mirrored structures, where the components are placed on opposite sides of the printed circuit board (PCB). This paper presents thermal cycling test (TCT) results in a temperature interval from −40 °C to +125 °C and simulation results of plastic ball grid array components mounted on one side [single side (SS) configuration] and in different mirrored configurations on a PCB. Anand’s constitutive model is used in the finite-element analysis software to calculate dissipated creep energy densities in the interconnections. Field lifetimes of the presented cases were also calculated on the basis of the dissipated creep energy densities and TCT data. In addition, supporting microscopic studies were done. The single-sided configuration had a longer lifetime than the mirrored configurations with components on opposite sides. The mirrored configurations had adverse simulated creep energies and TCT lifetimes, compared with the single-sided configuration. The simulations proved that the flexibility of the mirrored structures was hindered, thus increasing stress levels in critical interconnections.