This paper is a summary of a literature survey done on nonhermetic optoelectronic packages focusing on the cost and reliability aspects. The development of reliable non-hermetic lasers would not only lead to the elimination of the costs specifically associated with hermetic packaging but also lead the way for possible revolutionary low-cost optoelectronic packaging technologies. The paper starts with an introduction to the concept of hermeticity in optoelectronic packages, followed by a detailed review of the reliability issues and failure mechanisms observed in non-hermetic packages when compared to their hermetic counter parts. A hermetic seal in electronic packages is used to prevent the entry of air, foreign gases, contaminants, and most importantly moisture. To seal a package hermetically is particularly challenging and expensive. There are various techniques that have been investigated, to improve the reliability of non-hermetic packages. A few are discussed here such as facet passivation type, choosing a better optical glass and encapsulation techniques. Also accelerated testing is discussed. Some of the most common and important failure mechanisms like popcorn failure, adhesive degradation, laser misalignment and epoxy swelling are discussed.
A MEMS based device consisting of microactuators was modeled using finite element analysis. The temperature profile of the complete package was obtained and compared to experimental measurements. Good agreement was found between the modeling and measurements. Parametric studies of potential design parameters of the chip package to decrease the power requirements to the actuators have been studied. Increasing the gap between the handle layer and the device layer of the SOI (silicon on insulator) chip from 2 to 3 microns resulted in a reduction of 10% (0.2 Watts) per beam of the actuator. A glass top chip proved to be better at reducing the power requirements for the actuators when compared to a silicon top chip. Modeling shows that relief cuts in the substrate had a larger effect on the power reduction compared to those on the top chip since the heat conduction path to the substrate is a lower resistance path. The power reduction was as high as 50% (1.1 Watts) per beam of the actuator, when the relief cut in the substrate was 50 microns.
The required voltage and current to produce a desired displacement with a MEMS electrothermal actuator can vary considerably between devices due to manufacturing process variation. This article presents a case for using resistance change divided by the ambient temperature resistance (ΔR/R0) as the quantity sensed in a feedback system to control the displacement, and avoid either melting the actuator with over drive, or not producing the desired actuation with under drive. The electrical resistance of a MEMS thermal actuator was calculated using a resistivity model that includes both the extrinsic and intrinsic conduction mechanisms in silicon. The model also accounts for the change in resistivity due to the strain in the silicon resulting from the confinement of the actuator by the two mounting posts. Using the model, it was shown that ΔR/R0 is a unique function of the effective temperature over the operating temperature range. The displacement of the actuator was calculated from a simple model, and was used to generate plots of the resistance change versus displacement. These plots were compared with the nearly universal plot of ΔR/R0 versus displacement found experimentally for thermal actuators with a range of dimensions. The calculated and measured curves were in reasonable agreement.
A failure mechanism of pogo-type probe pin is investigated. A probing tester with actuation capable in three-axes is used to simulate the actual inspection process experimentally. Force required to break in surface oxides and develop electrical contact is measured. Contact resistance history reveals that pins mating to Sn surfaces fail earlier than SnPb surfaces. Through periodic inspection of pin using microprobe/EDS as a function of probing count, the general root cause of pin failure is turned out to be pin tip wear out associated with Sn oxide growth on its surface. The cause of earlier failure of the pin probing matte Sn surface is identified as severe wear out by a rough and abrasive characteristic of matte Sn.
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