Evaluation of Stresses in Flame Resistant Materials

K. Arun Vasantha Geethan¹ and S. Jose²

¹Department of Mechanical Engineering, Sathyabama University, Chennai, India.

²Alpha College of Engineering, Chennai, India.

DOI : http://dx.doi.org/10.13005/msri/070122

Article Publishing History
Article Received on : 15 Mar 2010
Article Accepted on : 25 Apr 2010
Article Published :
Plagiarism Check: No

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ABSTRACT:

Reliability prediction is fundamental to system design. It involves the quantitative assessment of systems reliability prior to development. For many years the reliability of an electronic system was based, to a great extent, upon the junction temperatures of the semiconductor devices. Substantial efforts were made in the fabrication methods, mounting methods, and cooling techniques of the electronic devices to reduce these hot spot temperatures below 100 °C. This has produced a significant improvement in the reliability and effective operating life of the equipment. However, the electronic failure rates are still too high. Additional reductions in the failure rates must be achieved to further improve the reliability of our electronic equipment. Some of the failure mechanisms that can cause malfunctions in electronic systems are examined in this paper Experience has shown that most of these failures are produced by a mismatch in the thermal coefficients of expansion (TCE) of the different types of materials typically used in electronic assemblies. The mismatch often generates high forces and stresses, which produce fractures and cracks in the electronic components and assemblies. An examination of a large number of avionics failures has shown that most of them are mechanical in nature. They typically involve fractures in solder joints, electrical lead wires, plated through holes (PTH), electrical cables, connectors, adhesive bonded joints, and hermetic seals. These failures are often produced by various combinations of thermal, vibration, shock, humidity, and salt environments, combined with poor manufacturing processes and poor design practices. These failures must be reduced in order to achieve a substantial improvement in the system reliability. The paper aims at presenting the numerical evaluation by finite element analysis of the common class of PCBs under the mechanical stress. This work contributes to the study of vulnerability in packaged electronics by comparing the numerical result with finite element analysis.

KEYWORDS: Reliability; Thermal coefficient of expansion; PCB; Malfunctions; Mechanical stresses; Finite element analysis.

Copy the following to cite this article:

Geethan K. A. V, Jose S. Evaluation of Stresses in Flame Resistant Materials. Mat.Sci.Res.India;7(1)

Copy the following to cite this URL:

Geethan K. A. V, Jose S. Evaluation of Stresses in Flame Resistant Materials. Mat.Sci.Res.India;7(1). Available from: http://www.materialsciencejournal.org/?p=2271

Introduction

Electronic assemblies utilize a wide variety of plastics and metals in the fabrication and manufacturing of their products. These materials can have significant differences in their thermal TCE, which may result in high strains and stresses in the lead wires, solder joints, and PTH if these factors are not understood or if they are ignored. Temperature changes will produce dimensional changes in almost all materials normally used in the assembly of electronic chassis and PCBs. Dimensional changes can occur due to power cycling where the power is turned on and off, which induces temperature changes within the electronic assembly. This can also be caused by thermal cycling, where the outside ambient temperature changes and the thermal lag within the chassis forces thermal gradients to develop because of different mass effects. These dimensional changes, which can occur along the X, Y, or Z axes of the electronic assemblies, can produce a wide variety of failures in the structural elements of these assemblies.

Problem Statement

A large number of components such as resistors, capacitors, diodes, flat packs, and hybrids are fabricated with axial leads. Wire-forming tools are then used to bend the leads 90° for insertion in through hole PCBs for flow soldering. During exposure to thermal cycling environments, the PCB typically ‘ has a higher TCE than the component in the X-Y plane, so the PCB will expand more than the component. Since the wire in an axially leaded component has a relatively low bending stiffness, virtually all of the deflection difference between the PCB and the component will be taken up by the bending in the wire, as shown in Figure below. A bending moment will be developed in the wire, at the PCB solder joint, which will produce a bending stress in the lead wire. This stress value should be limited to achieve a better operating life.

Figure 1: Expansion due to difference in Tce (through hole mount)