CHAPTER 9 CONCLUSIONS AND SCOPE FOR FUTURE WORK

Transcription

1 200 CHAPTER 9 CONCLUSIONS AND SCOPE FOR FUTURE WORK 9.1 CONCLUSIONS Trouble free functioning of equipment in severe vibration environments depends on the robustness of electronic systems which they use. Non-functioning of equipment (particularly in defense applications) due to sudden failure of electronic systems will lead to heavy losses of life and property. Therefore, it is necessary to test the electronic assemblies for their ruggedness or robustness to sustain severe vibration environments before they are put in field use. If they fail to survive the test conditions, then design engineers have to propose alternative methods to avoid early failure. The present research work focuses on the methods to reduce the peak acceleration, and transmissibility levels experienced by board level electronic packages in severe vibration conditions. Third chapter of the thesis was dedicated to study the influence of boundary conditions on the natural frequencies and the damping ratio of the PCB assembly. By using modal analysis; fundamental natural frequencies and damping ratios were determined for various PCB mounting conditions. Test results were validated by numerical simulation. From test results following conclusions are made:

2 The PCB assembly mounted on 2 screws and 4 screws (corner) showed minimum fundamental natural frequency compared to rest of the mounting methods. 2. The PCB assembly mounted on 4 screws (diamond), six screws and eight screws showed higher fundamental frequencies and damping ratio. 3. Placing the fastening screws at the centre of the PCB edges will have higher fundamental frequency of the PCB assembly. 4. Placing the fastening screws at the corners of the PCB will have lower fundamental frequency of the PCB assembly. 5. By increasing the number of point supports (number of screws) the fundamental frequency and damping ratio can be increased. Other objective of the research work was to reduce the amplification of acceleration, displacement levels and transmissibility ratios experienced by board level electronic assemblies at resonant frequencies in severe vibration environments by using a suitable vibration isolating material and having good damping properties. For this purpose rubber (neoprene) was selected as a spacer or stand-off (to mount PCB) material to investigate its suitability and effectiveness as a vibration isolator in reducing the vibration amplitudes in different vibration environments. Series of experiments were conducted on the DIP-PCB and PSOP-PCB assemblies in sinusoidal, random and mechanical shock environments by mounting the PCB assembly on the rubber spacers. The test results were compared with the results obtained by conducting similar tests on the two PCB assemblies when they were mounted on plastic spacers. The percentage of reduction in vibration amplitudes in different vibration environments obtained by using the proposed method is tabulated in

3 202 Table 9.1. Figures in the Table 9.1 indicate the percentage of reduction in the PCB assemblies response parameters when they are mounted on rubber spacers (in comparison to the response parameters of PCB mounted on plastic spacers). Table 9.1 Percentage of reduction in the PCB response parameters (PCB mounted on rubber spacers) Percentage (%) of reduction achieved in: Vibration Environment Peak acceleration levels G rms levels PSD levels Fatigue damage ratio DIP PSOP DIP PSOP DIP PSOP DIP PSOP Sinusoidal Random Mechanical Shock From the above table it can be seen that, considerable percentage of reductions in peak acceleration levels in case of sinusoidal and shock environments are achieved by mounting the PCB assembly on rubber spacers. Also, G rms and PSD levels in random vibration environment are reduced to a great extent when the PCB assembly is mounted on rubber spacers. By mounting the PCB assembly on rubber spacers, a considerable reduction in fatigue damage ratio of the component lead wires is also achieved. Also from Table 9.1 it is observed that the rubber spacers act as very good vibration isolators in sinusoidal environment in case of PSOP-PCB assembly and the same is not true in random vibration environments.

4 203 Other method proposed to reduce the PCB accelerations, displacements and transmissibility ratios was that of mounting the longer edges of the PCB assembly on rubber pads. The vibration test results have showed that, the rubber pads improved the life of both the electronic assemblies by reducing the peak acceleration levels in sinusoidal and shock environments and G rms accelerations in random vibration environments. The percentage of reductions achieved by mounting the PCB assemblies on rubber pads is summarized in Table 9.2 (with reference to the response of PCB assemblies mounted on plastic spacers). Table 9.2 Percentage of reduction in the PCB response parameters (PCB mounted on rubber pads) Percentage (%) of reduction achieved in: Vibration Environment Peak acceleration levels G rms levels PSD levels DIP PSOP DIP PSOP DIP PSOP Sinusoidal Random Mechanical Shock From Table 9.1 and Table 9.2 it can be seen that, the rubber spacers and rubber pads have reduced the PCB assembly responses and improved the life of the electronic equipment. Also, form the two tables it may be observed that, the PSOP-PCB assembly experiences lesser damage compared to the DIP-PCB assembly and this may be attributed to the smaller size of PSOP-PCB assembly. Therefore, by choosing miniature electronic packages having high package density, a smaller size PCB may be used which improves the natural frequencies and reduce the dynamic displacements.

5 204 The comparative statement of the percentage of reduction achieved using the proposed methods and the methods proposed by past researchers are given in Table 9.3 and same is shown with the help of a bar chart (Figure 9.1). Table 9.3 Percentage of reduction in vibration amplitudes obtained by different researchers Vibration Environment Ho C.V Veprik Silin Mark Peak Acceleration G rms Probst Eugene Sakri Ho Ho Veprik Sakri C.V C.V PSD Veprik Sakri Sinusoidal Random Mechanical Shock From data tabulated in Table 9.3 it can be seen that, the proposed methods are also effective in reducing the vibration amplitudes in sinusoidal, random and mechanical shock environments. Figure 9.1 Percentage of reduction in vibration amplitudes obtained by different researchers

6 Advantages of the Proposed Methods As the rubber spacers or pads are used to support the PCB assembly, entire PCB area is free for complete usage. Neoprene rubber material is easily available at affordable prices. Rubber spacer shapes and size can be customized to ones need and application. Servicing of PCB assemblies will be easy and quick Disadvantages of Proposed Methods One has to use proper and equal tightening torque to equally compress all the spacers. This needs initial trials to be conducted which consume time. The rubber spacers are less effective at higher inputs of mechanical shock loads. 9.2 SCOPE FOR FUTURE WORK From Table 9.3 and Figure 9.1, it is seen that the percentage of reduction achieved by the present method is relatively less compared to the methods used by past researchers. But, there is still lot of scope for further reduction in vibration amplitudes by using the following options. Experiments may be conducted by mounting PCB assembly on Silicone and Gel based rubber mounts to verify their effectiveness in reducing vibration amplitudes. Rubber spacer sandwiched between metallic washers may be used as PCB vibration isolation devices.

7 206 Alternate boundary conditions such as four-screws (diamond) may be used to mount the PCB assembly on Silicone and Gel based rubber mounts. Ribbed rubber pads may used to reduce the vibration amplitudes. Concentric rubber bush may be used as vibration isolators. Rubber springs (bellows) may be used as vibration isolators. Fatigue life estimation may be done by measuring the strain induced in the PCB assembly when it is mounted on conventional spacers (nylon spacers) and Rubber mounts.