CHAPTER 5 RANDOM VIBRATION TESTS ON DIP-PCB ASSEMBLY

Transcription

1 117 CHAPTER 5 RANDOM VIBRATION TESTS ON DIP-PCB ASSEMBLY 5.1 INTRODUCTION Random vibration tests are usually specified as acceptance, screening and qualification tests by commercial, industrial, and military manufacturers of electronic equipment. It has been shown that random vibrations more closely represent the true vibration environment in which the electronic equipment has to operate. Random vibration is also popular for performing accelerated life tests, to estimate approximate fatigue life of critical elements in electronic assemblies. Hence, random vibration continues to gain wide acceptance for use in many different testing programs associated with electronic equipment. The random vibration environments may be observed in airplanes, missiles, automobiles, trucks, trains, petroleum drilling machines, steel rolling mills, and numerically controlled machine tools. Random vibration test has been proved to be a very powerful tool for improving the manufacturing integrity of electronic equipment by screening out defective components and defective assembly methods, which result in a sharp improvement in the overall reliability of the system (Steinberg 2001). Therefore, electronic packaging designers and engineers must know the fundamental nature of random vibration and fatigue, in order to design, develop, and manufacture cost-effective and lightweight electronic structures that are capable of withstanding harsh random vibration environments.

2 118 Random vibrations are non-periodic and characterized in terms of the bandwidth of the frequencies as it consists of many different sinusoidal frequencies superimposed upon one another. All of the frequencies within the bandwidth are present simultaneously at any instant of time for every frequency. When the frequency bandwidth is from 20 to 1000 Hz, every natural frequency of every structural member between 20 and 1000 Hz will be excited at the same time. This includes every fundamental natural frequency, and every higher harmonic of every structural member within that bandwidth. Knowledge of the past history of random vibration is adequate to predict the probability of occurrence of various acceleration and displacement magnitudes, but it is not sufficient to predict the precise magnitude at a specific instant. Random vibration environments in the electronics industry normally deal in terms of PSD (indicated as P ) or mean squared acceleration density. However, random vibration can also be expressed in terms of velocity spectral density, and in terms of displacement spectral density. Accelerations, velocities, and displacements for random vibration are typically expressed in terms of root mean square (RMS) values. Many standards such as MIL-810-F, JEDEC JESD22-B103B (JEDEC, 2006) are used as qualification tests for electronic equipment. Many electronic package manufacturers such as Motorola, Intel, and AMD follow their own standards or adopt international standards to test their newly designed packages. This chapter deals with the random vibration experiments conducted on DIP and PCB assembly, and investigations made on the effectiveness of rubber spacers and pads as vibration isolating devices in a random vibration environment.

3 RANDOM VIBRATION TESTS WITH PCB ASSEMBLY MOUNTED ON PLASTIC SPACERS Experimental Procedure The test vehicle used for random vibration tests consists of a PCB and a DIP, having 16 lead wires (8 x 2 rows) as shown in Figure 4.2 (Chapter 4). The random vibration tests are conducted by exciting the PCB assembly in Z direction (perpendicular to PCB plane) using an electrodynamic shaker and random vibration software. The input acceleration PSD used for tests is the condition D as specified by the JEDEC standard and shown in Figure 5.1 and the frequency split up for this level is shown in Table 5.1. The test was conducted for the duration of 30 minutes. Figure 5.1 JEDEC test conditions

4 120 Table 5.1 Frequency breakpoints of PSD of level D of JEDEC Frequency (Hz) PSD level, (G 2 /Hz) Input G rms = 1.11 As the random vibrations are non-periodic and follows the Gaussian (normal) distribution, hence this was checked by collecting the random vibration signals (acceleration) in time domain using LabVIEW software and NI-4472 data acquisition (DAQ) card. From Figure 5.2 it is evident that, the distribution follows normal or the Gaussian nature. Figure 5.2 Time domain signals and corresponding normal distribution

5 121 The complete test setup for conducting random vibration tests on the DIP-PCB assembly is as shown in Figure 4.1 (Chapter 4). The PCB assembly was mounted on an aluminum fixture using four plastic spacers and steel fasteners. The natural frequencies of the PCB assembly and fixture were first determined using logarithmic sine sweep test and they are tabulated in Table 5.2. From the table it is observed that, the natural frequencies of PCB and fixture are not close to one another. Table 5.2 Natural frequencies of PCB assembly and Fixture Frequency (Hz) PCB Assembly Fixture The reference PSD profile as per JEDEC standard, was programmed using the random vibration test software and is as shown in Figure 5.3. The input acceleration PSD profile was monitored and controlled in a closed loop by using an accelerometer (B & K 4513, 8.6 g mass, 100 mv/g sensitivity) placed on the fixture. Another accelerometer (B & K 4517, 0.6 g mass, 10 mv/g sensitivity) placed near the component was used to measure the response of the PCB assembly.

6 122 Figure 5.3 Input acceleration PSD spectrum (control spectrum) The response of the PCB assembly measured near the component and in the Z direction is as shown in Figure 5.4. The area under the curve (Figure 5.4) represents the root mean square acceleration (G rms ) of the PCB assembly, and from the figure, G rms value is found to be 7.99, and the acceleration PSD at first resonant frequency is about 6 G 2 /Hz, whereas the input PSD at the same frequency is G 2 /Hz. Amplification of PSD level at resonant frequency will lead to increased G rms and induce more stresses in component lead wires. Figure 5.5 shows the transmissibility plot which is the ratio of input acceleration power spectral density and output acceleration power spectral density (PSD out /PSD in ). From this figure it is seen that, the acceleration PSD is amplified at resonant frequencies of 50 Hz, 210 Hz and about 500 Hz.

7 123 Figure 5.4 Response of the PCB mounted on plastic spacers Figure 5.5 Transmissibility plot of the PCB mounted on plastic spacers During 30 minutes of random vibration test, the failure of the lead wires was monitored using a failure detecting circuit as shown in Figure 4.3 (Chapter 4). If at any instant of time, any of the lead wire fails; the LED provided on the circuit will go off or glow intermittently due to opening of the lead wire.

8 124 The repeatability of the test was checked by performing five trials and the results obtained are tabulated in Table 5.3. Table 5.3 Random vibration test results Trial No. G rms The single amplitude displacement Z rms, near the component at resonant frequency f n is calculated using the Equation (5.1). Z rms 9.8G rms 2 f n (5.1) 9.8x7.99 = 0.79mm 50 2 The RMS displacement (Z rms ) experienced by the PCB assembly is found to be 0.79 mm and the same is validated using spectrum analysis in ANSYS ANSYS Simulation Results To validate the experimental results, ANSYS, commercially available finite element analysis software was used to simulate the random vibration tests. Three-dimensional geometries of the PCB and the DIP along with lead wires were modeled for the purpose of analysis. PCB, DIP body,

9 125 and lead wires were meshed using 10 node solid 92 elements. The material properties of the PCB assembly components used in the analysis are tabulated in Table 4.9 (Chapter 4). The spectrum analysis was performed by exciting the PCB assembly using an input random vibration test profile as shown in Figure 5.6 (level D of JEDEC standard). The excitation PSD profile was applied at the fixed nodes of the PCB holes in Z direction (perpendicular the PCB plane). A uniform damping ratio of was used during the spectrum analysis (corresponding to the PCB assembly mounted on plastic spacers). Figure 5.6 Input PSD test profile (level D of JEDEC) Figure 5.7 shows 1 RMS acceleration (G rms ) response of the PCB assembly, and the G rms acceleration at the centre of the PCB is 8.73 (85.77 m/s 2 ) which is close to the test data (7.99). Similarly, Figure 5.8 shows the 1 displacement of the PCB assembly due to particular random vibration input profile. The maximum 1 RMS displacement at the PCB centre is found to be 0.79 mm which is equal to the displacement value obtained from the experiment.

10 126 Figure RMS acceleration plot Figure RMS displacement plot The response acceleration PSD measured at the centre of the PCB is shown in Figure 5.9, and from this figure it is found that the PSD at first resonant frequency is about 7 G 2 /Hz which is again close to the experimental value.

11 127 Figure 5.9 Response PSD captured at the PCB centre The comparison of experimental and simulation results are tabulated in Table 5.4. From the tabulated data it is observed that, the simulation results are in close agreement with the experimental results. Therefore, the finite element analysis results such as stress induced in lead wires of the DIP due to random vibrations will be used to estimate the fatigue life of lead wires. Table 5.4 Comparison of Experimental and ANSYS results Parameters Experimental ANSYS 1 RMS acceleration, G rms RMS displacement, Z rms (mm) PSD at first resonant frequency (G 2 /Hz) 6 7

12 RANDOM VIBRATION TESTS WITH THE PCB ASSEMBLY MOUNTED ON RUBBER SPACERS Experimental Procedure In an attempt to reduce the excessive PCB deflection and corresponding damage due to random vibration loads, the PCB assembly was mounted on four rubber spacers (Figure 4.6, Chapter 4) and subjected to, D level random vibration test. Figure 5.10 shows the acceleration PSD response when the PCB assembly is mounted on rubber spacers. From this figure, the G rms acceleration and response PSD levels are found to be 2.72 and 0.7 G 2 /Hz. From the test results it is seen that, the G rms acceleration is reduced by 66%, the acceleration PSD is reduced by 87% compared to the corresponding responses obtained when the PCB assembly was mounted on plastic spacers. (Hz) Figure 5.10 Response of the PCB mounted on rubber spacers

13 129 Figure 5.11 shows the transmissibility graph when PCB is mounted on rubber spacers. Using the rubber spacers, the transmissibility ratio is reduced by 16 % (at first frequency). (Hz) Figure 5.11 Transmissibility plot of the PCB mounted on rubber spacers The RMS (Z rms ) displacement at the centre of the PCB when it is supported on rubber spacers is found to be 0.29 mm (Equation 5.1), on the contrary, when PCB was mounted on plastic spacers the RMS displacement was 0.79 mm. Therefore, from these results it can be concluded that, rubber spacers can be effectively used to reduce the damage to the PCB and the electronic packages mounted on it by reducing G rms and Z rms levels in a random vibration environment. Also, the fatigue life of PCB assemblies can be improved by mounting the PCB assembly on rubber spacers ANSYS Simulation Results Spectrum analysis, simulating the PCB assembly mounted on rubber spacers was carried out in ANSYS following the procedure as explained in section A damping ratio of (which is obtained from

14 130 sine sweep test) was used in this spectrum analysis. The 1 RMS acceleration (G rms ) value obtained from the simulation is as shown in Figure 5.12, from this figure the RMS acceleration at the PCB centre is found to be 3.1 (30.41 m/s 2 ). Figure RMS acceleration plot Similarly, 1 RMS displacement (Z rms ) plot obtained from the simulation is shown in Figure 5.13, from which the Z rms at the PCB centre is found to be 0.29 mm which is close to the experimental value. Figure 5.14 shows the PSD response captured at the centre of the PCB. From this figure the PSD value at fundamental frequency is found to be about 1 G 2 /Hz.

15 131 Figure RMS displacement plot Figure 5.14 Response PSD captured at the PCB centre

16 RANDOM VIBRATION TESTS WITH PCB ASSEMBLY MOUNTED ON RUBBER PADS In an another effort to reduce the PCB responses due to random vibrations, the longer edges of the PCB were made to rest on the longer faces of the two rubber pads and fastened to the fixture plate using fastening screws (refer Figure 4.14 in chapter 4). Now the PCB assembly is subjected to random vibrations as per JEDEC s D level (Figure 5.1) in a direction perpendicular to plane of the PCB. The response of the PCB assembly due to random vibration input is as shown in Figure 5.15, from which it is observed that, the RMS acceleration experienced by the PCB assembly is 1.95 and PSD level at first resonant frequency is 0.4 G 2 /Hz. Therefore, by using the rubber pads the RMS acceleration is reduced by 28% and PSD amplitude at first resonant frequency 43% when compared to the corresponding responses when the PCB assembly was mounted on rubber spacers. (Hz) Figure 5.15 Response of PCB mounted on rubber pads

17 133 The transmissibility plot as shown in Figure 5.16 is obtained when the PCB assembly was mounted on rubber pads. From this figure it is observed that, the transmissibility ratio at fundamental frequency is further reduced by about 20 % when compared to the transmissibility of PCB mounted on rubber spacers. (Hz) Figure 5.16 Transmissibility plot of PCB mounted on rubber pads PCB responses obtained from random vibration tests for the three mounting methods are tabulated in Table 5.5 for comparison. From the table it is observed that, the rubber spacers and pads have effectively minimized the RMS acceleration, RMS displacement and PSD amplitudes of the PCB assembly in a random vibration environment. Table 5.5 Comparison of random vibration tests with different mountings PCB Mounting method RMS acceleration (G rms ) PSD (G 2 /Hz) RMS displacement (mm) Four plastic spacers Four rubber spacers Rubber pads

18 RESULTS AND DISCUSSIONS Random vibration tests using level D of JEDEC standard as input profile in Z direction of the PCB assembly was conducted for 30 minutes. The test results showed that, the 1 G rms acceleration, response PSD, and 1 RMS displacement are 7.99, 6 G 2 /Hz and 0.79 mm respectively when the PCB assembly was mounted on plastic spacers. By mounting the DIP-PCB assembly on rubber spacers, the G rms acceleration was reduced by 66%, PSD level by 88% and RMS displacement by 63%. As the displacement values are reduced, the corresponding stress amplitudes in lead wires will also be reduced and enhance the fatigue life of electronic packages. Thus, the rubber spacers act as good vibration isolators in random vibration environment also and are effective over entire frequency band. Other method of reducing the PCB response was that of clamping the longer edges of the PCB assembly on rubber pads. By mounting PCB on rubber pads, G rms level was reduced by 66%, PSD level by 93%, and displacement by 99% (compared to response of PCB mounted on plastic spacers). Thus, from Table 5.5 it is evident that, by using rubber spacers or rubber pads, the PCB displacement is greatly reduced and the corresponding life of electronic packages is improved.