CHAPTER 6 MECHANICAL SHOCK TESTS ON DIP-PCB ASSEMBLY

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1 135 CHAPTER 6 MECHANICAL SHOCK TESTS ON DIP-PCB ASSEMBLY 6.1 INTRODUCTION Shock is often defined as a rapid transfer of energy to a mechanical system, which results in a significant increase in the stress, velocity, acceleration, or displacement within the system. The time in which the energy transfer takes place is usually related to the resonant frequency, or to the natural period of the system. Shock loads often excite many of the natural frequencies in a complex structure, which can produce four basic types of failures in electronic systems. These failures are due to (1) high stresses, which can cause fractures or permanent deformations in the structure; (2) high acceleration levels, which can cause relays to chatter, potentiometers to slip, and bolts to loosen; (3) high displacements, which can cause impact between adjacent circuit boards- cracking components and solder joints, breaking cables and harnesses; and (4) electrical malfunctions that occur during the shock but disappear when shock energy dissipates. Fatigue is usually not an important consideration in shock, unless a million or more stress cycles are involved. When less than a few thousand stress cycles are expected, fatigue stress concentrations are ignored because they do not have a great influence on how or when the structure will fail (Steinberg 2001). Isolation systems are often used to protect sensitive electronic equipment in severe shock environments. Care must be exercised to

2 136 allow sufficient sway space around the equipment to prevent impact against other surrounding structures. The objective of this chapter is to investigate the suitability of rubber spacers and rubber pads as vibration isolators to reduce the peak acceleration levels experienced by the PCB assemblies in a mechanical shock environment. 6.2 SPECIFYING THE SHOCK ENVIRONMENT Many different methods have been used to specify shock motion or its effects. The three most popular methods are (1) pulse shock, (2) velocity shock, and (3) shock response spectrum. Pulse shock deals with accelerations or displacements in the form of well-known shapes such as half sine wave, square wave, triangular wave and others (Figure 6.1). Pulse shocks are easy to work with because the mathematics is simple and convenient. However, pulse shocks do not represent the real world. The true shock environment is seldom a simple pulse. Figure 6.1 Shock pulse types

3 137 Velocity pulse is concerned with systems that experience a sudden velocity change, such as a falling package whose velocity abruptly goes to zero when the package strikes the ground. This is a common test called drop shock. Sometimes an inclined plane is used, where a package gains velocity as it slides down the plane and hits a rigid wall. The shock response spectrum deals with the way in which a structure responds to shock motions, rather than trying to describe the shock motion itself. The spectrum is a plot of peak acceleration response of an infinite number of single-degree-of-freedom systems to a complex transient wave form. The individual single-degree-of-freedom masses are usually specified as having a transmissibility of 10 when excited at their resonant frequency with a sinusoidal vibration. This method of analysis is more representative of a real world, but the mathematics is far more complex than the mathematics of the simple pulse. 6.3 RESPONSE OF PCBs TO SHOCK PULSES When PCBs are excited by shock pulses, they will respond by bending initially in the same direction as the pulse. When the pulse diminishes, the PCBs will then resonate at their own resonant frequencies, of which the fundamental, or lowest, resonant frequency is usually the most prominent. Sufficient clearances must be provided to account for tolerance accumulations in the thickness of the PCBs, the component sizes, component lead wire protrusions on the back side of PCBs, location tolerances, and possible displacement amplitudes of adjacent PCBs moving in opposite directions at the same time. It is important to keep the dynamic displacements low, so the dynamic stresses will be low and the chances for impact between adjacent PCBs will also be low.

4 138 Experience has shown that high shock acceleration levels can result in cracked solder joints and fracture lead wires on large or heavy electronic components. Large components such as transformers, DIPs, capacitors, and motors must be mounted very carefully to avoid failures in the support structures or in the mounting hardware. When large components are mounted on PCBs that exhibit large displacement amplitudes, the relative motion between the component body and the PCB can often produce high forces and stresses. The dynamic displacement Z 0 ( meters) of the PCB assembly due to shock loads may determined by using the Equation (6.1) Z * G f 2 n out (6.1) where, G out - Output acceleration f n - Natural frequency of the PCB assembly (Hz) Mechanical shock tests are usually conducted by using a free fall shock equipment, where the test vehicle is subjected to a very high acceleration level ( G). Usually the mechanical shock tests are conducted on free fall shock equipment, where the drop table carrying the test vehicle will be dropped from a certain height on to a strike surface and the response of the test vehicle is measured. But, quite often the electronic equipment are also subjected to a lower magnitudes of mechanical shock loads (up to 50G) when a vehicle moves over a small ditch or pot hole on the road. Therefore, in this thesis efforts are made to conduct mechanical shock tests on PCB assemblies using an electrodynamic shaker and classical shock software. The experimental procedure followed to conduct mechanical shock tests on DIP-PCB assembly using an electrodynamic shaker is explained in the following sections.

5 EXPERIMENTAL PROCEDURE Shock Tests on DIP-PCB Assembly Mounted on Plastic Spacers The printed circuit board many times experience light and moderate shock loads during transportation, handling, and accidental drops. The objective of the mechanical shock tests is to determine the response of PCB assembly when subjected to different magnitude of shock loads by mounting the PCB on plastic spacers, rubber spacers and rubber pads. An electrodynamic shaker is used to excite the PCB at desired shock loads. The overall size of the PCB used for the shock tests is 240 mm x 210 mm x 1.6 mm. A 16 pin DIP, is mounted at the centre of the PCB. The experimental setup for conducting the mechanical shock test is shown in Figure 6.2. The PCB assembly to be tested is mounted on an aluminum fixture using four plastic spacers placed at the corners of the PCB (encircled in Figure 6.2). The shock pulse used for the test is a half sine pulse which is mathematically defined by: G( t) Gin sin t T (6.2) where G(t) = Output acceleration at any instant of time t G in T = Input acceleration = Pulse duration (milliseconds) The half sine shock pulse was programmed using the classical shock software. The shock in terms of gravity units (G) is monitored and controlled by an accelerometer placed on the aluminum fixture in a closed loop. The response of the PCB assembly due to shock load is monitored using another accelerometer placed on the PCB (near the component). The

6 140 response of the PCB due to shock load is captured using the NI PXI-4472 data acquisition card and LabVIEW 8.2. The shock tests were conducted with peak acceleration levels of 20G, 25G, and 30G. The reason for not going beyond 30G of load is the limitations of the electrodynamic shaker. Accelerometers Fixture PCB Shaker Power Amplifier 4-Channel Signal Conditioner PC and Vibration Control Software Figure 6.2 Setup for conducting shock tests As mentioned earlier the shock pulse used for the shock tests is a half sine having pulse duration of 5 milliseconds and delay between two consecutive pulses was 800 milliseconds. A typical half-sine shock pulse as programmed using classical shock software is shown in Figure 6.3.

7 141 Figure 6.3 Typical half sine shock pulse The PCB assembly mounted on an aluminum fixture using plastic spacers placed at the four corners is excited in Z direction (perpendicular to the plane of PCB) at 20G shock. The output acceleration is measured by an accelerometer placed at the centre of the PCB. The response of the PCB assembly due to 20G shock load is as shown in Figure 6.4. From this Figure 6.4 it is seen that, the response acceleration (peak) experienced by the PCB assembly is 30G. The response also shows some disturbance at the peaks and this may be due to high transmissibility ratio. The response takes about 0.47 seconds to settle completely. The dynamic displacement of the PCB assembly determined by using Equation (6.1) is found to be 3.5 mm.

8 142 Figure 6.4 Response of the PCB mounted on plastic spacers due to 20G Similarly, the PCB responses due to 25G and 30G shock loads are shown in Figure 6.5 and 6.6 respectively. From Figure 6.6 it is seen that the peak output acceleration is 43G and it takes about 0.47 seconds to settle (oscillations to die out) completely. The peak response acceleration due to 30G load is found to be 60G and the PCB assembly takes about 0.46 seconds to settle completely. So, when PCB assembly is mounted on plastic spacers, the average time taken to settle is about 0.47 seconds. From the responses of the PCB assembly due to 20G, 25G and 30G shock loads it is observed that, the transmissibility ratio varies between 1.5 and 2. The shock transmissibility ratio will be reduced by making use of rubber spacers and their suitability as shock isolators will also be tested.

9 143 Figure 6.5 Response of the PCB mounted on plastic spacers due to 25G Figure 6.6 Response of the PCB mounted on plastic spacers due to 30G

10 Shock Tests on DIP-PCB Assembly Mounted on Rubber Spacers Form previous section it is seen that, the PCB assembly when mounted on plastic spacers experienced high transmissibility ratios. The electronic assemblies may fail early due to high transmissibility ratios and therefore it is necessary to reduce the transmissibility ratios using shock isolators. In this work, rubber spacers and rubber pads are used as shock isolators and their effectiveness to isolate shock loads is investigated. The shock tests were repeated by mounting the PCB assembly on rubber spacers (Figure 4.6, chapter 4) instead of plastic spacers. The peak acceleration response obtained due to an shock load of 20G, is shown in Figure 6.7. The response peak acceleration experienced by the PCB assembly is about 24G and the response dies out in 0.39 seconds. It may also be observed that the response of the PCB assembly is quite smooth compared to the response of PCB assembly to 20G and mounted on plastic spacers. This fact may be attributed to the damping mechanism introduced by the rubber spacers. The PCB assembly responses to the shock loads of 25G and 30G are shown in Figures 6.8 and 6.9 respectively. From these figures also it is observed that the PCB assembly response dies out in 0.38 seconds (average) which is 20% less than the time taken by the PCB assembly when mounted on plastic spacers. By mounting the PCB assembly on rubber spacers, the shock transmissibility due to different loads now lies between 1.2 and 1.5. The amplitudes of the peak response accelerations are also reduced due to the damping mechanism introduced by rubber spacers.

11 145 Figure 6.7 Response of the PCB mounted on rubber spacers due to 20G Figure 6.8 Response of the PCB mounted on rubber spacers due to 25G

12 146 Figure 6.9 Response of the PCB mounted on rubber spacers due to 30G The shock test results obtained by using plastic and rubber spacers are tabulated in Table 6.1 for comparison. From the responses of the PCB when mounted on rubber spacers, it is observed that, the peak response is smooth and the peak acceleration is reduced by 20% and displacement by 23% due to an acceleration of 20G. Similarly, the response accelerations (peak) due to higher shock loads are reduced to the tune of 33%. Thus, from the above data it is seen that, the rubber spacers may be effectively used as shock isolators to reduce the shock amplification, PCB deflection and improve the life of electronic assemblies.

13 147 Table 6.1 Comparison of shock test results Input Shock Level (G) Shock Response (G) Plastic Rubber Spacer Spacer PCB Displacement (mm) Plastic Rubber Spacer Spacer (20)* (23)* (33) (35) (35) (37) * Figures given within the bracket indicate percentage of reduction Shock Tests on DIP-PCB Assembly Mounted on Rubber Pads Now the DIP-PCB assembly is mounted on two rubber pads as shown in Figure 4.14 (chapter 4). The longer edges of the PCB are made to rest on the top faces of the two rubber pads and fastened to the fixture plate using fastening screws. Now the PCB assembly is subjected to mechanical shock loads of 20G, 25G and 30G. The response due to an of 20G is shown in Figure 6.10, and the peak output acceleration experienced by the PCB is 22.44G and it takes about 0.28 seconds to settle completely. Similarly, the peak output accelerations due to 25G and 30G are 25.52G (Figure 6.11) and 36.97G (Figure 6.12) respectively and again the time taken by the PCB to settle completely is found to be 0.28 seconds which is 26% less than the PCB mounted on rubber spacers. Decrease in the time taken by the PCB to come to equilibrium position indicates an increase in the damping ratio due to rubber pads. From these responses it is observed that, the peak output acceleration experienced

14 148 by the PCB is reduced by about 34% compared to the peak acceleration experienced when PCB was mounted on plastic spacers. Figure 6.10 Response of the PCB mounted on rubber pads due to 20G Figure 6.11 Response of the PCB mounted on rubber pads due to 25G

15 149 Figure 6.12 Response of the PCB mounted on rubber pads due to 30G A comparative statement of the shock test results conducted on DIP-PCB assembly using plastic spacers, rubber spacers and rubber pads is given in Table 6.2 and the same is represented with the help of the bar chart (Figure 6.13). Table 6.2 Comparison of shock test results of all PCB mounting methods Input Shock Level (G) Plastic Spacer Shock Response (G) Rubber Spacer Rubber pads (20)* (25)* (33) (41) (35) (38) * figures within bracket indicate percent of reduction with respect to plastic spacer data

16 150 Figure 6.13 Comparison of PCB responses 6.5 RESULTS AND DISCUSSIONS Mechanical shock tests were conducted on DIP-PCB assembly at shock loads of 20G, 25G, and 30G using an electrodynamic shaker. The half-sine pulse having pulse duration of 5 milliseconds was programmed using classical shock software. Experiments were conducted by mounting the PCB on four rubber spacers and the results are compared with the experimental results obtained by mounting PCB assembly on plastic spacers. Also, the longer edges of the PCB assembly were mounted on rubber pads and the responses were compared with those obtained by mounting PCB on rubber and plastic spacers. The peak acceleration experienced by the PCB mounted on rubber spacers due to 20G shock load was 24G, which is 20% less than the peak acceleration experienced by the PCB mounted on plastic spacers.

17 151 Similarly, the reduction in peak acceleration levels due to higher acceleration levels was found to be about 34% with the PCB mounted on rubber spacers (Table 6.1). The response of the PCB mounted on rubber pads showed that, the peak acceleration levels are reduced by 25%, 41% and 38% respectively due to 20G 25G, and 30G loads as shown in Table 6.2. The response of the PCB mounted on plastic spacers took 0.47 seconds to decay, whereas the responses of the PCB mounted on rubber spacers and rubber pads respectively took 0.38 seconds and 0.28 seconds indicating that, there is increase in the damping ratio of the system. Thus, from above discussion it can be concluded that the PCB assembly mounted on rubber spacers or rubber pads reduced the shock transmissibility due to which the system will experience less peak accelerations in a shock environment. Due to reduction in peak acceleration levels, the corresponding PCB deflection is reduced which will improve the life of the PCBs and components mounted on it.