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Figure 2: Four-point bend setup.
Figure 3 shows a test board in the fixture. Four AE sensors are under the clamp shown in Figure 3. To setup the 4-point bend testing, a bare board was tested to failure, plus a sample of each laminate was tested to electrical failure and AE failure respectively. With enough load, it can be expected that laminate boards will emit away from any stress rising due to solder balls. The first AE on boards without components was found to be at a deflection of 11mm. This load level is well above the load levels under test of the populated boards. Testing bare boards before the populated boards is necessary for any new board laminate/layup or the substitution of lower frequency sensors.
Figure 3: Test board in fixture and location of AE sensors.
All AE equipment used in this study is off-the shelf commercial offerings. A high end AE measurement system was used with pre-amplifiers which provided enough versatility for multichannel low noise captures, 100-nanosecond global clock resolution of arrival times.
AE sensors with resonant frequency of 650kHz and
Event criteria and location analysis
The three primary settings for AE detection in this type of AE system are threshold, rearm and duration discrimination testing. The former setting is to distinguish the beginning of the event, the latter two the end of the event. The transient capture allows a known beginning and end to an event, and detection threshold plays a role in both. Typical concerns of the settings are sufficient sensitivity, noise rejection of low level noise, managing the trade-off between distinguishing events and allowing overlap from reflected events and getting as accurate an arrival time as possible. Fortunately, most of these are not issues in this board, sensor and strain rate combination. Thresholds between 30-40 dB AE are all viable with little noticeable difference. Lower levels may detect more background noise and more care is required at 30 dB to keep out external vibrational noise. As the threshold for detection is increased, the beginning of the wave is not captured as well, causing a small amount of error in location. A good starting point is a 40 dB threshold until there is more familiarity with the AE measurements. The end of the signal is also affected by threshold level. With this test, the strain rate is very high and the potential for event overlap will occur more readily at lower thresholds. Rearm times in the ballpark of 1 millisecond can be used without compromising distinct AE hit determination. If a 300 kHz sensor was substituted, the lower range of thresholds would be less of an option due to susceptibility to low frequency noise and overlapping signals. With this test, unlike other AE applications, these parameters are non-critical, at least with the 650 kHz sensor.
The largest experimental factors affecting the location results are choice of sensor array. Figure 3 shows the placement of the sensor array. It is desirable for all damage locations to be within the array as location errors increase significantly outside the sensor array. It is also important to have redundancy in the measurements. It takes 3 sensors to locate in a plane, 4 sensors give a redundancy factor that can help mitigate errant sensor results. More sensors would improve on this further, but at the cost of slowing testing. For positions, the sensors were placed just inside the 4 point bend far enough so that at peak deflection the sensors do not contact the 4 point bend fixtures and generate noise. Two of the most likely weak points on the board itself, the corners of the J4 package, sit close to the edge of the four point span and even closer to, if not outside the sensor array, make the coordinate location in this region much more error prone than other parts of the board.
The velocity for the laminates was determined in pretest for both orthogonal directions. Event builder timeouts were several times the expected time delay across the sensor array. A representative velocity is used for x and y coordinate determination with the understanding that it is a compromise velocity with regard to the asymmetric nature of composites. Simulated AE (Pencil Lead Breaks or PLB’s) was performed in a test pattern, specifically the corners of all 3 chips (12 total positions), and showed consistent location within 1-3 mm of each corner. This was performed on all boards prior to testing. It is expected that simulated AE will locate better than real, damage based AE, although the converse can also be stated; that the real damage can almost always be expected to locate less well than the PLB. Figure 4 shows an example of the output of the AE test, with mapped location of AE events and signal amplitude vs time chart.
Figure 4: An example of AE events.
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