Flow Stages in Vibration Testing on Hi-Rel EEE Parts - EEE Parts Database | doEEEt.com (2024)

Flow Stages in Vibration Testing for space applications.

Depending on the intended application electrical components are exposed to either transitory or permanentdynamic mechanical stress which can impair the performance(e.g. mechanically inducedcrack in PCBs systemsor frettingcorrosion in connectors and others).

In order to ensure the proper system operation under such conditions,High-Rel EEE parts have to be submitted to thorough vibration test before their implementation in different areas. These tests must be carried out according to different specifications depending on the application areas, such as those included in the following table.

• Defense/Military
• Aerospace
• Automotive
• Space
• …

Space environment

Electrical equipment in rocket boosters and spacecraft are subjected to high dynamic loads and mechanical shocks of different nature. Space launch involves low-frequency vibrations generated during lift-off, motor excitation, buffet, and shut-offs. In addition, large shock-loads are experienced during the space trip, particularly in pyrotechnic separation stages which also involves high-frequency shock waves.

Such dynamic loads are responsible for different failures modes in EEE systems. For instance, excessive deformations and accelerations (g-loads) experienced by assembled-system parts cause heavy damage inconnectors,surface mounted devices (SMD), printed circuit boards (PCBs) and similar electrical components.

A clear example of a vibration induced failure in SMD systems is PAD cratering in BGA, CGA, and bottom termination components. Hence, the ability to simulate, in the ground, this mechanical stress or the capacity to investigate structural dynamic characteristics is critical for thesuccessful qualification of new EEE part for space applications.

Test flow

The main contributing factors include PCB thickness, size, resin type, CTE of the PCB and component, solder type, component position, assembly condition and post assembly handling. There are some strategies to mitigate the appearance of Pad Cratering failure, such as:

• Epoxy fillets at devices corners
• Increase board thickness to eliminate flexing
• Increase pad or trace widths

Since the initiation of pad cratering does not result in an instantaneous electrical signature, detecting the onset of this failure has been challenging. For instance units with pad cratering may not fail during functional test andMicrosectioning Analysisremains the most widely accepted means for analyzing the Pad integrity.

Types of Vibration Testing

Vibration Testing is the first approach to determine the mechanical weaknesses of the DUT the system is submitted to a sinusoidal stimulus of increasing frequency as illustrated in Figure 3. A logarithmic sweep rate is traditionally used to compensate the longer time required to complete a cycle at the lower frequencies.

As it is indicated in Figure 3 low-level sine-survey vibrations are conducted before high-level tests. This precaution is taken in order to analyze the mechanical impedance of the test-fixture/DUT assembly within the inspected frequency range. Such alow-level pre-test allows us to assess in a safe mannerthe natural frequenciesof the system in order toavoid excessive damage due to nonlinear responses.

Figure 3: Constant displacement sine sweep (time domain)

Moreover, using sine vibration to identify resonant conditions on the PCB or other systems is an effective way to understand how mechanical vibration propagates through a device and can help designers to improve the designs of the item by stiffening or soften elements and to reduce the probability of a fatigue failure.

Low-level sine-survey is also performed after each high-level vibration. This additional analysis is performed toreveal the effect of high dynamic loads on the mechanical properties of the system. Therefore, any difference detected between the pre-test and post-test low-level sine survey responses (e.g. natural frequencies shift of development of new resonances) must be taken as indications of damage.

In real applications, sine vibrations are rear and not representative of the real conditions experienced by the DUTs. For instance, dynamic stress generated in real scenarios is, in general, not periodic or predictable like sinusoidal waveforms. In fact, the majority of vibration environments where electronic devices are mounted applies random vibration conditions.

In order to simulate amore realistic dynamic stress, random vibration test simultaneously excites all the frequencies within a given range. The stimulus is applied in a semi-random way with a spectrum that constantly changes in amplitude and phase. Nonetheless, the process is not completely stochastic as the average level is adjusted to follow a given test profile as illustrated in Figure 5.By this way, the vibration level is maintained with the limits stated to ensure that the test reproduces the working conditions of the DUT and that the test is conducted under safe conditions.

In-situ electrical monitoring

Final inspection and measurements.

Inspections services: