Laser doppler vibrometry enables simultaneous measurement at dozens of points across populated PCBs, revealing component-level failure risks that accelerometers miss.

Printed circuit boards (PCBs) used in space, defense, aeronautics and transportation cannot tolerate in-service failures. Before deployment, these systems must pass environmental qualification tests, including vibration, shock, thermal cycling, radiation, electromagnetic compatibility (EMC) and ingress protection.

In vibration testing, PCB failures often arise from:

• Solder joint cracking
• Component detachment
• Pad delamination
• Trace fracture.

These failures are all linked to dynamic response under vibratory loading and can compromise an entire mission or system.

Understanding how PCBs actually deform and vibrate under load is essential to achieving uncompromised reliability.

Why Accelerometer-Based PCB Testing Falls Short

Because populated PCBs exhibit complex mass distributions, accurately modeling their vibration modes and the resulting stresses is inherently difficult. Standard tools like accelerometers provide overall board motion but cannot measure vibration at individual components, where failures originate.

Populated PCBs are not uniform flat plates. They have complex mass and stiffness distributions caused by components, solder joints and copper traces. This makes modeling and simulation highly uncertain.

A laser doppler vibrometer permits noncontact vibration measurement, but conventional scanning LDVs capture only one point at a time. This results in long acquisition times, repeated excitations and risk of missing localized behaviors.

Laser Vibrometry

A novel approach uses massively parallel laser doppler vibrometry, based on laser radar and interferometric measurement principles, to capture dozens of measurement points simultaneously. This method provides:

• Noncontact measurement (no mass loading, no wiring)
• High spatial resolution across the surface of the PCB
• Synchronous data across all points without stitching or repeated sweeps
• Clear visualization of vibration behavior at the component level.

It is particularly suited to environments in which PCBs must withstand random vibration, sine vibration, launch loads, weapon recoil, high-speed rail dynamics or continuous fatigue.

Test Setup: PCB Under Vibration Excitation

To demonstrate the viability of LDV, a populated PCB (Figures 1-2) was mounted on a linear shaker with vibration applied perpendicular to the plane of the board (Figure 3). Key setup parameters included:

• Vibration driven using the analog output of a novel LDV, amplified for sufficient excitation.
• To determine the eigenmodes of the PCB, it was excited with a chirp signal ranging from DC to 600Hz.
• 65 simultaneous LDV measurement points distributed across components and PCB surface.

Figure 1. Carrier board topside.

Figure 2. Carrier board bottom side.

Figure 3. PCB under test, imaged with integrated camera. The line of dots represents the positions of the 65 simultaneous measurement points used for vibration acquisition.

The frequency response revealed three distinct resonance peaks at approximately 150Hz, 200Hz and 250Hz. The shaker was excited individually at each of these frequencies, and the PCB vibration response was scanned. Figure 4 shows the resulting vibration velocity maps, overlaid with a semitransparent RGB image of the PCB for comparison.

Figure 4. Vibration modes of PCB when excited at 150Hz (a), 200Hz (b) and 250Hz (c).

Critical Observations

The vibration patterns differed significantly from those expected in an ideal flat plate. Components stiffened surrounding regions, altering local deflection patterns. Nodal lines were clearly visible at 150Hz and 200Hz (Figure 5). These high-density modal analyses reveal that component placement on a PCB must be carefully considered. In particular, components located along or across regions of maximum vibration velocity are more susceptible to detachment or solder failure.

Figure 5. Detail of the nodal lines measured at 150Hz and 200Hz.

Components positioned along high-velocity regions or near nodal lines face a higher risk of solder fatigue, pad delamination, or fracture during launch, flight, transport or impact conditions.

While this technique provides high-density operational deflection data, it is not yet a complete experimental modal analysis. It enables engineers to see how real hardware behaves under load early in the design and qualification process, however.

Why This Matters for Space, Defense, and Aeronautics

For teams developing spacecraft avionics, missile guidance systems, UAV electronics, railway signaling, defense radar systems or flight control computers, this approach offers tangible benefits:

• No accelerometers required. No added mass, no wiring
• Quantitative vibration data at the component level
• Detection of failure-prone areas before qualification testing
• Better correlation between simulation and hardware behavior
• Supports PCB layout optimization for harsh environments.

Figure 6. FFT magnitude at 200Hz, 600Hz, and 1MHz.

Figure 7. Baseline-subtracted average FFT spectrum with detected peaks.

Summary

In vibration-critical applications, understanding how a PCB deforms under excitation is essential. Using massively parallel LDV based on laser radar and interferometric measurement, engineers can now observe vibration velocities across components, not just board-level motion.

This high-density vibration-response data can serve as the basis for a subsequent full modal analysis with resolution and speed that exceed those of traditional tools.

Future technical notes will demonstrate full modal parameter extraction workflows.

Eladio Montoya, Ph.D., is sales manager and Óscar R. Enríquez, Ph.D., is product specialist at Ommatidia LIDAR (ommatidia-lidar.com); eladio.montoya@ommatidia-lidar.com.

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