Indice dei contenuti
What Causes Vibration Fatigue in PCB Solder Joints?
The solder joint supports two different types of loads: one is for electrically carrying current; the other is mechanically holding components in place. The mechanical aspect is attacked by vibrations and then the electrical function fails as a result.
Each time you flex the circuito stampato, there is some elastic (eventually inelastic) strain imposed upon the joint. After 10^5-10^7 repetitions of this flexing process, these strains can create cracks and cause the joint’s cross-sectional load bearing area to decrease to a level where it cannot support continuous flow, at which point the failure appears as an intermittent open rather than a complete short. This is vibration induced solder joint fatigue, a high cycle, stress driven fatigue process that has its own failure mechanisms.
Although they may be referred to as the same thing (“solder fatigue”) engineers who have treated vibration induced solder joint fatigue and thermal cycling fatigue interchangeably likely misapply their mitigation techniques. In this article, we explain how and why they differ, describe the quantitative differences between the two processes, and present various designs and testing practices based upon the actual physics.
PCB Solder Joint Vibration Fatigue: Failure Mechanism and Crack Path
A populated printed circuit board subjected to dynamic vibration acts like a distributed mass plate in which all masses are attached at different locations to the same neutral plane. As such, when an external vibrational spectrum (sinusoidal or random) includes frequency components close to one of the natural frequencies of either the overall board assembly or individual sub-assemblies; this results in a considerable increase in the amplitude of the out-of-plane deflection of the board due to its ability to convert input acceleration into large motions.
However, the component mounted onto the board does not move identically to the local surface of the board underneath it because each component also possesses mass and stiffness and has a center of gravity located away from the neutral bending axis of the board. Therefore, there exists differential motion between the component itself and the board to which it is mounted. This differential motion is resisted by the solder joints that join the component to the board.
At the joint level, these stresses take the form of cyclic shear and peel stresses that are localized to areas of geometric discontinuity (i.e., the heel of a gull-wing lead, the corner balls of a BGA/CSP array, etc.), where both local curvature and the mismatch in elastic moduli between solder, copper and the IMC layer can produce a stress riser even when there is an otherwise uniform load applied.
The solder joint itself does not have a uniform composition. A heterogeneous layer called an Inter-Metallic Compound (IMC) forms as a result of chemical reactions occurring at the copper to solder interfaces when the joints undergo reflow. The IMCs formed by soldering with SAC alloys are primarily composed of Cu6Sn5 (the eta phase), which is located adjacent to the bulk solder; and a smaller amount of Cu3Sn (the epsilon phase), which is located next to the copper and has a much lower thickness. In comparison to the eutectic or near-eutectic solder matrix, this IMC layer is significantly harder, less ductile, and also less resistant to cracking.
High cycle loadings via vibrations will most likely initiate cracks at the bottleneck location of the solder fillet or ball; however, once initiated, cracks will most likely propagate along the IMC / Bulk-Solder Interface prior to propagating through the bulk solder. These characteristics can be used to identify whether the joint failed due to vibration fatigue or thermal fatigue when examined using a Scanning Electron Microscope (SEM).
Quantitative models of vibration fatigue are based on Basquin’s equation which is the stress-based S-N relationship:
σa = σ’ f(2*Nf)b
where σa is the cyclic stress amplitude; Nf is the number of cycles to failure; and b is the fatigue strength coefficient.
The classification of vibration fatigue is as high-cycle fatigue (typically greater than 10^5 cycles); and because the deformation in each cycle is primarily elastic, it follows that there will be little or no plastic strain. The basis of most electronic assembly board-level stiffness design rules is to limit the maximum amount of allowable stress reversals over a specified time period at a level consistent with an assumed service spectrum, and therefore Steinberg has developed a vibration analysis methodology for electronic equipment.
Vibration Fatigue vs. Thermal Fatigue in Solder Joints
The two fatigue processes have several differences including driving forces; cyclic behaviors; numbers of cycles to failure; and crack morphologies. Combining the two fatigue processes is one of the primary causes of the incorrect diagnosis of root causes in this area.
Thermal Cycling and CTE Mismatch
Thermal cycling fatigue occurs due to the difference in the Coefficient of Thermal Expansion (CTE) values of the PCB laminate (FR-4 typically CTE ≈ 14 – 17 ppm/°C in plane) and the component package and the solder joints.
The differenace produces cyclic shear stresses in the solder joints when the entire assembly goes through temperature changes caused by environmental changes; i.e., ambient temperature swings, electrical operation (power cycling), and reflow-to-service transitions.
Low-Cycle Thermal Fatigue
When these stresses are high enough to produce plastic deformation in the solder joint it is called low-cycle fatigue (typically Nf < 10⁴ cycles).
Low-cycle fatigue is described using the Coffin-Manson equation which relates the number of cycles to failure (Nf) with the total plastic strain per cycle. Engelmaier has since modified this equation to include both the frequency of cycling and the dwell temperature effect present in many types of solders. In general, cross-sections of thermally fatigued solder exhibit ductile cracking throughout much of the volume of the solder whereas mechanically fatigued solder exhibits primarily interfacial cracking.
High-Cycle Vibration Fatigue
The behavior of vibrating fatigue is governed by the Basquin-S-N curve where there is relatively low peak stress per cycle but a very large number of cycles. For example, if a component vibrates at 200 Hz then approximately 1.7 × 10⁷ cycles occur per day. Due to the fact that all strains remain within the elastic region, the rate of growth of crack length will be slow but will still follow the harder, more brittle, Intermetallic Compound (IMC) interface.
Design Implications of Vibration and Thermal Fatigue
In terms of design impact, compliant stands-off or leads that help reduce thermal cycling induced
CTE-mismatch shear can not alleviate resonant amplification while stiffening a board to move it’s resonance away from input frequency can not alleviate CTE-mismatch shear during a thermal excursion.
Combined Vibration and Thermal Loading
Experimental evidence shows that combined vibro-thermal loading which is common in under-the-hood automotive applications and power module applications results in less time to failure than either load individually because thermally softened or partially cracked solder will generally have lower residual strengths against future vibration loads. Therefore, a complete failure analysis should start with identifying crack morphology and IMC involvement via fractography prior to assigning a root cause.
How to Prevent Vibration Fatigue in PCB Solder Joints
Increase PCB Stiffness and Shift Resonant Frequency
Move the resonant frequency away from the area where your vibration excites it. Boards that are thicker, have stiffening ribs or metal frames and have extra attachment points, increase the stiffness of the board or reduce the distance over which the board can bend, causing its first natural frequency to exceed the dominant vibrational frequency present in your input. Tests have indicated a deflection reduction of about 30% when moving from a 1.6 mm thick board to a 2.0 mm thick board subjected to the same vibratory input.
Reduce Stress from Heavy Components
Locate heavy components close to mounting points/constraints rather than mid-span for less deflection. Distribute mass evenly so there is no concentration of stress in any single location.
Use Staking and underfill for Mechanical Reinforcement
Stake down assemblies where inertia loads cannot be avoided. Stake-downs take the entire load off of the solder joint. Using an underfill under BGA/CSP packages distributes stress through the entire ball array instead of the four corners. This reduces the maximum amount of stress at the corners of each ball by approximately 50%. In addition, staking just the corners has demonstrated multiple fold increases in vibration fatigue life compared to unstaked assemblies during board-level tests.
Use Conformal Coating for Additional Damping
Add a protective/damping layer to help protect components. A silicone or polyurethane conformal coat (approximately .025-.075 inches) provides damping and helps restrict motion of individual components, however this method does not prevent resonance and should not be used as the main form of protection.
Select Flexible Termination Styles
Select termination styles that are flexible, if possible. Terminations that provide flexibility and allow the joint to stretch with the termination include gull-wing and J-lead termination types. Leadless packages (BGA, QFN, LGA) do not use a termination type and therefore require mechanical reinforcement as described previously.
Test Against the Actual Vibration Ambiente
Test against the exact vibration environment found in service. Test using random vibrations per IPC-9704, JEDEC JESD22-B103, or MIL-STD-810 Method 514 while using accelerometers placed both at the board and component levels prior to conducting a sine sweep/resonance search test followed by an endurance test.
Applications Where PCB Vibration Fatigue Matters
Automotive
The power train and under hood components of an automobile are subjected to repeated vibrations that occur as a result of both the operation of the vehicle (engine) and the physical travel of the vehicle along a surface (road), which is in addition to thermal cycling, the conditions described above for combined loading.
Aerospazio e Difesa
Aerospaziale and Defense products are subjected to broadband random vibration resulting from the operation of their engines, fluttering of the air frame, and other loads experienced during the launching phase. Most aerospace/defense companies qualify their products against military standards such as MIL-STD-810 and/or DO-160 specifications.
Heavy Industriale and Heavy Equipment
Sustained, large-amplitude, relatively low frequency mechanical vibration is commonly applied to pumps, compressors, and other types of industrial/heavy equipment throughout the duration of their duty cycle.
Rail and Marine
In these industries, both mechanical vibration and environmental factors including humidity and salt fog are present simultaneously, therefore requiring both fatigue resistance and corrosion resistance.
Consumer Electronics/Wearable Devices
Consumer devices/wearables experience lower amplitude of vibration; however, they still require reliable joints to withstand drops and physical stresses associated with use during a device’s entire lifetime.
When to Use Alternatives to Soldered Interconnects
The most appropriate method for attaching a connector or terminal to a PCB depends on the specific application.
Soldering provides an inexpensive way to connect devices that do not need to withstand large amounts of vibration or shock. However, when it comes to applications with large levels of vibration, such as those found in automotive systems, other methods may provide better reliability.
In these cases, crimped wire connections have shown to perform well due to their ability to spread force across the entire surface area of the termination. Additionally, press fit connectors use the compliant pins to deform into the plated-through hole and eliminate the solder joint from carrying stress, which can improve reliability and reduce failures.
Mechanical fasteners (i.e. screws) may also be used in some applications where a rigid connection is required. Finally, flexible board-to-board connectors made from an elastomer may also be used.
Domande frequenti
A1: Yes. Leadless packages are more susceptible to fatigue because the solder joint is the only compliant element in the package that carries the strain of vibration. Leaded packages share this stress with the leads.
A2: The crack paths. Vibration cracks follow the IMC/bulk-solder boundary and show brittle fracture and many, many high cycle characteristics. Thermal cracks cut through the bulk of the solder and have many large cyclic characteristics resulting from less number of large stress cycles.
A3: One needs to make this switch when the solder joint fails before reaching the desired service life during vibration testing, or if the mass of the components is so great relative to resonant amplitude that there is no way staking and underfill can offset these differences.
CONTATTO
Italian 
English 
Hungarian