Vibration Fatigue Of Solder Joints

A board that is mounted in an enclosure and exposed to vibration behaves as a plate with a mass attached to it, and the solder joints are the smallest and most highly stressed part of the structure. The loads are small compared with a drop, but they repeat millions of times, and a joint that survives a shock can still fail after an hour of resonance. Vibration fatigue is a cumulative damage process, and the failure appears as a crack that grows from the interface between the solder and the pad.

This article explains how a board responds to vibration, where the joints fail, which design measures reduce the strain, and how the behaviour is tested.

How A Board Responds To Vibration

A printed circuit board is a thin plate, and like any plate it has natural frequencies at which a small input produces a large displacement. The amplification at resonance is described by the quality factor, which for a populated board is usually between five and twenty, so an input displacement of a few micrometres can produce a deflection of a fraction of a millimetre at the centre of the board. That deflection bends the board, and every component that is mounted on it is strained by the curvature.

The two common test types produce different responses. A sinusoidal sweep finds the resonant frequencies as peaks in the response, and a dwell at one of those peaks is the most severe case, because the input is applied at the frequency where the board amplifies it most. A random vibration covers a band of frequencies with a defined power spectral density, and the response is computed from that spectrum; the stress is distributed across the modes rather than concentrated in one, which is closer to what a product sees in transport and in service.

The frequency range that matters depends on the product. A board in a vehicle sees road input at low frequencies and engine orders higher up, while a board in a rotating machine sees a narrow band at the rotational speed and its harmonics, and a portable product sees the spectrum of a drop that has been filtered by the housing. The first natural frequency of a typical board is often within the range that the product will experience, which is why a design that is never tested for it can pass every electrical requirement and still fail in the field.

Where The Joint Fails

The crack usually starts at an interface. For a leaded component the solder fillet at the lead is the point of highest strain, and for an area array package the outermost ball is the one that carries the largest displacement. The intermetallic layer between the solder and the pad is the location where the crack initiates in many cases, because it is thin, hard, and brittle compared with the solder around it, and the crack then propagates through the bulk of the joint or along the interface.

Another failure mode is the pad crater, in which the laminate beneath the pad fractures rather than the joint itself. That failure is associated with a high strain rate and with a weak resin, and it is not repairable, since the pad and the laminate under it are destroyed. The package type changes the failure location: a large body with stiff leads loads the joints at its corners, a chip component loads the fillets at both terminations, and a ball grid array distributes the load over the balls nearest to the package edge.

The Design Levers

The strongest lever is the stiffness of the board. A thicker board has a higher natural frequency and a smaller deflection for the same input, and the strain at a component falls roughly with the square of the thickness. Where the thickness cannot change, adding a mounting point or a stiffener reduces the effective span, which has a similar effect, and moving a heavy component away from the centre of the board and away from a mounting point reduces the mass that is accelerated.

The mass of the component matters as much as its position. A large, heavy package produces a larger inertia force for the same acceleration, so the joint strain rises with the mass, and the same package mounted near the board edge is far less stressed than one in the middle. The standoff of a component also matters, because a part that sits high above the board acts as a lever on its joints, which is one reason a leaded package with long leads is more vulnerable than a package with a low profile.

Material And Process Levers

The alloy changes the fatigue behaviour. A lead free tin silver copper alloy is stronger and stiffer than a tin lead alloy, which makes it more resistant to creep but also less able to accommodate strain, and its vibration fatigue life at a given strain is generally lower. Alloys with small additions of other elements have been developed to improve the fatigue resistance, and a bismuth bearing low temperature alloy is more brittle again and behaves poorly under repeated bending.

The joint geometry and the process also contribute. A fillet that rises to the correct height distributes the strain over the solder rather than concentrating it at the interface, an adequate solder volume provides more material to accommodate the strain, and a good bond between the solder and the pad avoids the weak interface where a crack would start. For an area array package, adding an underfill couples the package to the board and can raise the vibration life substantially, at the cost of reworkability and of moving the failure into the package or the laminate.

Testing The Behaviour

The test programme usually begins with a sine sweep at a low level to identify the modes and the frequencies at which the board responds. A dwell at each resonant frequency, or a random vibration to a defined spectrum, then applies the damaging load, and the electrical continuity of the joints is monitored during the test so that a crack is detected as a rise in resistance rather than after the test when the failure is already complete.

The result is a time to failure, and the analysis of the failed units shows where the crack started. That location is what points to the design measure: a crack at the outer balls of a package in the centre of the board suggests a stiffness or a position change, while a crack at a single corner suggests a mounting or a stiffener issue. The thermal and vibration loads are usually applied together in the qualification, because a joint that has been weakened by thermal cycling fails sooner under vibration. The acceptance framework is described under PCB design quality characteristics, the mounting features under board outline and mounting design, and the alloy context under lead free and leaded soldering.

FAQ

Why does the largest component matter most? Because the force on a joint rises with the mass it accelerates. A heavy package in the middle of a board combines a large mass with the largest displacement, which is the worst combination in the assembly.

Does a conformal coating help? It can, by damping the board and by mechanically coupling the component to the surface, but a stiff coating can also transfer more load into the joints. The effect has to be measured rather than assumed.

How is a crack detected during the test? By monitoring the resistance of a daisy chained circuit continuously. A crack that opens intermittently appears as a series of resistance spikes long before the joint fails completely.