A cracked joint conducts perfectly until it does not. When intermittent faults become field returns, the answer is usually a few hundred micrometres across.
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Solder joints fail in ways that are maddening in the field and unambiguous under a microscope. A joint with a fatigue crack propagating through it can pass every functional test at room temperature and open intermittently once thermal expansion pulls the crack faces apart — which is why "no fault found" returns so often turn out to be very real failures. The physics is well understood: solder is a soft metal operating at a high fraction of its melting temperature, so it creeps under sustained load and fatigues under thermal cycling, while the intermetallic layers that make the joint work also make it brittle as they grow. Cross-sectioning tells you which of those mechanisms actually occurred.
Most joint failures trace to a handful of mechanisms with distinct, identifiable signatures in cross-section.
CTE mismatch between component and board driving cyclic strain until a crack propagates through the joint — the dominant field mechanism.
Entrapped flux volatiles or outgassing leaving voids that reduce load-bearing area and concentrate stress and heat.
Excessive Cu-Sn or Ni-Sn intermetallic growth creating a brittle layer that fractures under shock or bending.
Board flexure, shock, and vibration cracking joints at the package corners where strain concentrates.
Cold joints, insufficient wetting, head-in-pillow, contaminated pads, or reflow-profile deviations built in at assembly.
Whisker growth from pure-tin finishes bridging adjacent conductors, and corrosion from flux residues or environment.
Solder joint work is a staged progression from whole-board imaging down to the microstructure of a single joint.
A systemic joint defect rarely stays contained to one unit:
Reflowing, desoldering, or "repairing" a suspect assembly destroys the crack morphology that identifies the mechanism. Quarantine failed units and their production and traceability records intact.
Because the component and the board expand at different rates. Every power or ambient cycle strains the joint that connects them, and solder is a soft metal operating near its melting temperature, so it creeps and accumulates fatigue damage rather than springing back. Cracks typically initiate where strain is highest — the outer corners of a package — and propagate through the bulk solder or along the intermetallic layer over hundreds to thousands of cycles.
A partially cracked solder joint is the classic cause. The crack faces stay in contact at some temperatures and separate at others, so the fault appears and disappears with thermal cycling, vibration, or board flexure — and often will not reproduce on a bench at room temperature. This is the mechanism behind a large share of "no fault found" returns, and X-ray plus cross-sectioning generally settles it.
Usually yes, because the signatures differ. Process defects such as voiding, cold joints, insufficient wetting, head-in-pillow, and excessive intermetallic thickness are built in at assembly and are visible in cross-section as-formed conditions. Service-induced failures show fatigue striations, progressive crack propagation, or corrosion consistent with the operating environment. Comparing failed units against unused ones from the same lot is often decisive.
Tin whiskers are thin conductive filaments that grow spontaneously from pure-tin finishes, sometimes millimetres long. They can bridge adjacent conductors and cause shorts, arcing, or intermittent faults, and they have caused failures in aerospace, medical, and automotive equipment. They became more prevalent after lead-free transitions, since lead had suppressed whisker growth. Identification is by SEM, and the mitigation is a finish and coating question.
Enough to separate a systemic defect from an outlier. A handful of failed units establishes the mechanism; comparison against unused units from the same production lot, and ideally against units from different lots or date codes, is what establishes scope. That distinction usually decides whether the matter is a warranty issue or a recall.
Technical briefings from our work in this area.
Solder joints have a fatigue life. Whether reaching it in service is a defect or an expected wear-out depends entirely on what the design assumed.
readArea-array packages hide every joint under the component. The defects that matter there are invisible to inspection and often pass electrical test at room temperature.
readUnits that fail in the field and pass every bench test are rarely mysteries. They are usually cracked solder joints that conduct at room temperature and open under thermal or mechanical load.
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