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thornbirdhsu
Joined: 30 Aug 2026 Posts: 3	Posted: Fri Mar 04, 2026 8:47 pm    Post subject: Thermal fatigue after IOL testing   Hi, Our company produces rectifiers (diodes), we found some SMD or plastic diodes parts had broken soldering joint after power cycling testing whatever the elec. parameters are good or bad. Is there someone to explain why the broken site occurres at soldering joint but not IMC and how thermal fatigue happens (the theroy)? Thanks! Thornbirdhsu
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Asi
Guest	Posted: Tue Mar 08, 2026 2:26 pm    Post subject:   Got this from http://www.ami.ac.uk Asi Quote: Fracture caused by Fatigue Having looked at failures caused by short-term overload, and by permanent loading producing creep, we move to the third and largest category of failures, the fatigue failures that result from cyclic loading. The major cause of such fatigue failure is the cyclic deformation of the joint by the stresses that result from temperature excursions combined with CTE mismatch. The extent of the deformation depends on the design of the joint, and whether or not there is any compliance in the system. Read Stress caused by thermal mismatch for guidance as to which designs can be expected to be most reliable when temperature cycled. Usually the deformation of the solder in the joints is rather large, of the order of 1%, and the movements are slow, with typical cycle times measured in hours. Referred to as ‘low cycle fatigue’, cracking from this type of deformation may be observed in all kinds of soldered joints, and this is the main cause of eventual crack formation in initially good joints. Where do the cracks start? During the fatigue process, successive metallurgical phenomena occur. As the strain in the joint exceeds the plastic limit, the solder will start to creep. However, there is the complication for tin-lead solder that the material consists of two separate phases: a-lead has up to 19% of dissolved tin, with a face-centred cubic crystal structure, and its CTE is the same in all three directions by contrast, the b-tin phase, containing up to 2.5% of lead, has a body-centred tetragonal structure, where the thermal expansion in the axis of maximum CTE is very close to that of a-lead, but only about half this along the minimum axis. This anisotropy causes internal stresses in the structure during slow thermal cycling. As a result of these stresses, the solder exhibits phase segregation and grain growth, and the weaker a-lead phase is where cracks start and propagate during continued thermal cycling. Before the cracks start, there is usually a change in the surface appearance of the joint, which becomes rough, often having the appearance of small steps which usually develop into cracks. What form does the fracture take? Slow cycle fatigue has been observed with most solder joints, but in general does not affect through-hole assemblies because of their higher margin of safety. The progress of the fatigue damage of leadless components may be seen as: - Start of the crack, generally under the component at the edge of the metallisation (difficult to observe in inspection) - Progression of the crack to the outer surface of the fillet, generally first visible at the corners of the metallisation - Growth of the visible cracks from the corners of the component to the middle of the joint - Sometimes, depending on the configuration, the cracks follow the interface between component and solder
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