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Solder Joint Reliability in Prototype Assembly

Reliability in Prototype Assembly

A key aspect of prototype assembly is assessing the quality of solder joints. Typically, a visual inspection of solder joints is performed by a trained technician with a multimeter and a torch to determine whether or not the joints are properly bonded. An inspection should start with the multimeter’s continuity or resistance mode, which is used to test for an uninterrupted connection between pads and traces. This is followed by a torch to examine the joint’s form to ensure that the solder alloy has melted and spread properly. In some cases, a non-concave shape indicates the presence of a cold solder joint and that the alloy was not heated enough to melt fully.

Thermo-mechanical fatigue is another common cause of failures in solder joints. This happens when mechanical loading in conjunction with thermal cycling causes microstructural changes that lead to failure. The traditional C-M approach fails to model these thermo-mechanical effects, which is why FEA is often used for predicting the fatigue life of ball grid array (BGA) packages.

When a BGA package experiences excessive stress in the form of a mechanical event, the resulting shear strain can break the components’ solder joints, especially if they are not designed for this kind of loading. To mitigate this issue, a designer can use a FEA simulation to determine the maximum stresses in their package and to optimize the component locations. In addition, they can consider using component mirroring to reduce the overall amount of package motion and thus the stress in the package’s solder joints.

Solder Joint Reliability in Prototype Assembly

Mechanical overstress failures generally manifest as pad craters or joint fracture along the intermetallic connection (IMC). This is because these areas are particularly susceptible to brittle damage, which can result from high shear loads in the region of the IMC. FEA can be a powerful mitigation tool for this type of problem because it enables you to simulate and iterate different mounting conditions without the need to perform extensive mechanical testing.

Thermal cyclic fatigue is also a leading cause of shear cracks in solder joints. Unlike mechanical overstress, which is driven by shear stresses, thermal cycles are caused by temperature fluctuations that lead to material deformation. This deformation is exacerbated by the mismatch between the IMC and bulk solder material’s coefficient of expansion.

FEA simulations have shown that the most effective way to prevent these cracks is to ensure that all components are placed within the appropriate temperature range. Moreover, it is essential that the profile of the PCB’s solder paste reaches and remains above the reflow temperature for at least a few seconds. In cases where the reflow temperature is insufficient, these defects can be corrected during rework by applying additional heat to the region of the defect. A successful reflow should produce a solid, well-bonded joint. In addition to proper reflow temperatures, a good rework process should include thorough cleaning of the surface of the PCB after reflow, which helps to remove impurities that can affect the bonding properties of the solder.

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