Solder joint reliability Lead-free impact

COMPONENT-TO-PWB RELIABILITY ESTIMATING SOLDER JOINT RELIABILITY AND THE IMPACT OF LEAD-FREE SOLDERS... 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Developing a lead-free SMT process requires planning and a close-working relationship with all suppliers. Understanding component and board compatibility issues with the use of higher temperatures is essential. Avoiding certain elements, such as bismuth and lead that may impact solder joint reliability also is important. 

It is well known that the microstructure of a solder alloy, like any material, has a very significant effect on its mechanical properties. The higher the application temperature in proportion to a material s melting point, the more rapid is the microstructural change, and hence the impact on solder joint reliability. Successfully migrating onto and implementing lead-free solders requires better understanding of how these solder alloys interact with the termination and lead materials... 

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