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REFLOW: The secret to a high tensile strength! (Part 4)

The final element of maximizing tensile strength through a proper reflow is the cool down.  Cool down is last line of defense against a poor solder joint.  This is because the cool down ramp, and it alone, controls the formation of the crystalline structure of the metal lattice.  The smaller, tighter and denser we can make the crystal lattice is, the higher the joint strength.  Because, it is along these facets of the crystal that the joint breaks, and the longer, larger and sparser the crystal facets are, the easier they are to cleave.




One way of visually investigating whether the solder joint is tight is to look at the post-reflow surface finish of the solder joint.  A joint that seems to have good wetting and good flow yet is grainy and gray may have been exposed to a slow cool down.  One way to test this is to heat it up with a soldering iron.  After it goes molten, remove the heat.  If it becomes brighter and shinier, it probably needs a faster cool down.  This may also happen if the joints around the perimeter of the board, or where the components are lightly populated, are bright and shiny and the densely populated areas have solders joints that are dull and grainy.  This is because the more densely populated areas take longer to cool off, and affect the cool down rate of the board.  I would reposition the thermocouples used in profiling to the denser area, and re-map the profile to meet their cooling needs.

More information may be found at Online Help: Indium Knowledge Base (IKB).