Lead-free fillet tearing
30 November 2009
It’s a fact that lead-free fillet tear in plated through hole joints is not uncommon. The images show two examples of this phenomena; optically and by using real time X-ray inspection.
These images show a typical example found on the surface of the solder joint after wave or selective soldering, and it can also be seen on intrusive reflow joints when solder paste is used to reflow and form a connection with through hole parts. The X-ray image shows visible evidence of tearing on joint surface and just below the surface.
In IPC-A-610D, the latest version of the criteria refers to this as a hot tear / shrink hole and is acceptable on lead-free joints if the bottom of the tear is visible and the tear does not contact the pad or lead on class 1, 2, 3 products.
It is a defect on each inspection class if it’s found on tin / lead, the bottom of the tear is not visible, or the tear touches the pad or lead. In reality, it is only seen on lead-free tin / silver / copper alloys and is normally very difficult to see the bottom of the tear – hence making it a defect, according to the standard – requiring rework.
Selected combinations of tin / silver / copper with a silver content of 3% - 4% are prone to cause this tearing during solidification of the solder. Tin / copper and tin / copper / nickel do not seem to feature this defect type. The solder is rough in appearance due to the shrinkage of the metal between the tin dendrites. There is no evidence currently that this has any impact on the reliability of the solder joints.
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