DEFECT OF THE MONTH
Age Causes Knee Damage
Soldering excursions can lead to visual process indicators.
THIS MONTH WE look at cracks in plated through-holes around the knee of the hole. FIGURE 1 shows very small via holes that were subjected to multiple lead-free soldering steps, then underwent thermal cycling with no failures but a little cracking.
The cracks visible in the microsection were found on via holes not after the initial two reflow steps and wave-soldering test boards, but after further temperature cycling at -55o +125oC. No electrical failures were detected, just the impact of repeated stressing of the copper. It is a good demonstration of how reliable a board can be, but all that stress does have some visual impact. Care must be taken during microsection preparation to see these indicators.

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grey, pink, and black cracks in a plated through-hole
Figure 1. Cracks in a plated through-hole.
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Bob Willis
is a process engineering consultant; bob@bobwillis.co.uk. His column appears monthly.