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.

We have presented live process defect clinics at exhibitions all over the world. Many of our Defect of the Month videos are available online at Find out how you can share our new series of Defect of the Month videos to explain some of the dos and don’ts with your customers via CIRCUITS ASSEMBLY:

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