Eliminating tombstoning or at least greatly minimizing it can be achieved in part through the solder paste recipe. First, use a solder paste with a thermally stable flux system that will maintain tackiness. Then, use a solder paste with metal particles with two different eutectic points: 50% melting at 179°C and the balance melting at 183°C - alloy after reflow: Sn62.5/Pb36.5/Ag1.0.
The angling effect caused by the wetting force in the joint that melts more quickly is mechanically hindered by solid particles of the alloy that melts at 183°C. This provides the alloy that melts at 179°C on the other pad with a fraction of a second more time to wet as well, thus restoring equilibrium.
In addition to using a solder paste with the abovementioned characteristics, tombstoning can in large part be prevented by following three rules of thumb:
1. Minimise ΔT in the board through control of the thermal (reflow) processing profile.
2. Control the tolerances on boards, components, and component placement.
3. Control the O2 ratio in N2 systems; the preferred level is 500 ppm.
Tombstoning is a preventable defect; attention to its causes and addressing each of them will reduce its incidence, resulting in higher yields, lower defect rates, and less rework overall.
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