![]() Longer pad extensions, i.e., those beyond the component, will increase the turning moment acting on the device by prompting the solder to fully wet out and reduce the contact angle on the component's termination. It is clear from practical experience that smaller passive devices are more vulnerable. ![]() Several physical factors can increase the risk of incurring tombstone defects, including higher surface-tension forces and lower component masses. However, once lifting starts, the forces that cause the tombstoning effect will increase. In normal circumstances where both ends of the component will be subject to the same forces working in the opposite direction, no undesirable movement will take place. Using the symbols in Figure 1, where the mass of the chip is M and g is the acceleration due to gravity, the expression becomes:įor larger components, the forces are closely balanced and lifting is not certain even when only one end of the component is wetted.F1 + the vertical component of surface-tension force acting under the chip is less than the vertical component of surface tension force acting on the top of the chip.Analysis shows that the surface tension of the solder will produce a turning moment thus, it is easy to demonstrate the conditions under which surface tension will be large enough to lift a component, i.e., tilting will occur if: F1 + F2 < F3. Figure 1 shows the forces acting on a component when it is in contact with molten solder during the reflow process. To understand the origin of tombstoning, it is necessary to consider all process and material factors. The primary cause of tombstone defects is the presence of unbalanced forces acting on the component arising from surface tension of the molten solder alloy. Partial lifting may be accompanied by lateral movement, which may make the defect more visible. In many ways, the latter is a more insidious defect than full-component lifting because it is more difficult to find via visual inspection. In less extreme forms, the component may undergo partial lifting at one end so that electrical contact with one pad is incomplete. In its most extreme form, "tombstoning" occurs when a component during soldering is raised on one end so that its axis, which should be parallel to the printed circuit board (PCB) surface, is perpendicular, effectively removing the chip from the circuit. ![]()
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