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IMDES CREATIVE SOLUTIONS
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4.1.3 Tombstone-Effect
The tombstone effect occurs if - under certain circumstances during the reflowing process of the solder paste and
its unilateral influence on the surface tension of the solder - the small bi-polar components start to lift (e.g. SMD
capacitors and resistors). The most important reasons for the rising of the components are:
1. The layout of the printed circuit board does not, or only badly suits the component geometry.
2. The applied solder paste is bad or unsuitable.
3. The solder paste print is uneven and/or badly positioned.
4. The amount of solder paste is too high. (Optimum approx. 0.15 mm thickness of the mask and pressure
reduction from 15 to 20 % with critical components of the size 08/05 and smaller).
5. The size of the stencil is not reduced. (Optimum is 10 to 15% reduction).
6. The mounting displacement is too high.
7. The metallization of the components and the contact pad is insufficient.
8. The components have ‘balled‘contact pads.
9. The solder resist is higher than the pad surface. It sometimes happens – should the solder paste have not or
only insufficiently dried - that the solvent evaporates violently during soldering. These small ‘explosions‘ may lift
the components. However, this will not occur if the dwell-times are correctly set and suitable solder pastes are
used. But the most frequent reason for the lifting is unsuitable pad geometry relative to its component geometry
as well as too big a solder paste quantity.
1.
The pads must not be too far apart - to prevent the surface tension of the solder to pull the component to one
side. This will make it impossible to produce a solder contact on its other side, due to an insufficient contact
area. The pads may, however, be positioned only to a certain degree under the components as they must still
produce a good solder contact (meniscus). The further the solder-joint lies below the component, the more
difficult it is to check its quality subsequently
.
2.
A further role, when designing pad geometry, is played by the metallization’s of the solder areas on the
components. Contrary to the SMD resistors, the SMD condensers possess metallization´s along the sides and
fronts. Due to the wetting forces additionally acting at these side areas, there is a certain compensation of the
unilaterally lifting force on the front side. Thus, tombstoning does not affect the condensers as much as the
resistors
.
Regretfully, there are at rare occurrences of tombstones in assemblies which meet all the above conditions. The
cause may be temperature differences in the printed circuit board itself. These temperature differences lead to an
uneven reflowing of the two pads. Even if one pad is reflow-soldered only marginally earlier, the forces of the
molten solder may pull the component vertical. Until now it has been generally recommended to improve pre-
heating.
Yet, this frequent advice to better pre-heating does not eliminate the temperature differences at the reflow point
temperature.
However, since the introduction of the variable temperature gradients in the vapor-phase process, these are
reflow-soldered homogeneously.
Another possibility to reduce the tombstone effect is to use solder pastes with a low wetting potential. An elegant
approach is also to use pastes which - instead of having an exactly defined melting point - feature a melting range
of approx. 5
K.
