Tombstone effect
The grave stone effect (Engl. Also Tombstone Effect or tombstoning ) or Aufrichteffekt is also Stonehenge or Manhattan called and is a common mistake during reflow soldering or vapor phase soldering SMD -Bauteilen. With this type of defect, small (mostly two-pole) components in particular stand up on one side during the soldering process. The second connection of the component is then not electrically contacted. The components then stand on a flat SMD board like a tombstone .
Description of the error pattern
With reflow soldering, the SMD components form an angle of 0 ° with the circuit board when they lie flat on the circuit board. In the event of a tombstone error , the components do not lie flat. An angle is formed between the axis of the component and the circuit board. This can be between 0 ° and 90 °. The tombstone effect occurs during reflow soldering and during vapor phase soldering of SMD components, although this effect tends to occur with a lower probability during reflow soldering.
This effect occurs with two-pole SMD components. Mostly it is about small designs of the components, z. B. 0805, 0603, 0402. Light components tend to be more affected than heavy components. In principle, this effect can also occur with multi-pole components, but the probability is extremely low.
mechanism
Various forces act on the component during the soldering process. During the soldering process, the solder paste on the two connection pins of the component melts at different times. The surface tension of the liquid solder acts on the side of the component with the first liquefied solder and pulls on the component. In this case, the surface tension of the liquid solder that lies outside the component tries to shorten the free path, which can then lead to the component being erected due to the forces acting (first force).
Furthermore, due to the surface tension below the connection metallization of the component, the liquid solder tries to keep it in the flat position. The plumb line also strives to shorten the free path. This force (second force) tries to keep the component in the flat position.
The weight of the component acts as the third force. This force acts in the center of gravity of the component and tries to keep the component in the horizontal position.
All three forces now act during the soldering process. The first force tries to erect the component, while the second and third force try to keep the component in the horizontal position. If the first force predominates, the component straightens up.
Furthermore, this effect can be explained by a different wettability of the two connection metallizations of the components.
Avoiding the effect
Due to the forces acting, effective approaches can be derived to prevent the components from erecting.
Higher weight of the components
If heavy components are used, the likelihood of standing up decreases.
Length of the connection metallization on the component
In the case of components, the surface tension of the solder below the connection metallization of the fused-on side inhibits the erection of the component. In the case of components with a longer connection metallization, there is therefore a tendency for the component to be erected with less probability.
In order for the inhibiting force to have any effect at all, the copper surface on the circuit board in the area below the component must be at least as large as the connection metallization of the component itself or be significantly larger towards the inside.
Copper pad on the circuit board
The pad on the circuit board should not protrude too much beyond the component in the longitudinal direction. The larger the copper surface protruding beyond the component, the more solder can collect in this area and thus lead to a very pronounced solder meniscus, which can then tend to support the component's erection again.
Connection of the lines to the component
The erection of the component is largely suppressed if the solder melts on both sides of the component at the same time and the forces acting thus keep the component in a position of equilibrium.
The most effective measure is to use connecting lines of the same size for both solder connections of the component on the circuit board. This prevents different amounts of heat from reaching the individual connections during the heating process during the soldering process. If both sides melt at the same time, the straightening effect is almost completely eliminated.
Contact pressure when placing the component in the solder paste
If the opposite terminal of the component only loosely adheres to the solder paste, the component can be lifted at this point. However, the influence of the contact pressure when fitting the component has only a very small influence on the alignment of the component.
Solder paste
The more solder paste there is, the larger the solder meniscus can develop on the solder joint. The surface tension of the meniscus ultimately leads to the erection of the component. If the meniscus is not excessive, the likelihood of the device erecting tends to be small.
It is also assumed that the type of solder paste has an influence on the erection of the components; Furthermore, that the duration between the solder paste printing and the subsequent soldering process has an influence. This could be explained by the fact that volatile components of the solder paste escape if the assemblies are left standing for a long time before soldering.
Furthermore, erecting can occur if different amounts of solder paste are printed on the two pads. This can be the case, for example, when one of the two openings of the component is partially closed with dried paste when the solder paste is stenciled, or the two connection pads in the stencil have a different size due to the design.
In addition, an unprocessed solder paste is assumed to be a further influencing factor. Before applying the solder paste, it must be moved a few times in the solder paste printer so that it is homogeneously mixed. If this is not done, the composition of the paste can be different for the different pads of the components. The mixing ratio of the paste (amount of solder, flux, ...) can change. As a result, the components can straighten up.
Gluing the SMD components
If the SMD components are glued with an adhesive point during assembly, the components cannot be erected.
control
If gravestones occur in production, they can, in contrast to other errors, be easily recognized by visual inspection or automatic optical inspection. The components on the corresponding circuit boards are then usually re-set and soldered by hand. The risk of faulty devices being delivered undetected is low.
literature
- Reinard J. Klein Wassink: Soft soldering in electronics . 2nd Edition. Eugen G. Leuze, Saulgau 1991, ISBN 3-87480-066-0 .
- Wolfgang Scheel (Hrsg.): Assembly technology of electronics . Verlag Technik et al., Berlin et al. 1997, ISBN 3-341-01100-5 .