Blurring
The blurring of SMD components is a fault in reflow soldering or vapor phase soldering . In this case, the component is not in the desired position on the circuit board after soldering . This error pattern in reflow soldering is closely related to the tombstone effect, whereby the tombstone effect is usually a stronger expression of the causes underlying the blurring.
Soldering process for SMD components
In the reflow process, an optimal solder joint is achieved if the following points are taken into account.
- There is a geometric adaptation between the size of the copper area on the circuit board and the solderable connection metallization of the component.
- The wettability of the component and the copper surface of the circuit board are sufficiently high.
- The amount of solder is correctly measured.
- The temperature profile when preheating and melting the solder paste is optimal.
If the points mentioned are met, slightly offset or twisted SMD components can even swim into the correct position despite a slight positional error during assembly. If the points mentioned are not met, soldering errors can occur.
Description of the error pattern
After soldering, the component is not in the desired position. For example, the component can be shifted in the X and Y directions. Furthermore, there can also be an error in the angle of rotation of the component. In this case, the component has rotated around its own axis.
Ideally, the connection metallization of the component is located in the middle of the copper surface of the circuit board . Depending on the displacement and rotation of the component, the following four cases can result as error patterns:
- The component is electrically connected, is not exactly in the middle, but is located with its complete connection metallization over the copper surface.
- The component is connected electrically, but with its connection metallization protrudes over the copper surface of the circuit board.
- The component is not connected electrically and its connection metallization projects over the copper surface of the circuit board.
- The component is not electrically connected and, with its connection metallization, lies completely outside the copper surface of the circuit board.
The blurring occurs with pure SMD components. In the case of THT reflow components, this type of blurring is technically not possible, since the connection pins are located in bores within the circuit board.
causes
Mismatch geometry
So that a correct soldering point with solder meniscus can develop, the copper surface on the circuit board must protrude all the way over the connection metallization of the component. If the length or width of the copper surface is smaller than the connection metallization, or if the copper surface is arranged in the wrong place, the component can become blurred during reflow soldering. In this case, the surface tension of the liquid solder cannot align the component by minimizing the acting forces.
If the copper area is significantly larger than the connection metallization of the component, the surface tension of the liquid solder cannot work properly. The component is usually within the copper surface, but is not optimally aligned in the X and Y directions and can also be twisted.
Low mass of the component
If there is an unfavorable relationship between the mass of the component and the surface on which the connection metallization rests, the component can be lifted by the liquid solder and blurred into a different position. This effect occurs to a greater extent if the component is not immediately wetted after the solder paste has melted. For this reason, particularly lightweight components or components with a large contact surface are threatened with blurring.
Offset when printing solder paste
A pronounced offset when printing solder paste can also cause the components to become blurred. If, for example, the solder paste is printed offset in such a way that it is printed in significant quantities outside of the copper surfaces on the circuit board, the liquid solder must retreat towards the copper surface during the melting process, as far as this is possible due to the surface tension. In this case there is also a one-sided oversupply of liquid solder, which in turn can lead to blurring of the component.
Placement offset
If components are fitted in the X or Y direction or with an incorrect angle of rotation, there is a placement offset. If the placement offset is very small and the component is still largely on the copper surface of the circuit board, it will usually float into the desired position. If dimensioned correctly, the surface tension of the liquid solder will align the component.
Direction of heat input during the soldering process
The direction of the heat input when the solder paste melts during the reflow process has an influence on the blurring of the component. If the sides of a two-pole component are heated differently, the solder paste also melts first in this area. In the partially already liquefied solder, the surface tension acts and tries to move the component into a position where the surface tension reaches a minimum. This position may also be outside the desired position. Under certain circumstances, the effect of the surface tension of the completely melted solder paste can no longer have sufficient force so that the component floats back from the shifted position back to the original position.
Oversupply of lot
If there is an oversupply of liquid solder, the component can float on the liquid solder, provided it is relatively light. The same behavior occurs as with components with a low mass (see chapter above).
Wettability
The uniform and sufficient wettability of the connection metallization of the component has a decisive influence on the blurring of the component. The wettability of the copper surface on the circuit board has the same effect. If the wettability on one side is different within the copper surface of the circuit board or within the connection metallization, the solder first connects the component and the copper surface in the area of this point. The surface tension will align the component in this position. As a result, the component is influenced by the surface tension and blurred in the respective direction. If the remaining surface of the component or the printed circuit board is then wetted, the surface tension will attempt to bring the component into an overall optimal alignment. If the component has shifted from its original and optimal position, there is a possibility that the force of the surface tension is no longer sufficient for the component to float back.
Rework of the assemblies
After reflow soldering , the soldering points and the positions of the SMD and THT reflow components on the assembled assembly are checked, for example, by means of a visual inspection or an automatic optical inspection . Blurry components can be found here. Depending on the severity of the mold, it may be necessary to rework the affected components. Here, the soldering points of the affected component must be heated at the same time so that the component can be aligned. Alternatively, it may also be necessary to unsolder and re-solder the component.
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 .