Wicking effect
The wicking effect , also known as solder suction and wick effect , is a defect in reflow soldering or vapor phase soldering of SMD components. The liquid solder flows off in the direction of the connection metallization of the components or in the direction of the copper tracks on the circuit board .
Description of the error pattern
With this defect pattern, the solder flows away from the soldering point in the direction of the connection metallization of the components. In some cases, the liquid solder rises against the force of gravity on the connection metallizations of the components. The liquid solder is withdrawn from the actual soldering point. Depending on the severity of the effect, the following forms are possible.
- In the simplest case, the solder joint is a bit lean, but the remaining amount of solder is still sufficient.
- In more severe cases, the soldering point is so lean that the remaining amount of solder still makes electrical contact between the component and the copper surface of the printed circuit board, but a clean solder meniscus can no longer be formed.
- In very severe cases there is no longer any contact between the component and the copper surface of the circuit board.
causes
The first possible cause is a large temperature difference between the copper surface of the conductor track, the connection metallization and the solder paste .
Usually components with a larger number of pins are affected. If the pins of the components are not all in one plane, individual pins are pressed deeper into the solder paste during assembly, while other pins hardly touch the solder paste or are only very weakly pressed into the solder paste.
The different strengths of contact between the pins of the components and the solder paste can also take place through twisting or warping of the circuit board, although the pins of the components are all in one plane.
Heating during the soldering process
During soldering, heat is added to the circuit board, the component and the solder paste from the hot ambient air. This applies both to pure SMD components and to THT reflow components.
The circuit board absorbs heat and transfers this heat to the solder paste via the copper tracks. The solder paste heats up and melts starting from the copper track on the first side (from below). The wider or thicker the copper track, the more heat is added to the solder paste in this way.
The component also absorbs heat from the ambient air and heats up in the process. The connection metallization of the component conducts the heat in the direction of the solder paste. In addition, the solder paste melts on the second side (from above) due to the heat supplied by the component. The larger the area of the connection metallization and the thicker the connection metallization of the component, the more heat is added to the solder paste via this path.
mechanism
The cause of the wicking effect is a greater temperature difference between the copper surface on the circuit board and the area of the connection metallization of the component during the melting of the solder paste. In the area of the solder paste depot where the temperature is highest, the solder paste melts locally first. Due to the surface tension of the liquid solder, it flows in the direction of the higher temperature.
If, in the present case, the heat input to the solder paste from the connection metallization is significantly greater than the heat input via the circuit board, then the solder paste mostly only melts from above. Due to the surface tension of the solder, part of the solder flows off in the direction of the connection metallization, since it is significantly warmer.
The wicking effect occurs more intensely with vapor phase soldering.
With the THT reflow components, however, the flatness of the connection pins is not the cause, since the connection pins are in this case in a hole in the circuit board. In this case, the hole itself is partially filled with solder paste. If the wicking effect occurs with THT reflow components, this is usually only due to the temperature difference when the solder paste melts.
Avoiding the effect
The cause of the wicking effect is to be found in the surface tension of the liquid solder in the temperature gradient. For these reasons, the probability of the wicking effect occurring cannot be completely eliminated, but only minimized. The following measures are suitable for this.
Better planarity of the components
An improvement in the planarity of the component connections means that all pins are evenly deep in the solder paste. This can support the even melting of the solder paste deposits.
Larger amount of printed solder paste
The thicker the printed solder paste deposit is in the wet state at the component connections, the more solder is present in the liquid state after melting. This also ensures that the pin is securely located in the solder paste when the component is populated. The larger amount of solder supports the formation of a solder connection between the pin and the copper track of the circuit board.
Contact pressure when assembling the components
When assembling the components with SMD, the contact pressure is selected so that the pins are pressed into the wet solder paste. A further increase in the contact pressure when equipping the components has no further significant influence on the wicking effect.
Temperature profile during soldering
The temperature profile when heating and soldering the assemblies in the reflow oven has a decisive influence on the wicking effect. If the assembly is heated evenly and not too quickly, there will not be large temperature differences between the components, the copper surfaces on the circuit board and the solder paste. This minimizes the probability of the wicking effect occurring by minimizing the temperature gradient.
Rework of the assemblies
After soldering, the soldered joints of the SMD and THT reflow components can be checked, for example, by a visual inspection or an automatic optical inspection . The components with missing solder at the soldering points can be re-soldered manually, provided that they are accessible.
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 .