Solder penetration
The solder penetration is a measure of the filling of a plated through sleeve when soldering a wired component in a two-layer or multi-layer circuit board .
In the case of a printed circuit board with copper cladding on both sides, there is a metallized sleeve in the bore for assembling wired components. The same applies if the circuit board has more than 2 copper layers. When a wired component is soldered into this circuit board on the solder side, the copper ring on the solder side is wetted by the liquid solder . In addition, the liquid solder fills the metallized sleeve of the circuit board. The fill level of the sleeve is known as the solder penetration and is a decisive criterion for the quality of a soldered joint. When soldering of wired components usually is wave soldering , the selective soldering , the dip soldering or manual soldering with a soldering iron used.
Limits of solder penetration
The soldering point on the circuit board is heated by the liquid solder. The insulation material of the circuit board, the copper surface and the metallized sleeve are heated. Due to the surface tension, the liquid solder wets the copper ring and the metallized sleeve. The surface tension also ensures that the solder pulls into the sleeve. The liquid solder rises in the metallized sleeve of a circuit board until it cools down to the liquidus temperature and solidifies. From this point on, there is no further soldering through. In some cases, this can result in insufficient solder penetration.
Risks with poor solder penetration
Exceeding the current carrying capacity of the metallized sleeve
Let us consider the power line from the bottom (solder side) to the top (component side) of an assembly . In the case of a 100 percent solder penetration, the current carrying capacity is distributed over the connection wire of the wired component, the metallized sleeve and the solder present in the sleeve. Without a 100% solder penetration, the load-bearing capacity of the component wire is not possible in the upper area without a solder penetration, since this is no longer bound by the solder on the upper side. Furthermore, the solder itself is eliminated because this area of the sleeve is not filled with solder. In this case, the current carrying capacity is only provided by the copper sleeve. With higher currents there is a risk that the maximum permissible current density of the sleeve can be exceeded and thus damaged.
Lack of mechanical stability
A 100% solder penetration connects the connection wire along the entire length of the sleeve. If the solder penetration is poor, the mechanical stability of the soldered connection also decreases. In the case of massive wired components, there is therefore the risk that the solder joint will be mechanically destroyed in the event of vibrations.
Measures to improve the solder penetration
The solder penetration can essentially be improved by the measures listed below. It should be noted here that individual measures may conflict with the requirements for the printed circuit board (e.g. low copper thickness with high requirements for current carrying capacity).
Stronger flux
Fluxes with a stronger effect improve the solder penetration by removing more oxides from the metallized surface of the circuit board and improving the wetting of the metallic surfaces. A stronger flux supports this effect.
Stronger preheating
By connecting inner layers of the circuit board, heat is dissipated into the inner layers during soldering . Increased preheating of the assembly during the soldering process also brings the inner layers to a higher temperature . Less heat is then dissipated into the inner layers during soldering. This has the advantage that the liquid solder can continue to rise in the sleeve before it cools down.
Increase in solder temperature
The solder is brought to the operating temperature in the liquid state by the soldering system. The higher the soldering temperature, the more heat can be given off to the environment without the solder cooling down. Technically, however, the maximum temperature of the solder is limited by the maximum permissible temperature load on the components.
Solder material with a lower liquidus temperature
If a solder material is exchanged for another material with a lower liquidus temperature , an improved solder penetration in the sleeve can be achieved. In this case, the liquid solder can give off more heat to the area around the solder joint without solidifying immediately. With lead-free standard solders, there is more and more poor solder penetration, for example these solders for lead-free soldering on a tin - silver- copper basis have higher liquidus temperatures than tin- lead solders.
Connection of only required copper layers
A multilayer circuit board has more than 2 copper layers. The more copper layers a printed circuit board has, the more difficult it is to solder, as all copper layers dissipate heat from the liquid solder during soldering. The solder penetration can be improved if the number of inner layers is reduced. However, if the inner layers are necessary for circuit-related reasons, the soldering behavior can also be improved by only connecting the electrically necessary inner layers to the copper sleeve.
Use of smaller copper thicknesses
The thicker the copper tracks, the more heat is extracted from the solder joint. The soldering behavior is improved by using the thinnest possible copper cladding. A copper thickness of 12 µm or 18 µm instead of a copper thickness of 35 µm is an improvement. Due to the low thermal conductivity of the thinner conductor tracks, less heat is withdrawn from the soldering point during the soldering process, which then leads to better soldering penetration.
Connection through narrow lines
Wide copper tracks on the circuit board conduct increased heat away from the soldering point. By connecting the conductor tracks to the sleeve with conductor tracks that are as narrow as possible, heat dissipation is reduced. This enables a better solder penetration to be achieved at the solder joint.
Single layer printed circuit board
In the case of a printed circuit board with a copper clad on one side, there is no metallized sleeve in the hole for assembling wired components. With this design there is no solder penetration because the metallized sleeve that is required for solder penetration is missing.
With this construction technique, the wired component can only be connected to the soldering side of the circuit board. In this case, a high-quality soldered connection is present when the copper ring is continuously wetted with solder through 360 °, a solder cone forms in the area of the connection pin and the hole is completely covered with solder.
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 .