Secure and stable soldered connections for high-vibration applications

The reliability of solder joints is particularly important in electronics when assemblies are continuously exposed to high mechanical stresses. This applies, among other things, to applications in rail technology, mining, and construction vehicles.

May 19, 2025

Voids

Voids as a Risk to Solder Joints Subject to Mechanical Stress

Voids are cavities or pores within a solder joint. Possible causes include organic inclusions in the surface metallization, flux components, or oxides dissolved in the solder.

The formation of voids is not caused by a single factor. Rather, it results from an interaction between:

Under the influence of vibrations or temperature differences, voids can compromise the stability of a solder joint. In the worst-case scenario, this can lead to a break in the connection between the substrate and the component.

To reduce this risk, the soldering process, solder alloy, and flux must be coordinated.

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SAC Solder

SAC solders place special demands on the soldering process

Compared to SnPb solders, SAC solders are more prone to porosity. The original article cites several reasons for this. First, the higher surface tension of SAC alloys makes it more difficult for trapped gases to escape from the solder. Second, the higher process temperatures lead to increased formation of potential gases from:

Added to this are the wetting properties of SAC solders. Because the difference between the process temperature and the melting point is relatively small, wetting can be difficult. This makes the coordination of solder, flux, and process parameters even more important.

Fluxes

Fluxes affect wetting and void formation

The surface tension between the liquid solder, flux, and the printed circuit board also affects the degree of wetting and, consequently, the formation of voids. Good wetting helps reduce the formation of voids.

Proper wetting helps reduce pore formation.

Flux plays an important role in this process. It helps remove oxides from the solder pads and create a clean, oxide-free surface. This promotes wetting by the solder and reduces the risk of inclusions and contamination.

Emil Otto's fluxes were developed for demanding applications and have been used for many years in the manufacture of electronic assemblies. Their composition has been continuously refined to ensure stable and durable solder joints.

Factors That Influence

Other Factors Affecting Void Density

In addition to the solder alloy and flux, other parameters can influence the number and size of pores within a solder joint.

Surface and holes in the printed circuit board:

Oxide layers can promote void formation.

Component Metallization and Oxide Adhesion:

Here, too, oxide layers can promote the formation of voids.

Alloy and layout of the printed circuit board

Time the THT component remains in the vertical position

Storage of Printed Circuit Boards and Components

Nitrogen Atmosphere in Selective Soldering

Preheating Temperature and Preheating Time

The formation of voids must therefore always be considered in the context of the interaction of several factors. A narrow focus on the flux or the solder alloy alone is insufficient.

Applications

Reliable soldered joints for demanding operating conditions

The combination of high-quality solder, a suitable flux, oxide-free solder pads, and optimized process control can positively influence the formation of long-lasting solder joints. This is particularly important in applications where electronic assemblies are continuously exposed to high mechanical stresses.

In railway engineering, the reliability of these connections contributes to the safe transport of goods and cargo. For construction vehicles and mining machinery as well, stable electronics are essential for systems to function reliably. The selection of suitable solder alloys and fluxes is therefore an essential component of assembly and joining technology designed for long-term stability.

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Emil replies

Which process parameters affect the void density in solder joints?

Relevant factors include the surface and drill holes of the printed circuit board, the PCB alloy and layout, storage conditions, component plating, preheating temperature and duration, the dwell time of the THT component in the solder, and the nitrogen atmosphere during selective soldering.

What role does the flux play in reducing voids?

The flux helps remove oxides from the solder pads, thereby contributing to a clean, oxide-free surface and better wetting. Good wetting, in turn, can help reduce the formation of pores within the solder joint.

Why are SAC solders more prone to porosity than SnPb solders?

SAC alloys have higher surface tension, which makes it more difficult for trapped gases to escape from the solder. In addition, the higher process temperatures can promote gas formation from the flux, the printed circuit board, and the components. The wetting properties of SAC solders also play a role.

What factors can cause voids to form in solder joints?

Possible causes include organic inclusions in the surface metallization, flux components, or oxides dissolved in the solder. Void formation is also influenced by the interaction between the printed circuit board, component metallization, solder alloy, and process control.

Why are voids a critical issue in solder joints subjected to vibration?

Voids can compromise the electrical and mechanical quality of a solder joint. Additional stress caused by vibration or temperature differences can destabilize the solder joint and, in the worst case, cause the connection between the substrate and the component to fail.