Abstract: Solder-ball adhesion is closely related to the degree of cross-linking in the polymer material used for the PCB solder mask. The most important parameters are often affected by day-to-day process variation during bare-board PCB production, which is difficult to control completely. As a result, PCB manufacturing must accommodate this variation, and the number of solder balls may change from board to board.
A common conclusion in electronics manufacturing is that changing a single parameter in the soldering system cannot completely eliminate solder balls. In addition to the solder mask, the choice of flux can have a significant effect. In sensitive areas, eliminating the solder mask altogether may also be an effective approach.
For everyone involved in electronic equipment production, solder balls appearing on the PCB surface during wave or selective soldering are a persistent problem. Discussions about their causes and prevention measures often continue indefinitely, and the soldering equipment is frequently blamed. This reaction is understandable because the solder balls become visible after production. However, their causes and potential effects involve several stages of the broader electronic manufacturing process, which makes the problem difficult to analyze.
The issue became particularly prominent after low-solids fluxes and inert-gas soldering systems were introduced. The Montreal Protocol, an international treaty for protecting the ozone layer, initiated a major change in electronics cleaning practices. After the agreement took effect on January 1, 1989, chlorofluorocarbons (CFCs), which had previously been widely used as highly effective solvents for removing flux residues from assembled PCBs, were prohibited.
1. How Solder Balls Form
Solder-ball formation occurs when liquid solder in the wave separates from a pad. This separation process is commonly called peel-off. Because molten solder has high surface tension, the separated material tends to form a spherical shape. The basic mechanism is therefore governed by physical laws and cannot be eliminated entirely.
High-speed camera observations show that a solder ball can form during nearly every separation event between a pad and the liquid solder wave. The small solder sphere acquires considerable kinetic energy and typically bounces once or twice between the PCB surface and the solder wave. The sequence below illustrates this formation process.
Figure 1 | Sequential images showing the formation of a solder ball
The trajectory of a solder ball depends largely on the actual location where peel-off occurs. For example, a connector pad may separate from the solder wave across the entire pad area in a similar manner. The solder ball rises along a ballistic trajectory and then strikes the PCB surface. Because the board is moving, the impact point is usually slightly behind the solder joint.
This type is referred to as a primary solder ball and should not be confused with a secondary solder ball. Secondary solder balls may be generated by solder splashing from the wave nozzle or by defective material in a customized carrier. Some carrier materials can absorb cleaning solvents, creating another possible source of solder-ball-related defects.
2. Solder-Ball Adhesion to the PCB Surface
As described above, the solder ball is generated at the instant the pad separates from the solder wave, and most solder balls initially travel with their leading side directed toward the PCB surface. The key question is whether the ball adheres at the point of impact or rebounds from the surface. This distinction is critical, but it is not easy to answer because the result depends on the condition of the surface at the moment of impact.
Figure 2 | PCB surface without solder-ball adhesion
Figure 3 | Solder ball adhered to the PCB surface
Before low-solids fluxes became common, fluxes often contained rosin, with solids content frequently reaching 20–30%. Solder balls were still generated during peel-off at that time, but adhesion was rarely a significant problem. The reason was that rosin-containing flux formed a liquid flux layer over the PCB surface during soldering. When a solder ball struck this liquid layer, it was deflected or carried away instead of adhering to the PCB.
The disadvantage of rosin-containing flux was the large amount of flux residue left after soldering. When low-solids fluxes were introduced, the surface conditions changed fundamentally. The solids content of no-clean fluxes is rarely higher than 3%, which is generally insufficient to form the same continuous liquid layer on the PCB surface during soldering.
Modern flux activators are also frequently based on organic acids rather than synthetic rosin, making the adhesion mechanism even more apparent. When a solder ball strikes a clean PCB surface covered only by solder mask, it may adhere directly to that surface. The problem therefore depends not only on the flux but also, to a substantial degree, on the properties of the solder mask.
3. Effect of PCB Solder Mask on Solder-Ball Adhesion
Modern bare-board PCB production commonly uses two-component solder-mask systems. The performance of these solder-mask systems depends strongly on how each component is manufactured and combined. An investigation of one solder-mask supplier found that even a small change in the ratio of rosin to catalyst—less than 1%—could alter the process behavior of the solder mask. That change could then have a significant effect during the subsequent soldering process.
The critical parameter is the degree of curing, or the degree of cross-linking in the polymer chains. If the rosin-to-catalyst ratio deviates slightly from the formulation, the solder mask may not cure completely. During soldering, heat can then soften the under-cured solder mask, making it easier for solder balls to adhere to the surface.
PCB manufacturers must therefore control solder-mask processing carefully. Before the coating is applied, the rosin and catalyst must be mixed thoroughly. The requirement is strict: air must not be introduced into the mixture, since air can carry bubbles and moisture into the material. The mixing operation must also be performed accurately and consistently so that all components are distributed uniformly.
Temperature and humidity during production are important process variables. The quality of the solder mask—especially its curing condition and polymer-chain cross-linking—depends on the environmental conditions present during PCB production. These conditions are difficult to control perfectly. Consequently, the practical focus shifts toward controlling the resulting solder-mask surface and the way the PCB is handled during soldering.
Some customers have attempted to evaluate the cross-linking condition of the solder mask on delivered PCBs and correlate the results with the number of solder balls detected by automated optical inspection (AOI). The following methods have been used:
- Using test inks with known surface tension to evaluate surface quality.
- Observing a cross-section of the solder mask under a microscope to evaluate the surface condition.
- Using IPC-TM-650 2.4.27.2 to assess the degree of solder-mask cure.
Over the course of one year, 16,000 samples were tested and analyzed. The results were significant: no single parameter with a clear correlation to the number of solder balls on the boards could be identified. During this investigation, the soldering parameters, flux, and solder were not changed. This consistency supported the reliability of the analysis.
Solder, flux, and soldering equipment do affect solder-ball formation, but their effects are relatively limited. Selecting a particular solder, optimizing the flux, or changing the soldering material may reduce the number of adhered solder balls. However, the primary influence is the PCB solder mask. A decisive improvement generally requires a change in the solder-mask system rather than adjustment of only the soldering equipment.
This conclusion was reported as early as 1999, and its central theoretical assumption remained consistent despite numerous customer tests conducted through 2016. Additional analysis has described the relationship between solder-mask roughness and adhesion. A smooth solder-mask surface has low roughness and provides a relatively large effective contact area for a solder ball. A matte solder mask has greater surface roughness, reducing the effective contact area and therefore lowering the adhesion force.
Figure 4 | Comparison of smooth and matte solder-mask surfaces
4. Effects of Soldering Parameters and Operation
The soldering system itself generally has a limited effect on solder-ball formation, although process engineers may still attempt to reduce the defect by adjusting equipment parameters. The flux used during soldering can have a somewhat greater influence. Some flux manufacturers have applied the principle that rosin-containing fluxes can prevent solder balls from adhering to the PCB.
One approach replaces rosin with a separating agent in a synthetic flux. The separating agent forms the necessary liquid layer during soldering and helps prevent solder-ball adhesion. This type of flux has been used in automotive electronics. However, it can create a cosmetic problem on the PCB surface. After soldering, the dried separating agent becomes inert and may remain clearly visible. Many users interpret this visible material as dried flux residue, although that interpretation is not technically correct. Even after efforts to reduce the visible residue, customers may remain reluctant to use such fluxes.
Comparisons of different fluxes show trends in solder-ball formation and adhesion to the PCB. The right-hand image shows the result obtained with a modified water-based flux containing a separating agent.
These test results are often specific to the product and soldering system under evaluation and therefore cannot be treated as universally applicable. They do, however, demonstrate the possibilities offered by modern flux formulations. Other influential factors include preheating, soldering temperature, and the atmosphere in the soldering system.
A lower soldering temperature may reduce the number of solder balls, while a lower residual-oxygen atmosphere may increase their number. These process observations cannot be applied universally. In practice, each PCB production process normally requires individual adjustment. The geometry of the soldering nozzle is another important factor that must be considered.
5. Additional Measures to Prevent Solder-Ball Adhesion
Various possible solutions have been evaluated, including changes to the PCB layout. One approach is to add a silkscreen coating in sensitive areas. However, silkscreen coatings are also polymer-based materials. At the high temperatures encountered during soldering, they may soften and behave like an unsuitable solder mask, creating another surface to which solder balls can adhere.
Figure 5 | Solder balls on a silkscreen coating
One highly effective way to avoid solder-ball adhesion is to remove the solder mask completely from selected sensitive areas of the PCB. This approach requires a substantial change to the PCB layout and must be evaluated against the electrical, mechanical, and manufacturing requirements of the design.
Figure 7 | Area around a plug-in connector with no solder mask
