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It all starts with the right soldermask and flux.

Tiny solder balls that stick to the solder side of the assembly after wave soldering are a type of soldering defect that has become more of a problem due to several new developments:

• Fine-pitch components and smaller distances between traces make the assembly more sensitive to shorts caused by solder balls.

• More SMDs on the solder side increases the frequency of selective soldering with pallets using a wave soldering machine.

• Higher solder temperatures due to Pb-free solder use.

• More nitrogen blanket systems are retrofitted to wave solder machines to reduce dross formation of Pb-free solders (a cost benefit) and improve the soldering process window.

In the past, the majority of solder balls were removed by cleaning the boards after soldering. No-clean fluxes reduced or eliminated the cleaning process, but solderballs remained. This is of concern in certain applications. For a number of control systems such as airbags and other automotive applications, the presence of solderballs (and the subsequent potential for shorts) can compromise safety.

Causes. Solderballs are formed when the assembly exits the liquidous solder. The solder pulls free from the board and bounces back into the solder bath. When this occurs, a snapping action takes place downward into the solder pot, forcing a sphere, or micro ball, of solder to be propelled toward the board (Figure 1). When designing the solder wave former and solder pot, it is important to reduce the fall height (gap or distance) of the solder. A lesser fall height reduces dross formation and solder spattering.

FIGURE 1: Micro balls between through-hole leads.

Nitrogen plays a role here. An inert blanket over the solder surface prevents the formation of an oxide layer on the solder bath and thus increases the potential for micro balls. Nitrogen also affects the surface tension of the solder.

A second cause of micro solderballs is outgassing of board material or soldermask. If there are cracks in the metalization of the holes, vapors can escape which can result in voiding or solder balls on the component side of the assembly.

A third cause of micro solder balls is related to the flux. Flux can be trapped underneath components or between an assembly and a carrier (when selective soldering with pallets). If the flux is not sufficiently preheated and completely dried before the assembly contacts molten solder, solder will splash and create solderballs. Respect the preheat recommendations of the flux suppliers per the flux datasheet.

Soldermask effects. Whether or not a solder ball will stick to the assembly depends on the board material. A solderball will bounce against the assembly and fall back into the solder unless the adhesion force between the board and the solderball is higher than the gravitational force.

In this scenario, the soldermask is an important factor. A rough soldermask will have a smaller contact area with the solder ball, making it less prone to solder balling. Higher solder temperatures (as in a Pb-free process) will soften the soldermask, making it more prone to balling.

Standards and criteria. Several standards define criteria for solderballing. The classifications vary from "no solder balls allowed" (MIL-STD-2000¹) to less than five solder balls per sq. in. (IPC-A-610C²).

IPC-A-610C considers solderballs located within 0.13 mm of lands or traces and larger than 0.13 mm in diameter as nonconforming process indicators (Figure 2). Such conditions require the manufacturer to get the process under control and take corrective action. IPC-A-610D, now rewritten to cover Pb-free soldering, is not clear on the subject of micro balls. The part that describes the solder ball criteria (five micro balls per sq. in.) has been removed. Automotive and military specifications do not permit any solderballs, so the assemblies must be cleaned, or solderballs must be removed manually.

FIGURE 2: Dimensions of solderballs can be determined with advanced microscopes.

Prevention. A study by a European group showed that soldermask has the most pronounced effect on the formation of solderballs (Figure 3). Thus, it is important to select the proper type.

FIGURE 3: Important parameters affecting solderballing.

Specially designed fluxes are available. Sufficient flux must be applied so there is some flux still available at the wave exit. The flux deposits an ultra-thin film on the board that prevents micro balls from adhering.

Compatibility between the flux and soldermask is a must, as is a tightly controlled flux deposition system such as a spray flux application.

A design of experiment (DoE) returned these recommendations for minimizing solderballs:

• Lower the solder temperature, if possible.

• More flux results in less solder balls, but also increases the amount of residue.

• A higher preheat setting is better, but keep within flux specifications; otherwise the flux activation period is too brief.

• A faster conveyor speed reduces the number of solder balls.

References

1. MIL-STD-2000, "Standard Requirements for Soldered Electrical and Electronic Assemblies," June 1995.

2. IPC-A-610C, "Acceptability of Electronic Assemblies," January 2000.

Bibliography

1. H. Bell and R. Zajitschek, "The Solder Ball Problem, Research and Results," August 1994.

2. TKB-4U.com, Reducing Solder Microballs in Inert Wave Soldering, tkb-4u.com/articles/soldering/reducingballswavesol/reducingballswavesol.php.

3. G. Schouten, Solder Balls in Wave Soldering, Vitronics Soltec information sheet 013, July 20, 2004.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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