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Lead-free: VOC-free Flux Deposition Reduces Defects
December 31, 1969 |Estimated reading time: 6 minutes
As the global push toward lead-free manufacturing gains momentum, much attention has been focused on its effect on assembly processes. While solder paste materials and reflow considerations have been researched extensively, the industry must consider flux chemistry and deposition for wave soldering.
By Tim Lawrence, Ph.D., Stanley Soderstrom, and Ian Wilding
Much attention has been focused on the effect of lead-free manufacturing on the assembly processes. While solder paste materials and reflow considerations have been researched extensively and often adopted, the industry must consider another process step in its quest to implement environmentally friendly manufacturing - flux chemistry and efficient flux deposition for wave soldering.
The widespread focus on solder paste for reflow soldering is not surprising, given this technique has overtaken wave soldering. The implementation of a lead-free wave soldering process also is considered more problematic. There are installation and maintenance issues associated with a lead-free solder bath running at elevated temperatures. Wave soldering remains an important part of most assembly operations, and must be considered.
Robust flux chemistry and efficient flux deposition technologies are intrinsically linked to the successful implementation of lead-free wave soldering. For manufacturers transitioning from tin/lead to lead-free, this step cannot be ignored. The role of flux chemistry is to remove tarnish/oxide from the substrate and prevent oxidation of the substrate and alloy during the soldering process. This process allows for intimate alloy-substrate contact, a prerequisite in solder joint formation.
VOC-free fluxes are different from alcohol-based, no-clean fluxes in that they are less volatile and less able to spread over the substrate. However, rapid fluxing action is created in the aqueous phase through improved acid dissociation. Because there are environmental and handling benefits, VOC-free fluxes are an appealing option.
VOC-free for Lead-free: Challenges
Because of environmental and handling advantages, developing a robust VOC-free flux for use with lead-free applications would seem to be a logical progression. There are challenges, however, with some VOC-free flux formulations with lead-free processes, as the flux must be capable of sustaining a controlled process. Lead-free solders have a higher liquidus (227°C for 99Cu alloy vs. 183°C for Sn63 alloy). Therefore, flux actives must be able to withstand higher temperatures without volatilization.
Figures 1 and 2. Bridge (l) and solder ball (r) showing typical wave soldering defects.
In wave soldering, when the level of flux activator on the PCB is diminished through wave contact, bridging and solder balling can occur (Figures 1 and 2). These defects become more prevalent in a lead-free process due to the higher melting point and the higher wave solder bath temperature. To minimize these problems, any flux activator package used with lead-free operations should exhibit low volatility. With VOC-free flux formulations, however, this is demanding because the solvent, which is invariably water, is not compatible with many activators used in traditional, alcohol-based liquid fluxes. Rosin, one activator component that provides ideal physical and chemical properties, has not been an option previously for VOC-free flux formulations because it is not water-soluble.
Rosin has unique chemical and physical properties and, in its molten state, provides a cleansing effect on the PCB’s metal substrate, which is then wet by molten solder to form a robust union between substrate and alloy. Low volatility from rosin also encourages sustained action. Rosin’s low water solubility delivers subsequent protection against corrosion from exposure to humidity. Dielectric properties reduce current leakage between adjacent conductors and provide high surface insulation resistance (SIR).
Rosin-based VOC-free Flux: The Solution
There have been two less-than-ideal methods to put rosin into a water-based product: the formulation of rosin soap and incorporating a water-soluble organic co-solvent. With rosin soap, the active acid in the formulation is neutralized to enable water-solubility. However, the reduced-acid functionality that results from the neutralization is detrimental to the fluxing process, given that it is the inherent acidity of rosin that enables it to function as an activator. Because rosin is comprised of many components, it is unlikely that complete dissolution will occur.
With a carefully selected co-solvent, however, complete dissolution is possible and allows wetting without the need for surfactants, which can impair the foam-fluxing capability of the product. This method will not provide the environmental VOC-free advantage, as the co-solvent will be manifest as a VOC. Two novel flux formulations were developed to address these challenges. One is a VOC-free product in which the activator package comprises dicarboxylic acids selected for low volatility. The other is a VOC-free formulation that includes rosin suspended in water. Testing of both products revealed compatibility with tin/lead and lead-free wave soldering processes. These unique materials deliver sustained activity through a low-volatility activation package.
Successful VOC-free Flux Deposition
Uniformity in the flux deposition process is critical for good results. In the ideal no-clean soldering process, most flux applied to the board is consumed during soldering, minimizing the level of post-soldering flux residue. Any remaining flux must be benign. One must be careful not to apply too much flux, resulting in post-soldering flux residue. In these circumstances, related issues are primarily cosmetic, although it should be noted that there is a slight potential for in-service problems to occur. Under the conditions of sustained activity from lead-free solder at high temperatures, there will be some post-soldering flux residue. However, rosinous flux residue is less problematic as it forms a conformal coating that protects the enclosed circuitry.
Figure 3. Ultrasonic nozzleless spray head.
Because of these issues, it is critical that the precise amount of flux is applied to the board. The most effective method to accomplish this is an ultrasonic, nozzleless spray-fluxing system that applies the flux uniformly to the bottom surface of the circuit board and into the thru-holes (Figure 3). To maximize resources, the system also should be able to maintain this process reliably and repeatably throughout a production shift. Flux manufacturer specifications provide the typical flux deposition range, which is generally specified in micro-grams of flux solids per square inch, such as a range from 750-1,500 µg/in2 of flux solids on the circuit board.
Because VOC-free fluxes do not wet as well as alcohol-based fluxes, fluxing systems cannot rely solely on wetting agents and must be able to deliver the flux to all metal surfaces to be soldered. Therefore, tight control of the flux deposition process becomes imperative, and ultrasonic, nozzleless spray fluxing is the most suitable method for uniform and repeatable application. Without uniform and repeatable flux application in lead-free processes, problems may arise. Generally, lead-free alloys are not able to wet as effectively as lead-bearing alloys. This is related to the fact that lead-free alloys cannot flow to the surface of the holes as easily, reducing the ability to create good solder fillets. Manufacturers must apply enough flux uniformly into the barrel of the hole to ensure lead-free solder can be drawn to the surface efficiently.
Conclusion
With the proven environmental and handling benefits of VOC-free fluxes, it is clear that a robust, lead-free-capable, VOC-free flux is an ideal material solution for modern electronic assemblers. The availability of such a material applied through the accuracy of spray-fluxing deposition yields benefits and reduced defect rates for those making the switch to lead-free wave soldering processes.
Tim Lawrence, Ph.D., technical capability manager, the electronics group of Henkel, may be contacted via e-mail: tim.lawrence@uk.henkel.com. Stanley Soderstrom, product engineer, Ultrasonic Systems, may be contacted via e-mail: sjsoderstrom@ultraspray.com. Ian Wilding, senior applications engineer, Soldering Products and PCB Protection, the electronics group of Henkel, may be contacted via e-mail: ian.wilding@uk.henkel.com.