Tailoring Flux Formulations to Enhance Solder Paste Performance

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By Kevin Gaugler, EFD Inc., a Nordson Company

Continued component miniaturization, the emergence of complex assembly methods, finer-pitch connections, changes in plating methods, nanometer-scale materials, and the requirements of new electrical generation and distribution technologies now challenge the performance of soldering processes at every level. Environmental initiatives launched at the beginning of the 21st century, such as RoHS, have also resulted in fundamental changes to both solder alloys and flux-based carrier systems.

Solder paste's primary purpose is to provide the metallic components and flux activity required to construct a solder joint. Solder paste users generally understand and appreciate the elementary characteristics and selection of metal alloy, particle size, and flux type.

Alloy composition affects reflow temperature, mechanical joint properties, visual fillet quality, component wetting, component dissolution (leaching), and suitability as a lead-free alternative. Powder size influences the paste's flow characteristics during both dispense and reflow operations. Standard flux formulations include mildly and fully activated rosins (RMA and RA), no clean (NC), and water soluble (WS).

RMA flux residue is clear, soft, non-corrosive, and non-conductive, while RA flux residue is more corrosive and should be removed to prevent damage to the assembly. No-clean flux has low activity; is suitable for easy-to-solder surfaces; and leaves a clear, hard, non-corrosive, and non-conductive residue. Water-soluble fluxes often contain organic acids and are available in a range of activity levels. WS flux residue is hygroscopic (absorbs water), making it both corrosive and conductive. Therefore, WS residue should be cleaned as soon as possible after reflow.

Many solder paste users, however, are not aware of how thoughtful selection of solder paste carriers and additives such as plasticizers, thixotropic agents, wetting additives, activators and activation carriers, stabilizers, solvents, and other formulation components can help resolve a range of process considerations and create opportunities to achieve significant improvements in the soldering process.

60512-efdtone.jpgTable 1. Flux classification challenges

Once the basic solder alloy composition and flux type are determined, many options are available to tailor the formulation to address specific application requirements, enhance paste performance during the soldering operation, and improve solder joint reliability in the field. For example, two paste formulations can differ greatly in performance despite having the same QQ-S-571E and J-STD-004 classifications, as shown in Table 1. Figure 1 shows how paste A (left) left a void in the hermetic seal, while paste B (right), with a gap-filling formulation, produced an acceptable seal.

The following examples demonstrate how different combinations of paste types and additives can be used to resolve application-specific challenges.

Solder paste spread after deposition and during initial heating results in loss of definition and poses a bridging risk when components or pads are closely spaced. Reduced slump additives will minimize these issues.

In RA, RMA, and NC formulations where the joint is visible or the spread of flux to surrounding areas might cause a problem, a restricted residue formulation will cause the residue to remain on or very close to the fillet after reflow.

Halides are materials that contain a chloride (Cl-), bromide (Br-), fluoride (F-), or iodide (I-) halogen, and are present in some flux activators. They assist in oxide removal due to their high free energy state. Halide-free solder pastes have an IPC J-STD-004A rating of zero for the fourth character (e.g., ROL0).

Low-residue formulations limit the quantity of flux residue remaining after reflow. Either there is less flux to begin with, or a larger percentage evaporates during the reflow process.

Hard-to-wet metals and oxidized surfaces can require a flux with higher activity or different activators that work more effectively with the metal involved. Aged components, nickel/iron alloys, and stainless steels are some examples that require special consideration.

Formulations used for bridging gaps, filling holes, or soldering joints on vertical surfaces hold the alloy in place until liquidus temperature is reached.

Rapid reflow refers to heating solder paste in less than 5 seconds. A rapid reflow formulation will not spatter, even when reflow times are a fraction of a second. Typical rapid reflow heating methods include laser, solder iron, hot bar, and induction.

Fluorescent dye can be added to assist in the verification of solder deposits at paste placement and reflow, aiding inspection with UV lighting and simple vision systems. The fluorescent additive becomes inactive during reflow, so that a post-soldering inspection can be performed to ensure that complete reflow has occurred.

Pin transfer or dipping is an application technique where solder paste is applied by dipping a component or pin into the paste to pick up a thin, consistent layer of solder paste. This technique is useful for applying solder to products that do not lend themselves to printing or dispensing, such as pin arrays. Dipping paste is also a useful way to apply additional solder paste to a through-hole component when the amount of paste that can be printed for paste-in-hole is insufficient due to stencil thickness.

IPC-7097A is the "Specification for the Design and Assembly Process Implementation for BGAs." The inspection criteria for BGAs and microBGAs often call for voiding of less than 20% of the area, with some fewer than 9% or even 4%, depending on the location of the voiding within the ball. A low-void solder paste is required to meet the very low voiding limits for Class 3 assemblies.

The data in Table 2 is presented to show that most additives are compatible with each other.

ConclusionSolder paste selection can make or break a solder joining process. Specialty solder pastes enhance and fine-tune performance, compared to off-the-shelf products. These are only some examples of details of flux performance that are important in the solder paste selection process. Using the best solder paste for a particular application will improve processing and the end product, making it prudent to call your solder product vendor to review your requirements.

Kevin Gaugler, solder group research and development manager, EFD Inc., a Nordson Company, can be reached at (401) 431-7177; kgaugler@efd-inc.com.

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