Synthetic Solder Paste — A New Class of No-clean Chemistry

Estimated reading time: 6 minutes

Just as synthetic motor oils have improved automotive engine performance, synthetic no-clean solder paste formulae will improve soldering performance, taking SMT process lines to higher speeds.

By Ajith H. Premasiri, Ph.D.

A new generation of no-clean solder paste delivers improved stability, lot-to-lot consistency, reduced scrap and rework, and higher hourly throughput. Organic-based solder pastes rely on natural rosins and partially hydrogenated resins. Change in temperature, humidity or air quality can cause isomerization of abietic acid-based rosins. Paste activation is achieved through carboxylic acids and halides. Release into the residue during reflow can result in electromigration failure.

Synthetically modifying the resin to derivatize its phenanthrene structure minimizes reactions caused by environmental variations. Additionally, long chain polymeric entities used in manufacture of traditional no-clean solder fluxes can result in precipitation without the proper solvent system. This can lead to non-homogeneity within the solder paste, a loss of paste tack and imparts a grainy look to the paste. Short chain polymers offer greater solubility in water.

After extensive research, development and experimentation, it has been found that a fully synthetic poly-adduct paste formulation can overcome conventional limitations.

Due to the rigid structural integrity of the poly-adduct components, isomerization of the resin is inhibited. A unique, aromatic solvent system homogeneously suspends the poly-adduct so that no crystallization occurs during paste processing. The flux's unique solvency enables higher metal loading: 91 percent in printing applications and 88 percent in dispensing applications. The high thermal stability of the poly-adduct targets it for use with many different alloys.

The chemical makeup of traditional paste activators causes polarity and the ready absorption of moisture. Synthetic no-clean solder paste features a unique poly-dendrimer activator that is olefinic, thus enhancing its hydrophobic character to eliminate paste splattering and outgassing effects.The synthetic solder paste activation system also involves organo-metallic chelation.

Within the activator are polymeric sequestrants containing multidentate ligand sites that chelate numerous metal ions (Cu++, Ni++, Zn++, Cd++, Pb++ and Ag+). Upon reflow, the poly-dendrimeric metal complexes are extracted into the flux residue.

Organo-metallic complex formation is governed by the cavity size of the multi-dentate ligand complexes of the poly-dendrimer and the ionic radius of the chelated metal ion. A particular metal-ligand complex must have a metal ionic radius that matches the cavity size of the ligand. One metal ion will fit into a particular ligand cavity. Because synthetic solder paste features a variable cavity size, it can encapsulate a wide variety of metal ions with different ionic radii. The extracted poly-dendrimeric metal complexes result in a clear, non-tacky post-reflow residue.

Use of a synthetic poly-adduct no-clean solder paste optimizes thermal profiling during the reflow process. Solder joint reliability also is enhanced by the wetting and spreading action exhibited during organo-metallic chelation. Metal ligand complex chelation is initiated around 140°C, and the fragmentation of the chelated poly-dendrimer may begin above 180°C. As poly-dendrimeric chelated metal complexes get extracted into the flux vehicle, they undergo a molecular reorganization.

Intermolecular interactions between the solder powder and the synthetic flux are less likely to adhere to stencil or squeegee blades, improving rolling character, paste snap off from blades and stencil cleaning during printing. Paste activation also is halide-free.

Efficient fluxing action enhances solder wetting and spreading by increasing the surface energy and surface tension of the substrate and component leads. Surface tension is the direct measure of the intermolecular forces acting at the surface. To wet or spread, the free energy (DG) of the newly formed system must be lower after wetting/spreading.

The elemental pure alloy powder is surrounded by a thin oxide crust, which protects powder particles, keeping them discrete and free-flowing. This is a thermodynamically stable state in powder, as the outer oxide coating is stabilized by the lattice energy. When this lattice structure is broken, the symmetrical bonding pattern gets perturbed, creating an unsymmetrical bonding environment in the oxide layer and causing the surface energy to rise. Creating a chemical disorder in the bonding pattern at the surface level, the system tries to lower its free energy (DG<0) or maximize its entropy (randomness) (DS>0).

This phenomenon occurs when fluxing action takes place in soldering. Identical phenomena take place at the substrate surface level and component leads as well. Before fluxing action takes place, the surface is saturated with molecular bonding in the oxide layer. This is supposed to be the minimum surface energy-containing state. After the fluxing action takes place, the surface tension increases readily as a result of rapid bond formation and cleavage at the surface level, creating unsymmetrical bonding patterns at the surface. For molten solder to wet the substrate, substrate surface energy must be higher than that of the molten solder prior to wetting. Once wetting is complete, the system returns to its most thermodynamically stable state. The greater the difference in the surface tension between molten solder and the substrates and leads, the more favorable the spreading and wetting.

Figures 1, 2 and 3 show the wetting action and angle of solder joints on organic solderability preservative (OSP) base material using Sn63/Pb37 alloy with synthetic solder paste.

Figure 1. Micrograph of a SOIC solder joint shows the ideal wetting angle.

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Figure 2. Micrograph of the same SOIC solder joint shows excellent wetting action and angle.

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Figure 3. Micrograph of a J-leaded PLCC component reveals good wetting between the component lead and the board, along with minimal solder void formation. The solder joints tend to be stronger than the adhesion of the Cu lands to the board, resulting in lifted lands on the J-leads during pull tests.

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Extended tack time is a direct result of solvent retention in the paste via hydrogen bonding between solvent molecules and the synthetic poly-adduct resin. Furthermore, the paste's flux components have enough nonpolar characteristics to be water repellent, which extends tack time and shelf life. Synthetic poly-adducts are photochemically and environmentally stable.

Figure 4 depicts the spectra of the differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) of the synthetic solder paste. The TGA is recorded by the weight (percent) lost against the temperature, while the DSC records the heat flow (w/g) vs. temperature. TGA's exponential decay with virtually no distortions indicates the product's stability. A major phase change occurs at 184.16°C when the solder melts, depicted by a sharp decline in the TGA slope. No other sharp variation in the TGA graph indicates the absence of sudden phase changes due to material weight loss. A sharp, narrow band emerges in the DSC at 180.12°C, representing the eutectic behavior of the solder exhibiting a sharp heat flow. Until such change in the solder paste phase at 180.12°C, no other phase change shows up in the DSC. Spherecity, particle size distribution and levels of impurities in the solder powder affect the quality and the integrity of the solder joint formation.

Figure 4. Differential scanning calorimetry (DSC) and thermo gravimetry analysis (TGA) spectra of the synthetic solder paste.

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Synthetic solder paste age spanning up to three years has been tested by spiral viscometry to evaluate the paste's shear strength on aging. Test results depicting Malcolm viscosity vs. time taken at 10 rpm at 25°C support the paste's physical and chemical stability during the testing period up to two years (Figure 5). Synthetic solder paste begins to show deterioration in shearability after two and a half years at ambient room temperature. After two years, the paste shows a marked rise in the viscosity. Spiral viscometric testing at 10 rpm at 25°C for 24 hours for samples up to two years exhibited no signs of paste degradation, thus resulting in a paste with creamy texture.

Figure 5. Test results depicting Malcolm viscosity vs. time taken at 10 rpm at 25°C.

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For a complete list of references, contact the author.

Ajith Premasiri, Ph.D., may be contacted at AMTECH Inc., 75 School Ground Road, Branford, CT 06405; (203) 481-0362; Fax (203) 481-5033; E-mail: apremasiri@amtechinc.com.