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No nitrogen assistance needed

Activation technique for no-clean soldering via organo-metallic chelation
No nitrogen assistance needed

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The no-clean paste NC-559 from Amtech uses organo-metallic chelation chemistry for activation with a dendrimer polymer as the activator. The solder paste is designed as a drop-in replacement that requires no nitrogen-environment assistance. It has low levels of residue that are non-corrosive, and the tack time is extended. The paste provides a wide process window and the opportunity for remarkable quality enhancements.

Ajith H. Premasiri, William R. Gesick, Amtech, Brandford, CT

The paste’s two parts are the resin and the activator system with its polymer, synthesized by a natural product that co-polymerized with a multidentate chelator compound. The activator is suspended in a trace of solvent, which, being high in polarity, is attracted to the polymeric network. The volatile fragments that result from activation are carried away along with a fraction of the consumed activator system, forming azeotropic mixtures with the solvent. In the soak zone of the oven, the polymer is activated and undergoes rapid chelation with the metal ions in the alloy. In the reflow zone, the resultant organo-metallic dendrimer polymer is consumed in a stoichiometrically controlled environment via a time and temperature dependent molecular reorganization process which yields a clear, hard, non-tacky, non-hygroscopic, low level residue. Depending on the length of the oven, resultant residue levels can be as low as 0.5%.
NC-599 versus traditional activator
In traditional solder paste technology, it is believed that the wetting of solder is achieved by chemical reduction of the surface of the substrate with halides, halogens, amines, and carboxylic acids in the activator system. By using such activators, chemical reduction causes a lowering of the surface tension of the substrate, resulting in the wetting by solder. By contrast, the paste NC-559 features a large number of multidentate reactive sites that contain hetero atoms and unsaturation (double and triple bonds). These engage in chelation with the metal ions in the oxide layer of the alloy while reducing the surface of the substrate to chemically enhance wetting characteristics. Since multidentate hetero atoms and unsaturated pi bonding belong to part of the polymeric network, the activator has a high number of reducing sites for electron donation to the surface so that the reduction may take place.
In traditional no-clean solder pastes, a limited concentration of halide ions, halogens, amines or carboxylic acids is permitted within the specifications of surface mount soldering guidelines. In the paste, one molecule of activator carries a large number of active reducing sites prevailing in a polymer network. Accordingly, the effective number of active reducing sites is higher. Another advantage provided by the activator is that the post reflow residue is non-corrosive, non-conductive and non-hygroscopic compared to traditional no-clean pastes, which often include corrosive and ionic species. A covalent nature is exhibited by the oligomer species found in the residue after reflow. The result is a hard, clear, non-tacky, non-hygroscopic, non-conducting and chemically benign surface whereas, in traditional no-clean pastes, ionic or polar materials frequently occur in the post-reflow residue that results in ionic contamination.
Activation mechanism
Using chemical modeling, a polydendrimer is synthesized. The monomer is co-polymerized with a naturally occurring material that may result in a highly branched and well ordered dendrimer polymer that consists of a large number of multidentate chelating sites that are highly flexible in coordinate or dative bonding. One of the key features of this molecule is its cavity size adjustability when chelation takes place with the metal ions. The final activator is polydispersed, possibly with different size oligomers having different molecular weights.
To date, the following solder paste alloys have successfully been used with the polymer activator system:
Sn62/Pb36/Ag2
Sn63/Pb37
Sn95/Ag5 – lead free
Sn96.5/Ag3.5 – lead free
Sn10/Pb88/Ag2
Sn43/Pb43/Bi14
Sn5/Pb85/Sb10
Sn60/Pb40
Sn95/Sb5 – lead free
Sn100 – lead free
Fatigue resistant solder (non-alloying dopant added)
The effectiveness of using these alloy systems with the novel activator may be traced to the ability of the dendrimer polymer to chelate effectively with metals having different ionic radii. The multidentate ligands of the activator polymer offer flexibility in encapsulating different size metal ions. Further, the pi orbitals and nonbonding lone electron pairs of chelating groups are spatially arranged for an effective chelation configuration. The body, tack time, and printability of a no-clean paste are dependent on its ingredients. Typically, the rosin or resin provides the desired features to the paste. The novel activator, however, is intended to provide an extended tack time together with fine-pitch print characteristics (0.005 to 0.008-in with T5 powder of 20 to 28µm particles). Also, with traditional no-clean solder pastes, the components must be placed within a few hours of printing the circuit boards. In contrast, the paste remains tacky because the solvent is chemically trapped in the three-dimensional polymeric network. No solvent dry-out is observed during printing or the subsequent time lag before placement of the components. Further, the activator’s gravitational force tends to slip paste through apertures of the stencil, yielding well-defined fine-pitch on printing.
Cross-sectional analysis
Cross-sectional analysis and pull-strength testing of various SMT solder joints enable one to determine their quality and integrity. Figure 1 is a cross-section view of a PLCC solder joint displaying good wetting to a lead and board with minimum void formation. Figure 2 shows the solder joint structures of a component housed in a 1206 case, and figure 3 presents a perfect SO16 joint. In all examples, pull tests indicated that the solder joints tended to be stronger than the adhesion of the copper lands to the board. Many of the PLCC tests resulted in lifted lands. Ion chromatographic tests performed for electromigration analysis of these joints confirmed the benign character of the post-reflow residues.
Quality enhancement
The solder paste is designed as a drop-in replacement that requires no nitrogen-environment assistance. Other characteristics of the organo-metallic polydendrimer-activated material are as follows: Low levels of residue depend on the length of the oven. The longer the oven, the lower the residue upon reflow. Post-reflow residue is chemically inert. Spectroscopic analysis after reflow has demonstrated its non-reactivity. Finally, since degradation of the organo-metallic polymer does not create ionic species, ionic contamination is not a hazard. The material’s extended tack time is due to the three-dimensional polymeric network, which acts as a barrier to solvent release from the paste.
This is an updated version of an article originally appearing in SMT magazine (March 1997) titled A Novel Activator for a New No-Clean Solder Paste.
Fax: +01-203-481-5033.
Zusammenfassung
Durch die Formulierung einer No-Clean-Lotpaste mit organo-metallischem Polymer zur Aktivierung lassen sich gute Ergebnisse bei der Benetzung und der Lötqualität erreichen. Die Paste läßt sich einfach als Ersatz für andere Produkte einsetzen, eine Schutzgasumgebung ist nicht erfoderlich.
La formulation d’une pâte à braser „no-clean“ avec un polymère organo-métallique pour l’activation permet d’atteindre de bons résultats d’imprégnation et de qualité de brasage. La pâte est simplement utilisable en tant que substitut à d’autres produits, un environnement en atmosphère protectrice n’est pas nécessaire.
Grazie alla formulazione di una pasta di brasatura No-Clean con polimero attivante organo-metallico è possibile raggiungere buoni risultati nella copertura e nella qualità della brasatura. La pasta può essere usata semplicemente in sostituzione dei prodotti comuni senza necessità di lavorare in ambiente protetto da gas.
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