Ein neues Aktivatorsystem für eine neue No-Clean-Lötcreme.

Ein organisch-metallischer Polydendrimer-Aktivator , mit verbesserten chemischen, physikalischen und rheologischen Eigenschaften. Diese komplexe technische Lösung wurde entwickelt, um die Standzeit, die Klebekraft und Klebedauer, sowie die Druckeigenschaften der Lötcreme zu verbessern.

Die chemisch-technische Abhandlung:

By Ajith H. Premasiri, Ph.D., William R. Gesick, Marcia F. Gesick, and Dana P. Imler

A new no-clean paste has been developed using organo-metallic chelation chemistry with a dendrimer polymer as the activator. The paste consists of two parts: the Resin and the activator system, which has a dendrimer polymer synthesized by a natural product copolymerized with a multidentate chelator compound. The activator is suspended in a trace of solvent, which is higher in polarity to attract to the polymeric network and to avoid forming an azeotrope with the consumed polymer species in the solder reflow process. Volatile fragments are carried away along with a fraction of the consumed activator system.
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 to yield clear, hard, non-tacky, non-hygroscopic, and low levels of residue. Depending on the length of the oven, resultant residue levels will be approximately 0.5 percent.

New vs. Traditional Activators
In traditional solder paste technology, 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 of solder. By contrast, the new paste features a large number of multidentate reactive sites that contain hetero atoms and unsaturated (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. The activator has a high number of reducing sites for electron donation to the surface so that the reduction may take place, since multidentate hetero atoms and unsaturated pi bonding belong to part of the polymeric network. In traditional no-clean solder pastes, a limited number of halide ions, halogens, amines or carboxylic acids exist within the specifications of surface mount soldering guidelines. In the new solder paste, one molecule of activator carries a large number of active reducing sites prevailing in a dendrite polymer network. Accordingly, the effective number of active reducing sites is higher.

Another advantage provided by the new activator is that the post reflow residue is non-corrosive, nonconductive and non-hygroscopic compared to traditional no-clean pastes, which include corrosive and ionic species. A covalent nature is exhibited by the polymer before reflow and oligomer species of the post-reflow residue. The result is a hard, clear, non-tacky, non-hygroscopic, and chemically benign surface vs. ionic or polar materials, which result in ionic contamination in the post-reflow residue of traditional no clean pastes.

To date, the following solder paste alloys have successfully been used with the polymer activator system:

Sn62/Pb36/Ag2 (type 3 and 4 powder)
Sn63/Pb37 (type 3 and 4 powder)
Sn95/Ag5 (type 3 powder) - lead free
Sn96.5/Ag3.5 (type 3 powder) - lead free
Sn10/Pb88/Ag2 (type 3 powder)
Sn43/Pb43/Bil4 (type 3 powder)
Sn5/Pb85/Sbl0 (type 3 powder)

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.

Print Characteristics
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 a tack time of several weeks together with finepitch characteristics (e.g., 0.005-0.008" with T5 powder of 20-28 µm 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 new 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 and pull-strength testing of various surface mount component solder joints enable one to determine their quality and integrity. Figure 1 is a crosssection view of a PLCC solder joint displaying good wetting to a lead and board with minimum void formation. Figure 2 is a close-up of the solder joint's heel area. 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 new solder paste is designed as a drop-in replacement that requires no nitrogen- environment assistance. Other characteristics of the organo-metallic polydendrimer- activated material include:
* 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. Spectroscopically monitored after reflow, it also has been found to be non-reacting. 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. SMT Magazine
* NC-559, Amtech Inc. Dr. A.H. Premasiri, W.R. Gesick, M.F. Gesick, and D.P. Imler may be contacted at Amtech Inc., 75 Schoolground Road, Branford, CT 06405; (203)-481-0362; Fax: (203)- 481-5033.

1. Tomalia, D.A, Naylor, A.M, Goddard III, W.A, Starburst Dendrimers: Molecular- Level Control of Size, Shape, Surface Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter, Angewandte Chemie, International Edition in English, 29, p.l38, 1990.
   
2. (a) Rao, C.N.R, Ultra-Violet and Visible Spectroscopy, 2nd Edition, Plenum Press, New York, Chapter 11, p.147, 1967.
   (b) Nakamoto, K, J. Am. Chem. Soc., 74, p.1739, 1952.
   (c) Andrews L, J. Chem. Rev., 54, p.713, 1954.
   (d) Rand, S, Strong, R.L, J. Am. Chem. Soc., 82, p.5, 1960.
   (e) Person, W.B, J. Am. Chem. Soc., 87, p.167, 1965.
   
3. (a) Poller, R.C, J. Inorg. Nucl. Chem., 24, p.593, 1962.
   (b) Cummins, R.A, Aust. J. Chem., 18, p.98, 1965.
   (c) Kawasaki, Y, Tanaka, T, J. Organomet. Chem., 6, p.95, 1966.

Figure 1. Micrograph of PLCC solder joint. Cross-sectional analysis reveals good wetting to lead and board.

Figure 2. Close-up micrograph of heel area of PLCC joint. Note minimal void formation.