Over the past several decades, an increasing number of electronic component manufacturers have begun using polyvinyl butyral as the temporary binder. With its efficient pigment wetting capabilities, tightly controlled molecular weight distribution, clean endothermic burnout and high oxide ceramic pigment binding capacity, polyvinyl butyral provides opportunities for further advances in oxide ceramic-based passive fabrication technology.
During the 1950s, Warren J. Gyurk teamed with Glen Howatt at Glenco Corp. in Metuchen, N.J., to develop some of the earliest prototype passives based on BaTiO3, ferrite and lead zirconate titanate (PZT).2 Afterwards, Gyurk joined the staff at Radio Corp. of America, Inc. (RCA) to collaborate on a military project called the Army Module Program. While working for RCA, he developed several application patents for laminating BaTiO3 films in multiple layers of 25.4 microns each.3 The slip formulation used to produce the ceramic film used a vinyl chloride-vinyl acetate copolymer as the binder. The new manufacturing process would serve as the foundation for the production of a new class of passives—multilayer ceramic capacitors (MLCCs).
During the 1960s, John L. Park, a researcher from the Chattanooga, Tenn.-based American Lava Corp., advanced the work of Howatt with the proposal to discharge the ceramic slip through a small orifice in a pool onto a flexible supporting tape. The patent suggested that the supporting tape could be based on polytetrafluoroethylene or glycol terephthalic acid polyester. Park was one of the first scientists to publish information about ceramic slip formulations based on polyvinyl butyral for use in tape casting applications.4
Today, thin ceramic films that use polyvinyl butyral as the temporary binder are incorporated into MLCCs, low temperature co-fired ceramics (LTCCs), lithium-ion membranes, metal brazing strips, bioceramics, solid oxide fuel cells, thick-film printable polymer insulator pastes, and many other electronic component fabrication processes. As many product engineers have discovered, polyvinyl butyral provides a host of benefits in these and other advanced ceramic applications.
Polyvinyl butyral is a thermoplastic resin manufactured through a three-step condensation reaction. First, vinyl acetate is polymerized to poly (vinyl acetate). The poly (vinyl acetate) is then saponified to poly (vinyl alcohol), and the alcohol is reacted with butyraldehyde to form polyvinyl butyral. For this reason, polyvinyl butyral should be considered a ter-polymer with functional pendant hydroxyl groups. When used as the main binder in the formulation of ceramic slips, polyvinyl butyral acts as a secondary dispersing agent for the ceramic particle. The polymer exhibits good solubility in low-cost alcohol and glycol ether solvents due to the presence of pendant hydroxyl groups on the polymer chain backbone. It is also soluble in most aromatic and aliphatic solvents.
In a submicron particle size environment, it is important that the binder adequately coat the large surface area of the particles to ensure that they are adequately separated within the green ceramic body. The thermolysis of the ceramic green body within a temperature range above the thermal degradation temperature of polyvinyl butyral (T = 170 degrees C), but well below the classical sintering temperature of 1000 degrees C, reveals that the concentration of the degradation products from polyvinyl butyral rises rapidly and then diminishes as the binders diffuse to the surface of the green body. The propensity of a binder to exhibit clean burnout is extremely important to prevent the development of flaws and to minimize carbon residues that may inhibit sintering. The minimization of carbonaceous residue within the ceramic body is especially important when the ceramic is co-fired with an oxidizing metal such as copper, since the reducing conditions may aggravate carbon formation. Systems based on polyvinyl butyral are known to exhibit clean burnout characteristics in low temperature co-firing systems with metallization.
In thin-film tape casting applications, both degradation reactions and volatile transport occur simultaneously during thermolysis; therefore, it is important to control the molecular weight of the polymer to improve the probability that the degradation byproducts will diffuse uniformly to the surface of the green body. During the manufacture of polyvinyl butyral, care is taken to ensure that the acetalization process of polyvinyl alcohol and butryaldehyde occur within a defined temperature range to generate a targeted molecular weight and distribution. Such control over the molecular weight distribution is necessary to ensure excellent solvent solubility at the individual polymer particle level and uniform burnout during pyrolosis.
Polyvinyl butyral can decompose by a cyclic-elimination mechanism to release butyraldehyde, with associated chain breakage, leaving aldehyde and unsaturated end-groups.8 In the late stages of this process, various products, including crotonaldehyde and unsaturated hydrocarbons, form by breakage at adjacent sites. Masia et. al. studied the effect of oxides on the burnout of binders.9 They concluded that the surface chemistry of oxides in a polyvinyl butyral ceramic oxide film has a strong catalytic effect on the activation energy and, hence, the decomposition rate of the polyvinyl butyral resin. In LTCC applications, a shift to the left of the polymer’s decomposition temperature profile can provide a ceramist more formulation latitude to incorporate more exotic metal compositions during the metallization process.
In addition, Masia found that selected polyvinyl butyral-containing ceramic green bodies undergoing thermolysis in an oxidative environment yield three decomposition phases, and that the degradation temperature of polyvinyl butyral shifts to lower temperatures depending on the type of ceramic used. Therefore, upon fabrication of the ceramic tape to a component, the burnout schedule should match the polymer/oxide ceramic combination to ensure a low probability for sintered part defects due to bubbling and warpage.
Polyvinyl butyral resin suppliers are devoting an increasing proportion of their overall research and development expenditures to developing the next generation of polyvinyl butyral resins that meet the higher mechanical and electrical requirements for use in specialized passive component applications. The push toward miniaturization of passive components through the use of submicron ceramic particles and the incorporation of crystallizing and filled glass particles into the ceramic matrix has re-established polyvinyl butyral as a leading binder of choice in critical, high-performance thin-film ceramic tape applications.
2. R.E. Mistler, “Tape Casting: Past, Present, Potential,” October 1998, www.ceramicbulletin.org/months/Oct98/cast2.html
3. W. J. Gyurk, “Methods for Manufacturing Monolithic Ceramic Bodies,” U.S. Pat. No. 3,192,086, 1965.
4. J. L. Park Jr., “Manufacture of Ceramics,” U.S. Pat. No. 2,966,719, 1961.
5. W.O. Hermann, GB Pat. No. 182,459, 1922.
6. W.O. Hermann, GB Pat. No. 184,442, 1922.
7. W.O. Hermann, GB Pat. No. 185,107, 1922.
8. F. Bakt, Pakistan J. Sci. Ind. Res. 26 (1), 1983, p. 35.
9. S.Masia, P.D. Calvert, W.E. Rhine, and H.K. Bowen, “Binder Burnout: The Effect of Surface Chemistry on Carbon Residue Elimination,” J. Mater. Sci., 24, 1907 (1989).
10. K. Niwa, N. Kamehera, H. Yokoyama and Kurihara, “Multilayer Ceramic Circuit Board with Copper Conductor,” Westerville, OH, 1986, p. 4.
*These resins are commercially available as PIOLOFORM, a registered trademark of Wacker Polymer Systems, L.P., Adrian, Mich.