Lamination isostatic

A much better approach to the elimination of uneven pressure application is the use of a relatively new technique isostatic lamination. In this process the layers are stacked on a fixture plate and sealed in a vacuum bag. The bag (or several bags) are then immersed in a pressure chamber that is filled with heated water or a mixture of water and glycerin. Most isostatic pressure chambers operate between 34.5 and 68.9 MPa (5000 and 10,000 psi). Typical temperatures range from 70 to 90°C and the time for the complete cycle is from 3 to 10 minutes. The manufacturers state the following advantages of isostatic lamination ... 

Isostatic Laminating Presses, ABB Autoclave Systems, Inc., Columbus, OH. 

In contrast, isostatic lamination uses a water-filled pressure vessel to apply heat and pressure to the ceramic layers, as illustrated in Figure 6.22. This process is more complex because contact between the water and imfired ceramic must be avoided. The pressure and temperature uniformity across the part, however, is superior to most uniaxial lamination systems. The tacked layers to be laminated are vacuum bagged to prevent introduction of water into the substrate, and metal plates may be used on either or both sides of the tape layers. A base plate is used to support the tape layers, whereas a second metal plate on top is referred to as the cover plate. Cover plates serve to distribute the pressure uniformly over the substrate, but may be omitted in some cases. As can be seen in the upper left-hand inset of Figure 6.22, the vacuum-bagged parts are often stacked inside the laminator on multiple shelves. Many parts can be laminated in one batch lamination cycle with this approach. 

An isostatic lamination system capable of laminating multiple multilayer ceramic substrates in one rapid process. As shown in the upper left inset, these substrates are normally vacuum bagged and stacked on shelves that are lowered into the presstjre vessel of the machine. (Courtesy of Avure Autoclave Systems, Inc.). 

The individual sheets of processed tape used in a substrate are collated and aligned by using tooling holes aligned to the via and circuit pattern in each layer of tape. The multilayer stack is then laminated under elevated temperature (70 to OO C) and pressure (2000 to 6000 Ib/m ) to bond the individual layers into one cohesive stack. Two distinct methods of lamination are used isostatic and uniaxial. Isostatic lamination is used to laminate substrates containing cavities since it is critical to attain uniform applied pressure to surfaces as well as bottoms of cavities. Uniaxial lamination is used for flat substrate designs. 

After all the layers have been printed, dried, and inspected, they are stacked using the holes previously cut in the comers for alignment. The circuits are then laminated under heat and pressure to form a monolithic structure. There are two lamination approaches that are commonly used uniaxial and isostatic. In the uniaxial approach, the circuits are placed between two heated parallel plates, and a force is applied in the normal direction. For better accuracy and lower distortion, the isostatic method is preferred. The circuit is enclosed in tooling and water at about 80°C, and 300 psi is applied to the tool. 

Isostatic Isotropic properties Uniformity No flow lines or laminations... 

The individual layers are laminated together by heated hydraulic presses. TypicaUy, isostatic pressure is achieved on the three-dimensional pattern with isostatic pressing or through the use of molds or conformal bladders or bags. Finally, the laminates are green cut to final or near-net expanded dimensions. The multilayer parts are then heated in a burnout and firing process to remove organics and densify the ceramic and metal materials. 

An uniaxial warm press was used to obtain the files at 70°C for 30 min with a pressure of 15 MPa. The rolls were produced by isostatically pressing the different green layers on a ceramic rod at 80°C. After sintering at 2150°C the reached densities were >97% in A tapes and -90% in B-tapes with a clear dependence with the SiC addition. The residual porosity of the rolls composed by five A layers is around 15 vol% as a non uniform lamination can cause the trapping of air between the layers and this effect could be enhanced by the constrain caused by the different curvatures and the yielding effect induced by the bending of the green layers. 

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