Laminates cycles

As mentioned earlier, the viscosity, tj, of the resin typically changes by orders of magnitude during a lamination cycle. Equation 5 below is a dual-Arrhenius type expression often used to describe the viscosity changes ... 

Control of the laminator cycle will be very flexible due to use of four independently adjustable timers and a 24 step industrial controller with 10 individual switch modules. 

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. 

Laminate Manufacturing Process Considerations. Historically, some printed circuit manufacturers had the laminate manufacturer bake the laminate prior to shipment, or would bake it themselves prior to use. The intent was to relieve any stress that may be stored within the iaminate prior to use in the circuit manufacturing operation. Aithough this process may help, the added material handling and cycle time does not usually justify the process. Instead, many laminate manufacturers reduce the lamination pressure at a specified point in their lamination cycle to minimize the stress that becomes stored in the finished product. 

Multilayer Lamination Processing. Multilayer lamination cycles must be designed around the specific material being used. Flowever, there are some general recommendations that can be applied to most materials ... 

If multiple lamination cycles consider moving up on chart for each additional cycle (treat as an additional reflow cycle)... 

Since hydranUc presses are by far the most common type of press, the following discnssion focnses on that process. The qnality of the ML-PWB lamination is affected by both the pressnre ramp and the temperatnre ramp. There are almost as many nniqne cycles (also known as press recipes) as there are fabricators. However, in general the lamination cycle can be divided into fonr regions B-stage melt B-stage flow B-stage cure and cooldown. Rgnre 27.37 shows a typical lamination cycle with these critical variables identified. 

A good lamination cycle produces a flat ML-PWB that is free of moisture or voids and has a fully cured substrate. All layers must be well registered. The ML-PWB must be free of warp and the thickness must be within the specification. Any controlled impedance layers mnst have the correct pressed dielectric separations above and below the signal layer. Each of these requirements puts special demands on the lamination process. To assist in setting control measures, the following process indicators (see Table 27.6) should be monitored with a Statistical Process Control (SPC) method. 

Although a bake will advance the cure, it is unnecessary if a proper lamination cycle is used (although polyimide is an exception check the recommended processing guidelines). As discussed earher, this can be verified by Tg measurements. However, additional baking beyond full cure degrades the material and reduces Tg. 

Cydohexanedimethanol, 1,4- dim ethyl o1 cycl oh exa n e, or 1,4-bis (hydroxymethyl) cyclohexane (8), is a white, waxy soHd. The commercial product consists of a mixture of cis and trans isomers (6). This diol is used in the manufacture of polyester fibers (qv) (64), high performance coatings, and unsaturated polyester molding and laminating resins (5). 

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

With a sufficiently long press cycle, a state of complete cure is reached. At this point, the laminate is cooled in the press, under pressure, and removed for finishing operations. If the press is opened at a temperature above the boiling point of trapped volatiles, vaporization occurs causing interlaminar blistering which mins the laminate. 

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

Manufacture of friction elements includes the impregnation of fabrics and subsequent lamination, the wet-dough process, and the dry-mix process. Elements from the last two are prepared by compression-molding the formulation for up to an hour at 150—175°C. Thick brake elements require a carefully controUed heating-and-cooHng cycle to minimize stresses created by expansion and contraction (see Brake linings and clutch facings Fillers). 

The type of varnish used in the process depends on the kraft paper manufacturer and basis weight of the papers the machine, temperature, and control (scraper bars, squeeze roUs) used the method of cutting the paper to size the laminate being produced (post-forming or regular) and the press-cure cycle (see Laminated materials, plastic). 

For smart cards, micro-robots and small precision instruments, thin laminated micro-cells are being developed. Some of these developmental thin-film devices—using an electrolyte of lithium, a copper cathode, and lithium again for the electrode—can charge and discharge up to 3 volts, and can be expected to tolerate up to 1,000 charge-and-discharge cycles. 

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