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63.10.5 Other Process The other processes such as blanking and stiffeners are very difficult except special designs with large volume production.
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63.10.6 Tape Processing Manufacturing process of TAB (Tape Automated Bonding) has been developed by reel to reel from the beginning for specific applications. The details of the technology will be considered in Chap. 66.
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63.11 DIMENSION CONTROL
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Due to increasing density requirements and inherently unstable film substrate characteristics, dimensional control has become a key issue for both circuit fabricators and their end customers. In order to achieve such densities while maintaining high production yields, careful dimensional consideration in both the design and manufacturing processes is necessary. An exact concept for dimension control of the whole manufacturing processes is required for high-density flex circuits.There are many factors that impact the dimensional stability of highdensity flex circuits. These factors can be categorized as follows:
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63.11.1 Materials Substrate material selection may be the most important item to consider for dimensional control of high-density flex circuits. (For a more detailed discussion of flexible circuit materials, see Chap. 61.) 63.11.1.1 Polyimide. Kapton HTM and Apical AVTM are standard substrate materials for traditional flexible circuits. Unfortunately, these films have high moisture absorption rates, which create excessive dimensional instability, making them unsuitable for high-density flexible circuits. Several newer polyimide films such as Upilex STM, Kapton ETM, and Apical NPTM and FPTM have been commercialized to satisfy the requirements for increased dimensional stability. A comparison of these base materials is shown in Table 63.8.Although these new polyimide materials have lower moisture absorption and CTEs than traditional polyimide films, their dimensional change is still not zero. In addition, process variations of 0.02 to 0.05 percent are common from batch to batch during film production. Generally, material manufacturers fabricate polyimide film with a wide web. Special conditioning is necessary to ensure uniform properties for the whole web. Unfortunately, there are usually deviations from one side to another. Because the master roll is slit in halves or thirds according to the customer s convenience, each roll has different mechanical properties depending on its location in the master roll. If the master roll is slit into three rolls, a roll in the middle may have more uniform properties than the others. If the master is slit into two rolls, relatively wide edges will be wasted. 63.11.1.2 Copper-Clad Laminate. Most manufacturers of flexible circuits initiate their processes with copper-clad laminates. During production of these laminates, mechanical
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MANUFACTURING OF FLEXIBLE CIRCUITS
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stresses are generated (by heat and pressure) that can become locked into the material. These stresses are partially released during the circuit manufacturer s etching operation and a certain amount of dimensional reversion takes place. In actuality, these stresses (or dimensional stability characteristics) differ in the machine direction (MD) vs. the transverse direction (TD). It is therefore important to review all the mechanical stresses in a laminated material and to consider some form of stress relieving prior to circuit fabrication. IPC-TM-650 recommends a method of testing dimensional stability in a 9 10-in rectangle for both MD and TD. One standard-size test sheet is too small to represent the whole web. Several samplings are required to evaluate the whole web.Yaw direction changes and local changes of dimensions that cannot be measured by the IPC test method are sometimes critical for high-density flexible circuits. Detailed investigation should be conducted. 63.11.1.3 Coverlayer Selection. Coverlayer selection should be considered the second important material factor for controlling dimensional stability. Generally this layer is adhered to the circuitry using heat and pressure, which can induce additional dimensional distortion. Careful consideration should be taken if effective dimensional stability is to be achieved.
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Circuit Design As discussed earlier, most laminate materials undergo mechanical stresses, which generate non-uniform dimensional changes during the etching process. After etching, an etched area with substrate only has different physical properties from a non-etched area with substrate and copper foil. During manufacturing, the laminate materials generate different dimensional changes. A uniform circuit pattern design is necessary to control dimensional stability. A dummy copper pattern in an empty area can help to maintain optimal dimensional stability.
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Critical Processes for Dimension Control The critical manufacturing processes for dimensional control of flexible circuits are shown in Tables 63.12 and 63.13. In general, a high temperature or a wet chemical process can alter the dimensions of flexible circuits. Increased mechanical tensions can also impact dimensional control. The major manufacturing steps for standard double-sided flexible circuits with throughholes are listed here. Each step should be considered carefully to determine its contribution to the overall distortion: 1. NC drilling Although NC drilling does not significantly impact the final dimension, certain scale factors based on assumed movements during the following operations may be factored into the drill pattern. 2. Through-hole plating Because the electric field in a through-hole plating bath is generally non-uniform, the copper thickness becomes uneven. An extremely thickly plated copper conductor will create mechanical stresses. During the etching process these stresses are released, generating some dimensional changes. In an extreme case, a copper-plated sheet cannot lie flat. An appropriate set of work fixtures and plating electrodes should be designed to create a uniform plating thickness. 3. Dry film lamination and pattern printing Due to the relatively low temperature and lack of wet processing (moisture), dry film lamination and pattern printing have little effect on dimensional control. Pattern imaging is, however, an opportunity to apply appropriate dimensional corrections for the following processes. 4. Etching and stripping of resists The most critical wet process in flexible circuit manufacturing is etching. Dimensional changes can occur when the by-products of the etching process are combined with the various materials. It is important to identify how to correct
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