Coverlay Processes

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

Unfortimately, it requires very complicated, labor-intensive processes, and it is difficult to introduce automatic manufactming systems. Another big issue is that the standard film coverlay process cannot make small openings with high dimensional accuracy because of the manud handling of thin film materials. 

The coverlay process could be the typical example. RTR lamination of film coverlay is almost impossible.The screen-printing for RTR processing has developed for relatively rough circuit patterns. The photo-imageable coverlay system could be the right solution for the RTR process of the coverlay of the flexible circuits. However, there have been several limitations to apply. 

Coverlay processes There are several coverlay processes that have been developed with varying process conditions for different concept materials. Film coverlay undergoes a manufacturing process with a temperature greater than 160°C and a pressure over 20 kg/cm. ... 

Lamination pressing for multilayers Like the coverlay process, lamination pressing involves high temperature and pressure. Due to the number of layers, however, dimensional changes are more complicated because each level needs to be considered. 

PI resins find use as coverlays. A coverlay is generally used to protect printed circuits during subsequent processing, primarily solder operations, or from environmental effects during use. The Pis can be used in combination with epoxies. In addition, additives, such as phosphorous-based flame retardants or adhesion promoters, may be used in the formulations. 

Flexible circuits. Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which is adhesively bonded a thin copper foil. The copper is then photoetched to form a circuit pattern using normal photolithography processes. A plastic film (coverlay) is then adhesive bonded to the etched... 

Usually the same films are chosen for the coverlay films as are used for the substrate layers. The films are coated with semicured acrylic or epoxy resin, which is used as an adhesive layer. The surfaces of the adhesive layers are covered with release films when the materials are delivered to circuit manufacturers. Semicured adhesive resins are unstable at room temperature, so film coverlay must be kept in refrigerators. The adhesive resins of the coverlay usually have only weeks of pot life even when they are kept in low-temperature environments, and exact conditioning is required for processing. 

The process of manufacturing with film coverlay is very complicated, which makes process automation difficult and increases the cost. The manual registration process and the use of unstable film material are major issues when considering film coverlay for HDI flexible circuits due to small hole capability and lower dimensional accuracy. Basic properties of the typical materials are shown in Table 61.14. 

The basic idea of the photoimageable coverlay is to satisfy both of fine resolutions of the opening and physical flexibility by one single material. Also, its simple manufacturing process should reduce costs. 

Most of the major materials for flexible circuits, such as copper laminates and coverlay films, are supplied in rolled form. The first process of manufacturing is to sUt and cut the rolled materials into workable sizes. It is possible to cut with a manual sharing tool however, an automatic machine is recommended to get an exact size without handling damage. 

The coverlay construction of flexible circuits demands another special process. It is very different from the solder mask used for rigid circuit boards. Furthermore, recent high-density flexible circuits need new coverlay technologies. Table 63.7 summarizes the comparison of traditional coverlay technologies and new technologies. Traditional film coverlay has an excellent balance of physical properties and can provide high reliability, especially for long dynamic flexible endurance. 

A typical coverlay mannfactnring process is shown in Fig. 63.26. Generally the same film materials are chosen for the base film and the coverlay film. A B-stage adhesive such as epoxy... 

It takes more than one and a half hours for the whole heat press cycle to complete the chemical curing reaction of the adhesive resins, therefore the batch size and the lamination equipment have been becoming larger to make the productivity higher. But it makes the process flexibility lower. A quick press system has been developed to have a better flexibiUty. One set of coverlay films and an etched flexible circuit panel is hold in a small vacuum press for less than two minutes as the preliminary lamination. After that, several numbers of panels are baked in a thermal oven to complete the chemical curing of the adhesive resin. This process needs a special epoxy resin for the adhesives, but it provides a high flexibility for the small volume and prototype productions. 

Basically, the same process and the same screen printer are available for applying liquid coverlay materiais as a solder mask on rigid circuit boards. The solder mask materials have to have appropriate flexibility to avoid the cracks during the bending.The only difference in the processes is the... 

FIGURE 63.27 Screen print process of liquid ink coverlay. 

There are several choices for appUcation processes of liquid ink photoimageable coverlay. 

FIGURE 63.28 An example of a manufacturing process for photoimageable coverlay (a) A schematic flowchart of a generic process using either dry film or liquid resist, (b)... 

FIGURE 63.30 Flowchart of process for laser drilling of film coverlay. 

An adhesive film is laminated on a rigid board with a release sheet first. The boards are then routed by punching or by NC router. It is a very simple process if the adhesive material is pressure-sensitive adhesives (PSAs). Each piece of stiffener is placed on the flex circuits with appropriate pressure, mostly by hand.The process is more compUcated when a thermo-set type adhesive material is required. A temperature higher than 160°C with a pressure higher than 20 kg/cm is required for more than 30 min. A similar heat press used for multilayer circuit boards or film coverlay is necessary. Dummy boards should be prepared to make the pressure uniform. 

Materials Design Manufacturing process Base film, copper, coverlay, bonding material Layer constructions, circuit density, drcuit balance Wet processes, heat processes, mechanically stressed processes... 

The conductive patterns are generated by the traditional etching process withont through holes. All traces are covered with plain coverlay films, which have no openings. The multiple covered flexible circuits are bonded with bonding sheets, which are aheady routed to make openings for nonbonding flexible areas. 

The rest of the processes are very similar to those used for multilayer rigid boards. Outer layer etching, coverlay (solder mask), and surface treatment are conducted in the same way. 

Figure 66.1 indicates two basic structures of dual-access conductors, in both the single sided and double-sided types of flexible circuits. Figure 66.2 shows actual examples of the fine flying leads built on the single side circuits and double side circuits. Basically, a coverlayer is applied to the side of the conductive layer opposite the substrate.The coverlayer can be processed easily to create the top-side exposure. The flying lead is created when the base substrate is also removed in the same area. 

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