Flexible circuits Manufacturing

The majority of flexible circuit manufacturers start the process with copper-clad materials. The properties of these materials depend on the capabilities of laminate manufacturers, even though the same base films and copper foils are used. To choose the right materials, manufacturers must carefully consider the basic properties of each laminate material. 

Etching and stripping of resists The most critical wet process in flexible circuit manufacturing is etching. Dimensional changes can occm when the by-products of the etching process are combined with the various materials. It is important to identify how to correct... 

FIGURE 66.5 Comparison of opening sizes of flying-lead flexible circuit manufacturing... 

The plastic film with interconnection layers, denoted (2) in Fig. 16.1, can be made by a process similar to that used to manufacture flexible circuit boards. First, plastic films coated with copper foil are processed by a numerically controlled (NC) drilling machine to make via holes. Plating is then used to make interconnections between top and bottom sides though via holes. Finally, the copper layers are patterned by conventional photolithography and etching. Gold plating is occasionally employed to improve electronic interconnections. 

This chapter presents an overview of performance plastic polymers in commercial planar and 3-dimensional circuit board products, and describes in detail one approach (two-shot molding) developed as an integrated 3-D circuit manufacturing technology. The distinctions between conventional planar (2-dimensional) circuitry, based on thermoset laminates and "subtractive etching processes, and the enhanced design flexibility afforded by expanded interconnection capacity in three axes are discussed. Specific examples of 3-dimensional interconnect protoypes and products are described and pictured. 

Furthermore, the overall properties of the base materials for circuit boards are determined essentially by the joint between the copper foil and the laminate. This joint can be realized by both an added adhesive and by the laminating resin itself, which additionally, acts as adhesive. An additional adhesive is needed in the manufacture of flexible circuit boards (e.g., polyimide/copper) or of paper-based rigid circuit boards. During this process the adhesive is deposited on the bottom side of the copper foil after... 

Glass-reinforced PBT has become popular for electromechanical parts and for minifans in computers and connectors, whereas flexible circuits are more often manufactured using polyesters and polyimides for the substrates, with the latter predominating in the USA. 

PEI resins have been extruded into film and sheet. Since they are thermoplastic, both sheet and film can be formed and drawn in secondary operations. PEI film can be used in many electrical applications, such as the manufacture of capacitors and flexible circuits. These films can also be laminated or coextruded to make various multilayer structures. PEI films are often laminated with metals, such as copper, which are then made into electric circuits. 

Roll forming is a process used to manufacture flexible circuits in very large quantities. This is the lowest-cost method for the manufacture of flexible circuits. However, it involves substantial tooling, so it is apphcable only for very high volume products. Examples of PCBs manufactured using this method are printer head connections, disk drive head connections, and the circuits used in cameras and camcorders. The PCBs can be single- or double-sided. 

Tape-automated bonding (TAB) has not been considered as a type of flexible circuit because of the slightly different manufacturing processes. However, the basic construction and materials of the final products are the same therefore, TAB is categorized as a type of flexible circuit in this book (see Fig. 61.2). 

Since 1990, many new materials for flexible circuits have been developed in conjunction with design and manufacturing processes to satisfy new requirements in HDI applications. These include ... 

HDI applications require finer traces and microvia holes in severe manufacturing and application conditions. For manufacturing convenience, flexible circnit prodncers prefer thinner conductors and substrates. However, physical performances of thinner materials may not be optimal. Thinner materials impact both the performance and manufacturing yield of the final flexible circuit. 

These hquid polyimide resins could be the major dielectric materials of special high-density flexible circuits that demand extremely high density, down to 5 micron pitches with 10 micron via holes. The cost of these materials is higher than that of polyimide film. However, they have broader capability for meeting nonstandard requirements such as ultrathin substrates with microvia holes. The properties are very dependent on the manufacturer. 

Historically, copper-clad laminates with acrylic or epoxy adhesives have been the major materials for flexible circuits. Each manufacturer has developed a special resin grade or special additives to ensure reliable flexibility and bond strength. Other adhesive materials such as phenol resin or silicon resin have been developed however, they have not become standard adhesive materials in flexible circuits. 

The same process is repeated for double-sided copper-clad laminates. (Some laminate manufacturers, however, have developed simplified manufacturing processes that can make the double-sided laminate in one single process to reduce manufacturing cost.) A well-conditioned aging process is important to achieving reliable bond strength and flexible characteristics of the laminates as the raw material of flexible circuits. 

Each has different manufacturing processes and advantages as the major materials of HDI flexible circuits. Table 61.13 shows basic performances of typical adhesiveless laminate materials. 

Currently, there is no perfect solution that satisfies all requirements including cost. Suitable adhesiveless laminate materials should be chosen according to the specifications of the final flexible circuits and the convenience of manufacturing processes. 

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. 

Managing the conditioning of the process is not difficult. Acryhc or epoxy/dry-film types are good for smaU-volume production because of the flexible manufacturing process. They may need some more improvements in electrical performance and chemical resistance if they are to be used in the general application of high-density flexible circuits. The present version... 

Process elements particular to flexible circuits are required to achieve a high manufacturing yield. Also, an appropriate total process design is required to achieve high productivity with high yield. 

A roll-to-roll (RTR) manufacturing system is a high-productivity solution for volume production of flexible circuits. However, the RTR lines are not flexible for a non-standard construction, and they are available for only the early steps of the long manufacturing processes. Also, RTR processes have many limitations to apply. 

High-density flexible circuits, which have been developed since the mid-1990s, have special constructions and therefore require supplemental processes. In addition, high-grade manufacturing technologies are required to generate finer circuits. Sometimes, they are called HDI... 

TABLE 63.2 New Manufacturing Technologies for High-Density Flexible Circuits ... 

Furthermore, ultra high-density flexible circuits have been considered recently as the necessary substrate materials for the next generation electronics packaging. Their circuit densities will be under 20 micron pitches with 20 micron diameter via holes. They need completely new manufacturing technologies, such as an additive process instead of traditional subtractive process. Ultra high-density flexible circuits will be considered separately. 

FIGURE 63.1 Standard manufacturing flow for double-sided flexible circuits. 

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