Hybrid and Advanced Applications

Molded Interconnect Devices. Following advances in control and capability of imaging and plating technologies, and commercialization of high-performance injection molded plastics, a natural development was the molded circuit board, or molded interconnect device (MID). Adhesion promotion principles developed for the decorative POP industry have been applied to devising adhesion promotion processes for a variety of engineering thermoplastics (Ref 39), permitting deposition of adherent electroless copper deposits that could be built up in thickness and imaged similarly to conventional PWBs.

In certain cases, conventional plastics such as ABS have been used as MID substrates. However, in order to survive board assembly temperatures, more advanced plastics are often required. These include polyetherimide, polyethersulfone, polyarylsulfone, liquid crystal polymers, and so on. Molding requirements are even more critical than for decorative POP, due to the need for higher levels of adhesion (typically > 1.1 kN/m, or 6 lb/in., peel strength, versus ~0.7 kN/m, or 4 lb/in., for decorative POP) and due to the stresses induced by plateup, imaging, and assembly processes. Certain of the plastics mentioned have proven more amenable than others to development of suitable adhesion promotion processes. Polyetherimide, in particular, has proven to be compatible with surface chemistry-altering pretreatments that produce very strongly adherent electroless copper (Ref 40), while not roughening the substrate surface excessively. The latter characteristic is particularly important when fine-line imaging processes are to be employed later in the process.

Depending on the MID design and process chosen, plateup of the initial electroless copper strike plate may employ either electrolytic copper or full-build additive copper (20 to 35 pmm). Imaging of three-dimensional substrates has necessitated development of inventive materials and processes (Ref 41). Electrodeposited photoresists, as well as novel methods of exposure and related equipment, have proven key in this effort.

At this time, the MID market has proven to be feasible and has achieved commercial success in several dedicated facilities. Many ingenious and cost-saving devices have been designed and are currently in production (Ref 39, 42). Figure 2 demonstrates a number of commercially produced MIDs. However, this market has not achieved the level of acceptance predicted in the late 1980s, due to the need for high volumes of a given design to amortize mold costs, the failure of materials costs to come down to required levels, and the tendency of electronic designers to employ more familiar methods, such as conventional PWBs, whenever possible.

Fig. 2 Examples of commercially produced molded interconnect devices. Courtesy of Shipley Co.

Composite Connectors. The composite connector application is a hybrid of two-sided EMI shielding with molded interconnects. These parts are currently made of aluminum, and for all the usual reasons, considerable interest lies in replacing the aluminum with a lighter material such as plastic. The physical requirements of the connector are such that only advanced engineering plastics, such as those used in the MID market, are suitable. The parts must be plated, not to form circuitry but to provide EMI shielding. Fabrication of these devices employs the same plastics and pretreatments for electroless plating that are employed for molded interconnects. Due to the durability, lubricity, and hardness requirements of the finished parts, the electroless nickel overcoat is built up to a rather higher thickness (~5 pm) than for EMI shielding of electronic cabinetry. Examples of some plastic composite connectors, processed through electroless copper and nickel, are shown in Fig. 3.

Fig. 3 Examples of plastic composite connectors plated with electroless copper and nickel. Courtesy of Shipley Co.

Multichip Modules. Progress in semiconductor technology continues to place increasing demands on interconnection and assembly technology. In the 1980s, the response to this demand on the PWB side was increasing numbers of layers and finer circuitry. However, this trend cannot be sustained at the needed rate (Ref 43), leading to the requirement for an intermediate level of interconnection onto which bare chips may be mounted (Ref 44). These devices, known as multichip modules (MCMs), may be fabricated using several approaches (Ref 45, 46, 47). However, a common feature is the use of full-build electroless copper (Ref 48) to build up the conductive traces.

An example of a fabrication process for an MCM is given in Fig. 4. The electroless copper subprocess generally follows along the lines of the PTH and POP processes (discussed in more detail in the section "Processes" in this article). Adhesion promotion for the metallization layer to the unique dielectric materials employed in MCMs can be a challenge in itself. Conventional "swell and etch" approaches are normally used; other approaches are also in development.

Fig. 4 A process flow chart for deposition of a multichip module on a conventional printed wiring board. Courtesy of Shipley Co.

Silicon Devices. Recently some interest has arisen in employing electroless copper for integrated circuit manufacture (Ref 49, 50, 51). It is felt that aluminum, which is used in providing the conductive path on chips, may not be sufficiently conductive at the very high resolutions required in future devices. Very thin films of electroless copper (~0.1 to 0.2 pm), deposited additively in channels between a photodefined temporary film, have been used.

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