Micromolding for microelectronics and micro-optics

Editor’s note: Micromolding entails special challenges beyond those of molding larger parts, including controlling extremely small shot sizes, parts handling, inspection, and measurement. Many different approaches to overcoming these challenges are evolving. In his Molding 2000 presentation, George Shevchuk, a member of the technical staff in the wireless packaging research department of Bell Labs, Lucent Technologies, asks, "Why should a micromolded part be a separate part?" He and his colleagues have explored the possibilities of incorporating a micro part as a feature of another part, or insert molding it into another component, thereby eliminating many of the special micromolding challenges. Copies of all Molding 2000 presentations are available from Executive Conference Management Inc., Plymouth, MI, phone (734) 420-0507, fax (734) 420-2280.

The constant miniaturization of most electronics systems has definitely influenced the design of many of the plastic components used in these systems. Within the realm of microelectronics and micro-optics, micromolded components are still not very prevalent, but their presence is increasing. Let’s review quickly how micromolded parts differ from what has become conventional molding: 

Two or more of these or other similar characteristics qualify the part as a micromolded part. In terms of size, micro parts are form-ed from about one pellet or less of resin.

Following well-established practices of design for manufacturing and assembly (DFMA), parts should be combined and/ or consolidated whenever possible. However, they do need to exist independently if there is a need for relative motion, if assembly or disassembly is required, or if materials need to be different. These guidelines hold especially true for micromolded parts. 

Thus, given that relative motion is required less frequently in microelectronics and micro-optics than in other types of products, micro parts often become features of other parts. What follows are three examples of micromolded parts that could not be integrated, along with some of their peculiar characteristics. It should be noted that these parts are more commonly found in the realm of optics than in electronics. 

Optical Attenuator
This first example meets all three DFMA conditions that require it to be an independent part. It is a small element of optical material that is placed in the light path between the ends of two fibers, acting as a resistor and attenuating some of the passing light. Designed for use with the newest generation of miniature optical connectors, the attenuator’s active portion must fit within a cylindrical alignment sleeve of 1.25-mm diameter. 

Its thickness is governed by the amount of attenuation desired, with the low end resulting in a thickness of only 50 µm and a tolerance of just a few microns. To keep it properly positioned in the sleeve, the attenuator includes a guide rail attached to its periphery that is positioned outside a split in the sleeve (Figure 1). The weight of the attenuator with the guiderail is approximately 1.6 mg in the thinnest case. 

Due to its size and weight, handling during assembly was difficult. For this purpose an additional block shape was added feeding a secondary gate to the attenuator. This block is used as an assembly and is broken off when the attenuator is in place. It also helps in molding by increasing the shot size with a part weight of 20 mg. 

Optical Detector Interface
The second example resides at the boundary between optics and electronics. It is part of a compact optical transceiver device and provides mounting capability for the photodiode receiver, along with alignment to a mating optical connector. This small device is molded separately to satisfy the "different materials" criterion. 

By insert molding a flex circuit into a precision mounting package, in essence, a 3-D circuit is formed. The detector assembly is shown in Figure 2 with the con-nector sleeve installed. While molding this subassembly is challenging because of the flexible insert, the resulting part is more readily handled than either subcomponent, and the need for more difficult assembly at a later stage is eliminated. 

Precision dimensions and thermal stability of the molded parts are critical in this application. The weight of the molded component itself is approximately 60 mg, using material (PPS) with a specific gravity of 2. 

Optical Plug Backbone
Most optical connectors use a cylindrical ferrule, typically made of ceramic, to provide the alignment and to protect the end of an optical fiber. However, this ferrule (Figure 3) requires additional features in order to be functional. As such, it is generally permanently assembled with a backbone that provides keying, positioning, and spring retention features, and fits the "different materials" criterion. 

In developing a new family of miniature connectors, a ferrule diameter of 1.25 mm was selected, replacing the predominant 2.5-mm size. Using adhesives on an assembly with a plastic backbone—one weighing about 20 mg—would have been very difficult. Therefore, mold tooling was developed to insert mold the ferrule into a backbone. Figure 3 shows a section of the subassembly. 

Challenges included molding a smooth, tapered lead-in into the 125-µm diameter capillary in the ferrule, accommodating ferrule length variations, and handling ferrules made of materials more fragile than ceramic. These challenges were accommodated with a patented tooling design that uses a floating core pin that seats gently against the ferrule and is locked in position using injection pressure during molding. 

While many of the differences between micromolding and parts weighing grams or ounces are becoming evident and are being addressed, there is still much to be learned. Look for developments in these areas in the near future: better, more accurate part- and dimension-measuring techniques; molding techniques that reduce scrap; and improved characterization of material properties of micro parts.