The photoimaging process accounted for about 15% of MID products in 1999. This procedure starts by creating an injection-molded substrate out of plateable resin. This step establishes the part's geometry. Next, a thin layer of electroless copper plating is deposited to promote full electroplating later. Then, a photosensitive polymer resist is applied over the entire part.
Using a trace mask, the resist coating is exposed to UV light to selectively harden the resist in noncircuit areas. The unexposed resist is chemically removed, revealing a circuit pattern with the electroless copper plating. This pattern is electroplated with copper or other metals to achieve the desired circuit performance. In the final step, the hardened resist is stripped and the underlying electroless copper is etched away.
Because the two-shot and photoimaging processes have different strengths and weaknesses, each one is more or less suitable for a given application (Table 2). Designers can determine the process used through their selection of an MID vendor. Some vendors provide access to both processes, while others offer only one.
Some software programs can take simple two-dimensional circuit diagrams and overlay them on a straightforward mechanical package in order to design an MID. As devices get geometrically complicated, however, this approach becomes inadequate. In such cases, MID design engineers must work closely with the customer to define the specifications of the interconnect device. When designing MIDs, engineers need to keep in mind some general guidelines that account for key mechanical and electrical factors.
Mechanical considerations include the injection-molding process. The first-shot material usually has a higher design temperature than the second shot. This is so that it doesn't become damaged when the second shot is injected. If the second shot covers a greater surface area than the first, steel must be used to support the first-shot part as it lies in the mold cavity. Typically, this support is in the form of pins or bosses.
Improving Bonding For a two-shot MID, the bond between the first and second shots is critical. Weak bonding will cause the two shots to separate. Or even worse, it will create a short circuit when the plating solution leaks into cracks. Better bonding is possible through the design of mechanical interlocks on the device so that the second shot encompasses the first shot. In this situation, the second-shot plastic can utilize its shrink to form a tight bond. Another approach to better bonding is through the formation of chemical bonds between the first- and second-shot materials. This solution, though, limits the material choices available.
Material thickness is another mechanical parameter. Design guidelines call for a minimum thickness of 0.080 in. for a two-shot part and 0.040 in. for a photoimaging part. These requirements are strictly enforced to establish complete material flow in the injection-molding process. In addition, to ensure product release from the injection-mold tool, it's important that all of the inside and outside edges and corners have a minimum radius of 0.01 in. (0.25 mm).
The injection-molding process also demands that holes have a 5 to 1 aspect ratio. This means the length of the hole shouldn't exceed five times its diameter. This rule reflects the limitations of the molding tool. As holes get longer, the pins used to make these holes become fragile and hard to maintain. But, this aspect ratio only applies to 0.02- to 0.06-in. diameter holes. Larger holes need larger mold pins, and those pins are more durable. Holes smaller than 0.02 in. require special tooling considerations and are difficult to plate. So, those are discouraged in MID design.
Plating concerns are the focus of electrical considerations for MIDs. Here again, hole design is important. The same 5 to 1 aspect ratio applies to any plated hole because small and long holes present difficulties for the chemical flow of the plating solution. With these types of holes, air and/or chemicals may become trapped during bulk plating. This situation can lead to incomplete plating of the hole. As before, the 0.02- to 0.06-in. diameter range applies because unlike large holes, small holes aren't easy to plate. Another important guideline is to avoid blind holes. To prevent obstructed flow of the plating solution, only specify holes that pass completely through the thickness of the MID.
MID vendors tend to use bulk plating, especially for large volumes. In bulk plating, it's important to not have interlocking features on the interconnect device so entanglement of parts doesn't hinder plating. For example, small pins on one piece may accidentally mate with large holes on another and prevent proper plating of those parts. Also, avoid flat surfaces that can plate together, producing bonded parts. Finally, anything that could easily be broken during bulk plating should be shrouded. For instance, a lone protruding pin can be protected by creating a wall around the pin.
Other than plating, electrical considerations relate mostly to the design of circuit traces. The width of the trace on a two-shot MID and the spacing between traces should be at least 0.02 in. This makes certain that the second-shot plastic flows completely during injection molding. The trace thickness and spacing for a photoimage MID is 0.007 in. Anything thinner than that will inhibit plating of the fine lines and edges necessary to avoid a short circuit.
Traces are commonly called on to handle large currents. To some degree, a trace's current rating can be raised by increasing the thickness of the trace's copper plating. The copper build on the traces can reach a maximum of 0.0020 in. (or 50 µm). Beyond this point, trace current ratings should be increased by raising the traces rather than by increasing the thickness of their plating. Raised traces have three plated sides that combine for greater current-handling capacity (Fig. 3).
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