You gain several benefits from a tangible
3-D enclosure. First, just as you have
to debug a simulated circuit and its software you have to assess the enclosure to make sure that the internal parts fit properly and with correct clearance and that you can assemble
the whole product without physical interference or poor access to mounting screws and tabs. Second, such an enclosure lets you assess your RFI/EMI situation and see what steps to take to reduce circuit emissions or sensitivity; these steps can include adding filters, gaskets, or shielding; moving components and pc-board traces; or spraying metallic
coatings on the inside of the enclosure. Third, a real enclosure makes it much easier for you to demonstrate to agement and target customers the product, its operation, and even your progress. It even inspires your design team!

The benefits of SFF go beyond having the tangible enclosure. You can also use it to make fittings, connectors, mounting brackets—anything you need to further verify your mechanical design and how it integrates with your circuitry. If you’re thinking about a sliding door to protect your product’s display, you can make one and try it.

Note that you don’t have to go to SFF directly from your solid-modeling design and simulation for the individual part by itself; you can get advanced-system solid-modeling packages that let you “assemble” your product on screen, so you can check for clearances, fit, balance, and the correctness of the sequence of assembly steps.But, like any simulation, the real product in hand often reveals things that you missed, which is especially important
when you consider the cost and leadtime of final production tooling for an enclosure or an internal part.

The tolerance specifications of SFF machines are impressive. Depending on the technology you choose, you can expect dimensional errors from 0.025 to 0.4 mm (1 to 16 mil). In contrast, a typical computer-controlled milling machine in today’s machine shop can produce parts with errors of less than half the 0.025-mm value. But the SFF specifications are often tight enough for a thorough first-pass analysis and may be sufficient for your final product as well.

The sizes of the parts that SFF machines make encompass most solid-part sizes you need in product development. You can make parts as long as about 300 mm (12 in.) on each side, though most parts made today with SFF are about half that size on each side. Completion time for parts ranges from 20 minutes to a few hours, depending on the technology of the SFF machine and the part’s size; the completion time is somewhat independent of the complexity of each layer the SFF builds.

The biggest down side to SFF is probably the cost. Even though prices of the controlling electronics and PCs are dropping, the machines still have a lot of complex, precise, and relatively costly parts. Prices range from $50,000 to 10 times that, depending on the underlying technology the SFF uses, the materials it forms, and other factors. These factors
make it too expensive for all but the largest companies.

However, local service bureaus fill the gap.You send them your file in suitable CAD format, and they deliver your part the next day for $1000 to $2000 for a representative part. This process is still much cheaper and faster than having a machine shop fabricate one part for you. Some of these service bureaus have more than one type of onsite SFF machine, so you are not restricted to one type of SFF technology or capability but can select the one that matches your part and application. Because SFF is a rapidly changing area and because many of the vendors and service bureaus are probably unfamiliar to electronic-design engineers, for additional information you should check the Web site of a publication with a strong mechanical-design focus, such as Design News (www.designnews.com) or
check out Reference 1.

SFF is affecting design and preproduction verification in other ways as well. SFF is surprisingly competitive in cost and completion time with traditional production techniques for runs of 10 to several hundred pieces because it requires no tooling, molds, and related setup. Taking the flexibility of the 3-D software tools even further, some of the application packages can generate a file for your final tooling, taking into account the normal clearances, material shrinkage, minimum radii, and support-spar thickness that the tooling needs to produce the part. For a final step, in some cases you can even use SFF to generate the tooling itself, which then lets you use conventional molding or casting techniques for low- to moderate-volume production runs.