Design News for Mechanical and Design Engineers

Drawing Circuits: Laser direct structuring, a new way of creating molded interconnect devices, uses laser energy to activate those portions of injection-molded carrier that correspond to the deposited circuitry.

Molded interconnect devices (MIDs), or circuits applied to a 3D plastic carrier, might seem like a good idea. By eliminating printed circuit boards and related components, a MID can shave space, cost, and weight from antennas, sensors, connectors, and other components that have both electronic and mechanical requirements. But molding these devices in the past required a complex manufacturing process that scared off some potential users. LPKF Laser & Electronics AG (www.lpkf.de) recently came out with a new technology that drastically simplifies MID production.

The company's Laser Direct Structuring (LDS) method can produce a MID in just three steps. The carrier is first molded from one of several new laser-sensitive plastics. A proprietary laser system working in the infrared spectrum then traces the circuit patterns onto the carrier. Finally, the part goes through an electroless plating process that deposits the circuit material onto those areas treated by the laser. "Our technology makes it much easier for the end user," says Nils Heininger, an engineer who manages LPKF's MID products.

He has a point, given that many current MIDs require a two-shot molding process. The first shot forms the bulk of the carrier in a conventional plastic while the second shot defines the circuit patterns using a plastic modified to adhere to plating. By doing away with that second shot, LDS promises an edge when it comes to tooling and molding costs. And Heininger adds that the new method also accommodates design changes easily. "You can change the circuit by changing the laser program rather than changing the tool," Heininger notes.

Another benefit has to be with the fineness of the circuitry. Heininger reports that laser can produce traces as small as 100 microns with a 150-micron gap versus 200 microns with a 200-micron gap in typical two-shot MIDs.

The trick to doing away with the second shot is the use of materials developed especially for the LDS process. Regardless of the base resin, these materials contain an organic metal complex rich in copper. When with the laser energy directed at the carrier, this complex breaks copper atoms off from the organics. These atoms later serve as the nuclei for the copper coating, Heininger explains. The laser also slightly ablates and microscopically roughens the areas it touches, leaving numerous anchor points for the plated circuitry.

And the fact that the laser handles the surface preparation gives it yet another edge over two-shot MIDs. Usually these require an additional etching step to prepare the plastic for plating. "With LDS, the laser does the etching," says Heininger.

All the suppliers report that the mechanical and dielectric properties of the materials are left essentially unchanged by the addition of the copper complex. But pay attention to other materials engineering fundamentals. "Seventy percent of MIDs go through solder reflow," Heininger says. So heat distortion temperature and CTE matter. So do flow properties since the MID geometries can be complex.

And for all its potential benefits, don't think that the new LDS process will take over all MID production. Heininger says that the two-shot molding can do a better job at the most complex MID and circuit geometries. For example, the laser can't reach all areas on complex parts or the backside of a double-side part without extra handling steps. And at very high production volumes—those exceeding 500,000 parts per year—the cost of extra handling can offset the cost of the two-shot tool. "There is still a place for the two-shot process," says Heininger.