New ‘Dry’ Process Puts Circuitry on Plastics
Close Up: Electronics
Laser ablation and plasma etching team up to apply thicker conductive layers without a ‘wet’ plating process.
The last 30 years have seen numerous technologies emerge for applying electrically conductive paths onto molded plastics. One of these is the Laser Direct Structuring (LDS) technique developed by LPKF Laser & Electronics AG in Germany (U.S. office in Tualatin, Ore.). LDS involves molding a special additive into the plastic, which is activated by a laser beam so that when the plastic is subsequently exposed to an electroless plating bath, the metal adheres only to the activated additive. A number of materials suppliers and compounders have been licensed to provide compounds for LDS.
The limitation is that electroless plating provides conductive paths cost-effectively only up to around 15 microns thick. Thicker circuitry requires an additional galvanic plating step, according to LPKF.
One fast-growing market that is creating demand for thicker conductive paths is LED applications in commercial or automotive lighting and a host of other uses. To meet this demand—and to satisfy sustainability efforts to find alternatives to wet plating chemicals—LPKF has come up with a new approach, called Laser Plasma Coating (LPC). LPKF is developing LPC together with atmospheric plasma equipment supplier Plasma Innovations GmbH of Austria.
PLASMA, LASER, PLASMA
At last fall’s Fakuma 2014 show in Germany, LPC was showcased at the booth of compound supplier PolyOne, Avon Lake, Ohio. The application highlighted there was heat sinks for LED devices. There are several steps in the process:
1. Injection Molding: The body of the heat sink is first injection molded using thermally conductive but electrically insulating compounds such as PolyOne’s Therma-Tech thermal-management materials. These are available with various base resins,
including PBT, nylon 66, and PPS.
2. Plasma Ceramic Coating: Next, the plastic part is plasma coated with an insulating layer of white ceramic powder. The aluminum oxide powder is melted in the plasma stream and deposited on the plastic.
3. Lacquer Coating: A liquid black lacquer coating is then sprayed over the ceramic layer on the part.
4. Laser Patterning: A thin laser beam burns off the lacquer coating where the conductive paths are meant to be, leaving the underlying ceramic layer exposed.
5. Plasma Copper Coating: The final LPC step is to apply the conductive paths. Copper powder is melted in a plasma stream and applied to the part. The copper adheres only to the ceramic exposed by the laser beam, not to the lacquer coating.
6. Assembly: LEDs can now be assembled onto the conductive paths on the plastic heat sink.
Besides the “dry” nature of the process (except for the liquid lacquer coating), LPC can deposit copper thicknesses up to 100 microns. In typical electronics applications, the copper layer is 35 to 70 microns, followed by a thin layer of another metal to protect
against corrosion.
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