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SelectConnect™ Process for Metallizing Circuits on Molded Parts and Components


By Richard Macary, vice president and general manager, Arlington Plating Company, Palatine, IL., and Robert Hamilton, Technology Manager, Molded Interconnect Devices, MacDermid, Waterbury, Conn.

3D-MID are designed to maximize space use where components are placed at different heights and in different declinations.

A three-dimensional molded interconnect device (3D-MID) consists of circuit traces set in molded thermoplastics to form electrical pathways and landings for surface mount components.

As opposed to two-dimensional printed circuit boards or a flexible circuit wrap, a 3D-MID can duplicate the surface of a molded component. This allows circuit paths to be created on non-conductors, typically used for the enclosure, chassis, and device structure. The electrical path is patterned in true 3D with x, y, and z-patterns following the contour of the molded component.

SelectConnect LDS. 3D-MIDs are created by first molding a component from thermoplastic resin doped with an organometallic material. Circuit layouts are patterned on the molded component with a process called Laser Direct Structuring (LDS), using a laser beam from the LPKF Microline 3D laser system to create the desired circuit paths. The laser defined pattern is then plated with electroless copper, electroless nickel, and immersion gold to enhance the electrical and assembly characteristics of the 3D-MID.

SelectConnect DS: 3D-MIDs can also be formed with “double-shot” molding, a process where the first shot is a non-plateable material (such as polycarbonate) and the second shot is a plateable grade (such as PC/ABS doped with palladium), which forms the base for the circuit path. During a chromic acid etching process, the surface of the platable second shot material is activated, then metallized with electroless copper, electroless nickel, and immersion gold due to the catalytic action of the added palladium.

SelectConnect Technologies,* a division of Arlington Plating Company, has acquired a Microline 3D laser system from LPKF and has installed electroless plating chemistry from MacDermid that allows us to provide these services to the North American market. Arlington Plating Company, located in Palatine, Ill., has been in the metal finishing business for more than 60 years. Currently, SelectConnect™ technology is producing more than 50,000 3D-MIDs per week, with additional capacity to produce 10 times that amount.

LPKF Laser & Electronics AG develops systems and process solutions for demanding tasks in printed circuit board technology and microelectronics. LPKF, based in Hannover, Germany, owns the patents for the use of a laser to make patterns on molded components. LPKF has currently sold more than 75 laser systems for this application, the majority being used for manufacturing cell phone antennas.

MacDermid has been developing and marketing plating chemicals worldwide for 80-plus years and holds a leadership position in developing chemical processes for producing 3-Dimensional Molded Interconnects. They supply electroless copper, electroless nickel, and immersion gold for the process of manufacturing 3D-MIDs.

The LDS system used for manufacturing 3D-MIDs is well established in Europe on a range of applications and is also widely being used in the Far East (China, Taiwan, Korea), predominately to support the world market for cell phone antennas. While the technology is more than 10 years old, application development and technology utilization in North America has lagged behind Europe and Asia due to the lack of installed laser systems and technical capability.

Plastics & Molding. A key benefit of the LDS process is the design freedom from utilizing injection molded thermoplastics as the base of the device. With a wide variety of thermoplastic material types available, the designer has many choices to optimize part performance. Typical thermoplastic materials include:

LCP (Liquid Crystal Polymer), based on Vectra® (Ticona GmbH)
  • high heat resistance and excellent flow for thin wall molding
  • excellent dimensional stability under thermal stress
PA6/6T (semi-aromatic polyamide), based on Ultramid® (BASF AG)
  • very high thermal shape stability, suitable for reflow soldering (also with lead-free solder)
  • very good mechanical properties
Thermoplastic polyester (PBT, PET and blends), based on Pocan® (LANXESS)
  • very good mechanical and electrical properties
  • very high thermal shape stability with addition of PET
Crosslinked PBT (polybutylenterephathalate), based on Vestodur® (Evonik)
 
  • migration-resistant fire protection equipment (VO–0.4 mm after UL94)
  • irradiation crosslinkable for high temperature resistance (all soldering processes)
PC/ABS (polycarbonate/acrylnitrile/butadiene/styrene)
  • very good processability
  • very good mechanical properties

There are certain design, tooling, and processing considerations for optimizing the creation of the circuit path on the thermoplastic part during the LDS process. There are several factors to consider; however, creating a smooth surface in the area to be plated is one of the most important.

The part should be designed to avoid sharp edged transitions in the area to be metalized. (Note that this is already a good rule of thermoplastic part design to reduce stress concentrations.) Minimum line width and spacing of the conductor paths should be observed based on the capabilities of the plastic material, the laser and the plating process. Also, conductor lines should be a certain distance away from perpendicular walls so they are not directly against them. Furthermore, if through-holes are being designed, proper angles must be specified to accommodate the laser.

Regarding tooling design, ejector pins should be located away from circuit path areas. Gates and runners should be designed to facilitate gentle, uniform filling of the cavity. Abrupt changes in flow direction (direct splashing) onto a plating surface should be avoided. Also, gate locations should be optimized to avoid weld lines in the areas to be plated. Note that a moldflow analysis can be helpful when designing the part.

In order to achieve a smooth surface, injection molding parameters need to be optimized to minimize surface defects. Since many of the material grades contain fillers, such as glass or mineral, or both, surface tooling temperatures need to be properly controlled to create a resin rich surface. Thus, a proper heating/cooling system needs to be incorporated in order to control the tooling surface temperature during the injection molding process.

Laser Patterning. Circuit traces are drawn on the component design using the CAD system used to design the part and translated from the CAD .STEP file of the component to the Microline 3D Laser System where the trace is rapidly written onto the part.

Metallization. Electroless plating is the method for forming electromechanical traces on the Laser Defined Structure or the double-shot-molded component. The sequence includes electroless copper plating (100 to 600 micro inches), electroless nickel plating (50 to 100 micro inches), and immersion gold plating (3 to 8 micro inches).

The first step in the process is critical: Depositing electroless copper on the laser-defined pattern or the catalyzed second shot material. In either process, deposition of the electroless copper in the desired areas is dependent on a catalytic contrast between areas to be plated and the rest of the part.

In the case of double shot molding, the contrast is enhanced by adding palladium to the second shot material creating an electronegative potential that initiates deposition. The result of laser ablation on the organometallic doped thermoplastic is a textured surface with metal particles, which also produces an electronegative potential initiating copper deposition.

There are several other factors to be concerned with plating electroless copper, electroless nickel, and immersion gold onto 3D-MIDs. Electroless copper chemistry requires constant analytical control for the individual components, including sodium hydroxide, formaldehyde (consumed by deposition and the Cannizzaro side-reaction), chelator, and copper along with several reaction stabilizers. An automatic chemical controller is utilized to keep the chemistry within tight parameters, assuring consistent and reliable plating performance. (This is supplemented with lab analysis.) The copper bath components are formulated to maintain proper stabilizer levels when replenished in this manner. This is particularly important in the copper bath, as it must be properly balanced so it will completely cover the catalyzed area while not plating “extraneous” copper on the non-catalytic background.

Most MIDs are then plated with electroless nickel, providing a more robust surface against oxidation. To accomplish this, the copper must be activated in some way, as hypophosphite reduced electroless nickel plating solutions are not catalytic on copper surfaces.

One common approach is to use an acidic palladium solution, which will put down a very thin immersion-displacement layer of metallic palladium on the copper. Ionic palladium is also catalytic, however. Therefore, care must be taken with the rinsing and a required acid dip afterwards to ensure that the nickel does not plate on the background. Another approach is to use an initial strike of boron reduced electroless nickel, because the higher reduction potential of these baths makes them catalytic on copper. Once a little nickel is on the copper, the subsequent nickel build bath will plate over it. This approach reduces the risk of extraneous background plating.

Once the electroless nickel (which may be of any available phosphorus content, depending on customer requirements) is built to sufficient thickness, the parts may be rinsed and dried. Alternatively, a topcoat of immersion gold may be applied. Other finishes are available, but rarely used for these applications.

3D-MIDs offer a combination of providing electrical and mechanical functionality. Electronic circuit traces are formed on the molding, which can also be used as a housing, a chassis for additional electromechanical functions, and the frame for mounting chips, resistors, and electronic connections. Most importantly, utilization of a 3D-MID in an existing design can lead to miniaturization, component reduction, a more environmentally friendly product design, and a lower cost.

MID applications are the focus of the automotive and medical device industry. In such products, circuits are directly applied onto the surface of injection molded plastic parts or are integrated into parts of these products. In a 3D-MID, surface mount device components are normally on multiple levels.

The intent of these MIDs is to gain design freedom, rationalize the production process and increase environment protection by reduction of the material mix. 3D-MID are designed to maximize space use where components are placed at different heights and in different declinations. This enables the increase in functional density of products.

Applications for 3D-MIDs are increasing due to a combination of wider availability of manufacturing sites and the need to miniaturize, eliminate waste, and reduce costs.
Existing Applications for 3D-MIDs include:

1. IV regulators (medical)
2. Barometric pressure sensors (industrial)
3. Laptop antennas (consumer)
4. Cell-phone antennas
(consumer)
5. Integrated connectors
(industrial)
6. Automated pipets (industrial)
7. Glucose meter (medical)
8. Steering wheel hub (automotive)
9. Forward control switch (motorcycle)
10. Security shields (consumer)
11. Dental tools (medical)
12. Radar filter (military)
13. Motion sensor (industrial)
14. Lead frames (electronics)
15. Brake sensors (automotive)
16. LED lights (motorcycle)
17. Positioning sensor (automotive)
18. Hearing aids (medical)
19. Pincer (medical)
20. Temperature diagnostic pen (medical)
21. Rinsing unit (dental)
22. RFID antennas (industrial)
23. Insulin chassis (medical)

REFERENCES
  1. Radeck, A, Lanxess Deutschland GmbH, Germany, “Thermoplastic Polyesters for Laser Direct Structuring” 
  2. Heininger, N., LPKF Laser & Electronics AG, “3-Dimensional Circuit Design Rules”
  3. Grande, J. Plastics Technology, “MIDs Make a Comeback”

 

ABOUT THE AUTHORS

Richard L. Macary, is the General Manager for SelectConnect Technologies, a division of Arlington Plating Company. He attained a Bachelor of Science from Charter Oak College and an MBA from Keller Graduate School. Macary holds two patents for applications of chrome plating called Chrome Accent and plating on Magnesium. Prior to SelectConnect Technologies, Macary worked for Enthone and Atotech.

Robert Hamilton, Technology Manager Molded Interconnect Devices at MacDermid, studied Chemical Engineering at Yale. He started in the printed circuit business in 1978 and has been in PCB's, plating, and surface finishing ever since. A 22-year veteran of MacDermid, Hamilton has been working on MIDs for seven years.

*For more information, contact SelectConnect online or via phone: (847) 359-1490. For more on this article, please see the March edition of Metal Finishing.

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