New circuit board technologies meet the challenge of digital power

& -July 15, 2013

MiB - Opportunity for Innovative Design
“Metal in the Board” or “MiB” includes a number of approaches where MiB building blocks are combined to provide effective high-current solutions. The "DWPCB" (discrete wire pcb) type is one of the most versatile.

One of the commercially available versions of DWPCB is "HSMTec," developed by the Austrian pcb manufacturer Häusermann GmbH. HSMTec uses 0.5mm diameter copper wire and rectangular sectioned 0.5mm thick copper strips ("profiles") to provide discrete low resistance electrical and thermal pathways in the board as shown in Fig. 2.

Figure 2: "Discrete Wire" process - bonding high current elements to inner layer.

There are a number of advantages to this solution compared to conventional thick copper or metal core boards:

  • Conventional pcb processes ensure consistently high reliability
  • Enhanced thermal and current pathways only where needed
  • Cost of MiB limited to those nets needing MiB
  • Wiring densities up to and including HDI enable logic and power integration
  • FR-4 materials reduce CTE mismatch common to aluminum based substrates
  • Board may be folded during assembly, providing photometric solutions for LED luminaires and eliminating daughterboards/connectors

The profiles and wires which make up the MiB components in a DWPCB board are bonded to tracks etched on innerlayer cores, basically forming a sandwich consisting of the etched track and the bonded elements. This patented process ensures a consistent bond line between the track and the wire/profile which is essential for uniform heat spreading and consistent conductor cross-sectional area. It also simplifies the layout task and/or conversion from conventional designs, as placement of the high current MiB components: the wires and profiles will be done on what are essentially enlarged tracks on an inner or outer layer.

This arrangement provides a great deal of flexibility in stackup configuration. Profiles/wires are bonded to a routing track, and thermal dissipation planes can be located on the same layer or on a facing layer co-axial with the MiB track. Facing layer thermal planes have been seen to improve thermal performance as shown in Fig. 3.

Figure 3: Effect on heat spreading of thermal dissipation plane.

Design guidelines include ampacity tables based on actual thermographic observations of different layup configurations. As shown in Fig. 4, the DWPCB type is suited for "medium" power applications with nominal currents up to about 140A (40°C temp rise).

Figure 4: Temperature increase for various profile cross-sections and current in Amperes.

In combination with thermal vias or inlays this value can run to over 300A depending on duty cycle. Other MiB types covered in BPA's report are designed for high current applications (250 - 1000A) typical of Hybrid/Electric Vehicle and high power rectification.

The medium current capability, heat-spreading characteristics, and design versatility of the DWPCB type make it a cost-effective alternative to logic boards, busbars, and cable in an expanding range of applications.

Design Example: Electromobility Powertrain
The Lithium-ion battery pack shown in Fig. 5 provides a mean value of 100A with peaks to 300A for a light electric vehicle.

Figure 5: Li-Ion battery pack/motor controller package for light electric vehicle.

Both weight and size are critical in this application as any increase in mass directly affects vehicle range. To save both, a motor controller consisting of control logic and eight DirectFET power devices is mounted right on the front of the battery pack. A DWPCB solution proved ideal for integrating the control logic with the power section, reducing two pcbs to one and eliminating the associated connectors and cabling.

The design challenge involved routing 100/300A through an FR-4 circuit board to the DirectFETs and out to the motor bus. Häusermann's versatile DWPCB technology enabled several different options to be considered, ranging from single-layer power distribution to a multi-level design with integrated thermal vias and dissipation planes.

The most cost-effective design is shown in Fig. 6.

Figure 6: MiB layout for DirectFET package using HSMTec.

In this layout, the profiles have been bonded to an inner core consisting of High Tg FR-4 clad with 2 oz (70µ) copper. Since the profiles are all on the same reference plane, setup and run time for the bonding equipment is optimized and the subsequent mass-lamination process has a less complex embedding task compared to a design where the profiles are on several different layers.

The embedded profiles also free up real estate on the surface of the board: the area needed for the high-current tracks is confined to the via arrays used to access the embedded profiles while the wide tracking necessary to support the profiles is confined to the internal layer.

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