The Benefits of Copper Clip Over Wire Bond Packaging

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Demand for high performance and functionality from ever-smaller power electronics form factors is presenting significant challenges for engineers.

Power electronics have high voltages applied to their pins, plus a large amount of current flowing in and out, all of which may be limited by the electrical resistance and thermal impedance of the package. A traditional wire-bonded approach to make electrical connections from the device or integrated circuit (IC) to the outside circuit or circuit board is widely used, but requires a high number of source wires - typically made of aluminum, copper or gold - to reduce conduction losses or increase power density due to drain-source on resistance, or RDS(on). Wires and other parts of the package that carry current can contribute up to 90% of device’s electrical resistance.

Copper Clip Comes of Age

Figure 1: Copper clip offers industry-leading performance in an innovative package. Source: NexperiaTo meet demand for improved product performance, copper clip technology is fast becoming the preferred interconnect. In clip-bonding technology a solid copper bridge attached to the die surface replaces multiple bonded wires. Copper clip offers several benefits, the most important of which is that it significantly reduces the overall package resistance of the product when compared to copper wire.

There are other performance factors driving copper clip adoption, including:

• RDS(on) losses will result in temperature rises in the device and elsewhere and MOSFETs may be damaged or destroyed by excessive temperature. Copper clip contact areas are much larger than with wire bonding. This increase in area results in better thermal performance by providing efficient heat dissipation from die top to the lead frame. It helps to reduce the maximum junction temperature during operation and extend the device’s operation life and reliability. Copper clips also help conduct generated heat to the package exterior and provides the option of two-sided cooling.
• When a copper-clip package is combined with a heat sink, more heat can be transferred to the air through natural convection or forced air cooling. Depending on the size of the heat sink used on the top-side of the package and any air flow that may be present, the thermal resistance can be further improved as it allows the package to run cooler or draw more power while maintaining a stable junction temperature.
• Copper clip bonds also do not introduce the long conduction path of wire bonding; the leads themselves have an impact. These wires are very long and thin and have parasitic inductance, especially at high frequency that can affect both speed of operation and efficiency.
• Mechanical bonds introduce a new failure mechanism and reduced product reliability.
• Given that today’s high performance DC/DC converters switch as high as 400- 600 kHz, package parasitics become an increasing concern. One of the advantages for copper clip bonding is the possibility of stacking the control MOSFET’s source to the synchronous MOSFET’s drain internally via copper clips. The stacked configuration virtually eliminates the parasitic inductance and resistance between high- and low-side MOSFETS. In general, reducing or even eliminating the converter’s internal parasitic allows the system to switch faster and work at higher frequencies because of the reduced switching losses.
• Another concern about wire bonding: having to increase the number of source wires reduces manufacturing productivity and increases material cost (particularly with gold wire).

New Solutions

Unlike competing Power-SO8 package types that are constructed using wire bonding, Nexperia’s LFPAK56 uses a copper clip that is soldered in a single operation to the gate and source. This reduces spreading resistance and gives LFPAK56 superior electrical and thermal characteristics, as well as increased reliability.

The LFPAK56’s copper source clip design overcomes the limitations of SO8 and Power-SO8 packages and has been fully qualified to the stringent AEC Q101 standard for discrete devices, thereby demonstrating its superior ruggedness and reliability in the toughest conditions.

Different rates of expansion and contraction of the PCB and the MOSFET package can cause cracking of the MOSFET molding, as well as solder joint failures causing degradation and early failure of the MOSFET. LFPAK’s construction allows the gate and source pins to flex and safely absorb the mechanical stresses that occur when a device is rapidly heated and cooled.

The exposed leads of LFPAK are available for detailed solder joint inspection and assessment and, if necessary, it is possible to rework an LFPAK device using simple, low-cost tools.Figure 2. An exploded view of an LFPAK56 package. Source: Nexperia

Additionally, due to its exposed gull-wing leads, LFPAK 56 is the only Power-S08 type package that can be wave soldered.

For situations where space is at a premium a new range of 80 V dual Power-SO8 MOSFETs in the LFPAK56D package may fit the bill. The LFPAK56D offers two isolated MOSFETs in one Power-SO8 package. It occupies 77% less PCB space than two DPAKs or 50% less space than a single Power-SO8 device, offering significant savings in space, weight and cost. Nexperia’s portfolio of such devices now ranges from 30 to 100 V.

Automotive power MOSFETs are also available from Nexperia in the new LFPAK33 package, which has a footprint of 10.9 mm², more than 80% smaller than industry-standard devices. Because no wires or glue are used internally, maximum junction temperatures up to 175^o C are possible. Devices can handle up to 70 A, and the product portfolio includes devices that range from 30-100 V and an RDS(on) as low as 6.3 mΩ.

In summary, when the evidence is laid out, there are a lot of advantages for copper clip packaging over multiple copper wires: lower total device resistance RDS(on), higher power density and high frequency switching. Applications range from telecommunication, computing and battery management to such automotive uses as next generation engine management systems, chassis and safety technology, LED lighting, infotainment, navigation systems and ADAS.

For more information, visit https://efficiencywins.nexperia.com/