Dual Thermal Paths Double Power Handling For Surface-Mount MOSFETs

Improvements in silicon processes have steadily produced more-efficient power MOSFETs. These MOSFETs have propelled dc-dc converters, such as the synchronous buck converters used to power microprocessors, to higher levels of power density. But as MOSFETs have gotten better, the package's electrical characteristics—in particular, surface-mount styles—have become much more significant.

As a result, over half of the R_DS(ON) of the best available MOSFETs is now attributed to the package rather than the silicon. And because existing MOSFET packages introduce significant thermal resistance as well, they also limit the power dissipation of MOSFETs, because of their inability to transfer heat efficiently away from the chip.

International Rectifier's DirectFET is a surface-mount package that improves MOSFET performance by lowering both the package's electrical and thermal resistance. It does so with a design that permits direct attachment of the die to the customer's pc board via solderable pads on the chip and through attachment to a copper drain clip that allows double-sided cooling (Fig. 1). The latter feature is an industry first for a surface-mount power package, according to Carl Blake, an IR marketing manager. He notes that DirectFET differs from previous package innovations, which produced relatively minor improvements in performance. Says Blake, "This is the first power package developed from the ground up."

DirectFET is being introduced in an SO-8-sized footprint, which naturally warrants its comparison with that industry-standard package. Because of its improved electrical and thermal design, IR's new package can carry twice the current of a standard SO-8. The DirectFET boasts a die-free package resistance that is 86% less than the SO-8. In terms of R_DS(ON), one of the first DirectFET packaged devices—the IRF6603—measures just 3.8 mΩ at V_GS = 10 V, a value that is 45% less than IR's best SO-8 MOSFET, the IRF7822.

At the same time, DirectFET's thermal resistance is much less, with values of 3°C/W junction-to-case on top and 1°C/W junction-to-pc board. For the SO-8, these same parameters would be 18°C/W and 20°C/W, respectively (Fig. 2). So given the same power levels, a DirectFET-packaged MOSFET operating under full load should run more than 35°C cooler than an SO-8.

Although the bottomless SO-8 developed by Fairchild also achieves a 1°C/W thermal resistance from junction to pc board, its thermal resistance from junction-to-case on top is similar to a standard SO-8. Consequently, it transfers all of its heat to the pc board, which can complicate board design. Double-sided cooling also means better thermal performance for the DirectFET when compared to other existing surface-mount packages (Fig. 3).

To achieve these performance levels, DirectFET trades plastic packaging and wire bonds for solderable die pads and a copper clip, while eliminating overmolding in favor of passivation (Fig. 1, again). Gate and source connections are made directly to the pc board (without solder balls) via wide-area pads on the die. These pads provide the good thermal interface needed to achieve the 1°C/W noted previously, while also reducing electrical resistance.

At the same time, the drain connection is made through a copper clip that simultaneously provides low electrical resistance to the board and low thermal resistance from the top side of the chip to the case. That in turn permits greater heat dissipation up top with the option for heatsinking and forced-air cooling.

Eliminating wirebonds not only lowers the R_DS(ON) associated with packaging, it also permits the package to house a 30% larger die than an SO-8. Another benefit of the DirectFET construction is its low package height. It stands 0.7 mm tall, versus 1.75 mm for the SO-8.

The primary innovation in DirectFET is a proprietary passivation scheme that serves two purposes. The passivation acts as a solder mask when the device is mounted to the pc board, preventing the gate and source from shorting. It also seals the die against potential sources of contamination—humidity, solder flux, board cleaners, and others—while leaving the die pads exposed for soldering.

Producing a die with solderable metallization is itself a difficult task. To do so requires a complex scheme of multiple metal layers, which provide a robust electrical and mechanical connection, while preventing metal migration. IR developed its techniques for producing solderable die pads with its previously introduced FlipFET package.

The DirectFET technique of using solderable top metal to attach the MOSFET die to the pc board differs substantially from ball-grid-array (BGA) and flip-chip technologies, which use solder balls to attach a device to the board. Compared with DirectFET's solderable die pads, solder balls provide limited contact area with the board and are therefore much less efficient at transferring heat from the die to board.

Although DirectFET's construction is very different from standard surface-mount packages, its requirements for board-level assembly are similar to those of standard packages. DirectFET is compatible with conventional pick-and-place equipment, solder masks, and fluxes. Nevertheless, the company will offer an application note advising designers how to size pc-board pads, determine thickness of solder, and which solder fluxes have been tested with the package. The DirectFET package is free of both lead and bromides.

Double-sided cooling can be used to advantage in a variety of ways. If an SO-8-packaged MOSFET is simply replaced with a DirectFET type, and no additional cooling is provided, the junction temperature of the chip will decrease under similar loading. That fact may be used to improve the transistor's reliability or to allow an increase in the MOSFET's current output.

These points are made clear in tests conducted on the first DirectFET product offerings. International Rectifier is initially offering the DirectFET package with two pairs of matched devices for synchronous buck dc-dc converter applications. Fabricated in the company's Advanced Planar silicon process, the IRF6601 sync FET and the IRF6602 control FET are 20-V devices that target servers. The IRF6603 sync FET and IRF6604 control FET—30-V parts fabricated in IR's Stripe Trench process—are optimized for notebook computers.

In one design example, a synchronous buck converter switching at 300 kHz converts 12 to 1.3 V using 20-V MOSFETs (Fig. 4). When the converter is constructed with DirectFET-packaged MOSFETs (IRF6603/IRF6604) and cooled with a heatsink and 200 lfm of forced air, it achieves current levels of 35 A per phase. In contrast, a version of this converter that's built with similar silicon in SO-8 packages (IRF7811/IRF7822) yields performance of 18 A per phase.

The DirectFET's doubling of dc-dc converter current density is achieved though reductions in the number of MOSFETs re-quired and in the amount of pc-board real estate occupied by these devices. Consider that a multiphase converter capable of 30 A per phase requires five of the best SO-8-packaged MOSFETs per phase, compared to only two dual-side cooled DirectFET MOSFETs. This directly reduces part count by 60%. In addition, the use of a thermal pad to transfer heat away from the device to the system chassis permits DirectFET devices to be laid out in a smaller area on the board (see the table).

Down the road, there are plans to introduce DirectFET in footprints other than the SO-8. One possibility is a replacement for the D²Pak.

DirectFET samples are available now, with full production expected this summer. Requirements for second sourcing are also being addressed. The company plans to license the DirectFET technology to other vendors and expects there will be a second source for the new package within a year.

International Rectifier, 233 Kansas St., El Segundo, CA 90245; (310) 252-7019; www.irf.com.