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.
Price & Availability
Pricing for DirectFET devices will start at $0.90 each in quantities of 10,000. Although pricing for DirectFET components is generally expected to be higher than for existing package styles, the company projects that, in time, the cost per amp for DirectFET-based systems will be significantly less than that of any other solution.
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.