High-Tech Packaging Comes to Power Supplies

The power distribution architecture for high-speed digital products is changing, and the revolution is being enabled by a new packaging approach for power modules.
 
Core and I/O voltage levels are dropping, yet the total current draw to chips is increasing. As the current draw to ICs switch because of the clock, I/O lines and changing micro code, any impedance — especially inductive impedance — between the regulator and the chip will cause a voltage drop or rail collapse. More than ~5% voltage drop and the digital device may fail.

The solution to this problem is "to distribute an intermediate bus voltage to the vicinity of the chip and regulate locally," said Dennis Roark, chief technology officer and vice president of engineering of the Silicon Power Systems Division of Power-One Inc. (Camarillo, Calif.).

This new intermediate bus architecture (IBA), using local regulation, has been termed point of load (POL). An intermediate voltage bus, with a range of 3-12 Vdc, is distributed throughout the board, and low-voltage regulators are co-located with the chips that need it.

But board area is expensive in high-density, high-speed digital products. The cost of placing a regulator in the proximity of the chip that needs the low-voltage, high-current supply is directly related to the regulator's surface area. A small-form-factor dc-to-dc converter is ideally suited for distributed local regulation applications.

An MCM-based dc-to-dc converter for local power regulation. (Source: Power-One)

In evaluating alternative packaging approaches, the objective was optimized thermal management at the lowest cost in this form factor. After surveying the contract equipment manufacturers (CEMs), Roark and his team settled on the extensive infrastructure established for organic substrates.

The MCM is fabricated on a conventional two-metal-layer BT substrate, with a silicon controller, two power MOSFET chips and more than 17 passive components. The active devices are adhesively attached and wire bonded to the substrate. The assembled module is overmolded. I/Os are off the bottom in a land grid array (LGA). The MCM is intended to be surface-mounted to the board adjacent to the chips requiring local regulation.

The MCM technology generally uses off-the-shelf capabilities. To keep the substrate isothermal, extra thick copper layers are used. The significant challenge, Roark said, "was reliably mounting the die, while maintaining good thermal management, low electrical conductivity and keeping the stress low in the package, without cracking or delaminating the die, and keeping the cost low." The development of a reliable substrate design and assembly process took one year and another year for product roll out. It was a joint development between Power-One and the package foundry.

Key to the successful implementation, Roark said, was a close relationship with the packaging supplier. "You have to have the technical competence on your staff to take responsibility for the development, or you are at the mercy of the subcontractors and their experience. This is a pretty key issue and this is how you form a really tight relationship by jointly working the issue." The first production units of this power regulator MCM are scheduled for release in July 2003.

As supply voltages continue to drop and currents increase, local regulation will become more important. The smallest-form-factor regulators will be the most cost-effective. This can only be enabled by packaging technology. As Roark said, "Packaging and packaging innovations is where we have to be going forward to get these modules smaller yet."

For additional information on assembly and packaging, go to www.semiconductor.net/assembly