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Stop Burning Money: Forget Prevailing Wisdom

Counterintuitive thinking often saves money in electronics

Randy Frank, Contributing Editor -- Design News, November 22, 2004

Integrated circuit (IC) technology provides added features and improvements with each advancement in processing and design methodology. Since available board space can be costly and, in some instances, non-existent in next generation designs, taking advantage of the higher integration in the latest ICs to eliminate components is normally an excellent design strategy. A potential downside comes when all the features of the IC are not required for an application. Each added feature normally requires more pinouts, which in turn means a larger, more expensive package.

An IC with just the right amount of integration can use a package with fewer pins and provide a significant reduction in cost for both the product and a significant savings in board space. An example of this approach is a microcontroller (MCU) that fits into a six-pin SOT-23—a semiconductor package initially developed for a single discrete transistor. Microcontroller packaging has been reduced from14 pins to eight pins for the more cost-effective, limited function MCUs and the eight-pin packages have continuously shrunk during the past decade. A SOT-23 six-pin MCU such as one from Microchip's Pic10F family is packaged in a 2.8 × 2.95 mm package that occupies only 28 percent of the footprint of a 6.01 × 4.89-mm SOIC-8 package that was state of the art just a few years ago. The MCU still provides functions such as:

• Eight special function hardware registers

• 4 MHz precision internal oscillator

• 3 I/O pins with individual direction control, one input-only pin

• High-current sink/source for direct LED drive

• 8-bit real-time clock/counter (TMR0) with 8-bit programmable prescaler

• One comparator

Among other possibilities, these functions allow the chip to provide a soft-start controller for switching power supplies with only two additional components—a hefty cost savings.

Getting the heat out of power MOSFETs frequently involves a heatsink attached to a back-plate. To stop the baseplate from getting electrically hot, an electrical isolation pad with good thermal conductance, such as Bergquist's Sil-Pad, is used between the MOSFETs and the heatsink. The problem with this approach is that attaching the leaded TO-220 power MOSFETs to a printed circuit (pc) board requires hand soldering of the leads—a processing cost adder that can be significant.

An alternative solution uses a metal-backed board, such as Bergquist's Thermal Clad Insulated Metal Substrate (IMS) material mounted to the back plate and stand-offs to connect to the printed circuit board. (See the comparison in the brown box at left.)

The thermal performance of both methods is about the same. The MOSFET is the same as the initial case, but in a surface mount version. By automating the assembly process, an estimated cost savings of $0.50 labor per heatsink assembly is expected. For units with several heatsink assemblies there is a significant payoff. For example, with six of these units in a brushless dc motor and an annual volume of 10,000 units, the cost savings can be $0.50 × 6 × 10,000 =$30,000. The tradeoff for this approach is the standoffs also carry high current. If this isn't a problem, take the money and run with this one.

Connection: Using a standoff to electrically connect the surface-mount MOSFETs on an IMS board to the rest of the circuitry on the PC board reduces cost for both the components and assembly.

Smaller, smaller, smaller: Microcontroller packages shrink with each new silicon process improvement, allowing the same or more functionality to be available in increasingly smaller footprints. Eliminating two pins does require sacrificing some performance, but for many designs there is still more than enough MCU capability. Those applications are ideal candidates for cost savings.

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