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Packaging becomes problem-solving tool

Posted: 01 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:automotive packaging? thermal conditions? temperature? mosfet? smart power?

Automotive devices run at high temperatures for long periods of time and may experience junction temperatures above 200C for short times during load dumps. Modern package designs minimize package contributions to Rds(on), thus reducing the normal operating temperatures. The package also improves thermal resistance, further reducing junction temperatures.

As with all electronics, automotive electronics is being driven to miniaturization. Integrating control circuitry within the power device package minimizes size, but adds complexity to an already challenging task for the package. Not only does the package have to provide good thermal dissipation for the power die, it must also electrically isolate the control die from the high voltages and currents. The drive to make things smaller also makes the thermal-management task more difficult as the area for thermal dissipation decreases, although the amount of power remains the same or even increases.

Running hot

Not only is the thermal density increasing; electronics are being used in higher temperature locations within the car. The environment can range from inside the transmission at 200C to being on a spark plug with ambient of 165C, to mounted in the engine compartment with ambient of 150C, to the relatively benign environment of the passenger compartment at a maximum of 80C. An automobile is estimated to have 6,000 cold starts over its lifetime, where the temperature could cycle from 40C to 150C within the engine compartment. Protecting the silicon from extreme environments and associated stresses is part of the package function.

Packaging has moved well beyond merely a chip carrying and chip-board interface element to become an even more powerful tool in solving problems.

With the drive for circuit miniaturization and higher temperature environments comes the need to understand the thermal limits and thermal management of power semiconductors. This will ensure that designs that will continue to provide the reliability required by the automotive market.

Temp effect on silicon

Increasing temperature adversely impacts the performance of power devices.

For MOSFETs:

The Rds(on) goes up as temperature goes up, causing more power loss;

The BVdss (brake down voltage) goes up as temperature goes up;

Leakage increases exponentially as temperature goes up;

The threshold voltage goes down as temperature goes up, making it harder to turn off the gate at high temperatures.

For PIN diodes:

The forward voltage drop goes down as temperature goes up;

The reverse recovery charge and time go up as temperature goes up.

For punch-through IGBTs like those used in ignition systems:

The Vce(sat) goes down as temperature goes up;

The threshold voltage goes down as temperature goes up;

Switching time with inductive loads goes up as temperature goes up;

Leakage increases exponentially as temperature goes up;

The BVdss goes up as temperature goes up.

From a power-device perspective, Tj is the most critical factor. Most failures result from forcing Tj too high. Equation 1 summarizes this point: T= Rth {(Von x Ion)+ ( ? V(t) * I(t) dt) f} (1)

The T is the degrees above a content temperature at some distant infinite heat sink. For the vehicle, that infinite heat sink is the incoming air, and the content temperature used for vehicle designs is the classic 122F (50C). But that air is used to cool the engine via the radiator. In general, the electronics modules see a far hotter infinite heat sink temperature. For the power device in most modern power-train designs, the infinite heat sink is the 105C air moving across the module's heat sink.

In the past few years, power MOSFETs have improved their specific on resistance. For the silicon, die size is a large factor in the (Von * Ion) and Rth portions of the Equation. The improved on resistance enables a smaller die to have the same resistance of a larger, older MOSFET. But this smaller MOSFET will have a higher thermal resistance. The onset of trench structures and the resulting improvements to this technology have led to significant advancements in specific on resistance (Figure 1). This means that power density has gone up by almost an order of magnitude in the past 10 years. Unfortunately, the thermal performance of the interface for the power device in the engine control unit (ECU) has not kept pace. In fact, the desire for SMDs has grown and modern ECUs no longer have power devices connected directly to the heat sink. Where once a power MOSFET was in a TO-220 connected to a heat sink, that same function today is most likely performed with a DPAK soldered to a PCB with vias to an isolation pad to a heat sink.

Process separation

Smart power devices need to process both power and data. It is often more cost-effective to use a silicon process optimized for signal processing for the smart functions of the device and use an entirely different silicon process optimized for power devices. The separation of processes leads to the need to reintegrate these different dies into a package that provides interconnects between the power die and the signal-processing die and to the external circuitry.

The package provides power handling, die interconnect, power and signal connections and possibly die substrate isolation, along with the physical support and environmental protection. Low thermal impedance from the power junction to the case of the package is required to allow thermal cooling of the power devices. The thermal resistance impact is represented in the Equation by Rth. Low thermal resistance is achieved by having the metal lead frame that the power die is attached to extend to the surface of the package. Solder die attach is required to offer the lowest thermal and electrical impedance to the back surface of the power device. Use of a non-conducting epoxy or a polyimide tape provides electrical isolation of the control die from the electrical potential of the back of the power switch die's back.

Figure 2 shows the inside of a three-paddle, five-die assembly. This multidie packaging provides isolation between die substrates, low thermal resistance for the power die and the ability to interconnect two separate smart power devices. In this device, the two control die have 12 interconnects, each using small gold bond wire to minimize control die size. The control IC is isolated from the power die by using non-conductive adhesive die attach. The power devices use thick aluminum bond wires for current handling and are solder die attached for high power dissipation. The solder connection of the power die to the DAPs and the DAPs to the circuit board provide minimal thermal resistance from the power die to heat-dissipating surfaces.

Qualification requirements

Automotive products are typically qualified for requirements found in AEC specifications Q100 for ICs or Q101 for discrete devices. Tests include operating life, temperature/humidity/bias testing such as HAST or H3TRB, power cycling, temperature cycling, high-temperature reverse bias. In addition to reliability stress, characterization of the package material is required to understand performance trade-offs. Characteristics such as mold compound ionics, Tg, moisture absorption, and modulus at room and elevated temperatures are some of the characteristics of interest.

Package technology provides improved electrical and thermal performance for today's single-die power products. As products transition to smart power components, integration of optimized silicon processes in a single, small smart power package provides the size, electrical, thermal and environmental performances required for automotive electronics.

Alexander Craig

Stephen Martin

fairchild semiconductor Inc.




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