Advanced plastic packages break heat barrier, key for high-reliability and aerospace applications

By Ross Bannatyne, VORAGO Technologies

Engineers have traditionally opted for ceramic packages when choosing integrated circuits (ICs) for high-reliability applications such as aerospace. Ceramic is a proven solution in extreme temperature applications, in applications where many temperature cycles are likely to trigger thermal expansion effects and when a hermetic package is required. Avionics, space, and downhole electronics typically use integrated circuits in ceramic packages or die-based assemblies.

The ceramic package preference has been established over many years. Although there are still certain applications that plastic packages would still not be considered for, there are now many applications that are using new technology high-performance plastic packages. One of the biggest driving factors here has been cost constraints. Plastic packages are less expensive, so necessity has become the mother of invention in enhancing the reliability of plastic packages.

Plastic packages are lower cost because the raw material cost is lower than ceramic, manufacturing a plastic package is simpler and benefits are gained from the scale economies of high production. In addition, there are some other benefits of plastic packages over ceramic.

Plastic packages are lighter weight than ceramic, by approximately a third. In some applications like Aerospace where thrust-to-weight ratio is a key metric, this is important. Even though the incremental weight benefit of a single component is not in itself meaningful, the aggregate is important and can affect lifetime costs due to fuel consumption.

The availability of the 200 degree Celsius plastic package is a big step in the right direction for applications, such as avionics FADEC (Full Authority Digital Engine Control) systems, that need to provide high-temperature operation with robust components at competitive costs. A block diagram of a FADEC system is shown in Figure 1. The system distributes the electronic controls around the aircraft and requires high temperature operating components such as microcontrollers that are located next to sensors in high temperature zones. Enabling FADEC systems reduces wiring and connectors, reduces weight and simplifies maintenance.

Figure 1 – Full authority digital engine control block diagram

Although plastic packages are not hermetic (that is, they are not airtight), they are less brittle than ceramic packages and the nature of the molding adds protection against physical shocks and g-forces. Figure 2 illustrates a plastic package side elevation with some of the key internal features highlighted.

The plastic (epoxy resin) molding compound encompasses both the die and the bond wires. This “glob” prevents the bond wires from being displaced due to physical movement and stops them from shorting out against each other. In a ceramic package, there is a lid that sits above die and bond wires, but no encompassing glob that protects the assembly (in a similar way to an airbag) in the event of a high-g shock.

In contrast, a ceramic package has an open cavity in which the die and bond wires are located. This is shown in Figure 3.

Figure 3 – Ceramic quad flat package side view

An IC package contains many different materials and layers in the package, substrate, and encapsulation materials that are connected to each other. Each material has a different thermal expansion coefficient. As temperature changes, these materials expand and contract at different rates and the interconnections between them are stressed. There are also interactions that occur between different materials at high temperature. These differences in materials have traditionally been the source of problems with plastic packages operating at extreme temperatures.

The molding compound chemistry itself has evolved considerably in recent times and we have now seen plastic packaged VORAGO Technologies microcontrollers qualified to operate at 200˚C. In addition to this extreme temperature qualification (following JEDEC and MIL-STD-883 standard), the devices were subjected to 4000-hours continuous operation at 200˚C and 2000-hours continuous operation at 225˚C. This high temperature is possible due to advances in molding compound chemistry and advanced metallurgy to ensure that high-temperature cycling does not provoke damage due to temperature coefficient mismatches or delamination.

Following the 4000-hour stress, an analysis was performed to assess the Cu – Al (Copper – Aluminum) bonding to observe the difference between a “fresh” raw stock device and a device that had been subjected to 4000 hours of oven time at 200˚C. The devices were de-processed by grinding the package back to the pad – bond interface. The de-processed devices were then photographed using a Scanning Electron Microscope (SEM) at 1000X and 9000X magnification to observe possible damage from voids, cracks, oxidation, or corrosion. The bond area of the package stressed at 200˚C for 4000 hours is shown in Figure 4.

Figure 4 – Plastic package following 4000-hour 200˚c stress

The image, under 1000X magnification shows an intermetallic Cu - Al layer clearly visible. This is a strong alloy bond that has been forged over 4000 hours due to the temperature stress. Despite the lengthy high-temperature stress, no signs of significant voids that might impact the integrity of the bond or present other reliability risks are observed.

It has been a source of frustration for many years that advances in silicon technology continually improve performance and reduce cost, but conventional packaging technology has not evolved as quickly. The evolution to higher temperature plastic packages is overdue and is being welcomed warmly by engineers of high-reliability systems.

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