Near-Hermetic, Thermoplastic MEMS Packages Advance

Epoxies are the workhorse polymer for electronics found in most plastic packages, encapsulants and underfills. These compounds are notable for their balance of properties, since they typically contain more filler than epoxy resin to tame the high coefficient of thermal expansion (CTE) mismatch among packaging materials and silicon. Epoxies belong to the thermoset class of polymers because they are "set" by polymerization, typically triggered by heat. While the packaging industry has settled on epoxy molding compounds for non-hermetic packages with years of infrastructure in place, MEMS might offer alternative materials solutions.

The other major polymer class is thermoplastics. Thermoplastics have long molecular chains that are independent and not linked, so when sufficient heat is applied, the chains can move and the plastic can flow and be shaped, but upon cooling, the material is returned to the solid state with no significant material change.

Thermoplastics are getting a second look now because of some inherent advantages over epoxies. The first criterion is high temperature stability compatible with lead-free processes. The liquid crystal polymer (LCP), in particular, offers a softening point of ~300°C, is produced by some very large resin makers, and has a long history of use in fiber-optic connectors.

Also, when LCP is compared with epoxy in packaging, some environmental aspects make it compelling. LCP has no bromine or other halogen. It is intrinsically flame-retardant and only contains the friendly elements of carbon, hydrogen and oxygen. Moreover, LCP, among other thermoplastics, does not generate scrap in production. Epoxy waste is being classified as hazardous in many locales, and disposal is costly.

Thermoplastics and MEMS research have conspired in the search for a "quasi-hermetic" package, a practical alternative to the expensive fully hermetic packages made of metal or ceramic. In fact, industry has already begun to explore the "near-hermetic" package. Silicon Bandwidth (San Jose) has molded packages with LCP. RJR Polymers (Oakland, Calif.) has worked with LCP packaging by inserting molded metal leadframes and targeted some optical applications. Recently, start-up Quantum Leap Packaging (Wilmington, Mass.) has begun working on LCP molded packages and leadframes for MEMS and other applications.

One new concept molds very simple discrete electrical conductors into the package bottom to make it a surface-mount device. Ken Gilleo at ET-Trends LLC (Warwick, R.I.) has begun work with a precision plastic molder, Matrix Inc. (Providence, R.I.), to discover whether it is possible to mold metal spheres into a plastic cavity package. The top of the sphere protrudes inside the package cavity while the bottom projects through the package "floor" for outside connection to the printed circuit board. This "low ball" package solves interconnect, assembly and cost issues in a cavity BGA package. And since a cavity is ideal for MEMS, the package is MEMS-specific.

The molding process takes metal balls of copper or nickel, and robotically places them into a two-up mold with matching depressions in its base. The result is two complete cavity BGAs ready for MEMS assembly. In high volume, the mold produces either strips or arrays of packages to accommodate standard package assembly production.

Finished parts ship to a packaging foundry or the MEMS fab. Final assembly follows variable MEMS processes involving die attach, wire bonding, and lid seal. The inside section of the ball is flat, or coined, to facilitate wire bonding and better seal the metal-plastic interface. The lid can be sealed ultrasonically, with adhesive or with a laser. A prototype mirrors MEMS gyroscopes that require 16 I/Os used for anti-rollover technology in SUVs, as well as other consumer applications.

The low-ball thermoplastic, near-hermetic package features interconnections molded into the floor of the package cavity. Once assembled with die, wire bonds and sealed lid, it becomes a ready-made surface-mount device. (Source: ET-Trends LLC)

The low-ball BGA is undergoing further assembly testing with copper wire bonding to copper spheres. While this initial package may be a path to low-cost, near-hermetic packaging, other thermoplastic packaging concepts are in R&D. A rerouting structure can be added to packages by selective plating techniques using a molded interconnect device. The package can also be turned into a display or photonic type by using a glass lid, or molding in fiber ports.

The recent semiconductor upturn has been characterized by packaging innovation. As telecom recovers and MEMS find more practical applications outside the R&D halls, near-hermetic packaging offers a cost-effective breakthrough. Thermoplastic materials and creative package construction undoubtedly will hasten practical solutions if Einstein's golden rule, "Everything should be made as simple as possible, but not simpler," is obeyed.