Choosing the right material for RF packaging

Choosing the right material for RF packaging

New compounds and wide-spread demand for high-performance devices and are altering the chip-packaging landscape

DICK ROSS and JOHN ROMAN
RJR Polymers, Oakland, CA
http://www.rjrpolymers.com
and EDSON ITO
Ticona Engineering Polymers
Florence, KY
http://www.ticona.com

While semiconductors tend to capture people’s attention far more than their packaging, a die and its packaging are inseparable. The success of a device — especially an RF or microwave device — often depends on both.

The choice of packaging material for interconnects involving printed circuit boards and devices, often referred to as Level 2 and 3 packaging, depends on many factors: in addition to assuring a viable interconnect, it may also need to provide for downstream testing and processing, compatibility with other components, in-use thermal management, and environmental-stress protection, among other functions.

Level 2 and 3 packaging is usually made from epoxy-molding compound (EMC), ceramic, metal, or engineering thermoplastic. EMC is used to provide complete chip encapsulation, while the other materials create open-cavity packages when dies cannot be encapsulated.

The choice of material is most often driven by economics and the need for optimum yield and performance. EMC transfer-molded packaging, which accounts for nearly all Level 2 and 3 packaging, is often the least expensive option.

But in increasingly more instances, air-cavity packaging is the best option. Though a relatively small portion of today’s market, air-cavity packaging’s share is forecast to grow by more than 30% annually over the next five years. Driving this growth is the trend to higher-power devices and those that work at frequencies above 1 GHz, as well as the need to decrease attenuation and frequency shift and limit thermal loss.

Several environmental factors have also been affecting material choice. The rise of Pb-free soldering has elevated processing temperatures by about 40°C, to between 240° and 260°C or more, pushing the practical limits of EMC and most plastics and opening the door for higher-temperature engineering plastics. In addition, relatively recent environmental regulations have worked against the use of EMC, as has the growth of devices that cannot be encapsulated, such as sensors based on MEMS for automotive, aerospace, military, medical and consumer products.

Also, the trend toward thinner-walled, more intricate packaging is creating demands that exceed the inherent design limitations of metal, ceramic, and EMC, and favor the use of injection-molded polymers.

farcrjrticona_nov2007 The desire for more intricate RF packages to optimize board area and meet dimensional constraints is changing packaging material choices.

Epoxy-based packaging

Transfer molding using thermoset epoxy accounts for more than 90% of all semiconductor packaging. This is due to its economy, the large installed base of equipment for this process, and the industry’s comfort with it.

EMC packaging begins by wire bonding a die to a metal leadframe and placing it in a mold. The compound, which contains epoxy, filler, flame retardants and other additives, is heated to 150°C so it can flow through runners and gates into the cavity and around the array. The EMC then polymerizes to a solid. The molding process generally takes 30s to 20 min, depending upon cure requirements.

Encapsulation with EMC typically costs just one-third of a U.S. cent per lead. It works well with most silicon dies in end uses like phone amplifiers, automotive switches, motherboard and peripherals elements for personal computers, and most other common consumer devices. Its use has been growing at a rate of 7% to 10% per year.

Transfer molding has its limitations, however. It can cause attenuation, does not accommodate devices that need high heat dissipation and, because EMC is a poor moisture barrier material, does not protect microchips from water vapor.

Waste is also an issue with EMC. As a thermoset, its polymerization cannot be reversed, so all runners, out-of-spec product and other items must be discarded. This can raise material loss to 50% or more and make higher-cost thermoplastics cost competitive with it. In addition, EMC may use substances, such as bromine flame retardants and mold release agents deemed hazardous, that make disposal difficult.

Metal and ceramic

Metal and ceramic are traditional open-cavity package materials. Metal packaging accounts for less than 1% of the market and is most often found in military, aircraft and space applications. It can offer a total barrier to gases and moisture and estimated lifetimes to 20 years or more. Although metal is relatively inexpensive, fabrication which involves machining, electroplating and other steps pushes costs for metal package well above those for others.

Ceramic has been the dominant open-cavity material for much of the past four decades. It was developed for hermetic, high-end packaging where EMC cannot work. Today it is used to package MEMS, radio frequency and microwave devices, and many other components.

Ceramic, primarily alumina, is fired at 1,600°C. It may be co-fired with tungsten so leadframes can be attached, since this metal survives the firing temperature. Low-temperature co-fired ceramic, which is half glass and half alumina, processes at 700° to 800°C and allows the use of copper, silver or gold for printed metallization.

Ceramic shrinks significantly during firing and often must be lapped to form flat mating surfaces. This raises ceramic packaging’s cost significantly and, tied with other cost factors such as secondary plating and brazing the leadframe during assembly and one-unit-at-a-time fabrication, has resulted in flat growth.

Premolded air-cavity

With the move to Pb-free soldering, LCP (liquid crystal polymer) became the primary material for premolded air-cavity packaging. This thermoplastic can withstand assembly temperatures well beyond the abilities of polyester, polyethersulfone, and other engineering polymers. Only LCP and PEEK (polyetheretherketone) can tolerate 260°C or more without melting or distorting, and PEEK is more expensive, more difficult to bond, and much harder to mold in thin cross-sections than LCP.

Not only has LCP begun displacing more expensive ceramic packaging equivalent LCP packages typically costs about 40% less but it is also replacing less-expensive EMC in high-performance applications, such as those that call for high power density and high frequencies.

Manufacturing with LCP differs from other packaging processes. LCP packages are injection molded around leadframe inserts, often at mold cycle times under 10 s. This method lends itself to high-efficiency approaches, such as a continuous process that molds strips of metal leadframes in multi-cavity molds. Alternatively, conductor patterns can be placed on LCP after molding by such automated processes as laser direct structuring, in which a laser draws a circuit pattern on a platable LCP that is then metallized in an electroless plating bath.

Once a die is attached and wirebonded, the air-cavity package can be sealed with a metal, ceramic, glass, or plastic lid. Sealing often involves an adhesive such as epoxy, although heat, laser, or other means can be used. LCP works well in premolded LCP packaging because many grades of this polymer withstand temperatures above 300°C. It is inherently flame retardant and meets UL 94V-0 flammability criteria without the addition of halogen, phosphorous, or other flame retardants. And, as a thermoplastic, it is recyclable.

LCP has 10 times the water vapor barrier of epoxy and absorbs just 0.02% moisture, so it can create near-hermetic packaging. It has excellent dimensional stability in molding and creates precision parts having flat surfaces right out of the mold, without rework. It also has good electrical properties: for instance, it has a low dissipation factor and its dielectric constant is 3 to 4, compared to 9 to 13 for ceramic.

Comparison of packaging materials

For advanced packaging designs, LCP offers greater functionality and flexibility than ceramic. While ceramic packages can be shaped in two dimensions, they have trouble with three-dimensional shapes or complex contours. Injection-molded LCP does not, and thus lets many elements connectors, inductors, and other structures normally placed on boards be integrated into complex packages. This will reduce demand for board space and lower component, processing, and assembly costs. ■

For more on packaging, search keyword “package” at http://electronicproducts-com-develop.go-vip.net/packaging.asp.