Prototype Packaging Solutions for IC Designers
05/01/2003
Prototype packaging option alternatives
BY DAVE HYPNAROWSKI, ELLEN EMERY, BILL LAWRENCE AND ROBERT BLUE
Being the first to market with new integrated circuit (IC) designs is essential to the survival of a fabless company, and it is significant to the growth of established semiconductor manufacturers. Establishing a time- and cost-effective strategy to package new IC designs for rapid prototype evaluation and verification has never been more important.
This article explores options for prototype packaging and discusses the advantages and disadvantages of each alternative. Package types can include standard ceramic open cavity, production mode plastic or a rapid turnaround version of the actual plastic package intended for production. Typical process flows for both ceramic and plastic package assemblies are shown in Figure 1.
Ceramic Packages
Sometimes ceramics are used as a substitute for the production package in prototyping. Unfortunately, the selection is limited and, in many cases, a ceramic package that matches the physical dimensions of the plastic package is unavailable. Using a ceramic package usually requires modification of the test sockets and test boards for evaluation. Although the initial cost of a ceramic package may appear attractive, the cost in testing and usefulness in die analysis can negate any initial savings on the package itself. Because a ceramic package does not have the same physical and electrical characteristics as a plastic package, the ceramic option may introduce performance variables in such a way as to alter the results of the evaluation. As a result, additional design revisions may be needed and additional prototype passes built to remove the undesirable characteristics of the alternative package.
Plastic Package Equivalents
Some ready-to-use premolded plastic open cavity format packages are available in the industry; however, the selection is limited. Even if the desired style is available, this pre-molded package is not identical to a production part. Electrically and mechanically, the pre-molded package may exhibit performance variations when compared to the final molded package. Custom tooling for a specific package type not currently available also is cost prohibitive for most users.
Open Cavity Packages
Assembling prototypes in the same plastic package as intended for production is an intriguing option. Open-cavity plastic IC packages present a quick-turn solution for rapid evaluation of IC prototype designs. New designs can be evaluated and verified in the final, production-level package. The design engineer can select any plastic package available on the market, and because this package is mechanically and electrically identical to the production package, many prototyping variables are simplified or eliminated. Multiple versions of a package style can be tried to determine optimal die performance. The design also can be evaluated in newly introduced package types such as the latest higher lead count quad flat no lead (QFN) package. Local (onshore) assembly can be performed, providing the ability to obtain assembled units in as little as 24 hours. Assembly can be performed with the die fully encapsulated by opaque or clear epoxy or with the die exposed. These methods provide ideal situations for rapid and straightforward die probing, focused ion beam (FIB) work and thermal testing.
 Figure 1. An illustration of prototype packaging process flow. |
null
As IC designs evolve to higher frequencies, lower operating voltages and greater noise sensitivities, it becomes more critical to evaluate new chip designs in their final package type. Designs are simulated with specific values for package performance characteristics of resistance, inductance and capacitance. Circuit operating points are matched to the package parameters. If an attempt is made to evaluate circuit performance in a package dissimilar to its final type, the product will not perform as expected. The designer will not know if the problem lies with the design point, or if it has been induced by the behavior of an improperly selected prototype package. The open-cavity plastic option avoids this undesirable situation by using the correct production-level package from the beginning.
 Figure 2. Cost per chip of various packaging alternatives. |
null
The open-cavity process is universal and can be applied to any plastic IC package. No package type, lead count, size or orientation is excluded. The process works on packages with a die (such as test rejects or excess inventory) and on "dummy" packages (no die inside). The package's supply source is not an issue because typically the same supplier will be used for prototyping as for production. The process is robust on many popular designs. Thin small-outline package (TSOP), small-outline integrated circuit (SOIC), micro-lead frame (MLF), quad flat no lead (QFN) and ball grid array (BGA) packages are examples of the styles that can be opened with the process.
During processing, an existing transfer molded package is reworked. The plastic compound, old die and wires (if present) are removed, exposing the bonding pad and lead fingers. No residues are left on the reprocessed surfaces or package, nor is the silver plating damaged. This is critical to the ability to successfully wire bond a new die in the package. The structural integrity of the package is maintained and a new die can be wire bonded in the package.
Assembly Locations
In addition to the package style and supplier, the IC designer is confronted with the choice of assembly alternatives. New designs can be assembled at a design facility, by a local assembly house, or an overseas production line facility. Each choice of location has its own advantages and disadvantages.
Offshore Assembly - One option for the design engineer is to route the wafer from the foundry to an overseas assembly house. There, the wafer may be thinned, diced, die attached and wire bonded into the package of choice, typically the one intended for final production. This procedure usually takes three to five weeks. The assembly house process time is subject to many variables, and will increase when the factory is in high demand. In addition to the long wait for assembly, the designer will have to deal with shipping costs and lot charges, and must address suitable substrate and lead frame availability. The process can be challenging to monitor and difficult to control. The provider of the services will drive the completion schedule. Companies with an urgent need to get initial prototype units may even have staff members fly overseas to hand carry and personally manage critical wafers. This overseas assembly option is very costly in terms of engineering time and travel expenses (Figure 2).
Onshore Assembly - Alternatively, die/package assembly can be performed onshore at the location of the designer's choice—the design center, in-house assembly facility, local contract assembly house or an open-cavity package supplier. Generally, onshore assembly will be much quicker than if done overseas. Communications are faster, and the need for hand carries typically can be avoided by overnight shipping services. Local assembly suppliers are more responsive to particular customer needs, such as small lots, quick-turns, on-the-fly updates and unique die/wire bond layouts.
A particularly useful advantage of quick-turn assembly is the much faster feedback, which can be provided to the wafer fabrication facility.
Many Choices
A range of choices is available to IC designers for prototyping new designs. The open cavity package offers designers timely and cost-effective solutions for new design verification. Prototyping services in as little as one day present the best option for new IC designs. Prototypes can be assembled onshore, and the designer can add flexibility and control to the design program.
Dave Hypnarowski and Ellen Emery may be contacted at Quik-Pak, 10987 Via Frontera, San Diego, CA; 92127; (858) 674-4676; fax (858) 674-4681; e-mail: [email protected]; [email protected]. Bill Lawrence and Robert Blue may be contacted at Quik-Pak, 39 River East, Essex Junction, VT; 05452; (802) 879-5313; fax (802) 879-5318; e-mail: [email protected]; [email protected].