What, I Need a Package?
A typical phone call to a package manufacturer begins with: “I have this chip, what package do you have that will fit?” Next comes the basic questions: “What size is your chip? How many pads does it have? Does it need to be hermetic? Do you want it to be surface mountable? Leaded? Ball grid array? Oh, and by the way, how fast does your chip operate?” Depending on the answer to the last question, everything else becomes moot.
Package requirements are being designed around high-speed and high-frequency chips, which call for a whole new set of parameters. Once frequency reaches above 2 GHz, the package no longer works in the “traditional” electrical manner, but moves into the microwave realm. Although basic packaging needs remain the same - protection from environmental factors, thermal dissipation, handling, and durability for the chip - a whole new set of electrical requirements exist. Ideally, the chip designer should work jointly with a packaging company to design an optimum package for the chip instead of choosing one off-the-shelf. Because there are a number of good standard packages that work in excess of 50 GHz, an off-the-shelf option may exist, but that might not be the case. Either way, it’s still important to bring the chip designer together with the package designer to reach the best technical and economic solution.
With new generations of chips routinely working in microwave and high-speed digital applications, electrical package loss can become significant factors in the system’s success. Typically, chip designers optimize the electrical system to operate at 50-Ω impedance, which requires a similar design for inputs and outputs of the system. Most standard packages do not have their ports optimized for 50 Ω, causing large electrical losses to occur at both the signal entrance and exit of the package. If chip frequency exceeds 2 GHz and electrical losses are critical, specialized packages are required. However, many options still exist depending on the other application requirements.
If good electrical performance is critical, then the application will require a ceramic-based package. Glass or plastic packages can be used for low-frequency or other applications where electrical loss is less critical. Depending on upper frequency, both leaded and lead-free surface mount configurations are available, but when hermeticity is required, ceramic, glass-sealed metal packages or glass-sidewall packages must be used. When the frequency exceeds 40 GHz, current package designs are limited to lead-free, “drop-in” configurations, where ports must be mounted on the same plane as the bonding pads of the circuit used to hold the package. Surface mount designs cause too many losses at these ultra-high frequencies.
High-power chips require the use of thermally enhanced bases to dissipate the heat generated during operation. These bases must have similar thermal expansion characteristics of both the package body material and the chip. These packages tend to be more expensive than traditional commercial products because of these specialized metals.
How much will it cost to package these chips and how long will it take to get them? The answer depends on the answers to all of the previous questions, the number of chips being packaged, and whether a standard, custom, or semi-custom design can be used. Standard package outlines are available with a variety of input and output configurations and cavity sizes, and can be used for most applications. The chips may not be a perfect fit in these packages, so the cost of a custom-length, 50-Ω line to minimize any excess cavity space must be figured in. Even with this additional cost, the standard package will almost always be the most cost-effective solution for annual production rates below 10,000 pieces, and will also have the shortest lead time. Semi-custom packages are the next best choice for applications in excess of 10,000 pieces, or where standard mechanical configurations don’t meet the chip needs.
Faster chips are driving the need for high-performance packages that accommodate the specialized electrical, thermal, and mechanical requirements of semiconductor materials. Although there are a number of standard products available for this specialized segment of the industry, it is more important than ever for the chip designer to work with their packaging vendor very early in the development cycle.
TIM GOING, president and chairman of the board, may be contacted at StratEdge, 4393 Viewridge Avenue, San Diego, CA 02123; 858/569-5000; E-mail: email@example.com.