By Casey Krawiec
More packages are being designed for high-speed, high-frequency chips, especially with the recent licensing of spectrums above 50 GHz. These packages require a whole new set of parameters. Once you move above 2 GHz in frequency, the package stops working in the “traditional” electrical way and starts moving into the microwave realm. The basic requirements for the package remain the same. The package must protect the chip from the environment, allow for interface to the next level of assembly, dissipate the heat, and provide ease of handling. For these packages above 2 GHz, we now have a whole new set of electrical requirements thrust upon the package.
JEDEC standards are available for lower-frequency packages, but not for packages that need to operate at 5 GHz and above. There are no standards because the demand hasn’t been as great. These packages don’t run in high volumes. The more complicated electrical performance in these higher frequencies also makes it hard to develop standards and standard packages. So what are the options? How do you select a package when there aren’t any standards? Does every package have to be customized? Won’t that make the package even more expensive, and what does that mean in terms of delivery time?
Although many companies manufacture application-specific packages that work with a narrow range of frequencies, it is better to select a company with open-tooled packages that operate in a broad range of frequencies. Ideally, they operate from DC to as high of a frequency as possible. Package designers should take a particular package technology and stretch its performance as far as physics will allow. Also, work with a company with more than one basic packaging technology. Different package technologies offer a range of economic and performance choices for factors such as thermal dissipation and various levels of electrical loss. Delivery times will be faster if the company is accustomed to manufacturing and delivering high-frequency packages. It is even better if they are accustomed to doing it in high volumes.
Some package companies provide assembly services specializing in high-frequency chips. The services include eutectic or epoxy die attach, passive component attach, automatic wire bonding, package lidding, lead trim and form, and testing. It’s important to know the requirements for the assembly of high-frequency chips are different from assembly of low-frequency chips. For instance, the connection between the chip and the package can be critical. The type of wire and how it is bonded can greatly affect electrical performance.
Using a standard package is especially important for testing. A fixture that provides good, repeatable contact with the package is a must for testing high-frequency chips. Bad fixtures give false results that will drive technicians crazy. You end up troubleshooting the fixture rather than the packaged chip. Custom packages require custom test fixtures. Custom fixtures, especially fixtures for high-frequency testing, are expensive. The more standard the package, the better chance that a previously designed fixture exists. The availability of a standard fixture will save time and money.
There are standard off-the-shelf packages that work in excess of 50 GHz and these may become the defacto standard. There are differences in technologies, materials, and designs and each yields different performance. A designer needs to look at the performance of the chip and make sure the performance of the package, especially the electrical performance, and the dimensions match that of the chip. You’ll probably not be lucky enough to find a package that is a perfect fit for your chip. That’s all right because you can use a custom length 50-Ω line to minimize the gap between the chip and the package. A custom 50-Ω line is a lot cheaper than a custom package. Once your chip design is fixed and volumes increase, then it’s an economic decision as to whether or not you should customize the package to eliminate the 50-Ω line.
The key is to find an existing package that gives you the electrical performance that you need. In working with chips above 2 GHz, you want to select a package that is transparent to the performance of the chip. Find a package with low insertion loss and adequate return loss. Once you achieve good electrical performance, you can begin tackling the other issues.
Chip designers usually look for a package that works “good enough.” The question is “What is ‘good enough’?” The answer is it’s a package that will work at that high speed or high frequency. All the engineer really wants is an off-the-shelf high-frequency package.
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CASEY KRAWIEC, director of North American Sales, may be contacted at StratEdge Corp., 4393 Viewridge Ave., San Diego, CA 92123; 858/569-5000 ext. 103.