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MUNICH, Germany, Nov. 21, 2002 — Carsten Bahle of Wicht Technologie Consulting doesn’t mince words when it comes to the need to rethink MEMS packaging.
“The biggest stumbling block to commercial success is the lack of general, simple and effective packaging techniques,” he said in remarks made during a special forum on MEMS packaging held last week during the Electronica trade fair in Munich.
The reason, everyone agrees, is simple: cost. Packaging of a MEMS device generally takes up 50-90 percent of its cost, with 80 percent the norm. Although these microsystems are getting smaller all the time, that doesn’t mean packaging costs are shrinking as well. In fact, maintaining the high level of precision necessary at ever smaller scales tends to push prices up.
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Despite that, there can be no scrimping on packaging when it comes to MEMS. Together with the functional unit (sensor, micromechanical component or integrated circuit), the surrounding package is the most important element of the device and plays several roles. It has to protect the sensitive functional unit from environmental factors that could affect its performance, like moisture, high temperature, vibration or corrosion. It also has to provide the component’s connection to the outside world through electrical, optical and other types of interfaces. Finally, it cannot hinder function in any way.
Yet if packaging is so important, it seems strange that it has it been treated like a neglected stepchild for so long.
“There are historical reasons for that,” said Erik Jung of the Fraunhofer Institute for Reliability and Microintegration (IZM), and one of Germany’s leading experts on packaging issues. “MEMS evolved from the microelectronic industry and took over an infrastructure that already existed. That is, design the device first, packaging is an afterthought.”
But that has been the wrong approach, he and others say.
“Simply put, design has to be tackled early on in the process,” said Katrin Persson of IMEGO, a microsensor systems company based in Gothenburg, Sweden. “Some 80 percent of breakdowns are due to packaging problems. We need to dedicate more attention to the outside of a component.”
Integral functions are part of the solution, according to Fraunhofer’s Jung. Packaging, he said, should be designed to do more than just protect. It should also add to the final component’s functionality.
“Packaging should be a value-added process,” he said.
For example, a microfluidic sensor package would add this value if it contained a tiny pipeline to bring the media to be measured to the device. Other concepts have the package forming part of the sensing structure itself, becoming part of the device’s own complex system instead of just a dead casing around it.
“We’re seeing people start to think in this direction,” Jung said. “Not as much as I’d like them to, but it’s a start.”
He said that over the past few years engineers on both the MEMS and the packaging sides of the equation have begun talking to each other earlier in the design process.
“But the minute commercialization begins,” he laughed, “the talking stops.”
Another area in which Jung wishes a lot more talking would be going on is in standardization, which he said would help solve some of today’s packaging and manufacturing problems. But that is not easy, given that MEMS environmental parameters are very diverse. Some devices must exclude light while others must allow it onto the die surface. Some packages exist in a vacuum, while others must pipe gases or liquids around a chip.
But it is highly desirable that the industry define a standard package for each application category. Come up with a reasonable standard regarding inputs and outputs, for example, and you could have one MEMS package that is appropriate for several different devices.
One group in Germany that Jung thinks could serve as a standardization model for the MEMS community is the AMA German Trade Association for Sensor Technology. The 400-member group association is developing sensor element modules with interfaces that are compatible with various types of components, such as transformers and microcomputers.
“It will save on resources. When you get, say, a new pressure sensor in the future, it will mean you don’t have to replace your entire sensor mechanism, just that module,” said Dirk Rein, a member of the AMA management.
Experts predict that packaging issues could be the make-or-break issue when it comes to RF (radio frequency) MEMS switches, which are used in wireless applications. Current RF MEMS packaging increases a device’s size by a factor of 10. The final cost of the component breaks down to 5 percent device and 95 percent packaging. If RF MEMS are going to live up to their potential, the packaging price has to be brought down without sacrificing reliability, according to Robert Aigner, director of MEMS R&D at Infineon Technologies AG.
“Packaging is the turning point where RF MEMS will win or lose the battle against conventional components,” he said.