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Feb 5th, 2014
 
The future of MEMS packaging
 
New adhesives push miniaturization
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MEMS sensors are gaining ground and are now used in many everyday products. Consumer electronics and automotive applications in particular use many MEMS elements. Already today, a smart phone contains a total of more than 12 MEMS chips, which is expected to rise to 20 elements soon. The requirements on the systems, which are only a few millimeters in size, are tough, and a further trend towards miniaturization has become apparent. In view of the continuously increasing process requirements, previously used adhesives, such as silicones or epoxy resins, will soon have reached their limits. The smaller the MEMS element is, the more difficult it is to bond it.

MEMS: Where are adhesives used?

MEMS packaging presents four key application areas: cap bonding, glop top, ASIC die attach and MEMS die attach. As a consequence of ongoing miniaturization especially in cap bonding, glob top and MEMS die attach, adhesives face new challenges.

When bonding the protective cap, the most MEMS applications require electrically conductive adhesives (ICAs) to ensure electromagnetic shielding. This is essential as electromagnetic radiation may interfere with the signal of MEMS device. As miniaturization is also progressing in this field, bonding is the joining method of the future and the best alternative to soldering. When packages are becoming smaller, the risk is growing that flux material covers the pad during soldering, which makes wire bonding more difficult. Special adhesives for cap bonding prevent this and provide excellent adhesion to gold, nickel and stainless steel at the same time. In addition, these products are highly electrically conductive, have high impact resistance, and enable short process times by fast curing at +130 °C within just 5 min. In addition, preliminary light fixation is possible if high positioning accuracy is necessary, for example for optical MEMS packages.

The progressing miniaturization brings new challenges also in glob top applications. As a consequence of the ever smaller and thinner packages, the glob top must be adapted as well to be very flat. Nevertheless, it should still perform its task of covering and protecting the wires. Therefore, the adhesive used must have optimized flow and processing properties. The flowability of DELO’s glob top products is easy to adjust to the specific chip size, resulting in reliable protection of the chips and wires, even if the chip dimensions are very small. In addition, they feature high flexibility, can be jetted and have short process times at low temperatures (15 min at +100 °C).

MEMS die attach presents the greatest challenges. It is especially important that the adhesive used is highly flexible to prevent tensions from arising in the micromechanical structure during packaging and upon temperature changes as tension input would change the signal characteristics. At the same time, the adhesive must have high die shear strength. Newly developed products provide much better properties than previously used silicones and epoxies. Although silicones are very flexible, in general, they only have low die shear strength, which may cause difficulties with ever smaller chips. Their processing properties and flow capability are also disadvantageous. Epoxies, in turn, have high die shear strength, but lower flexibility, and therefore equalize tensions poorly.

Figure 1: Cross-section of MEMS microphone package and magnification of the bonding tasks. (©DELO)

High flexibility and high strength at the same time
DELO has succeeded in combining high flexibility and high die shear strength. This combination of properties makes these MEMS die attach adhesives unique in the market. The one-component, heat-curing adhesives impress with best performance in MEMS packaging. They are highly flexible, give high die shear strength, and are easy to process. In addition, low curing temperatures down to just 100 °C minimize stress during the packaging process.


Figure 2: Properties of DELOMONOPOX adhesives. (©DELO)

Summary
New adhesives promote the growing trend towards miniaturization in MEMS sensors. Thus, they help integrate more exciting MEMS applications in smartphones to design ever thinner devices.

About DELO:

DELO is a leading manufacturer of industrial adhesives with its headquarters in Windach near Munich, Germany. In the past fiscal year ending on March 31, 2013, 350 employees generated sales revenues of more than EUR 50 million. The company supplies tailor-made special adhesives and complementary equipment for applications in special lines of business – from electronics to the chip card and automotive industry, as well as in glass and plastic design. DELO’s customers include Bosch, Daimler, Festo, Infineon, Knowles and Siemens. DELO has a network of worldwide distributors and sales partners.

Contact: DELO Industrial Adhesives
DELO-Allee 1, 86949 Windach, Germany
Phone +49 8193 9900-0
Fax +49 8193 9900-5212
E-mail info@delo.de
 

 
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