Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
The Influence of Adhesive Materials on
Chip-On-Board Packing of MEMS Microphone
Cheng-Hsin Chuang *1 , Yi-Hsuan Huang 1 and Shin-Li Lee 2
1 Department of Mechanical Engineering, Southern Taiwan University
No. 1, Nantai St., Yung-Kang City, Tainan, Taiwan, ROC.
2 Micro System Technology Center, Institute Technology Research Institute Southern, Tainan, Taiwan, ROC.
*E-mail Address:chchuang@mail.stut.edu.tw, Tel:+886-6-3010081 and Fax:+886-6-2425092
Abstract- Adhesive material is commonly used for attaching die
onto the printed wiring board (PWB) in the Chip-on-Board (COB)
packaging of MEMS devices. However, the polymer-based
adhesive usually possesses large difference in the coefficient of
thermal expansion (CTE) between silicon chip and PWB.
Therefore, the mismatch of CTE could lead to the
thermally-induced stress and coupling deformation of multilayer
structure in the reflow process as surface-mount technology
(SMT) on printed circuit board (PCB). In this study, three
different adhesive materials, namely 2025D, 3140RTV and
SDA6501, and two different cap materials, namely liquid crystal
polymer (LCP) and nickel (Ni), were evaluated the influences on
the thermally-induced stress in the ploy-silicon diaphragm of
MEMS microphone based on Finite Element Analysis (FEA).
According to the results, we obtained the following two findings:
(1) The CTE mismatch of LCP cap and the metal (Ni) cap caused
different type of thermal deformations of PWB and lower
thermally-induced stress and deformation were found in the case
of LCP cap, however, different cap materials less affected the
thermal stress in the diaphragm. (2) Soft adhesive materials
(3140RTV and SDA6501) have better mechanical isolation of
PWB thermal deformation due to the buffer layer effects. On the
contrary, hard adhesive material (2025D) could be affected by
PCB thermal deformation when the thickness of adhesive was less
than 30μm, thus, a lower stress in the diaphragm existed due to
the stress compensation by PWB thermal deformation. In
general, present study provides the basis of selection of adhesive
material for COB MEMS packaging.
Keywords: Diaphragm, Adhesive, Die attach, Thermal analysis
I. INTRODUCTION
Chip-on-Board (COB) packaging technology is directly
bonding a device chip to a second level substrate with
adhesive material. Currently, COB packaging has been
adopted by semiconductor manufacturing as well as MEMS
foundry due to multiple advantages including low thermal
resistance, high cost efficiency, ideal design flexibility, etc.
However, the mismatch of coefficients of thermal expansion
(CTE) between multi-laminated materials may introduce
thermal stress and deformation when the COB device is
further mounted onto a printed circuit board (PCB) surface by
surface-mount technology (SMT). During SMT process, a
heating process of solder reflow is necessary to produce a
high quality of solder joint between COB device and PCB.
The soldering processing involves four steps such as preheat,
activation, reflow and cool down, the highest temperature
during reflow usually gets up to 260℃ and the total time is
about 360 seconds from oven entrance to the end of reflow
stage. Therefore, it’s necessary to evaluate the thermal
influence on COB devices as assembling by SMT process.
Several researchers have investigated the thermal influences
of IC packaging based on COB method [1-3]. Tom Tuhus and
Are Bjomeklett [1] indicated a soft adhesive material could
bring the stress relaxing effect but may lead to fatigue of the
adhesive layer as repeat cyclic temperature changes. Qing’an
Huang, et al.[2], proposed a 2D theoretical model of COB
packaging for evaluating the coupling deformation and stress
under thermal load. They found less thermal influence when
the silicon die attached on a ceramic substrate instead of an
organic substrate. Andrew A. O. Tay and K. Y. Goh [3]
addressed the delamination phenomenon might occur during
solder reflow in the COB packaging device. As we knows, the
packaging of MEMS devices is quite different with regards to
IC packaging due to internal moving parts and external
environmental exposure for sensing purposes. The moving
parts in an MEMS device usually are the most important and
fragile structures relevant to its performance, e.g., sensing or
actuating; therefore, the thermally-induced stresses and
distortions of the moving parts could affect overall
performance after reflow process. Recently, COB packaging
has already been used in MEMS devices and found
significant influence on sensor performance after adhesive
curing or reflow process. Zhigno Sun et al. [4] experimentally
revealed the residual stress after adhesive curing could be tens
of MPa for a piezoresistive pressure sensor packaged by COB.
Furthermore, the offsets of pressure sensor output varied with
different kinds of adhesive materials and adhesive thickness
have been investigated by several studies [5-7]. Consequently,
the selection of adhesive material and its thickness could play
an important role for reduction of the thermal influence under
thermal load. In this study, we tried to numerically evaluate
two different cap materials and three adhesive materials with
different Young’s modulus and CTE for a silicon MEMS
microphone attached on printed wiring board (PWB) as
shown in the Fig. 1. Two cap materials are nickel and liquid
crystal polymer (LCP), and three commercial adhesive
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