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Research Papers

An Overmolded Pressure Sensor Package Using an Ultrathick Photoresist Sacrificial Layer

[+] Author and Article Information
Lung-Tai Chen

Institute of Electro-Optical Engineering, National Sun Yat-Sen University, 804 Kaohsiung, Taiwan; Micro-System Technology Center, Industrial Technology Research Institute, 709 Tainan, Taiwan

Wood-Hi Cheng1

Institute of Electro-Optical Engineering, National Sun Yat-Sen University, 804 Kaohsiung, Taiwanwhcheng@mail.nsysu.edu.tw

1

Corresponding author.

J. Electron. Packag 131(3), 031013 (Jul 31, 2009) (8 pages) doi:10.1115/1.3144156 History: Received September 22, 2008; Revised March 27, 2009; Published July 31, 2009

This study presents a novel technique for an overmolded package of piezoresistive pressure sensors using an ultrathick photoresist sacrificial layer. A 150μm photoresist block is placed just on the silicon membrane of the pressure sensor and removed after the molding transfer process. The removal of the photoresist block exposes and reserves a sensing channel in the overmolded pressure sensor package. Experimental observations reveal that the silicon membrane of the pressure sensor is completely free of any epoxy molding compound contamination after the transfer molding process. The effectiveness of the photoresist block in shielding the silicon membrane of the pressure sensor was confirmed. Experiment and finite element model results reveal that the packaged pressure sensor has similar sensing characteristics to those of an unpackaged pressure sensor at room temperature. However, the packaged pressure sensor exerts a thermomechanical stress on the silicon membrane of the pressure sensor, resulting in an undesired output voltage drift. Employing a proper package design can reduce the output voltage drift. The proposed packaging scheme has a small package volume and surface mount device compatible features, making it suitable for portable commercial devices.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Piezoresistive pressure sensor chip: (a) upper surface showing four piezoresistors and (b) Wheatstone-bridge layout of four piezoresistors

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Figure 2

Top view of the unpackaged pressure sensor

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Figure 3

Packaging process of the overmolded pressure sensor package

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Figure 4

Photograph showing an encapsulated organic panel substrate following photoresist removal and a detailed view of the exposed sensing channel

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Figure 5

(a) Quarter FE model of the packaged pressure sensor and (b) a close-up view of detail A

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Figure 6

Comparison of the output voltage between the FE model and the experimental results for an unpackaged pressure sensor (a) under various pressure loadings at 25°C and (b) under various temperature loadings at 0 Pa

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Figure 7

Comparison of the output voltage between FE model and experiment for a packaged pressure sensor (a) over various pressure loadings at 25°C and (b) over various temperature loadings at 0 Pa

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Figure 8

(a) 3D image of the overmolded pressure sensor package with a sensing channel and (b) its cross-section image at section A-A

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Figure 9

Hysteresis curve of the packaged pressure sensor at 25°C

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Figure 10

Output voltage of the packaged pressure sensor as a function of the pressure at various temperatures

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Figure 11

Sensitivity of the packaged pressure sensor as a function of the temperature

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Figure 12

Output voltage of the packaged and unpackaged pressure sensors (a) as a function of the pressure at 25°C and (b) as a function of the temperature at 0 Pa. The dashed and solid lines correspond to a second-order polynomial fitting of the experimental results.

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Figure 13

Calculated output voltage as a function of the temperature for the packaged pressure sensors with various sensing channel openings at 0 Pa

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