Fabrication and Characterization of Flexible Substrates for Use in the Development of Miniaturized Wireless Sensor Network Modules

[+] Author and Article Information
Bivragh Majeed

 Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Irelandbmajeed@tyndall.ie

Kieran Delaney, John Barton, Niall McCarthy, Sean C. O’Mathuna, John Alderman

 Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland

J. Electron. Packag 128(3), 236-245 (Nov 24, 2005) (10 pages) doi:10.1115/1.2229221 History: Received February 13, 2005; Revised November 24, 2005

In this paper we describe the materials-related challenges in applying folded flex packaging technology to wireless sensor networks and propose solutions for implementing miniaturized 5mm cube platforms. The focus is to apply thin silicon stacking methods using thin flexible substrate interconnect and in particular to investigate the behavior of the selected materials. Both commercial and in-house polyimide substrates, in the thickness range 25μm down to 3μm (each with 4μm of sputtered copper) were analyzed for appropriate electrical, chemical, and mechanical properties. The characterization highlighted that in flex of thickness below 10μm, a dramatic decrease in stiffness occurs and the polyimide wrinkles due to stresses generated by the copper sputtering process. An evaluation determined that specific steps, such as polymer support ring formation, could be employed to eliminate impact of wrinkling on the process of developing the 5mm cube prototypes.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Cross section of a two-die folded stack implemented by Tessera (see Ref. 9)

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Figure 2

Schematics showing varying bonding techniques for folded chip assembly by Tessera (see Ref. 9)

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Figure 3

Graph showing the impact of flex thickness on the overall effective volume of thin packages

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Figure 4

A schematic showing the overall experimental and characterization work for this program

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Figure 5

Spin curve for PI5875

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Figure 6

This shows polyimide samples being released using a mechanical technique

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Figure 7

Two checkerboard patterns used to chemically etch the backside of the silicon carrier wafer

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Figure 8

A schematic of flex release mechanism that applies when using UV laser ablation

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Figure 9

A 16-μm thick flexible polyimide circuit successfully delaminated using laser ablation

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Figure 10

A 3.9-μm thick flexible substrate after release showing a significant level of wrinkling

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Figure 11

A wrinkle-free 3.9-μm polyimide flexible test circuit constrained using a polymer ring

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Figure 12

FTIR transmittance spectrum of polyimide samples (labeled SAM1-4) where SAM1 is a commercial 25μm sample, SAM2 is 3μm in-house sample, SAM3 is a post-HF buffer solution release sample, and SAM4 is a post-KOH treated 3μm sample

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Figure 13

A graph of stress in cured polyimide samples as the thickness is varied

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Figure 14

Tensile strength and Young’s modulus for increasing thickness of polyimide substrate

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Figure 15

This graph shows the “percentage elongation at break” as the polyimide thickness decreases

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Figure 16

This graph shows measured polyimide material stiffness as substrate thickness decreases




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