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Research Papers

Advanced Methodologies for Developing Improved Potted Smart Munitions for High-G Applications

[+] Author and Article Information
Nien-Hua Chao

U.S. Army, ARDEC,
Picatinny Arsenal, NJ 07806

John A. Dispenza

Design Results, LLC,
Long Valley, NJ 07853

Mario DeAngelis

U.S. Army, ARDEC,
Picatinny Arsenal, NJ 07806

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received August 20, 2012; final manuscript received April 12, 2013; published online June 4, 2013. Assoc. Editor: Stephen McKeown.This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Electron. Packag 135(3), 031002 (Jun 04, 2013) (14 pages) Paper No: EP-12-1077; doi: 10.1115/1.4024301 History: Received August 20, 2012; Revised April 12, 2013

Potted electronics are becoming more common in precision-guided smart munitions designs due to the requirements for miniaturization and structural-robustness. In most of these applications, the potted electronics are inactive for most of their lifetime and may be stored without environmental (temperature and humidity) controls for up to 20 yr. The uncontrolled environment for smart munitions however makes the thermal management task especially difficult due to the coefficient of thermal expansion (CTE) mismatch that can exist between the potting material and the electronic components. In this paper, we will do the following: (1) present a methodology being developed for reducing the thermal stresses to the potted electronics used in uncontrolled environments by encapsulating the circuit board assembly (CBA) with a thin polymer layer which has been precisely formed to conform to the imprecisely shaped, as-populated, CBA. The protective polymer layer will be both flexible and soft enough to protect the CBA components from damage caused by thermal expansion mismatches, but not degrade the structural support that the potting provides during high-g force projectile launches, (2) discuss how the protective polymer layer methodology can also be used to lessen in-circuit board crosstalk, improve shielding from external RF interference, control tin-whisker growth, and enhance moisture barrier properties and thermal management for CBAs, and (3) demonstrate how to improve the smart munitions survivability under extreme high-g applications through the use of syntactic foams and material characterization before and after accelerated temperature-cycling and thermal-aging tests.

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Figures

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Fig. 1

(a) Potted munition component and separate housing and CBA before potting and (b) an exploded view of a component

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Fig. 2

A process of forming polymer layer(s) to CBA with vacuum using CBA duplicate

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Fig. 3

A process of layering polymer layer(s) to CBA by using conformal mold

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Fig. 4

A CBA production sample fully populated

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Fig. 5

A protective polymer layer tightly fit to CBA

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Fig. 6

Soft, synthetic rubber, conformal cavity mold

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Fig. 7

FE model of a MEMS attached to PCB, frame, can, cover, and potting materials [1]

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Fig. 8

MEMS device with and without protective polymer (elastomer) layer

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Fig. 9

HALT temperature profile 150 °F to −50 °F Δ = 9 °F/min

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Fig. 10

Seal maximum principal stress comparison (without and with protective layer). (a) MEMS seal without protective layer (stress range: −12420 psi to 7160 psi) and (b) MEMS seal with protective layer (stress range: −80 psi to 5060 psi).

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Fig. 11

Seal Tresca stresses comparsion (without and with protective layer)

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Fig. 12

Temperature cycle profile (°F versus second)

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Fig. 13

Experiment thermal setup and HALT test recorded data

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Fig. 14

Alchemix (unfilled versus filled) tensile stress versus strain (nonthermal conditioning)

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Fig. 15

Alchemix (unfilled versus filled) tensile stress versus strain (after HALT test)

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Fig. 16

Alchemix (unfilled versus filled) tensile stress versus strain (after HAST test)

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Fig. 17

The projectile launch and penetration profiles. (a) Projectile launch profile and (b) projectile penetration profile.

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Fig. 18

Sensitivity analysis of potting material tensile modulus versus density

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