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Rev. Sci. Instrum. 91, 109501 (2020); https://doi.org/10.1063/5.0030318 91, 109501
© 2020 Author(s).
Breakthrough instruments and products:
Biocompatible epoxies for medical device
manufacturing
Cite as: Rev. Sci. Instrum. 91, 109501 (2020); https://doi.org/10.1063/5.0030318
Submitted: 21 September 2020 . Accepted: 24 September 2020 . Published Online: 09 October 2020
Rohit Ramnath , and Venkat Nandivada
Review of
Scientific Instruments NEW PRODUCTS scitation.org/journal/rsi
Breakthrough instruments and products:
Biocompatible epoxies for medical
device manufacturing
Cite as: Rev. Sci. Instrum. 91, 109501 (2020); doi: 10.1063/5.0030318
Submitted: 21 September 2020 •Accepted: 24 September 2020 •
Published Online: 9 October 2020
Rohit Ramnatha) and Venkat Nandivadab)
AFFILIATIONS
Master Bond, Hackensack, New Jersey 07601, USA
a)Author to whom correspondence should be addressed: rohit@masterbond.com
b)venkat@masterbond.com
ABSTRACT
This article discusses the use of biocompatible, two-part epoxies in medical devices. When used as adhesive encapsulants, these products
improve the ruggedness of wire-bonded, chip-on-board microelectronic assemblies. Biocompatible products from Master Bond include
EP42HT-2MED and the enhanced EP42HT-4AOMed Black product.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0030318
., s
I. INTRODUCTION
Microelectronics devices composed of integrated circuits are
complex and pose many engineering challenges. A careful design
must be employed to allow heat to be dissipated from the device
and mitigate thermally induced stresses. Adhesive encapsulants have
been developed to seal and protect the sensitive electrical compo-
nents and connections from contaminants and assist with thermal
management. Encapsulants provide mechanical support, distribute
stresses, and protect sensitive connections from mechanical shock.
Typically, encapsulants with ceramic fillers provide enhanced ther-
mal conductivity and improve the heat transfer and heat dissipation
properties of the design, while a reduction in the thermal coefficient
of expansion of the encapsulant mitigates stresses from thermal mis-
match. Depending on application demands, encapsulant adhesives
can be formulated for a wide range of viscosities and provide var-
ious thermal, mechanical, and environmental resistance properties.
Products may be engineered for application specific approvals such
as ISO 10993-5 and USP Class VI for use in medical devices, NASA
low outgassing requirements, and cryogenic serviceability.
II. USES AND THERMAL PROPERTIES OF
ENCAPSULANTS FOR CHIP-ON-BOARD ASSEMBLIES
In microelectronics, methods are used to form the electrical
connection between an integrated circuit chip, also called a die, and
the substrate. The methods of wire-bonding and flip-chip are pre-
dominantly used today.1An adhesive encapsulant is commonly used
in these assemblies to protect the integrity of the electrical con-
nections. The potential for thermal mismatch can be assessed by
evaluating the coefficient of thermal expansion (CTE) of the mate-
rials used in the assembly. CTE may be conveniently measured as
ppm/○C. Of relevance are the CTE of silicon used in the die (2.6
ppm/○C–3.0 ppm/○C), solder (21.5 ppm/○C–24.6 ppm/○C), and the
substrate itself. A widely used organic substrate such as FR-4 exhibits
a CTE of 14 ppm/○C–17 ppm/○C. As different materials thermally
expand at different rates, stresses can accumulate. Even small dif-
ferences in CTE can result in premature device failure during tem-
perature excursions. Encapsulants and underfill provide a means
to mitigate thermal mismatch by distributing stresses across the
coupled area and thereby minimizing the magnitude of stresses at
critical solder joints. The CTE of the encapsulant itself can be opti-
mized depending on the design requirements. Encapsulants must
be electrically non-conductive and provide strong adhesion to the
substrates. For demanding applications, a low CTE and moderate
thermal conductivity aid in thermal management.
The function of the adhesive encapsulant can be visualized in
Fig. 1 for a wire bonded assembly. The die is physically bonded to the
substrate often with a suitable die adhesive; the connections between
the die contact pads and the underlying board circuitry are then
made through the wire bonding process. This particular case sees
Rev. Sci. Instrum. 91, 109501 (2020); doi: 10.1063/5.0030318 91, 109501-1
Published under license by AIP Publishing
Review of
Scientific Instruments NEW PRODUCTS scitation.org/journal/rsi
FIG. 1. Wire bonded assembly with glob top epoxy encapsulant, dam, and
fill.2No part of these images may be reproduced or transmitted in whole
or in part without the express written permission of Master Bond, Inc.,
https://www.masterbond.com/industries/glob-top-epoxies, accessed August 2020.
Copyright 2020 Master Bond, Inc.
the use of a dam to contain the uncured encapsulant adhesive prior
to final cure. From this figure, it can be seen that the encapsulant
effectively seals the chip and protects the fragile wire bonded con-
nections between the chip and the board substrate. The encapsulant
utilizes physical mass to seal and mechanically secure the assembly
while distributing any mechanical stresses resulting from thermal
mismatch of the components.
Flip-chip assemblies offer many advantages over wire-bonding
including higher interconnect density and speed improvements
owing to the shorter interconnect lengths. Other benefits include
smaller size, greater ruggedness, and lower cost when manufactured
at high volume. An example of a flip-chip assembly is shown in
Fig. 2—in this fabrication method, the chip is flipped or inverted
with its solder balls facing the contacts of the substrate; thereafter,
the solder is remelted to form the electrical connection. The gap
present beneath the chip is then underfilled with adhesive encapsu-
lant often through capillary action. For these applications, a viscosity
less than that used in encapsulation of wire-bonded assemblies is
necessary to enable a fast flow-rate and prevent entrapment of air.
The stiffness or modulus of the underfill must be optimized; too low
a modulus may result in excess strain at the solder joint, while too
high a modulus may overly redistribute stresses to the chip and result
in cracking.3
As with many organic materials, an unfilled epoxy has a high
CTE of 66 ppm/○C–72 ppm/○C, but it can be reduced by com-
pounding with fillers, such as aluminum oxide (8.1 ppm/○C) or silica
(0.5 ppm/○C). Addition of either filler increases modulus with silica,
providing a more potent affect. Due to its higher thermal conduc-
tivity, 35 W/m K for Al2O3vs 0.363 W/m K of epoxy, Al2O3filler
FIG. 2. Flip chip assembly with underfill. No part of these images may be repro-
duced or transmitted in whole or in part without the express written permission
of Master Bond, Inc., “Enhancing reliability of flip-chip assemblies with underfill
encapsulants,” White paper. Copyright 2020 Master Bond, Inc.
increases the thermal conductivity of the composite while main-
taining a suitable degree of electrical non-conductance.4Aluminum
oxide is then preferred for applications requiring greater thermal
conductivity, while silica is preferred if higher modulus is required at
reduced filler loading. For epoxy filled with Al2O3, above a thresh-
old volume fraction, thermally conductive paths form between the
filler particles, leading to an abrupt increase in thermal conductiv-
ity. This is similar to the percolation threshold that is observed in
electrically conductive adhesive systems loaded with electrically con-
ductive particles. The particle size of the filler also impacts the load-
ing response of the composite; at a fixed volume fraction, smaller
particles of silica result in a greater decrease in CTE for an epoxy-
silica composite.5The mechanism of this can be attributed to the
greater interfacial surface area that comes with a decreased particle
size at constant loading. As the particle size decreases, the inter-
facial area between the high CTE epoxy matrix and the low CTE
filler increases, resulting in a greater restriction of the epoxy and
less thermally induced expansion of the bulk composite.6Overall,
optimization with respect to particle size and desired viscosity is
necessary as viscosity generally increases with decreased particle size
due to similar reasoning with regard to interfacial area and friction
forces in the uncured resin. The aspect ratio or shape factor of the
filler particles influences the degree of CTE isotropy; spherical par-
ticles generally result in an isotropic thermal expansion, whereas
fiber or disc-like particles, especially when aligned in the matrix,
result in anisotropic expansion properties.7For flip-chip underfill,
the maximum particle size is restricted by the gap size between the
chip and substrate, and the maximum particle size must be selected,
often below 1/3 of the gap size, to prevent impeded flow during the
underfill process.1
III. MASTER BOND EP42HT-4AOMED BLACK
Master Bond EP42HT-4AOMED Black is a two-component
ceramic-filled epoxy adhesive, sealant, coating, and casting system.
The objective for this product was to improve upon a previous
legacy system, EP42HT-2Med, by including a carefully selected filler
package at appropriate loading to reduce the CTE while maintain-
ing ISO 10993-5 and USP Class VI compliance for use in medi-
cal device applications. This product has exceptional thermal and
chemical resistance properties standing up to repeated autoclav-
ing, radiation, and chemical sterilants. Figure 3 demonstrates an
almost fourfold improvement in weight loss when subjected to 100
autoclave cycles when compared with a standard, two-component
ambient cure epoxy. Filled with aluminum oxide, this product has
increased thermal conductivity (>1 W/m K) and lower CTE (18
ppm/○C–21 ppm/○C) than an unfilled epoxy while maintaining elec-
trically non-conductive properties (volume resistivity: >1014 Ω cm).
The addition of filler provides enhanced dimensional stability, very
low cure-induced shrinkage, and high modulus. The wide service
temperature range of 4 K to +400 ○F (4 K to +204 ○C) is an addi-
tional benefit of the product. When bonding surfaces are appro-
priately prepared, EP42HT-4AOMED Black bonds well to a wide
variety of substrates, including metals, composites, glass, ceramics,
rubber, and plastics. For ease of use, the product employs a forgiving
100:40 by weight mix ratio, and it readily cures at ambient temper-
ature or more rapidly at elevated temperatures. The optimum cure
Rev. Sci. Instrum. 91, 109501 (2020); doi: 10.1063/5.0030318 91, 109501-2
Published under license by AIP Publishing
Review of
Scientific Instruments NEW PRODUCTS scitation.org/journal/rsi
FIG. 3. Resistance to repeated autoclave cycles measured by percent weight loss for EP42HT-4OMed Black vs standard two-component, ambient cure epoxy.
schedule is overnight at ambient temperature (75 ○F/24 ○C) followed
by 2 h–3 h at 150 ○F–200 ○F (66 ○C–93 ○C). The working life or open-
time after mixing is 75 min–120 min allowing for flexibility on the
production line.
IV. APPLICATION EXAMPLE: IMPLANTABLE NEURAL
PROBE WITH BOND-WIRES SEALED VIA
BIOCOMPATIBLE EPOXY ENCAPSULANT
In this application, Raducanu et al.8constructed an implantable,
active neural probe capable of performing in vivo neuroscientific
studies on laboratory animals. The etch process used to form the
electrodes on the chip’s implantable shank was optimized to provide
a high degree of smoothness to avoid damage to the animal’s neural
tissue. The high uniformity and electrode density of the recording
sites coupled with the small electrode size allowed the researchers to
capture the signal from individual neurons with a high spatial reso-
lution. The chip’s probe shank, 8 mm in length, 100 μm in width, and
50 μm in thickness contained a 4 ×336 array of 20 ×20 μm2elec-
trodes with 12 larger reference electrodes measuring 20 ×80 μm2to
provide improved recording quality.
Implantable devices such as these face limitations with regard
to power dissipation—heat dissipated from the device into the ani-
mal’s neural tissue should be kept at or below 1 ○C for long-term
studies. To mitigate these constraints, auxillary circuitry was moved
off-chip to a small, lightweight printed circuit board (PCB) head-
stage unit weighing 1.25 g. The headstage unit was then connected
via a flexible and lightweight cable to a back-end field programmable
gate array enabling data capture while allowing the animal to eas-
ily move about during experiments. Connections from the chip
probe were wire-bonded to a small, thin PCB that was then con-
nected to the headstage. The bond wires were finally protected and
sealed with Master Bond EP42HT-2MED, a biocompatible epoxy
used as an encapsulant for wire-bonded chip-on-board applica-
tions requiring ISO 10993-5 or USP Class VI compliance. Mas-
ter Bond EP42HT-2MED is a legacy version of EP42HT-4AOMED
Black; the latter product having been formulated with aluminum
oxide to further improve the CTE and thermal conductivity while
maintaining biocompatibility and not compromising on sterilization
resistance.
V. SUMMARY
Master Bond EP42HT-4AOMED Black and the legacy product
EP42HT-2MED are biocompatible two-part epoxy adhesive systems
for use as encapsulants in wire-bonded, chip-on-board microelec-
tronic assemblies. Master Bond products are formulated to provide
the desired thermal, electrical, and mechanical properties needed to
protect sensitive wire-bonded chip-on-board assemblies from mois-
ture, dust, and other contaminants while providing mechanical and
thermal support to assure reliable, long-term performance. In addi-
tion to these products, Master Bond offers a full line of products
for the electronics industry, including those with properties and vis-
cosities optimized for capillary underfill, conformal coatings, and die
attachment.
REFERENCES
1Enhancing reliability of flip-chip assemblies with underfill encapsulants, Master
Bond, White paper.
2See https://www.masterbond.com/industries/glob-top-epoxies for glob top
epoxies, Master Bond; accessed August 2020.
3C. A. LeGall, J. Qu, and D. McDowell, “Some mechanics issues related to the ther-
momechanical reliability of flip chip DCA with underfill encapsulation,” Trans.
ASME EEP-222, 85–95 (1997).
4A. Agrawal and A. Satapathy, “Experimental investigation of micro-sized alu-
minum oxide reinforced epoxy composites for microelectronic applications,”
Procedia Mater. Sci. 5, 517–526 (2014).
5J. Qu and C. P. Wong, “Effective elastic modulus of underfill material for flip-chip
applications,” IEEE Trans. Compon. Packag. Technol. 25(1), 53–55 (2002).
6C. P. Wong, M. Vincent, and S. Shi, “Fast flow and novel no flow underfills: Flow
rate and CTE,” ASME—Adv. Electron. Packag. 19(1), 301–306 (1997).
7K. Y. Lee, K. H. Kim et al., “Thermal expansion behavior of composites based on
axisymmetric ellipsoidal particles,” Polymer 48, 4174–4183 (2007).
8B. C. Raducanu, R. F. Yazicioglu, C. M. Lopez, M. Ballini, J. Putzeys, S. Wang,
A. Andrei, V. Rochus, M. Welkenhuysen, N. van Helleputte, S. Musa, R. Puers,
F. Kloosterman, C. van Hoof, R. Fiáth, I. Ulbert, and S. Mitra, “Time multi-
plexed active neural probe with 1356 parallel recording sites,” Sensors 17(10), 2388
(2017).
Rev. Sci. Instrum. 91, 109501 (2020); doi: 10.1063/5.0030318 91, 109501-3
Published under license by AIP Publishing
