W3P.082

EPOXY-LESS PACKAGING METHODS FOR ELECTRICAL CONTACT TO PARYLENEBASED FLAT FLEXIBLE CABLES
Christian A. Gutierrez, Curtis Lee, Brian Kim and Ellis Meng
University of Southern California, Department of Biomedical Engineering, Los Angeles, CA, USAABSTRACT
We present two methods for establishing rapid epoxyless electrical connectivity to Parylene-based flat flexible
cables (FFC). The first utilizes commercially available
zero-insertion-force (ZIF) connector technology and thesecond utilizes a custom-fabricated connector with anacrylic/polydimethylsiloxane (PDMS) interface featuring
screen-printed contacts. A contact pitch of 0.5 mm wasachieved using both connector methods. Thesetechniques are simple to implement, reversible, scalable
and unlike epoxy-based approaches, do not require
manual intervention to secure individual contacts.

KEYWORDS
Parylene C, flat flexible cable (FFC), zero-insertionforce (ZIF), screen printing.

BACKGROUND
Parylene has emerged as a popular substrate formicroelectrode technology due to its biocompatibility
and mechanical strength (Young’s modulus E ~ 4 GPa).
Compared to traditional FFC materials (e.g. polyimide, E~ 7.5 GPa), Parylene is considerably softer, thusachieving robust electrical contacts to Parylene-based
devices is a challenge. The use of thin-film metallization,
as is often the case in Parylene FFCs, also makes themsusceptible to scratching/delamination and requires carewhen establishing connections. The combination of thesechallenges and a tendency to place a secondary emphasison connectivity/packaging often results in methods thatlargely employ manual application of conductive epoxies
to secure connections [1-3]. Although these methods canbe effective, they limit the ability to scale experiments,
can be time intensive, can introduce error and/or noise
and for most practical applications, are not a viable
solution. Here, we present two epoxy-less methods forestablishing simultaneous and reversible electricalconnections to Parylene-based FFCs.

Figure 1: 8-channel Parylene FFC with ZIF-style termination

evaporator and patterned by lift-off. A second insulating
layer of Parylene was then deposited (2 μm) andpatterned by reactive ion etching (RIE) in oxygen plasma
to expose contact pads and electrodes (100 W, 100 mT).
The cables were then released by deep reactive ion
etching (DRIE) of the cable perimeter using a switchedchemistry process [4].
Method 1 – ZIF Interface
A commercially available flip-lock ZIF connector(Hirose, FH12-8S-0.5SH, 0.5 mm pitch, bottom contact)
was selected and consisted of raised gold-plated
phosphor bronze contacts. Simple and rapid connections
were established with this connector type by using its
hinged actuator which closes and locks onto the cable fora secure, reversible connection (8 or 18 channel, Fig. 2).
Commercially available ZIF connectors require cablesof a minimum thickness in order to achieve a proper andsecure interface with FFCs. Significant deviations fromthe recommended thickness results in unreliable
connections and may cause damage to the cable. Thiskind of connector is typically utilized with polyimide
FFCs with thicknesses generally greater than 200 μm. In
contrast to polyimide, Parylene is deposited in a roomtemperature chemical vapor deposition process withMETHODS AND FABRICATION
Parylene FFC Fabrication
Parylene FFCs consisting of 8, 18 and 30 contacts
were designed for characterization purposes. One end ofthe FFCs includes contact pads designed to interface witheither a (1) ZIF or (2) custom connector, while the otherend comprised open electrode sites for continuity
checking and cable resistance measurements (Fig. 1). The
FFCs were fabricated by first coating a standard siliconwafer with Parylene-C (10-14 μm). Thin-film platinum(2000 Å) was then deposited in an electron-beam

978-1-4577-0156-6/11/$26.00 ©2011 IEEE

Figure 2: Profile view of flip-lock, bottom-contact, ZIF
interconnect.

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Transducers’11, Beijing, China, June 5-9, 2011

Figure 3: Parylene FFC-ZIF connectors. 8 and 18-channel
Parylene FFC cables were mated to 8 and 18-channel ZIF
connectors which were adapted back-to-back with a 32channel ZIF connector for ZIF-to-ZIF implementation. ZIF
connectors used standard surface mount contacts.

typical deposition thicknesses ranging from 1-20 μm.
The significantly thinner Parylene cables thus require an
additional backplate or stiffener around the connection
interface region as a means of providing greaterthickness.
A PEEK (polyether-ether-ketone) backing (254 μmthick) was affixed by means of cyanoacrylate adhesive tostiffen the cable. The etched border of the Parylene cable
was traced onto the PEEK using a razor blade to create
the exact dimensions for insertion into the ZIF connector.
The final thickness of the inserted composite cable was320μm.
Standard ZIF connectors of this size are typicallyutilized to interface FFC with circuit boards and as suchgenerally feature surface-mount contacts. In the case ofstand-alone Parylene-based FFC this may not bedesirable as cable lengths will tend to be limited bysubstrates on which the Parylene FFCs are fabricated.
Hence, we also demonstrate the ability to create a ZIF-toZIF adaptor by soldering two ZIF connectors back-toback. This provides the capability of interfacing andextending the generally short Parylene FFCs with astandard FFCs, (eg. polyimide), for added flexibility inimplementation (Fig. 3).
Method 2 – Custom Interface
For applications that require complete flexibility indesign, it is desirable to have a means of realizing acustom connection interface of arbitrary geometry toParylene FFCs. This is especially useful in applications
where, for example, contacts are placed around theperimeter of a Parylene film or if non-symmetric contact
spacing is required. This custom interface was realized
by compression of screen printed silver paste (~30 μmthick) patterned on a compliant PDMS sheet (~500 μmthick) onto contacts on the Parylene FFC (Fig. 4). Thesilver paste provides a conductive link between the FFC
pads and the conductive leads which make contact withthe paste via drilled holes in the acrylic plate. Unlike
conventional ZIF connectors, the silver paste/PDMS
composite provides a very compliant yet conductive
contact surface for mating against the Parylene FFC. This
reduces the risk of scratching or delamination of fragile
thin-film metal contacts.
The FFC, silver paste and PDMS were sandwiched

Figure 4: Parylene FFC (30-channel, 0.5mm pitch) placed andclamped inside customer acrylic/PDMS connector. The
photograph on the right shows a detail of the Parylene FFCand PDMS connector interface. Alignment marks and guide
posts are shown.

Figure 5: PDMS sheet with screen-printed silver-paste andParylene cable within clamped acrylic assembly. The upper
right shows the parts of the custom interface including the twoacrylic plates and PDMS connector.

between two laser-machined acrylic plates and secured inplace with screws. The screws also provide a means forcontrolling the amount of compression applied.
Alignment of the connector with the FFC wasaccomplished through the use of matching alignment
marks on the FCC and PDMS as well as guide posts
integrated into the acrylic plates (Figs. 4 and 5).
A lithographically-patterned brass sheet was etched toproduce a stencil for screen printing silver paste onto thePDMS sheet (Fig. 6). The brash sheet was secured by anacrylic frame and placed over the PDMS substrate forprinting. A precision metal-blade squeegee was thenutilized to press the silver paste through the openings inthe brass sheet, thereby transferring the pattern onto theunderlying PDMS substrate. The patterned paste wasthen cured in an oven prior to use (130 °C, 5 hours). We
have previously applied this method to the fabrication ofPDMS-based carbon thick-film strain sensors anddemonstrated a printing resolution down to 75 μm line
width using this technique [5].

2300Figure 8: Optical micrographs of (a) Parylene cable pads
prior to insertion, (b) Parylene cable pad after 1 insertion.
[note: indentation due to ZIF connector], and (c) Parylene
cable pad after 10 insertions: [note: deep indentations due toZIF connector].

Figure 6: (Top): Etched brass screen used to screen-print
silver paste onto PDMS sheet (top-right). (Bottom) lasermachined acrylic plates with alignment guide posts.

RESULTS
Method 1 – ZIF Interface
Electrical characterization of the interface wasperformed by measuring end-to-end cable resistance,
optical observation of FCC contact pads and pull-out
force measurement to assess connection robustness.
Successful connections were established across all
contacts as verified by resistance measurements(495±21.5 Ω, n=80). This verified that the composite
cable/stiffener thickness and alignment within the ZIF
connector were appropriate for establishing repeatablesecure connections. The effect of cable orientation was
also assessed by applying varying degrees of curvature to
the Parylene FFC (0°, 90°, 180°, 360°) (Fig. 7). No
significant change to the in the average measured cable
resistance was observed under any of these conditions.

The effect of multiple insertions was assessed byrepeating the insertion process (consisting of insertion ,closure, release and removal) of the FFC ten times. The
condition of the FFC contact pad was tracked optically todetect any mechanical deterioration. It was found that theParylene cable (8-channel) maintained stable andrepeatable connections over the ten cycles as evidenced
by consistent resistance measurements; however clear
depressions at contact points between the ZIF connector
and Parylene cable were observed (Fig. 8).
A failure mode was identified after 15 insertion cycles(on a single FFC pad) due to mechanical damage from araised contact point within the ZIF connector. Although
all pads exhibited some wear at 15 cycles, the added
fatigue induced by the raised contact point resulted in theearly failure. This non-uniformity was likely due toexcessive heating of the plastic housing during soldering
causing the metal contact to shift within the ZIF
assembly. Robustness of the connection wasdemonstrated in pull-out force measurements (Fig. 9).
The ZIF assembly was fixed to a hard surface with aParylene FFC inserted. A proof mass was then attachedand hung vertically from the other end of the cable. The
supported proof mass was incrementally increased until
failure was observed. Failure of the cable occurred byfracture and ripping of the Parylene material and not at
the ZIF connection interface. This was observed first for
the 8-channel cable at a 120 g load followed by the 18channel cable at 150 g load (Fig. 10). This failure mode
occurred prior to cable pull-out, demonstrating the abilityof ZIF connection interface to withstand significantloads.
Method 2 – Custom Interface
The lead resistance of a 30-channel Parylene FFC wasmeasured from end tip to connector using
micromanipulators connected to a multimeter. Thisresistance was typically 1 kΩ per lead. The average
resistance of the silver paste leads, which is in-line withthe cable resistance, was measured to be 15.75±6.66
Ω, (mean±SD, n =27) (Fig. 11).

Figure 7: Cable orientations (0°, 90°, 180°, 360°). Resistancemeasured across 4 channels. Maximum deviation of 0.7%
among the different cable orientations.

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through the etched openings in the Parylene film) also
ensure that the FFC lies securely within the connectors
and prevents pull-out of the cable. The failure mode forthis connector is expected to be ripping and/or fracture of
the Parylene cable. It is also expected that this method
will enable a high number of connection cycles without
any noticeable wear to the FCC contact pads. A morethorough lifetime and fatigue study is required to validate
this hypothesis.

CONCLUSION

Figure 9: Experimental setup for pull-out force measurements.

The design, fabrication and characterization of twomethods for establishing rapid epoxy-less electricalconnectivity to Parylene-based flat flexible cables (FFC)
was presented. Both the ZIF and custom acrylic/PDMS
interface achieved a contact pitch of 0.5 mm. ZIF
connections were robust and outlasted the Parylene FFC
in pull-out force measurements. These techniques are
simple to implement, reversible, scalable and unlike
epoxy-based approaches, do not require manualintervention to secure individual contacts.

ACKNOWLEDGEMENTS
This work was funded in part by Engineering
Research Centers Program of the NSF under Number
EEC-0310723, an NSF CAREER Award (ECS0547544), and the Bill and Melinda Gates Foundation(CG). The authors would like to thank Dr. Donghai Zhu
and the members of the USC Biomedical Microsystems
Laboratory for their assistance.

Figure 10: Maximum pull-out forces occurred prior to failureof Parylene FFC for both 8- and 18-channel connectors.

REFERENCES:
1b’-15b’
1a’-15a’
1a-15a1b-15bFigure 11: Layout of conductive silver paste leads. There are atotal of 30 leads.

Once aligned, the PDMS slab was clamped andpressed onto the FFC contact pads and a robust
connection was obtained. This procedure was repeatedmultiple times with no noticeable wear on the Pt pads.
Conductive pins were inserted through holes in theacrylic layer and were used to make contact to enlarged
silver pads on the PDMS. These pins enable direct
measurement and provide the ability to solder wires orleads for downstream interfacing with the connector.
Resistance measurements from pin to cable electrodewere typically 1.3−1.4 kΩ which is in agreement
measured values for each portion of the connector.
Alignment of the connector layers to the Parylene
cable is the most critical step for this approach and isfacilitated by the clear acrylic material and alignment
guide posts. The alignment guide posts (which pass

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CONTACTC.A. Gutierrez, tel: 213-821-3949; cagutier@usc.edu

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