Monday, June 25th, 2018

Poster Number: 93

Spinal Cord Stimulation

INTERCONNECT STRATEGIES FOR JOINING INTEGRATED CIRCUITSTO HIGH-DENSITY POLYMER NEURAL INTERFACESJames J. Yoo1 and Ellis Meng1
1Department of Biomedical Engineering, University of Southern California, Los Angeles, CA

Background: High density silicon neural probes for large scale recording achieve compact formfactors using multiplexing and low-level processing via application-specific integrated circuitry(ASIC), employing both passive and active approaches. However, the chronic immune responseto rigid probe implants poses a challenge for reliable long-term recording and motivates the useof compliant polymer substrates. This necessitates a new strategy for joining rigid ASIC chips.
Taking into account the lower thermal budget and yield strength of polymers, four interconnect
strategies to join Parylene probe arrays and ASICs were investigated: conductive epoxy, wedge
wire bonding, anisotropic conductive film (ACF), and polymer ultrasonic bonding on bumped chips(PUB). The first three were adapted for use with Parylene, and the latter is a new method
specifically developed for this purpose.
Materials and Methods: Test structures representing the interconnection region to the neuralinterface were fabricated from a Parylene C-Ti/Au-Parylene C sandwich, and in place of the ASIC,
surrogate chips were made of glass with Ti/Au traces. Both Parylene and glass test structurescontained 28 matching interconnect regions of varying bond pad size and pitch, with the smallest
matching that of commercially available bare-die Intan chips. Alignment was performed with acustom-built polyether ether ketone (PEEK) jig and off-the-shelf rotation and translation stages.
Conductive epoxy (Epo-Tek H20E) was manually applied using a polyurethane block. ACF(Dexerials CP13341-18AA) was applied manually and cured in an oven, using a custom jig toapply pressure via screws. Wedge wire and PUB bonding was performed using a Hybond Model
572A. Bonds were assessed by measuring electrical continuity and resistance. Yield wascalculated as the fraction of successful bonds.
Results: Successful bonds were achieved with all techniques, but as the spacing between traces
and bond area decreased, bonding yield decreased. For the smallest bonding area (100 µm pitchby 70 µm width), bonding via conductive epoxy produced an average yield of 66.7%; wedge wire
66.7%, ACF 87.5%, and PUB 87.5%. Failures with conductive epoxy were associated withinsufficient clearance beyond the bonding area, since similar experiments on wider chips
produced higher yield. Wedge bonding (25 µm wire diameter) becomes untenable for the tightest
features because the resulting wedge extends into adjacent bonds with the tool used. ACF failures
may be related to uneven pressure application; in one device, cracking was observed that
disconnected bond pads. Failure modes associated with PUB bonding are under investigation;
one possibility is a relationship between insulation thickness and ultrasonic transmission, asfailures appear to be thickness dependent.
Conclusions: We achieved successful bonding between thin film Parylene and glass test
structures towards producing a strategy to incorporate ASICs into polymer-based high density
recording arrays. Initial results indicate that ACF and PUB bonding result in the highest yield at
the finest pitch, making them likely candidates for high-density polymer ASIC integration.
Funding: This work was supported by the National Institutes of Health under the BRAIN Initiative
Grant No. U01 NS099703 and the University of Southern California Provost Fellowship.