{'fraction_non_alphanumeric': 0.015807962529274005, 'fraction_numerical': 0.00234192037470726, 'mean_word_length': 5.3277777777777775, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
MEMS Assembly and Wafer Level Packaging – Adapting New Applications to Stencil Printing Technology

Optoelectronic interconnection has depended upon polymer adhesives since its inception. Epoxy materials are commonly employed for the attachment of lenses on the ends of fiber optic cables, encapsulation or protective coatings, and for die attach of laser diodes to various heat dissipating substrates, to name only a few. MEMS devices and sensor components also represent an emerging new market that will depend heavily on epoxies for assembly in years to come. In many instances, the importance for cleanliness and low temperature processing precluded consideration of solder attachment. Epoxy materials are typically applied within individual packages using automatic dispensers, but the intense commercial need to reduce cost is pushing the industry to develop wafer level packaging alternatives. Wafer level packaging represents the maximum achievable economy-of-scale, depending of course on die size verses wafer size and die yield. Though dispensing technology has achieved significant innovation and improvement over the past decade, the dispensing rates are not expected to be fast enough for processing whole wafers. The issue is similar to the problem facing gold-wire stud bump bonding. Wire bond technology has improved dramatically over the years and bond rates approaching 20 connections per second may soon be achievable. Even at these rates, a single LED or RFID wafer with 400K bond pads could consume an entire production shift of machine-time. A more practical process involves depositing epoxy bumps on the surface of whole wafers using a stencil-printing technique. Screen and stencil printing technology has been advancing over the years also. Both stencil and screen manufacturing materials and equipment have matured along with the electrically conductive and non-conductive (dielectric) epoxy materials specifically tailored for this type of processing. Assembly techniques are constantly being driven and adapted for the varied applications that appear limited only by the imagination of MEMS and sensor designers. For example, one client needed to assemble a MEMS mirror array on a silicon substrate with an integral mechanical stop on the surface of the substrate. The solution involved a doubleprinting process, wherein a pattern of 5-micron high dielectric epoxy bumps were first stencil printed and cured, followed by a separate print depositing an outer pattern of electrically conductive epoxy bumps for attaching the mirror array to the silicon surface (Figure 1). Achieving the required 5-micron bump height was a matter of choosing the right combination of stencil aperture and printing parameters for the particular viscosity of the dielectric epoxy used. Though never previously attempted, the subsequent print of the conductive epoxy bumps on the same surface proved fairly simple in practice. The conductive epoxy in this case was a b-staged, solvent-based, thermo-plastic material that needed to withstand the water sprayed on the surface during the dicing operation. A variety of b-staging cure cycles were tested to find the optimal trade-off between bump cohesiveness, capable of withstanding the wafer dicing operation, yet still containing sufficient solvent to be compliant enough to enable the devices to be subsequently thermocompression bonded to the mirror array.