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PCB-Aug2017

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46 The PCB Magazine • August 2017 a high density of features, such as in advanced flip chip packages, require substrates with low CTE, high dimensional stability, high thermal conductivity and suitable dielectric constant. Glass offers a number of advantages in this re- gard [1] , including that it is very stable in terms of electrical properties, moisture absorption, and aging, and has a CTE similar to that of silicon, making it ideal for IC packaging. Furthermore, the dielectric constant of glass is, in some in- stances, lower than that of FR-4. This, coupled with a low loss tangent, and low materials cost compared to high-performance materials, make glass suitable for high-frequency applications. Many different approaches have been taken toward the realization of conductive plating of glass substrates, including: chemical vapor de- position, evaporation and sputtering [2,3] ; chem- ical, mechanical, and laser roughening to im- prove electro- and electroless plating, laser di- rect-write techniques (vide infra), including sintering of metallic powders [4] ; and using self- assembled monolayers to better adsorb catalysts for electroless plating [1] . Difficulties with glass metallization arise from chemical and mechan- ical incompatibilities between brittle, stiff glass and the metal, such as CTE incompatibility and strong interfacial stresses. Smooth glass surfaces present no possibility of mechanical interlock- ing, so metal films can easily separate from the substrate. We report here a novel method for metal- lization of glass dielectrics involving laser-in- duced forward transfer (LIFT) of metallic foils to seed electroless plating, thereby forming strong- ly anchored conductive patterns. The specific techniques examined here allow for plating of conductive traces and vias, multilayer all-glass structures, and multilayer mixed-material struc- tures. In LIFT, the desired material for deposi- tion is adhered to a transparent carrier; this sub- strate is referred to as the "donor." A laser is fo- cused through the transparent carrier layer of the donor onto the material, resulting in trans- fer of the material to a "receiving" substrate. The LIFT technique applied to conductive metals was first described in 1986 by Bohandy et al., for the forward transfer of copper onto silica substrates using an ArF excimer laser [5] and has since been applied to deposit a variety of materials onto many different substrates [6,7] , including organic and biological materials [8] . Printing of conductive inks and nanopastes has been a focus of recent research in LIFT applica- tions [9] . Techniques that utilize conductive inks offer the promise of a high degree of shape and size control for the deposited material (for ex- ample, using spatial light modulators), but the inks themselves have conductivities several or- ders of magnitude less than their bulk counter- parts, some of which can be mitigated through in situ laser curing of the deposited ink [10] . LIFT has also been used for preparing em- bedded components, by direct-writing conduc- tive inks to make connections between already embedded components [11] , or by using LIFT to place the components themselves [12,13] . Copper beams can be laser cut, bent, and deposited us- ing LIFT, but require conductive glues for adhe- sion [14] . Most similar to the technique described in this report is an approach that uses LIFT to deposit palladium droplets [15] , in which excimer lasers were used to decompose a palladium ac- etate film on a transparent substrate to palla- dium particles and deposit them on quartz, ce- ramic, and polymer substrates. The palladium droplets can act as catalysts for the plating of copper, nickel, and gold. This approach is limit- ed by the low abundance, and correspondingly high cost, of palladium (around $650 per ounce at the time of writing). Plating on Glass The seeding methods described below can be carried out on flat, smooth glass substrates, but the metals plated on these surfaces lack me- chanical stability and dimensional control. As a result, a typical approach requires laser abla- tion of the glass substrate to produce unplated features (pads, wires and vias), followed by ap- plication of the following seeding method. La- ser glass ablation was carried out on a compa- ny system, employing a laser operating at 515 nm with pulse duration of 800 fs. A typical glass substrate used in this work is a microscope cov- er slide (either soda lime or borosilicate glass), cleaned by rinsing with methanol and wiped dry using a lens wipe, and handled only with gloved hands. The 150 μm glass slides used in this work for LASER PATTERNING AND METALLIZATION TO REDUCE PROCESS STEPS FOR PCB MANUFACTURING

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