There is rapid progress in manufacturing electronic devices that are smaller and thinner with more powerful features. SLP (Substrate Like PCB) and HIDI-PCB (High Density Interconnect) are “next level” circuit boards taking advantage of higher density wiring and pad connections. There is also system integration technology called SiP (System in Package) to increase semiconductor performance. The technology of interposer wiring, deep RIE (Reactive Ion Etching) and laser processing technology are all headed towards even narrower pitches. The accuracy and refinement of printed electronics using gravure offset offers a method of producing not only fine traces but solder bumps on both chips and micro-LED’s. Since an additive printing process has a gentler environmental footprint than plating or photolithography methods that are traditionally used, it is also likely to incur a lower manufacturing cost and potentially offer a faster overall process. Therefore, this report examines printing very small solder bumps with tight pitches using the gravure offset printing method. Successful printing at this level of refinement requires discipline throughout the process. Essentially gravure offset encompasses four main areas of concern; the intaglio (gravure) plate, the blanket and underlayment used for the transfer, the composition of the conductive paste or ink and the structural integrity of the overall mechanism.
Several different materials and processes were tested in the manufacturing of the intaglio plate with each have different characteristics. The actual shape and depth of the engraving also plays a role in deposition size. Different aspect ratios can be used to predict the final bump size. A PDMS blanket with an underlayment is used to transfer ink from the intaglio plate to the final substrate. This blanket should have the desired properties of absorbing solvent from the paste and provide accurate, repeatable placement of print position. Different types of underlayment were tested to determine their effect on the elastic properties of the blanket and resulting print position precision. The ink or paste used in this case was Type 9 with a center particle diameter of 3 microns suitable for micro-LED’s. Of import is the amount of solvent in the paste which must be optimized to maintain positional accuracy. At every stage of testing the mechanical accuracy of each component of the printing process had to be checked including the flatbed holding the plate, the blanket cylinder and the substrate plate, the press displacement during the process of removing ink from the plate and applying it to the final substate and finally checking the print position accuracy in both the circumferential and axial direction. As a result we can report the minimum bump size possible with existing pastes and the positioning accuracy on the final substrate.