Non-traditional Applications of Jet Dispensing
by George A. Riley, Contributing Editor
While jetting of fluids has become common in semiconductor packaging, it is finding new applications in emerging fields. At the recent SMTA Pan Pacific Symposium, Alec Barbiarz of Asymtek described jetting opportunities in medical analytics, high-intensity lighting, active-matrix displays, green energy, and 3D assemblies. In most of these applications, the jetted materials are not just part of the packaging, they are an essential component of the device.
For example, the growing life sciences field of on-chip blood analysis requires depositing chemicals into wells in each die of a semiconductor wafer. Both the amount dispensed and the location must be carefully controlled, while the large number of die per wafer calls for high-speed dispensing. That combination — carefully controlled, high speed — defines the territory of today’s jetting.
The blue LED has lighted the way to high-brightness white illumination, but to produce white light, the blue must first be covered by a yellow filter. The color temperature of the resulting white light is critically dependent upon the composition, thickness, and the yellow phosphor layer’s uniformity of distribution. Slurry-dispensing leaves a thicker coating of phosphor granules at the LED’s center, thinning towards the edges as the granules settle. Jetting gives a uniform, repeatable layer.
A future replacement for our ubiquitous liquid-crystal flat panel displays may be active-matrix organic light-emitting diodes (AMOLED). A fluid sealant must be dispensed around each display element before glass lamination, so the largest current AMEOLED display requires nearly 900 individually-dispensed seals.
The higher speed of jet dispensing substantially lowers seal dispensing costs by replacing multi-head needle dispensers with a single jet dispenser. An even greater time and cost saving results from jetting filler material inside the seals. The piezoelectric jet dispenses multiple small drops uniformly spaced inside the seal boundary before lamination, saving hours compared with needle-dispensed fluid penetrating along an edge.
Green energy applications of fluid dispensing include both photovoltaics and fuel cells. Photovoltaics require fluid dispensing from beginning to end. The wafers must be uniformly sprayed with a dopant before entering the drive-in furnace. The backside coating of the wafers is also printed or sprayed.
Electrically conductive “buss bars” may be jet dispensed across the wafer to join the cell conductor traces and form a panel. This application is similar to the replacement of bond wires with jetted conductive adhesive edge connections on 3D parallel or offset chip stacks, as shown in my October, 2008 Advanced Packaging article “How 3D is Stacking Up.”
Direct methanol fuel cells (DMFC) are a challenging emerging application for jetting, since they must handle both volatile and corrosive fluids. Holes etched in a silicon wafer permit water and methanol (the fuel) to reach an anode-side catalyst layer where they are reduced, allowing protons to pass through a permeable proton exchange membrane to the cathode side. A counter-current of electrons flows externally from anode to cathode, completing the electrical circuit.
The proton exchange membrane is a DuPont perfluorosulfonic acid polymer, which can be jet-dispensed with a volatile solvent. The catalyst layer may be carbon black, dispensed as an ink. Complications include that the carbon black can settle and clog a jet, the ink may not properly wet the surface, and the carbon forms a brittle layer when it dries. The polymer solvent may evaporate, sealing the jet nozzle, and the wetted portions of the jet must resist attack by sulfonic acid.
The automated dispenser includes a mass flow calibration function. When operated in a dam-and-fill mode it has produced uniform dry film thicknesses of 40µm +/- 5.
In summary, the enhanced capabilities of fluid jetting are opening up new, non-traditional electronic and semiconductor assembly applications.