Oft-Forgotten Stencil Solution ‘Lights’ the Way

Multiple advanced printing capabilities converge for next-gen LED technology.

WHAT’S BEHIND THAT TV panel? Light-emitting diodes – or LEDs. LEDs are found in just about every display, from flatscreen televisions to high-end desktops and all kinds of products in between. The most common type of display – the liquid crystal display, or LCD – relies on light from LEDs for illumination, as an LCD pixel cannot illuminate itself, unlike an organic LED (OLED). Behind the LCD screen, areas are divided into zones of LEDs that are switched on to backlight the colors. And, in areas where the screen is to remain black, no light is emitted through those zones. Older designs or lower-end LCDs, however, may suffer from zone leaching. This is when the light in a specific zone may be switched off, but light from an adjacent zone crosses into the black, unlit zone. This results in a haloing effect.

For high-end desktops, ultra-high resolution flatscreens, and other display electronics, black must be black to deliver tight contrast. And, as with all things electronic, the answer seems to come from miniaturization. To overcome the definition dilemma, a newer technology, Mini Backlight LEDs – or Mini LEDs – provide a solution. Ranging in size from 50μm to 150μm, these small devices provide the tight dimensions needed for exceptional contrast and fine lines. Instead of the coarseness of the previously described traditional LED zones, Mini LEDs can be individually turned on or off to provide intense granularity and sharpness.

“How does this relate to SMT?” you ask. Currently, classic SMT processing is the most efficient and effective way to manufacture Mini LED panels, but it requires reviving some historical stencil technology and combining it with current advanced printing platform capabilities. Although the panel tends to be homogenous – unlike heterogeneous SMT – the dimensions present some obstacles. The tiniest of these LED apertures measures 50μm with a 50μm interspace. Coupled with ultra-high-density assemblies, a standard stainless steel, laser-cut stencil will not get the job done. The repeatable aperture size limit for most laser-cut stencils is about 100μm. The thinnest piece of steel foil available today is around 50μm. Enter electroform, or Eform, stencils. Once thought the answer for miniaturized SMT, the higher cost and manufacturing time (compared to laser-cut stainless steel) for Eform stencils pushed them to the exotic application/semiconductor side of the ledger. But this stencil technology is proving to be a solution for Mini LED processing.

For those unfamiliar, Eform stencils are produced using an additive process where the nickel stencil is grown on a mandrel in an electroplating process. This enables ultra-fine aperture features, remarkably smooth aperture walls, and very thin stencils, down to 23μm or thinner in some cases. With Eform, area ratios can be maintained even with the tiny aperture dimensions and tight pitches, permitting traditional SMT processes to be used for production. While the cost for an Eform stencil may be slightly higher than a traditional stainless-steel stencil, the economies-of-scale achieved by the ability to mass produce these Mini LED panels are tremendous.

Other considerations for stencil-printing Mini LEDs are the solder paste materials and, naturally, the printing equipment capability. Because this is SMT, conventional protocols apply. A Type 6 solder paste is necessary to adhere to the five-ball rule, which means the particle size needs to be such that five particles will fit side by side in the narrowest aperture. And, because Type 6 solder paste materials are quite a bit more costly than their counterparts, optimizing material use with paste management tools like paste roll height monitoring and automatic paste dispensing will help keep the cost benefits of high-volume production palatable.

Second is the print platform alignment accuracy. Mini LEDs are super small, and there are gracious plenty of them across each panel, so precision, precision, precision is the name of the game. A supremely accurate printer is a necessity. The industry once thought +/-12.5μm print accuracy was overkill. Not anymore! In addition to material waste-reducing technologies and alignment, the machine’s handling capabilities must be state-of-the-art. Some substrates for Mini LEDs are glass, so a near-touchless handling system, including board stops, has to be achieved optically and not physically. The machine’s stability, coplanarity, and the flatness of the tooling solution are critical because, if everything is not perfectly flat, the substrate can fracture or break. A tooling solution such as porous aluminum will work for rigid substrates, whereas a flexible substrate may require third-party custom tooling.

Suffice it to say, there is much to consider for Mini LED processing. But one thing is clear: SMT processing is the most viable mass production approach. Who would have predicted that Eform would be lighting it up again for SMT? Full circle, indeed.

Clive Ashmore is global applied process engineering manager at ASMPT (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.