Laser imaging module for PCB direct imaging: the 405 nm exposure head that writes circuit patterns into dry-film resist without a phototool

01

A DMD at 405 nm: array temperature, mirror duty cycle and diffraction orders

Texas Instruments' application note on using DLP DMDs down to 400 nm names the DLP7000BFLP, DLP9500BFLN, DLP6500BFLQ, DLP9000BFLS and DLP9000XBFLS as devices specified for that range and says 405 nm light can be used with them. Because the DMD modulates by reflection from aluminium micromirrors, TI says it tolerates short wavelengths better than modulators that rely on organic molecules, which degrade under such light. [1]

TI asks for the DMD array to be kept below 30 °C, preferably 20 to 25 °C, by passive cooling or a thermoelectric cooler, with no gradient above 10 °C across a Type A package. Mirrors should average close to a 50/50 landed on/off duty cycle; a pixel run at 62/38 for four hours can be driven at 38/62 for four hours when idle. Illumination should be shuttered at the source, not by the DMD, whenever no pattern is being written. [1]

With laser illumination the DMD acts as a grating. Because micromirror tilt can vary by ±1°, a given device can fall anywhere between a blazed condition (one order with nearly three-quarters of the energy) and an anti-blazed one (four orders with roughly a sixth each), so TI recommends output optics that capture at least the four brightest orders: at least 4.4° angular aperture at 405 nm for a 7.56 µm pitch device, 6.2° recommended, plus ±2° adjustment of the illumination angle. [1]

TI's direct-imaging article pairs the DLP9000X, with more than 4 million micromirrors, with the DLPC910 controller at up to 60 gigapixels per second, and says the DLP7000UV and DLP9500UV extend to 363 nm. [2]

Ask the module maker which DMD and wavelength band it uses, how the array is cooled and monitored, and what aperture the projection optics give at 405 nm.

02

The resist sets the dose the head must deliver

DuPont's Riston DI9300 data sheet rates the film for 355 nm, 405 nm and 375–405 nm dual-band LDI machines and gives exposure energy against steps held on a Riston step tablet (RST): [3]

The film is offered in 20, 25 and 30 µm thicknesses. DuPont adds that steps held can vary by ±1 depending on the development breakpoint, that the energies come from lab testing, and that lab resolution was 18 µm lines and spaces in 25 µm resist on both 355 nm and 405 nm LDI machines (20 µm in an optimised production environment). Panels need at least 30 minutes after exposure before development, and up to 24 hours as a guideline. [3]

Editorial calculation: at the top of the DI9330 405 nm range, 27 mJ/cm², each watt of optical power reaching the panel exposes 1 / 0.027 ≈ 37 cm² per second; at the bottom of the DI9325 range, 12 mJ/cm², about 83 cm² per second. The figures assume all delivered power lands in exposed areas and ignore scan overhead; they show why a slower resist needs more head power for the same throughput. [3]

Give the module or system supplier the resist type, thickness and its LDI dose window so source power can be checked against the panel rate wanted.

03

What the finished imager quotes: minimum feature, registration and wavelength

KLA lists its Orbotech Nuvogo direct imagers for HDI, flex and multilayer boards by minimum feature size, feature-to-grid registration and laser wavelength, all with an XL option for imaging up to 25 × 32 in. [4]

KLA attributes depth of focus to its Large Scan Optics and wider resist compatibility to its MultiWave Laser technology, and lists fully automated systems for lines down to 8 µm and 15 µm in its IC-substrate and HDI range. [4] Riston DI9300's 18 µm lab resolution sits between the 15 µm and 18 µm Nuvogo classes, so the resist and the imager both bound the finest line. [3] [4]

04

Information for an LDI head or module enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Application: [inner layer; outer layer; solder mask; IC substrate]
Resist: [type; thickness; dose window mJ/cm²]
Wavelength: [405 nm; 375 + 405 nm; 355 nm]
Resolution: [minimum line/space; registration]
Modulator: [DMD model; pixel pitch]
Throughput: [panel size; sides; panels per hour]
Cooling: [DMD and source temperature control]

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