Master your densitometer, minimize film problems: fine-tuning the film process means knowing what this oft-misused equipment is saying.
A densitometer measures the optical density of "clear" (non-image) and "opaque" (image) areas of a phototool film. Most units in the field are transmission densitometers; that is, they measure the amount of light being transmitted through different areas of the film. The measurement of the clear area is referred to as the D-Min and that of the opaque areas is the D-Max.
Densitometers can measure either diazo phototool films (with selective filtration), silver phototool films or both. I most often encounter the X-Rite 369, an instrument that measures diazo and silver films. A densitometer should be calibrated daily--as well as any time the unit is moved or bumped. Use the calibrated density "Standard" as provided by the manufacturer. This is a small piece of film used to verify and adjust the readings. To prevent fingerprints or other contaminates from adversely affecting the readings, hold the Standard along the edges.
What do D-Min and D-Max numbers mean? Let's start with D-Min. (I am focusing on diazo phototool films that are universally used for exposing LPI and dry-film soldermask materials.) The starting D-Min of a properly processed diazo phototool ranges between 0.08 and 0.11. That's the rating in the photolab before the film is sent to production. In production, D-Min will increase as the film sees multiple exposures. It is important to examine the phototool after every 100 exposures and measure the increase in D-Min.
Many shops report they discard the phototool when it reaches a D-Min of between 0.20 and 0.24 (note: it is nearly impossible to place an exact D-Min number on this, because the type of ink or soldermask used will require different exposure values and processing requirements). The key factor for replacing film is when exposure time to soldermask must be increased to achieve the same step reading on processed boards (panels). Depending on the soldermask used, this can occur anywhere between 100 and 500 exposures, again depending on the amount of exposure energy the soldermask requires.
D-Max is the measurement of the image on the film and its ability to block UV light. When properly developed, the image on silver film (as well as on diazo) will block all UV light. But silver film--with its metallic silver emulsion--blocks not only UV light but also all visible and IR light (energy). In contrast, diazo blocks light selectively; it blocks all UV light, but permits both visible and IR light to pass through the diazo emulsion. The key factor is that the IR energy passes through the diazo image, but is blocked by the silver image. With a silver film, this IR energy causes the metallic silver particles to heat up. Under extreme conditions, the heat buildup can distort the image.
One major advantage of using diazo is the ease of registering the film to the board/panel within the soldermask department. Another is the fact that the photolab cannot overdevelop the diazo emulsion during processing. When in doubt--and before sending film to production--simply pass the film through the diazo developer again! The D-Max of a fully developed diazo film should measure 4.5 D-Max or higher. However, be aware that when reading numbers greater than 4.0, the densitometer is trying to measure 1--1/100th of 1% of UV light transmission through the image (0.01% transmission). So a 4.0 reading means the image measured is blocking 99.99% of the UV light. Transmission of light through the image:
1.0 = 10% transmission (90% blocked)
2.0 = 1% (99%)
3.0 = 0.1% (99.9%)
5.0 = 0.001% (99.999%)
Officials at the X-Rite Co. told me they guarantee Model 369 densitometer readings to 4.0 +/-0.04D. Readings greater than 4.0 will be less accurate because the instrument is attempting to measure such a small amount of UV light transmission. Fabricators should establish a realistic specification.
Individual densitometers can vary greatly when measuring greater than 4.0. So just like determining when to replace a phototool that has seen a rise in the D-Min area, consult with the soldermask supplier on the phototool's D-Max requirements. With diazo, the 2 mm aperture is recommended for the most accurate readings.
After exposure and development of the diazo phototool, but before measuring the D-Max, it is imperative that photolab exposes the film through the base side--and not through the emulsion side--at 3x the original exposure time or units. This simple burn-back phase--in which undeveloped diazo emulsion is burned away--is important for avoiding possible false density readings. An accurate and stable reading is absolutely critical. Too often, technicians expose and develop film and immediately take a reading instead of performing a burn-back.
Always take readings with the film's emulsion up, with the film base side to the lamp and emulsion to the reading head. This also serves as a way to test developer performance.
Densitometers require little maintenance. A 5-minute warmup is required. Keep it free from dust and dirt, and place it where it won't be moved or bumped. Clean the glass filter on the reading head monthly using alcohol to remove any oxidation. Other chemicals such as glass cleaner can contain optical brighteners that can compromise the instrument's readings. Don't forget about cleaning the aperture and beam splitter below the aperture using a cotton swab and alcohol. Blow air into the aperture housing to remove dust and lint.
JEFF JARVIS is senior territory manager at InteliCoat Technologies (intelicoat.com). He can be reached at email@example.com.
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|Title Annotation:||Better Phototooling|
|Publication:||Printed Circuit Design & Manufacture|
|Date:||Mar 1, 2004|
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