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Technical Information

Safe Lighting for RISTON®Photopolymer Dry Film ResistsTechnical Bulletin TB-0169

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Printed Circuit Materials

IntroductionOne inherent property of UV-sensitive negative

working photoresists is that they polymerize when ex-posed to ultraviolet (UV), violet, or blue light. Thepolyester coversheet absorbs UV radiation at wave-lengths below approximately 320 nanometers (nm) sothat the resist does not “see” this part of the spectrum.Photoresist sensitivity peaks near 365 nm and extendsmore or less to 430 nm in the blue portion of the spec-trum depending on the composition of the resist. Straywhite light and UV radiation from photoprinters, ex-amination tables, windows, and light fixtures may causeunwanted polymerization.

Using Safe Lights“Safe lighting” is the room light illumination a

photoresist is exposed to during the time period requiredto handle and process unexposed resist (and exposedresist prior to development) that will keep unwantedpolymerization below a level that is detrimental to theresist’s performance. A marginally unsafe lighting situ-ation is often not recognized, and troubleshooting tendsto focus on potential development or etching problems.Keep in mind that there is no absolute level of saferadiation intensity or safe exposure time since it is theproduct of time and intensity that determines the totalradiation energy the resist experiences. In addition, thephotosensitivity of the resist and the degree to whichthe safe light spectral emission matches the photo-re-sponse curve of the film do play a role.

A light source is typically considered “safe” foruse in areas where panels are laminated, exposed, and

developed if it does not emit UV radiation, violet, orblue light, and if the “yellow light” flux intensity isless than 70 foot-candles (753 lux). The “flux” is ex-pressed in foot-candles (fc) which is equivalent to “lu-mens per square foot”. The metric equivalent unit isthe “lux”, or “lumen per square meter”. The conver-sion factor is: lux = fc x 10.76 ft2/m2 = 10.76 lumen/m2. Some fabricators deem the 70 foot-candle recom-mendation to be unrealistically low for today’s workenvironment. Shops have run successfully at illumina-tion levels of 100 “foot-candles” (1076 lux) after care-fully characterizing the sensitivity of the photoresist inuse and adjusting work flow to minimize exposure timein the yellow room.

Radiation sources shielded with a “gold shield”or Photoresist Gold Lamps that cut off radiation below530 nm are considered particularly safe. Such safe lightlamps should not be confused with inexpensive butunreliable yellow-colored decorator tubes so often seenin yellow rooms that are not made to meet safe light

Fig.1: Spectral Sensitivity Curve, Resist A

UV-Light Sensitivity/Absorption (Resist A)

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standards and may leak white light. Some photoresists,notably high-speed laser direct imaging (LDI) resists,are more sensitive to UV-radiation, and most likely,are more sensitive to violet and blue light. Resist A(Fig. 1) shows little or no sensitivity in the range of410-430 nm, i.e., there is no “step held”-response (bluecurve) of the resist to radiation passing through filtersthat allow transmission in 10 nm increments in thisrange (see Ref. 1). Resist B (Fig. 2), on the other hand,shows some polymerization when exposed to the 400-

UV-Light Sensitivity/Absorption (Resist B)

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430 nm range radiation. Such sensitive films may re-quire a combination of shorter hold times during lami-nation, exposure, and development, a protective coverunder black plastic during holding steps, or lower in-tensity safe lighting. “Safe white lights” have gainedpopularity in recent years, but they are generally notrecommended, unless their “safe light” characteristicshave been verified with the particular resist in use un-der realistic process flow conditions (see Safe LightCheck Section below). These “safe” white lights typi-cally cut off radiation below 400 nm, but potentiallydamaging violet and blue light is emitted as part of thewhite light spectrum. Such “safe” white lights have beenused where white light is preferred for aesthetic rea-sons (as in museums) but where long-term UV expo-sure might fade colors. Note, that there are “comple-mentary color” wavelength pairs in the visible spec-trum which complement each other to appear as whitelight. Yellow and blue are such a pair; thus filtering outthe blue light makes the “yellow room” appear yellow.

As a rule of thumb, one can rank “not safe” lightsources in order of UV radiation emission, going fromthe highest to the lowest UV emission as follows:

Fig.2: Spectral Sensitivity Curve, Resist B

• Direct sun from windows & skylights• Mercury street lamps• White fluorescent lamps• Incandescent lamps• High pressure sodium lamps

Safe lighting can be obtained by• using fluorescent safe lights that come equipped with

shield, filter, and protective end caps, or gold fluorescenttubes, such as Illumination Technology’s Photoresist GoldLamps F040T12 (40 watt) or F032T8 (32 watt) that arepermanently sealed in a filter, or by installing UV sleevesthat cover fluorescent lamps, such as EncapSulite TypeG10.

• laminating UV filter foils over window panes, or replac-ing window panes with amber, type 2422 Plexiglas® sheets1/8 in. (3.2 mm) to ¼ in. (6.4 mm) thick, and

• using UV laminated filter panels for mounting in recessedceiling fixtures.

Exterior or hallway windows may also be cov-ered up with UV absorbing polyimide foils such asDuPont’s Kapton® polyimide, 2 mils (0.05 mm) orthicker. Thicker film is easier to handle but is moreexpensive.

Safe Lighting CheckTo determine whether there are sources of unsafe

white light in the production area, turn off all yellowlights and look for any remaining white light. This checkis of course only valid if there are no white light leaksfrom the yellow light fixtures.

Some lights that appear to be yellow or ambermay not be safe. For example, high-pressure sodiumlamps look yellow but also emit blue and UV radia-tion. Use the following procedure to check whether thelighting is safe:

1. Contact the manufacturer for the spectral output of thelamps in question or the absorption spectrum of the fil-ters. If the lamps emit light below 450 nm, or the filterspass light below that wavelength, they are generally notconsidered safe.

2. Laminate two panels with the photoresist to be used. Ifrunning more than one type of photoresist, laminate twopanels for each photoresist used.

3. Expose the panels with a fine resolution pattern that ischaracteristic of the highest resolution requirements.

4. Store one test panel in the dark, e.g., in a black plasticbag. Expose the other test panel to the normal lightingconditions in the panel storage area. Keep both panels fora realistic, longest hold time typically encountered between

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lamination and development.5. Develop both panels6. Flash solder or tin plate both panels. Areas of incomplete

plating indicate the presence of resist residue caused byexposure to light that is outside the “safe” range. An alter-native, but more subjective, test method is to give bothpanels a brief acid microetch. Copper areas free of resistwill turn brown; areas protected by resist residue will re-main “pink”. It is also possible to strip the resist and in-spect for the smallest surface etched feature under a mi-croscope. In photochemical machining, flash etching, strip-ping of resist, and microscopic inspection for the smallestsurface etched feature is a common test.

Alternatively, a more quantitative test consists ofimaging, after the post-lamination hold time, a fine lineproduction job or fine line test pattern such as the 3 mil(75 micron) line & space IPC-9251 test pattern. Afterdevelopment and etching (e.g. ½ ounce copper sub-strate) the panel is AOI-inspected for evidence of etchretardation (small copper protrusions).

A light meter (such as Extech’s Model 407026 orequivalent light meters available in photographer sup-ply stores) can be used for direct measurement of theactual yellow light intensity. Make sure to remove thediffuser cover from the light meter before measure-ments. The light meter integrates radiation over theentire range of the visible spectrum with a sensitivityresponse curve that peaks in the 550 nm region, mim-icking the sensitivity curve of the human eye, but notthat of the photoresist. Thus, the light meter is not agood indicator of the radiation the resist “sees” andwill of course not detect harmful UV-radiation.

Suppliers of safe lights also provide a very usefultool for a quick, qualitative check for blue/violet radia-tion: it consists of a colored, transparent foil, usuallysupported by a cardboard frame, that the operator holdsup against the light source. The detection of a bluishcolor through the foil is an indicator for the emissionof blue/violet light from the light source.

Although photoresists do not polymerize undersafe light conditions, some do exhibit changes in color,loss of photospeed, or signs of “lock-on” when exposedto yellow light for a long period before photoprinting.“Lock-on” refers to the formation of resist develop-ment residues that interfere with etching in print andetch processes or cause copper-copper peelers in pat-tern plating. Loss of photospeed was more commonwith older resists and is likely to be seen with high-speed photoresists. A peculiar behavior that has beenobserved with some photoresists is the initial slightincrease in photospeed after lamination, before the onset

of the steady decline in photospeed with time.

Other ConsiderationsThe exposed photoresist that remains on panels

after development will continue to polymerize at a slowrate when exposed to white light. Although some lowintensity exposure is not harmful, prolonged or highintensity exposure may lead to slow stripping and pho-toresist embrittlement. For production areas where de-veloped panels may be exposed to white light for ex-tended periods of time, install lights with low UV out-put to minimize unwanted polymerization.

Diazo phototool processors may be found in yel-low rooms. One has to keep in mind that diazophototools are sensitive to white light, and what hasbeen said about white light and photoresists applies toprocessing and storing of diazo phototools as well.

The degree of white light polymerization is af-fected by the light intensity, duration of exposure, thedistance between the panels and the light sources, panelshadowing in racks or stacks, and the photoresist type.

Good practicesCover the panels to prevent direct yellow light

exposure if the hold times between lamination, print-ing, and development exceed the hold times recom-mended for the resist.

Yellow light examination tables that are used tocheck the phototool-to-panel registration may have highintensities. A few minutes of exposure to that light maycause changes in the photospeed. Therefore, minimizethe time that panels are on these tables.

Vendor ListInformation on some of the products mentioned

in this bulletin is available from the following vendors.Similar products may be available from other vendors.

Plexiglas®Rohm & Haas Co.Independence Mall WestPhiladelphia, PA 19106-2399Phone (215) 592 3000

Kapton®DuPont Product Information: 1-800-441-7515

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Safe light systems, filters, sleeves, panels:EncapSulite International Inc.12603 Executive DriveSuite #810Stafford, Texas 77477Tel: 281 240 2130Toll Free: 1-800 227 8756Fax: 281 240 2899

Illumination Technology, Inc.12010 Industrial Park DriveBishopville, MD 21813Tel: 410 352 5016 or 1-800 631 1170Fax: 410 352 5062e-mail: Illtech@erols.com(supplier of safe gold lamps to Sylvania,GE Supplies, Phillips, and Wesco)

Extech Instruments Corporation285 Bear Hill Road, Waltham, MA 02451Phone (781) 890-7440 • Fax (781) 890-7864

Reference1. Fine Lines in High Yields, (Part LVII): Spec-

tral Sensitivity of Photoresists, Karl H. Dietz,CircuiTree Magazine, June 2000, pg. 50

TB-0169 03/01 Edited byKarl .H. Dietz and Robert L. Seyfert

(Supercedes H27628 12/91, edited by Ceferino G. Gonzalez and William L. Wilson)

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The information set forth herein is based on data believed to be reliable, but the DuPont Company makes no warrantiesexpress or implied as to its accuracy and assumes no liability arising out of its use by others. This publication is not to betaken as a license to operate under, or recommendation to infringe, any patent.

AmericasDuPont Printed Circuit Materials14 T.W. Alexander DriveResearch Triangle ParkNC 27709-4425E-Mail: americas.pcm@usa.dupont.comTel: Customer Service: 800-243-2143Tel: 919-248-5000Fax: 919-248-5550

AsiaDuPont Singapore PTE LtdElectronic Materials1 Maritime Square #07-01World Trade CentreSingapore 099253E-Mail: asia.pcm@usa.dupont.comTel: 65-2773626Fax: 65-2726065

Europe, Middle East & AfricaDuPont de Nemours International S.A.Electronic MaterialsP.O. Box 502, Chemin du PavillonCH-1218 Le Grand-SaconnexGeneva, SwitzerlandE-Mail: europe.pcm@usa.dupont.comTel: 41-22-717-5507Fax: 41-22-717-5661

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