Technical Information
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Printed Circuit Materials
Safe Lighting for RISTON®
Photopolymer Dry Film Resists
Technical Bulletin TB-0169
Using Safe Lights
“Safe lighting” is the room light illumination a
photoresist is exposed to during the time period required
to handle and process unexposed resist (and exposed
resist prior to development) that will keep unwanted
polymerization below a level that is detrimental to the
resist’s performance. A marginally unsafe lighting situation is often not recognized, and troubleshooting tends
to focus on potential development or etching problems.
Keep in mind that there is no absolute level of safe
radiation intensity or safe exposure time since it is the
product of time and intensity that determines the total
radiation energy the resist experiences. In addition, the
photosensitivity of the resist and the degree to which
the safe light spectral emission matches the photo-response curve of the film do play a role.
A light source is typically considered “safe” for
use in areas where panels are laminated, exposed, and
developed if it does not emit UV radiation, violet, or
blue light, and if the “yellow light” flux intensity is
less than 70 foot-candles (753 lux). The “flux” is expressed in foot-candles (fc) which is equivalent to “lumens per square foot”. The metric equivalent unit is
the “lux”, or “lumen per square meter”. The conversion factor is: lux = fc x 10.76 ft2 /m2 = 10.76 lumen/
m2 . Some fabricators deem the 70 foot-candle recommendation to be unrealistically low for today’s work
environment. Shops have run successfully at illumination levels of 100 “foot-candles” (1076 lux) after carefully characterizing the sensitivity of the photoresist in
use and adjusting work flow to minimize exposure time
in the yellow room.
Radiation sources shielded with a “gold shield”
or Photoresist Gold Lamps that cut off radiation below
530 nm are considered particularly safe. Such safe light
lamps should not be confused with inexpensive but
unreliable yellow-colored decorator tubes so often seen
in 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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UV Density
Last Step Held
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Optical Density
One inherent property of UV-sensitive negative
working photoresists is that they polymerize when exposed to ultraviolet (UV), violet, or blue light. The
polyester coversheet absorbs UV radiation at wavelengths below approximately 320 nanometers (nm) so
that the resist does not “see” this part of the spectrum.
Photoresist sensitivity peaks near 365 nm and extends
more or less to 430 nm in the blue portion of the spectrum depending on the composition of the resist. Stray
white light and UV radiation from photoprinters, examination tables, windows, and light fixtures may cause
unwanted polymerization.
Last Step Held (SST 41)
Introduction
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300 310 320 330 340 350 360 370 380 390 400 410 420 430
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Wave Length (nm)
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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 of
410-430 nm, i.e., there is no “step held”-response (blue
curve) of the resist to radiation passing through filters
that allow transmission in 10 nm increments in this
range (see Ref. 1). Resist B (Fig. 2), on the other hand,
shows some polymerization when exposed to the 400Fig.2: Spectral Sensitivity Curve, Resist B
UV-Light Sensitivity/Absorption (Resist B)
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Steps Held
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Last Step Held (SST 41)
UV Density
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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 fluorescent
tubes, such as Illumination Technology’s Photoresist Gold
Lamps F040T12 (40 watt) or F032T8 (32 watt) that are
permanently sealed in a filter, or by installing UV sleeves
that cover fluorescent lamps, such as EncapSulite Type
G10.
• laminating UV filter foils over window panes, or replacing window panes with amber, type 2422 Plexiglas® sheets
1/8 in. (3.2 mm) to ¼ in. (6.4 mm) thick, and
• using UV laminated filter panels for mounting in recessed
ceiling fixtures.
Exterior or hallway windows may also be covered up with UV absorbing polyimide foils such as
DuPont’s Kapton® polyimide, 2 mils (0.05 mm) or
thicker. Thicker film is easier to handle but is more
expensive.
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310 320 330 340 350 360 370 380 390 400 410 420 430
Wave Length (nm)
Safe Lighting Check
430 nm range radiation. Such sensitive films may require a combination of shorter hold times during lamination, exposure, and development, a protective cover
under black plastic during holding steps, or lower intensity safe lighting. “Safe white lights” have gained
popularity in recent years, but they are generally not
recommended, unless their “safe light” characteristics
have been verified with the particular resist in use under realistic process flow conditions (see Safe Light
Check Section below). These “safe” white lights typically cut off radiation below 400 nm, but potentially
damaging violet and blue light is emitted as part of the
white light spectrum. Such “safe” white lights have been
used where white light is preferred for aesthetic reasons (as in museums) but where long-term UV exposure might fade colors. Note, that there are “complementary color” wavelength pairs in the visible spectrum which complement each other to appear as white
light. Yellow and blue are such a pair; thus filtering out
the blue light makes the “yellow room” appear yellow.
As a rule of thumb, one can rank “not safe” light
sources in order of UV radiation emission, going from
the highest to the lowest UV emission as follows:
To determine whether there are sources of unsafe
white light in the production area, turn off all yellow
lights and look for any remaining white light. This check
is of course only valid if there are no white light leaks
from the yellow light fixtures.
Some lights that appear to be yellow or amber
may not be safe. For example, high-pressure sodium
lamps look yellow but also emit blue and UV radiation. Use the following procedure to check whether the
lighting is safe:
1. Contact the manufacturer for the spectral output of the
lamps in question or the absorption spectrum of the filters. If the lamps emit light below 450 nm, or the filters
pass light below that wavelength, they are generally not
considered safe.
2. Laminate two panels with the photoresist to be used. If
running more than one type of photoresist, laminate two
panels for each photoresist used.
3. Expose the panels with a fine resolution pattern that is
characteristic of the highest resolution requirements.
4. Store one test panel in the dark, e.g., in a black plastic
bag. Expose the other test panel to the normal lighting
conditions in the panel storage area. Keep both panels for
a realistic, longest hold time typically encountered between
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lamination and development.
5. Develop both panels
6. Flash solder or tin plate both panels. Areas of incomplete
plating indicate the presence of resist residue caused by
exposure to light that is outside the “safe” range. An alternative, but more subjective, test method is to give both
panels a brief acid microetch. Copper areas free of resist
will turn brown; areas protected by resist residue will remain “pink”. It is also possible to strip the resist and inspect for the smallest surface etched feature under a microscope. In photochemical machining, flash etching, stripping of resist, and microscopic inspection for the smallest
surface etched feature is a common test.
of the steady decline in photospeed with time.
Other Considerations
The exposed photoresist that remains on panels
after development will continue to polymerize at a slow
rate when exposed to white light. Although some low
intensity exposure is not harmful, prolonged or high
intensity exposure may lead to slow stripping and photoresist embrittlement. For production areas where developed panels may be exposed to white light for extended periods of time, install lights with low UV output to minimize unwanted polymerization.
Diazo phototool processors may be found in yellow rooms. One has to keep in mind that diazo
Alternatively, a more quantitative test consists of phototools are sensitive to white light, and what has
imaging, after the post-lamination hold time, a fine line
been said about white light and photoresists applies to
production job or fine line test pattern such as the 3 mil processing and storing of diazo phototools as well.
(75 micron) line & space IPC-9251 test pattern. After
The degree of white light polymerization is afdevelopment and etching (e.g. ½ ounce copper sub- fected by the light intensity, duration of exposure, the
strate) the panel is AOI-inspected for evidence of etch distance between the panels and the light sources, panel
retardation (small copper protrusions).
shadowing in racks or stacks, and the photoresist type.
A light meter (such as Extech’s Model 407026 or
equivalent light meters available in photographer supGood practices
ply stores) can be used for direct measurement of the
Cover the panels to prevent direct yellow light
actual yellow light intensity. Make sure to remove the
exposure if the hold times between lamination, printdiffuser cover from the light meter before measureing, and development exceed the hold times recomments. The light meter integrates radiation over the
mended for the resist.
entire range of the visible spectrum with a sensitivity
Yellow light examination tables that are used to
response curve that peaks in the 550 nm region, mimcheck the phototool-to-panel registration may have high
icking the sensitivity curve of the human eye, but not
intensities. A few minutes of exposure to that light may
that of the photoresist. Thus, the light meter is not a
cause changes in the photospeed. Therefore, minimize
good indicator of the radiation the resist “sees” and
the time that panels are on these tables.
will of course not detect harmful UV-radiation.
Suppliers of safe lights also provide a very useful
Vendor List
tool for a quick, qualitative check for blue/violet radiaInformation on some of the products mentioned
tion: it consists of a colored, transparent foil, usually
supported by a cardboard frame, that the operator holds in this bulletin is available from the following vendors.
up against the light source. The detection of a bluish Similar products may be available from other vendors.
color through the foil is an indicator for the emission
Plexiglas®
of blue/violet light from the light source.
Rohm & Haas Co.
Although photoresists do not polymerize under
Independence Mall West
safe light conditions, some do exhibit changes in color,
Philadelphia, PA 19106-2399
loss of photospeed, or signs of “lock-on” when exposed
Phone (215) 592 3000
to yellow light for a long period before photoprinting.
“Lock-on” refers to the formation of resist developKapton®
ment residues that interfere with etching in print and
DuPont Product Information: 1-800-441-7515
etch processes or cause copper-copper peelers in pattern plating. Loss of photospeed was more common
with older resists and is likely to be seen with highspeed photoresists. A peculiar behavior that has been
observed with some photoresists is the initial slight
increase in photospeed after lamination, before the onset
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Safe light systems, filters, sleeves, panels:
EncapSulite International Inc.
12603 Executive Drive
Suite #810
Stafford, Texas 77477
Tel: 281 240 2130
Toll Free: 1-800 227 8756
Fax: 281 240 2899
Illumination Technology, Inc.
12010 Industrial Park Drive
Bishopville, MD 21813
Tel: 410 352 5016 or 1-800 631 1170
Fax: 410 352 5062
e-mail: [email protected]
(supplier of safe gold lamps to Sylvania,
GE Supplies, Phillips, and Wesco)
Extech
Instruments
Corporation
285 Bear Hill Road, Waltham, MA 02451
Phone (781) 890-7440 • Fax (781) 890-7864
Reference
1. Fine Lines in High Yields, (Part LVII): Spectral 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 warranties
express or implied as to its accuracy and assumes no liability arising out of its use by others. This publication is not to be
taken as a license to operate under, or recommendation to infringe, any patent.
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