TECHNICAL DATA / INDUSTRIAL MATERIALS GROUP

Dimensional Change
Characteristics for Printed Circuit
Board Films

Pc
Printed Circuit
Board Products

October 2010 • TI-2530

INTRODUCTION
Kodak films for PCB phototooling have the following
component layers:• a plastic base
• a photosensitive emulsion layer (silver halide)• and a backing layer
The 3 layers interact to cause dimensional change.
Kodak films for phototooling are coated on ESTAR Thick
Base. This plastic base is polyester material, made ofpolyethylene terephthalate (PET).
Thickness of film for phototooling use is 0.18 mm(0.007 inch, or 7 mil). Thickness of the emulsion andbacking layers are usually about 0.005 mm (0.0002 inch,
or 0.2 mil).
The emulsion layer is composed of gelatin and polymer
that act as a binder for silver halide and other chemicals.
The backing layer is composed of gelatin and otherchemicals, including matte particles.
Kodak’s premium red film (APR7) lacks the gelatinbacking layer. Antihalation dyes are located directly underthe emulsion layer for improved imaging preperties.
Elimination of the gelatin backing layer also reduces
dimensional size change due to changes in relative
humidity.
The four factors that cause size change of photographic
films are:• Relative humidity• Temperature• Processing• Aging

Relative Humidity
The polyester base itself absorbs and desorbs moisturefrom the ambient environment. As the plastic absorbs
moisture, it expands. As the plastic releases moisture, it
contracts. The process is slow. It takes one hour per
0.0254 mm (0.001 inch, or 1 mil) of thickness to reach
99% equilibrium with the ambient environment. The
humidity coefficient of expansion for polyester base is
0.0008%⁄% RH. The humidity coefficient is a constant,regardless of the thickness of the polyester base.

©Eastman Kodak Company, 2010

Gelatin in the emulsion and backing layers also absorb
and desorb moisture. Both layers are tightly bonded to theplastic base. As the gelatin layers expand and contract,they exert tremendous forces on the tightly bonded plastic
base, causing expansion and contraction. Films with highgelatin concentration (thick layers) will exert greater
forces than films with lower gelatin concentration (thinlayers). The gelatin layers coated on polyester base
therefore will increase the humidity coefficient from
0.0008%⁄% RH to a higher value, depending on theconcentration of gelatin. The process is very fast. Gelatin
absorbs and desorbs moisture very quickly. It takes lessthan 2 or 3 minutes for gelatin layers to reach 99%
equilibrium with the ambient environment.
The process of absorption and desorption by both thebase and the gelatin layers is considered to be completely
reversible. Effects of hysteresis are negligible with today’s
films.
Humidity Coefficient is expressed as % Size Change per% Change in Relative Humidity (% / % RH). Note thatprocessed film will always have a lower humidity
coefficient than unprocessed film.
Table I: Humidity Coefficient for KODAK ACCUMAX
Photoplotter Film APR7
Unprocessed Film

Processed Film

0.0011% / % RH

0.0009% / % RH

Preconditioning
Photographic film can be preconditioned prior to exposurein order to reduce the size change that will occur following
exposure and processing. The size change is caused by thedifference between the photoplotter room RH and theequilibrium RH of the photographic film as manufactured.
Preconditioning is accomplished by exposing individual
sheets of the raw film to the environment of thephotoplotter room. A period of 4-7 hours is generally
required to produce acceptable results. Refer to Pg 7 fordetails concerning preconditioning.
Kodak's premium red film (APR7) is preconditioned
during manufacturing to an equilibrium RH of 50%. Users
operating in a 50% environment can enjoy the benefits ofpreconditioning without the effort and risk of film damage.

Temperature
Photographic film expands as the surrounding air
temperature goes up, and contracts as the temperaturegoes down. The temperature coefficient of expansion forpolyester base is 0.0018%⁄° C (0.001%⁄° F). The
temperature coefficient is a constant, regardless of the
thickness of the polyester base.
The equilibration rate is fast. The entire film structure
will equilibrate to ambient air temperature in less than
2 to 3 minutes.
Expansion and contraction with temperature is areversible process. However, if film is heated to 60° C(140° F), a permanent change will occur. The polyester canshrink or expand to relieve minor stresses remaining frommanufacture. The magnitude of these changes is ±0.01%,
not easily controlled, and they are permanent.
Temperature Coefficient is expressed as % Size Change
per Degree Change in Temperature (% ⁄ ° T). Note that allpolyester based films have the same temperature
coefficient and it is the same for both processed andunprocessed film, and is independent of thickness.
Table II: Temperature Coefficient for All Films
Unprocessed Film

Processed Film

0.0018 % / ° C (0.001% / ° F)

0.0018 % / ° C (0.001% / ° F)

Processing
Film processing will reduce the humidity coefficient of theunprocessed film (see Table I). Film processing changes
the gelatin structure by removing some of the chemicalcomponents of the raw film. The unexposed silver halide isremoved from the image-forming layer by the fixer
solution. The antihalation backing layer also contains
chemicals that are removed during processing. Removal ofthese chemicals lowers the strength of the gelatin and thus
reduces the forces on the base. There is an additional
processing dimensional change (PDC) that is associatedwith the drying temperature. Films that are over-dried will
be over-sized. Films that are under-dried will be
under-sized. The magnitude of this PDC with current
0.18 mm thick films is within ±0.02%.
Coefficients of humidity expansion are an indicator but
cannot be used to predict the exact extent to which films
change size as a result of processing. With preconditioned
film that has been dried at the optimum dryer temperature,
the PDC can be too small to measure.
There is a dryer temperature that will produce a zero
PDC. This zero-point drying temperature will varydepending on the type of film. For a given film, the ideal
drying temperature is highly dependent on the ambient
relative humidity, and on the relative humidity of the air
within the dryer of the machine processor. Therefore theideal dryer temperature must be empirically determined
under the actual conditions of use. Use the following chart
to find an approximate drying temperature range and thenadjust the drying temperature up or down depending on
the size changes observed.

Table III: Guide to Dryer Temperature for Minimum Processing
Size Change
Film Group

Ambient
Relative
Humidity

Dryer Temperature

APR7

40%

27 - 32° C (80 - 90° F)

RED7, ABG7

40%

41 - 46° C (105 - 115° F)

APR7

50%

32 - 38° C (90 - 100° F)

RED7, ABG7

50%

43 - 49° C (110 - 120° F)

APR7

60%

35 - 41° C (95 - 105° F)

RED7, ABG7

60%

46 - 54° C (115 - 130° F)

Aging
Size changes due to aging of ESTAR Thick Base (0.18 mm)
are relatively insignificant. Polyester base can show a±0.01% change over a period of 5 to 10 years. Changes are
dependent on the humidity and temperature of storage.
Storage at high humidity will probably result in a slightswell. Storage at high temperature will result in a slightshrinkage (as much as 0.02%).

DIMENSIONAL STABILITY
Kodak Films for PCB Phototools
Films can be grouped into 2 classes: films that do not havea gelatin backing layer and films that do have a gelatinbacking layer.
• Films With No Gelatin Backing– KODAK ACCUMAX Photoplotter Film APR7
• Films With Gelatin Backing– KODAK ACCUMAX Photoplotter Film RED7
– KODAK ACCUMAX Photoplotter Film ABG7
Table IV: Film Listing of Humidity and Temperature CoefficientsFilm

Humidity
Coefficient% / % RH
Unprocessed

Humidity
Coefficient% / % RH
Processed

Temperature
Coefficient% / % °T
Unprocessedor Processed

APR70.00110.00090.0018% / ° C(0.001% / ° F)

ABG70.00130.00120.0018% / ° C(0.001% / ° F)

RED70.00150.00140.0018% / ° C(0.001% / ° F)

ISO 6221:1996(E)
ISO Reference:
Both the Humidity Coefficients and the Temperature
Coefficients are measured in accordance with ISO
6221:1996(E), “Photography—Films and Papers—
Determination of Dimensional Change.”

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Dimensional Change Characteristics for Printed Circuit Board Films • TI-2530

Humidity:
Coefficients are determined between 15 to 50% RH, at atemperature of 21.1° C (70° F). The procedure conforms to
the range of relative humidity specified in ISO
6221:1996(E). This procedure is relatively severe, since
size change is not linear with humidity, but is steepest at
low humidity extremes. This coefficient will be lower for agiven film if it is determined over 30 to 60% humidity
range and still lower at a higher range such as 50 to 80%
humidity range. An example of the unprocessed humidity
coefficient of a typical film on 7-mil (0.18 mm) ESTAR Basetested over these three ranges is shown the table below.
Table V: Humidity Coefficient of Linear Expansion
Example of Humidity Coefficient
Measured at Low, Medium, and High Humidity
Unprocessed (% per % RH)
Low Range
15 to 50%

Medium Range
30 to 60%

High Range
50 to 80%0.00140.00120.0010

When comparing humidity coefficients of various films,
it is important to know the humidity range that was used todetermine them.
Temperature
Coefficients are determined between 21.1° C and 48.9° C(70° F and 120° F) at 20% RH. The procedure conforms to
the range of temperatures specified in ISO 6221:1996(E).
Thermal coefficient is very similar for all ESTAR Base films,
regardless of support thickness, or whether the film isunprocessed or processed (see Table II).

Effect of Silver Content—Amount of Exposure,
Negative vs. Positive
The amount of developed silver can influence the sizechange of film. Negatives having a high ratio of black area
vs. clear area will have very slightly less size change
compared to positives that have a low ratio of black area vs.
clear area. High silver content does affect the strength ormodulus of the gel. High silver content makes for astronger gel. The magnitude of this effect is very small—about ±10% of the total effect. If there is a 10-micron
average change, there could be 1-micron variation,
depending on the silver content. Within this magnitude ofvariation, minor differences can be measured with somesophisticated methods.

Table VI: Humidity Equilibration Rate for 0.18 mm and 0.10 mm
Thickness Films
Percent Equilibrium
To Relative
Humidity

Time Required
Film Thickness
0.18 mm
(0.007-inch)

Time Required
Film Thickness
0.10 mm (0.004-inch)

50%

1 hour

0.4 hour75%

2 hours

1 hour90%

4 hours

2 hours99%

7 hours

4 hours

Note: To fully equilibrate in the time shown, films must becompletely separated, with both sides of the film exposedto the ambient humidity. This is most easily accomplishedby placing individual sheets of film on separate shelves in aconditioning cabinet.
Temperature:
Film equilibrates to ambient temperature within
1 to 3 minutes.

Non-Uniaxialism—Difference in X and Y
Dimensional Change
A perfectly uniaxial film base will have identical properties
in X, Y, and diagonal directions within the plane of the sheetof film. ESTAR Base is intentionally stretched during
manufacture in both length and width directions. The
degree and rate of stretch during extrusion is carefully
controlled so that the finished support displays as high a
degree of uniaxialism as possible. The manufacturing
process of polyester base is not absolutely perfect. These
are not random variations, but are measurable andcontrolled levels of non-uniaxialism, as it is called. During
manufacture, the polyester is stretched and molecularly
locked together in a definite plan or pattern within the rollof extruded polyester. With very precise and accurate
measurement, large sheets of film can show slightly
different dimensions of size change in X and Y direction.
The dimensional properties of ESTAR Base may varyslightly in different directions within a sheet; thedifferences that may exist, however, are not always equalin both the length and width directions. Differences in sizechange between length and width should be within±10 percent of each other. If there is a 10-micron averagechange, there could be 1-micron variation in X and Y
dimensions.

Equilibration Rate
Humidity:
Time required to reach equilibrium with relative humidity
depends on the thickness of the film. It takes 1 hour per
0.001 inch (0.0254 mm) of film thickness to reach nearlycomplete equilibrium to relative humidity. Table VI shows
the difference between 0.18 mm and 0.10 mm films.

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Dimensional Change Characteristics for Printed Circuit Board Films • TI-2530

Hysteresis

Practical Steps:

1. Control temperature and humidity to same
conditions in all areas where film is used.
2. Precondition film to the operating
conditions, for at least 4 hours (90%equilibrium) or to 7 hours (99%equilibrium).
3. Standardize the dryer temperature of thefilm processor. See Table III. These
starting point dryer temperatures will benear optimum for achieving a zeroprocess dimension size change. Exactdrying temperature must be establishedempirically.
4. Determine post-process equilibration timeby measuring at 5-minute intervals until
size change stops.

Conditioning
Cabinet:Must be light tight when closed.
Filter should be replaced or cleaned on amaintenance-scheduled frequency.
Examine interior for dust, using flashlight; cleaninterior as needed using house vacuum or withclean-room vacuum cleaner.
Racks or shelves should be examined carefully
for any burrs that could cause scratching.

Conditioning:

Generally, the film must be free to exchange
moisture with the air. This is most easily
accomplished by placing individual sheets offilm on separate shelves in a conditioning
cabinet.

Separating Films
for Humidity
Equilibration:

A stack of film, open to the air, may require
hundreds of hours to equilibrate. It may takeweeks or months depending on the size andstack of film. There is no practical way tocondition stacks of film. Methods such as
fanning a stack of film, or tricks such as
interleaving sheets of film with previously
conditioned paper, are contrivances not worthyof endorsing or considering because of theunpredictable outcome.

Factory
Pre-conditioning:

Kodak's premium film APR7 is preconditioned
during manufacture to an equilibrated 50% RH
position, using a clean roll-to-roll process. Thisis ideal for any customer operating in anenvironment close to 50% RH. Since the film isalready pre-conditioned by Kodak andpackaged in a moisture-proof bag, there is noneed for the customer to pre-condition. Thisnot only eliminates the cost and hassle ofpre-conditioning, it also eliminates the risk ofdamage that can occur when individual sheets
are handled.

Hysteresis refers to a non-reversible humidity size changethat is “humidity-directional” in origin. This“humidity-directional” effect means that a film will notcompletely return to original size, and depends on whether
the film is coming from a low humidity, or is coming from ahigh humidity. The following is an example:
Two Films, Film A and Film B start with the same size at
50% RH.
Film A is taken to a very low humidity, and then returned
to equilibrium at 50%.
Film A will show a very slight increase in size comparedto the original size.
Film B is taken to a high humidity, and then returned toequilibrium at 50%.
Film B will show a very slight decrease in size comparedto the original size.
These phenomena have been well documented in thetechnical literature. With older generation films,
constructed with thick gels layers, and low modulus base,hysteresis was a noted factor and important to consider.
Hysteresis effects are greatly reduced with newer films
that are made with thin, low gel layers, and with highpolymer content. With new films today, the effect is of nopractical consequence. The magnitude of this effect is lessthan 10% of whatever size change can occur. If the size offilm were to show a 10-micron change in dimension, thedirectional or hysteresis effects might cause a 10%
variation of this (or ±1 micron). With newer films, theeffect is much less than this and is rarely measurable.

Preconditioning of Film for Phototooling
Recommendation:

Film should be preconditioned from4 to 7 hours prior to photo plotting.

Why Precondition:

Preconditioning films for phototooling will
improve reliability and consistency of film size
holding.

More Control:

Preconditioned film will provide greater control
and thus minimize sources of variability as
follows:• Sheet to sheet variation due to differences inlength of time film has been exposed to theatmosphere (moisture) prior to plotting.
• More predictable post-process size change.
Usually faster post-process equilibrationtime.

Troubleshooting:Control over this source of variation makes it
easier to trouble shoot and identify cause(s) of
out-of-control situations.

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Dimensional Change Characteristics for Printed Circuit Board Films • TI-2530

Equation for Calculation of Size Change
Humidity Coefficient:

0.0009 % / % RH

Temperature Coefficient:0.0018 % / ° C

Size Change

Humidity Coefficient---------------------------100= Original Size x+ Original Size x

x Change in % RH

Temperature Coefficient----------------------------------- x Change in ° C
100

Example #1
Film Size:

500 mm (millimetres)

Change in % RH:+ 4%Change in ° C:

- 2° C

Size Change =0.0009500 mm x ---------100=

0.018 mm=0 (no change)+x4+0.0018500 mm x ---------100x -2(-0.018 mm)

Example #2
Film Size:

500 mm (millimetres)

Change in % RH:+ 4%Change in ° C:

+ 2° C

Size Change = 500 mm x

0.0009---------100=

0.018 mm=0.036 mmx4++

500 mm x

0.0018---------- x 2
1000.018 mmPc

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Dimensional Change Characteristics for Printed Circuit Board Films • TI-2530

Dimensional Change Characteristics for Printed Circuit Board Films

Summary—Pearls of Wisdom
Pearl

WisdomFilm Base

Know the limits of 7-mil polyester film.
All polyester films have humidity coefficient of 0.0008 %⁄% RH
All polyester films have temperature coefficient of 0.0018 % / °C
"Will film base meet the required tolerance?"
Understand the rheology of the material.

Environment Control

Humidity and temperature control are a Must!

Precondition Film

Condition and plot film at the same humidity and temperature to be used in resist imaging.
Sheets must be conditioned separately.
Expose both sides to moving air.
Stacked sheets will take weeks to equilibrate.
APR7 Film is pre-conditioned at the factory.

Drying Conditions andSize Change

Process Dimensional Change (PDC).
Determine dryer temperature for Zero PDC.
—Under dry is under size.
—Over dry is over size.

Equilibration

Equilibrate film to new conditions before measuring.

Time to Equilibrate

Humidity: emulsion equilibrates rapidly.
Temperature: emulsion equilibrates rapidly.
Humidity: PET base takes 1 hour per mil thickness.
Temperature: PET base equilibrates rapidly.

Film Storage

Store film flat. Do not roll film.

Reversible Changes

Size changes due to changes in relative humidity and temperature are considered reversible, except above 60° C(140° F) (leads to permanent change).

Four Causes of Size
Change(1) Relative humidity(2) Temperature(3) Processing(4) Aging (> 3 years fairly negligible)

Emulsion Gel Layers

Exert compressive forces on the base with absorption and desorption of moisture.

Polyester Base

Expands and contracts with absorption and desorption of moisture.

X + Y Axialism
Dimensions

Changes in dimension should be within 10% of each other in X-Y dimensions.

Post Process
Equilibration

1⁄4 hour is adequate if film was preconditioned to same environment before exposing and processing.

NOTICE: While the sensitometric data in this publication are typical of production coatings, they do not
represent standards which must be met by Kodak. Varying storage, exposure, and processing conditions willaffect results. The company reserves the right to change and improve product characteristics at any time.

Kodak, Accumax, and Estar are trademarks.

Revised 10-10
Dimensional Change Characteristics for
Printed Circuit Board Films
KODAK Publication No. TI-2530

Industrial Materials GroupEASTMAN KODAK COMPANY • ROCHESTER, NY 14650-0505