Printed electronics - Part 302-2: Equipment - Inkjet - Imaging-based measurement of droplet volume

IEC 62899-302-2:2018(E) specifies the method for determining accurate inkjet droplet volume based on images obtained by drop-in-flight measurement systems. It does not apply to imaging systems using interference fringes, such as holography or phase doppler anemometry. This document is not limited to drop-on-demand inkjet systems, but might not be applicable to continuous inkjet or liquid dispensing systems. This document includes a description of the issues concerning such measurements and consideration of the limits to measurement accuracy.

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Publication Date
06-May-2018
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IEC 62899-302-2
Edition 1.0 2018-05
INTERNATIONAL
STANDARD
Printed electronics –
Part 302-2: Equipment – Inkjet – Imaging-based measurement of droplet volume
IEC 62899-302-2:2018-05(en)
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IEC 62899-302-2
Edition 1.0 2018-05
INTERNATIONAL
STANDARD
Printed electronics –
Part 302-2: Equipment – Inkjet – Imaging-based measurement of droplet volume
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 19.080; 37.100.10 ISBN 978-2-8322-5671-8

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 62899-302-2:2018 © IEC 2018
CONTENTS

FOREWORD ........................................................................................................................... 3

1 Scope .............................................................................................................................. 5

2 Normative references ...................................................................................................... 5

3 Terms and definitions ...................................................................................................... 5

4 Droplet volume measurement .......................................................................................... 6

4.1 General ................................................................................................................... 6

4.1.1 Overview ......................................................................................................... 6

4.1.2 Volume measurement and droplet shape equalization processes ..................... 6

4.1.3 Imaging optics ................................................................................................. 7

4.1.4 Image shape processing .................................................................................. 7

4.1.5 Calibration ....................................................................................................... 7

4.1.6 Uncertainties ................................................................................................... 7

4.2 Processes for measurement of inkjet droplet volume ............................................... 8

4.2.1 General ........................................................................................................... 8

4.2.2 Process for measurement of inkjet droplet volume – Method 1 ......................... 8

4.2.3 Process for measurement of inkjet droplet volume – Method 2 ......................... 8

Annex A (informative) Key considerations for in-flight droplet volume measurement ............ 10

A.1 Jetted droplet volume in printed electronics .......................................................... 10

A.1.1 General ......................................................................................................... 10

A.1.2 Image resolution ............................................................................................ 10

A.1.3 Greyscale-to-binary image conversion ........................................................... 11

A.1.4 Absolute droplet volume ................................................................................ 13

A.2 Formulae for inkjet droplet volume ........................................................................ 14

A.3 Results ................................................................................................................. 15

Bibliography .......................................................................................................................... 16

Figure 1 – Representation of greyscale drop size 1 (“native drop”) to size 7 ........................... 5

Figure A.1 – Magnified droplet grey image ............................................................................ 10

Figure A.2 – Threshold value influence on binary image: on the left, a threshold of 25;

on the right, a threshold of 75 ............................................................................................... 12

Figure A.3 – Apparent image height of objects imaged near the focal plane (FP) using

a conventional lens ............................................................................................................... 12

Figure A.4 – Example of percentage size distortion in image plane for a conventional lens ... 13

Figure A.5 – Shadowgraph of inkjet-printed droplets, ligaments and satellites in-flight .......... 14

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IEC 62899-302-2:2018 © IEC 2018 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PRINTED ELECTRONICS –
Part 302-2: Equipment – Inkjet –
Imaging-based measurement of droplet volume
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

International Standard IEC 62899-302-2 has been prepared by IEC technical committee 119:

Printed Electronics.
The text of this International Standard is based on the following documents:
FDIS Report on voting
119/204/FDIS 119/216/RVD

Full information on the voting for the approval of this International Standard can be found in

the report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

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– 4 – IEC 62899-302-2:2018 © IEC 2018

A list of all parts in the IEC 62899 series, published under the general title Printed electronics,

can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
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IEC 62899-302-2:2018 © IEC 2018 – 5 –
PRINTED ELECTRONICS –
Part 302-2: Equipment – Inkjet –
Imaging-based measurement of droplet volume
1 Scope

This part of IEC 62899 specifies the method for determining accurate inkjet droplet volume

based on images obtained by drop-in-flight measurement systems. It does not apply to

imaging systems using interference fringes, such as holography or phase doppler

anemometry. This document is not limited to drop-on-demand inkjet systems, but might not be

applicable to continuous inkjet or liquid dispensing systems. This document includes a

description of the issues concerning such measurements and consideration of the limits to

measurement accuracy.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
droplet volume

amount of jetted fluid from an inkjet print-head nozzle measured by single event imaging

Note 1 to entry: For a single event drive pulse designed to produce sub-drops that are intended to merge in-flight

to form a larger droplet, for example to form a specific greyscale image value on deposition, droplet volume refers

to the large merged droplet and not to smaller component sub-drops.
3.2
native drop volume

amount of fluid within the smallest sub-drop jetted from a greyscale inkjet print-head used to

create images with droplets formed by multiple sub-drops within a single event

Note 1 to entry: Native drops (or threads or satellite drops) might be too small for accurate measurements by

flash imaging, but satellite drops that have merged in-flight might be large enough for drop analysis systems. (See

Figure 1 for a representation of the relative sizes for greyscale droplets in-flight.)

IEC
Figure 1 – Representation of greyscale drop size 1
(“native drop”) to size 7
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– 6 – IEC 62899-302-2:2018 © IEC 2018
4 Droplet volume measurement
4.1 General
4.1.1 Overview

This document concerns accurate determination of inkjet droplet volume from high speed

flash images of inkjet droplets travelling in-flight from inkjet print-head nozzles. Accurate

relative (rather than absolute) droplet volumes are useful for industrial inkjet-printed

electronics applications. Short flash durations avoid significant motion blurring in droplet

images. Two widely used scenarios, for flash imaging as applied to measurement of inkjet

drop speed, are considered in this document because they produce slightly different

information about droplet volumes. Images that contain superposed nominally identical and

similarly placed droplets can provide an average size for volume measurements, whereas the

single event images give measures of the size and variations of the size and centroid location

produced during volume measurements. Clause A.1 gives further information about specific

instrumentation limits.

Shadowgraph imaging can readily determine individual inkjet droplet sizes if the fluid bodies

appear dark against a light background [1 ]. However, droplets should be well-focused, and

the optical images have a suitable pixel resolution and background intensity with low intensity

variations, and image blur due to droplet motion during the flash should have minimal effect

on droplet size determination. For liquid droplets, background intensity level, refraction and

diffraction can alter apparent image size, and if the liquid is not opaque refraction often

produces a bright central spot. As spherical droplet shapes are preferred for the most

accurate and rapid online image analysis and conversion to volume, all in-flight

measurements should be made only where any sub-drops (and satellites) from the same

single event pulse have merged, and also any droplet shape oscillations have fully damped

out. Droplet volume is then inferred from the diameter (pixels) or area (number of pixels

covered) of the dark region by assuming spherical geometry and image symmetry about the

focal plane and linear calibration of the optical system (in µm/pixel). Accurate fitting

algorithms determine droplet size to sub-pixel levels, but also depend on an assumption about

where the droplet boundary is located in an image. This can provide an absolute droplet

volume if calibrated using a suitable traceable method. For example, the measured weight of

a known number of drops of known density gives the average drop volume which may be

compared with that deduced by the drop measurement system [2].

More commonly, reference objects of known dimension(s) placed in the drop measurement

system are imaged and analysed using the same optical conditions as for the inkjet droplets.

Relative volume comparisons can be made between droplets without an absolute calibration.

4.1.2 Volume measurement and droplet shape equalization processes

In principle, once droplets have been ejected from an inkjet print-head nozzle, their volume

does not change if evaporation losses or drop merging are negligible. However the resultant

droplet shapes can alter markedly from their jetted shapes until they relax towards their final

shapes (see Figure A.5). Accurate in-flight measurements always analyse spherical shapes,

avoiding long thin jet shapes [1] (or spinning non-spherical blobs of liquid) that might not

actually lie symmetrically in the (2D) image focal plane and hence might not be convertible

into volume. Some inkjet fluids used in printed electronics and other applications do not

always form fully smooth or spherical droplets under all jetting conditions [3] and in such

cases the drop analysis system provides less accurate (or even misleading) results. Examples

are highly shear-thinning high viscosity fluids, gels and fluids with large particles. Accurate

droplet volume measurement systems using multiple event imaging also require very stable

jetting by the inkjet print-head and avoidance of first droplet and burst printing effects in inkjet

printing [4, 5].
___________
Numbers in square brackets refer to the Bibliography.
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IEC 62899-302-2:2018 © IEC 2018 – 7 –
4.1.3 Imaging optics

Drop-on-demand inkjet drops move at 5 m/s to 10 m/s and typically need sub-microsecond

high power flash illumination and also high resolution digital cameras with 10X magnification

(or more) for in-flight measurement of droplet volume using shadowgraph imaging.

Tele-centric optical designs and high power LED flash can deliver (background) illumination

and imaging conditions with a depth of focus sufficient for accurate inkjet droplet volume

measurements using drop analysis systems. Appropriate flash delay times can locate droplets

near the centre of the optical field of view for accurate droplet volume determination. Optical

field of view is determined by the magnification and camera sensor area and spans typically a

few hundred micrometers. Multiple-event imaging increases the background image level

where the single event flash intensity is limited, at the cost of achievable droplet volume

accuracy. Background intensity levels, expressed as a greyscale intensity level, are ideally

specified for the drop analysis system, as is apparent from recent studies of image analysis

errors [6]. Proper focusing of drop analysis systems can reduce unwanted blurring of droplet

images, which can otherwise cause inaccurate analysis of the droplet shape, size and

volume.
4.1.4 Image shape processing

One or more regions of interest, within the total field of view of the drop imaging system, may

be set (by a user or automatically) to contain the particular droplet images for analysis. This

assists automatic identification of droplets and speeds up the analysis and presentation of

results. Either axial or spherical symmetry of droplet images should be assumed by the drop

analysis system so that droplet volumes can be computed from individual shadowgraph

images. The image shape processing used by the drop analysis system may involve

thresholding, edge detection, boundary location, circles, ellipses, equivalent circular

diameters, maximum lengths and widths, area, sliced drums or cones, or even evolving

shapes [6, 7, 8, 9]. Accurate determination of droplet volume requires sub-pixel techniques,

as shown in Clause A.1.
4.1.5 Calibration

The linear calibration of the optical field of view should be established with grids, lines or

objects of suitable known size and spacing under similar light conditions and flash duration;

the threshold value used for such calibrations should equal that used for drop measurements.

Calibration factors of 1,00 µm/pixel or less are typical for drop analysis systems accurately

measuring drop-on-demand inkjet droplet volumes, using camera pixel sizes of 10 µm or less.

An approximate (~1 %) calibration factor can be conveniently found for some measurements

by imaging several inkjet nozzles or emerging jets within the same field of view

(i.e. > 100 µm) and comparing the (nominal) nozzle pitch with the apparent pixel separation.

However this is often not feasible for industrial inkjet print-heads because they have a

shielded nozzle plane.
4.1.6 Uncertainties

Imaging-based measurements of droplet volume depend on the cube of the linear calibration

factor, so that droplet volume uncertainty is three times that of the calibration factor

uncertainty. Thus the minimum uncertainty in accurate droplet volume measurements is ± 3 %

for a linear calibration factor known to ± 1 %. As a typical example, the calibration factor of

1,00 µm/pixel in 4.1.5 has no error shown, so that the minimum assumed linear uncertainty is

± 0,01 µm, i.e. ≥ ± 1 %, and therefore the minimum assumed absolute volume uncertainty is

≥ ±3 %. Traceable standards should normally have an absolute uncertainty more than ten

times lower than this. Uncertainties in droplet volumes appropriate to relative comparisons of

images recorded with a drop analysis system are given in Clause A.2.
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– 8 – IEC 62899-302-2:2018 © IEC 2018
4.2 Processes for measurement of inkjet droplet volume
4.2.1 General

Inkjet droplet volume shall be measured by using one of the two following methods, unless

there is an alternative user and supplier agreement. These two methods principally differ in

the use of single- or multiple-event-mode images to determine droplet diameter, but may also

differ in calibration procedures: single-event mode is appropriate for highest accuracy and

absolute measurements while multiple-event mode provides smeared “droplet size” measures

often used for relative comparisons.
4.2.2 Process for measurement of inkjet droplet volume – Method 1

This process describes the in-flight method for inkjet droplet volume measurements using the

single-event mode: individual droplet images are recorded for analysis without any

superposition of other droplets.

1) Commence inkjet printing with the desired specifications (frequency, ink selection,

waveform, etc). It is recommended to record jetting conditions, including these
specifications and other relevant details, as described in Clause A.3.

2) Ensure that the desired jets and merged droplets are “in focus” at the focal plane of the

imaging optics, adjusting the delay time of the single event light flash such that fully

merged and equalized droplet shapes are located in the analysis region of interest. This

process may be performed automatically or manually, and it may be combined with a

measurement of instantaneous droplet speed using double flash, as described by

IEC 62899-302-1 (analysis method 4) [13]. The duration of the flash should not be greater

than a few hundred nanoseconds to avoid image blur of the droplet. The (double) flash

intensity should be sufficient to allow discrimination between the drop edge and the

background, without saturating the image.

3) Extract the single event mode droplet diameter D (pixel) by suitable analysis of the

recorded image. The average and statistical variance of repeated single-event-mode

droplet diameters are used to represent the average droplet diameter with a standard

error free from smearing by any velocity and timing variations. This process may be

performed automatically or manually, and it is recommended that this and the number of

drops used in the average also be recorded, as described in Clause A.3.

4) The calibration factor F (μm/pixel) should be determined at the optical magnification used

by the drop analysis system to determine droplet volume (picolitre). This calibration is

preferably made using traceable standards placed in the focal plane, or using the average

pitch of inkjet nozzles in array print-heads. It is recommended that all details of the

calibrations used be recorded, as described in Clause A.3.
4.2.3 Process for measurement of inkjet droplet volume – Method 2

This process describes the in-flight method for inkjet droplet volume measurements using the

multiple-event mode: different droplets are recorded as a single image and analysed as if they

were a single droplet.

1) Commence inkjet printing with the desired specifications (frequency, ink selection,

waveform, etc). It is recommended to record jetting conditions, including these
specifications and other relevant details, as described in Clause A.3.

2) Ensure that the desired jets and merged droplets are “in focus” at the focal plane of the

imaging optics, adjusting the delay time of the single event light flash such that fully

merged and equalized droplet shapes are located in the analysis region of interest. This

process may be performed automatically or manually, and it may be combined with a

measurement of drop speed, as described in IEC 62899-302-1 (methods 1 to 3) [13]. The

duration of the flash should not be greater than a few hundred nanoseconds to avoid

image blur of the droplet. The flash intensity should be sufficient to allow discrimination

between the drop edge and the background, without saturating the superposed image.

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IEC 62899-302-2:2018 © IEC 2018 – 9 –

3) Extract the multiple-event-mode droplet diameter (pixel) by suitable analysis of the

recorded image. This gives an average droplet diameter (potentially) smeared by any

velocity and timing variations. This process may be performed automatically or manually,

and it is recommended that this and the number of drops used in the average is also

recorded, as described in Clause A.3.

4) The calibration factor F (μm/pixel) should be determined at the optical magnification used

by the drop analysis system to determine droplet volume (picolitre). This calibration is

preferably made using traceable standards placed in the focal plane, or using the average

pitch of inkjet nozzles in array print-heads. It is recommended that all details of the

calibrations used be recorded, as described in Clause A.3.
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– 10 – IEC 62899-302-2:2018 © IEC 2018
Annex A
(informative)
Key considerations for in-flight droplet volume measurement
A.1 Jetted droplet volume in printed electronics
A.1.1 General

Accurate measurement of jetted droplet volume is an important issue in printed electronics

applications because the amount of material deposited on the substrate is directly related to

device performance. The jetted droplet amount should be consistent (uniform) for all nozzles

in a multi-nozzle head to ensure device uniformity. This requires reliable measurements of the

droplet volume at all times and for all nozzles for use in printed electronics applications.

Reported inkjet droplet volumes may not be reliable or traceable to any national standard.

Droplet volume measurement issues identified for drop analysis systems are discussed below.

A.1.2 Image resolution

There has been strong demand for the generation of smaller droplets with higher printing

frequencies. Unless the digital camera used to record the image of a smaller droplet has

correspondingly improved pixel resolution there is increasing difficulty to measure droplet

volume accurately. As an example, for an inkjet print-head generating 20 μm diameter

droplets, the typical image resolution setting of 1μm/pixel implies the droplet grey image will

be only 20 pixels across. Figure A.1 (for a droplet with a diameter of 28 pixels) illustrates how

a one-pixel uncertainty due to image resolution limits the determination of droplet diameter.

IEC
Figure A.1 – Magnified droplet grey image
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IEC 62899-302-2:2018 © IEC 2018 – 11 –

A one-pixel resolution limit for twenty pixel droplet diameters would itself imply a minimum of

± 5 % diameter error and ± 15 % volume errors; these are completely unacceptable

uncertainties in inkjet droplet volume determination for use in printed electronics. Even for

comparison purposes inkjet droplet images should have more pixels/diameter (i.e. higher

resolution and/or higher optical magnification) and a higher raw background greyscale level

(i.e. higher intensity or longer flash duration or lower optical magnification) than for

Figure A.1, which clearly shows individual square pixels. Accurate drop analysis systems

should also use sub-pixel techniques rather than
...

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