EN 14571:2005

SLOVENSKI STANDARD
01-julij-2005
Kovinske prevleke na materialih z nekovinsko osnovo – Merjenje debeline
prevleke – Metoda mikroupornosti
Metallic coatings on nonmetallic basis materials - Measurement of coating thickness -
Microresistivity method
Metallische Überzüge auf nichtmetallischen Grundwerkstoffen - Schichtdickenmessung -
Mikro-Widerstand-Verfahren
Revetements métalliques sur matériaux non-métalliques - Mesurage de l'épaisseur des
revetements - Méthode utilisant la microrésistivité
Ta slovenski standard je istoveten z: EN 14571:2005
ICS:
17.040.20 Lastnosti površin Properties of surfaces
25.220.40 Kovinske prevleke Metallic coatings
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 14571
NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2005
ICS 25.220.40; 17.040.20
English version
Metallic coatings on nonmetallic basis materials - Measurement
of coating thickness - Microresistivity method
Revêtements métalliques sur matériaux non-métalliques - Metallische Überzüge auf nichtmetallischen
Mesurage de l'épaisseur des revêtements - Méthode Grundwerkstoffen - Schichtdickenmessung - Mikro-
utilisant la microrésistivité Widerstand-Verfahren
This European Standard was approved by CEN on 3 March 2005.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Central Secretariat or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
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© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14571:2005: E
worldwide for CEN national Members.

Contents page
Foreword.3
1 Scope .4
2 Measurement principle.4
3 Factors affecting measurement uncertainty.6
4 Calibration of instruments .7
5 Procedure .8
6 Accuracy requirements.9
7 Test report .9
Annex A (normative) Method for determining the critical current path width.10
Bibliography .11

Foreword
This document (EN 14571:2005) has been prepared by Technical Committee CEN/TC 262 “Metallic and other
inorganic coatings”, the secretariat of which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an identical text or
by endorsement, at the latest by October 2005, and conflicting national standards shall be withdrawn at the latest
by October 2005.
This document includes a Bibliography.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark,
Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
1 Scope
This document describes a method for nondestructive measurements of the thickness of conductive coatings on
nonconductive base materials. This method is based on the principle of the sheet resistivity measurement and is
applicable to any conductive coatings and layers of metal and semiconductor materials. In general, the probe has
to be adjusted to the conductivity and the thickness of the respective application. However, this document focusses
on metallic coatings on nonconductive base materials (e.g. Copper on plastic substrates, printed circuit boards).
NOTE 1 This method also applies to the measurement of through-hole copper thickness of printed circuit boards. However,
for this application a probe geometry different from the one described in this document is necessary.
NOTE 2 This method is also applicable for thickness measurements of conductive coatings on conductive base materials, if
the resistivity of the coating and the base material is different. This case is not considered in this document.
2 Measurement principle
The sheet resistivity method uses the so called four-point probe as shown in Figure 1. A row of four spring-loaded
metal tips are placed in contact with the surface of the conductive coating. The tip distances between the outer and
inner tips S and S are equal. Usually a constant current is passed through the two outer contacts (4 and 7). The
1 3
introduced current penetrates the conductive material of the coating with the resistivity ρ. The resulting voltage
drop is measured across the two inner contacts (5 and 6).
In general, the flow of the introduced current is non-uniformly distributed over the cross-section of the coating and
is not parallel to the coating (see Figure 2). The current density decreases with increasing distance from the direct
line between the contacts 4 and 7 (with depth and width). If the current is effectively limited by the thickness of the
coating, the voltage drop between 5 and 6 is a measure of the thickness.

Key
1 Outer contact of the probe
2 Inner contact of the probe
3 Conductive coating
4 Nonconductive base material
t  Coating thickness
Figure 1 — Schematic representation of the sheet resistivity method
Key
1 Outer contacts of the probe
2 Inner contacts of the probe
3 Conductive coating
4 Nonconductive base material
t Coating thickness
Figure 2 — Schematic representation of the non-uniformly distributed current within the coating

The measured voltage drop depends on the resistivity of the metallic coating, on the probe geometry (distance of
the 4 probe contacts S , S , S ), the applied current and the thickness of the coating. If the resistivity of the coating
1 2 3
can be expected to be homogenous and the thickness is sufficiently small, the measured voltage drop is
determined only by the unknown thickness and the applied current. In general, there is no simple and practical
equation to calculate the thickness as a function of the material resistivity, the probe geometry and the measured
voltage and current. However, there are some well known approximations for practical use in certain cases.
Especially in the case of equal tip distances (S =S =S = S) and for a thickness to probe spacing ratio t/s < 0,5 the
1 2 3
coating thickness, t, in micrometres, can be calculated using the equation:
()
I ln 2  t 
t =ρ when < 0,5 (1)
 
V π S
 
where
ρ is the resistivity coating, in ohm.m;
V is the potential difference across the inner probe tips, in Volts;
I current passed through outer probe tips, in amps;
S is the equal probe tip spacing (S=S =S =S ).
1 2 3
Usually the supplied current I is held constant. Therefore, the coating thickness is inversely proportional to the
measured voltage :
C
t = (2)
V
where
C  is a the constant 0,221ρI
Equation (2) is the basis for many applications in the above case. In general suitable correction functions for
Equation (2) are necessary if the prerequisite of a ratio t/s<0,5 or an equal probe tip spacing is not satisfied.
Because the introduced current decreases with increasing penetration depth, a sufficiently thick coating does not
limit the current and the coating appears to be infinite to this method. The wider the probe spacing the deeper the
current penetrates into the conductive material. Consequently, the measurement range is determined by the probe
spacing for a given coating material. The probe geometry (tip spacing) has to be adjusted with respect to the
conductivity and the expected thickness range of the application of interest. Furthermore, the sensitivity of this
method decreases with increasing thickness.
The application of Equation (2) is also limited by very thin coatings because the resistivity is expected to be
constant and not a function of the thickness. However, for very thin thicknesses the resistivity starts to increase and
below a critical thickness this increase of the resistivity is strongly pronounced. Typical values of this critical
thickness are in the range of approximately 10 nm to 300 nm for metals. For measurements in this range and below
this critical thickness a special calibration or additional correction functions are necessary.
Because the introduced current decreases with increasing distance in width, the current flow is not affected by a
sample width wider than a critical width. Therefore, the sample width has to be wider than this critical width.
Otherwise, the measured thickness becomes a function of the sample width and the sample width has to be
considered in addition. The probe spacing also determines the value of the critical width for a given coating
material.
3 Factors affecting measurement uncertainty
3.1 Range of measurement
The measurable thickness range is determined by the probe geometry (tip distance) and the conductivity of the
coating. The probe geometry has to be adjusted to the thickness range of interest.
Usually the manufacturer provides the uncertainty of the respective probe for the recommended thickness range.
3.2 Coating resistivity
Measurements will be affected by the resistivity of the coating if the resistivity of the coating differs from the
resistivity of the calibration standard(s) used to calibrate the instrument. A 5 % difference in resistivity will result in
a 5 % error unless this difference is accounted for in the calibration procedure.
Furthermore, a homogenous resistivity throughout the coating is expected for this method. The measurement will
be affected by a resistivity variation of the coating. This can be caused by composition variation of the coating, by
coating defects (e.g. cracks, porosity, voids, inclusions) or by a surface preparation or contamination.
3.3 Width of the sample
Below a critical width, determined by probe design (tip spacing) and to a lesser degree on the electrical conductivity
of the metallic coating, the coating thickness measurement becomes dependent upon the width of the electrical
current path (e.g. conductive track width of printed circuit boards). The instrument shall therefore be calibrated
using calibration standards of the width to be measured or appropriate correction functions shall be used.
NOTE 1 An exact positioning of the probe in the middle of the sample (e.g. conductive track) and parallel to its direction is
necessary to avoid measurement errors. Usually special probe positioning systems or probe guides are provided by the
manufacturers.
NOTE 2 If the critical path width is not known, or for some reason is unobtainable, it may be obtained using a number of
reference standards having the same thickness (made from the same piece of uniform material), but of different known widths
(see Annex A).
3.4 Curvature
Sharp or small radii of curvature will greatly affect the thickness measurement. This effect is minimised if the probe
is placed on the surface so that its axis is parallel to that of the curved surface. Alternatively, calibration standards
of the same curvature can be used. The influence decreases with increasing radii of curvature.
3.5 Surface roughness
Measurements are affected by surface topography of the metallic coating. Rough surfaces can cause thickness
measurement errors. In such cases it is strongly recommended to perform a sufficient number of measurements at
different locations on the sample and using the mean together with the standard deviation as a representative
thickness value of the coating.
3.6 Temperature
A temperature change between calibration and measurement causes errors of the measured thickness because
the resistivity of the coating varies with temperature. This temperature influence is important especially if the
-1
resistivity temperature coefficient of the coating material is high (e.g. Cu : α=0,0039 K ). Therefore, the
temperature of the sample should be measured and the thickness should be corrected with respect to temperature.
Some manufacturers provide instruments with a temperature sensor and an automatic temperature correction for
this purpose.
3.7 Probe
...