IPC-4556 印制板化学镍钯浸金（ENEPIG）规范ENG - 第35页

In the case of immersion Pd however , we can normally assume this ef fect is insigniﬁcant for two reasons: a) The Pd thickness is very low where the thickness measurement is not very dependent on the layer purity . b) Th…

(2) A second source of potential error lies with the electroless nickel layer. Proportional counter based XRF instruments

cannot measure phosphorous content in the Ni layer directly. Therefore it is generally assumed in the calibration of the

instrument that the Ni layer contains some constant amount of phosphorous composition, typically about 8%. However,

if the sample does not contain 8% phosphorous in the Ni layer, errors in electroless Ni thickness will occur. As a very

rough rule of thumb, one can expect about a 4% thickness error for each 1% difference in phosphorous content between

the sample under test and the standards used for calibration.

The electroless nickel thickness measurement will be erroneously high if the sample phosphorous content is less than

the calibration standard. The measurement of thickness will be erroneously low if the sample contains more phospho-

rous than the standards used for calibration. Since most platers maintain the phosphorous content at approximately 8%,

the electroless nickel thickness measurement error is normally minor.

Although electroless Ni thickness errors due to phosphorous content variations are normally less than 10%, it is pos-

sible to correct for changes in sample phosphorous content if they are known. Some XRF software does offer such cor-

rections which allow the user to enter the known % P content in the nickel layer and the thickness measurement will

automatically be corrected. Alternatively, suppliers of such XRF instrumentation should be able to provide correction

factors for changes in sample % P which can be entered as ‘‘density’’ factors that can correctly compensate for changes

in sample % P.

(3) Measurement of the immersion Pd layer is subject to two possible errors. The more signiﬁcant potential error is caused

by variations in the x-ray background level in the spectral region where the Pd K-α peak is detected. The Pd K-α peak

is added to this background level. Typically, XRF plating thickness measuring instruments integrate this area to obtain

Pd intensity and relate that intensity to Pd thickness. In the case of immersion Pd where thicknesses are in the order of

0.05 µm - 0.15 µm [2 µin - 6 µin], the spectrum background level is often as intense or more intense than the Pd peak

itself. If the background level is constant, this inﬂuence may be included in the calibration. However, the background

level may, in fact, vary. This potential background level variation is a function mainly of the Cu layer’s thickness below

the Pd.

The background scatter originates mainly from the epoxy substrate. The W or Mo target x-ray sources provide a broad

band of x-ray energies to the sample during the measurement. It is mainly the higher energy x-rays that will penetrate

the thick Cu-clad laminate, scatter back from the epoxy through the Cu layer to be detected and seen in the measured

spectrum. Lower energy x-rays originating from the source, do not have enough energy to penetrate the Cu layer twice

and be detected. Therefore background ‘‘noise’’ is only an issue in the higher end of the x-ray energy spectrum.

It is in this high end of the spectrum that the Pd K-α appears (~ 21.1 keV). The amount of background scatter that

reaches the detector is again a function of the Cu thickness since the scattered x-rays are shielded primarily by this rela-

tively thick layer. As with the Br interference with Au, the background scatter will be a function of the underlying Cu

thickness as well as the spatial resolution of the x-ray beam and the position of the beam relative to the edges of the

sample plated area.

Again, to be prudent and to optimize accuracy of Pd thickness measurements, the potential varying background level

should be compensated for to obtain net Pd intensity information. Most XRF instruments are equipped with background

correction software to deal with this issue and obtain reasonably accurate net Pd intensities. The user should be famil-

iar with how to access this feature if its use is optional, or know if automatic background correction is always used by

their XRF instrument to measure plating thickness. There are a wide variety of background correction schemes that are

available, some of which are more effective in this case than others.

Other approaches to mitigate Pd measurement errors due to varying background levels include the use of primary beam

ﬁlters to attempt to suppress the background level to a magnitude far below that of the Pd K-α peak intensity. This

method can work well when using, for example, a Mo ﬁlter with a larger collimator. Since primary ﬁlters reduce over-

all signal intensity the ﬁlter material and thickness must be selected with an attempt to balance the gains from reduc-

ing the background and the loss due to reduced Pd signal intensity. Typically, primary ﬁlters are effective only when

using larger x-ray beam collimators. Users should consult with the manufacturer of the instrument before attempting to

use a primary ﬁlter.

Less signiﬁcantly for the commonly used XRF instrument is the error bias that may be introduced by the fact that

immersion Pd layers are not pure Pd. Instead, it is common that such layers contain about 2% phosphorous. Like the

electroless nickel measurement, the level of phosphorous in the plating has some effect on the thickness measurement.

IPC-4556 January 2013

In the case of immersion Pd however, we can normally assume this effect is insigniﬁcant for two reasons:

a) The Pd thickness is very low where the thickness measurement is not very dependent on the layer purity.

b) The Pd x-ray energy is almost 3X higher than Ni so that the absorption of Pd x-rays within the layer by phosphorous

is much smaller than it is for nickel. This combined with (a) above will result in errors of about 5% assuming the cali-

bration standards are pure Pd (which is typically true).

Much of this error originates with the density difference between immersion Pd and pure Pd used for calibration standards.

By requesting standards with the Pd certiﬁed to a density of 11.6 g/cc instead of the conventional 12.0 g/cc, the bias error

is reduced to less than 2%.

Long term stability of XRF calibrations using proportional counter detectors for ENEPIG applications is often compromised

primarily by the use of the peak deconvolution method for the Au. This method places higher demands on the detector elec-

tronics stability, making the measurement more sensitive to instrument drift. To avoid additional error due to this effect, one

should check calibration standards frequently, at least twice a day, to assure that the instrument is still within calibration

tolerances. Use supplier provided drift correction features to correct for drift if standard measurements are outside their tol-

erances. If drift correction fails to restore the calibration to within tolerance, the instrument should be recalibrated.

Summary for Common Plating Thickness Measuring XRF Instruments The predominantly used XRF type in the PCB

industry to measure ENEPIG coatings should be calibrated with the following considerations in mind:

1. Use of peak deconvolution for the Au layer measurement.

2. Use of an appropriate background subtraction model for the Pd layer.

3. Use of appropriate corrections for differences in phosphorous content between calibration standards and test samples for

the nickel layer and, less importantly, the Pd layer.

4. Use of calibration standards with similar thicknesses to the speciﬁed ENEPIG thicknesses which are to be measured. If

possible, request Pd thickness certiﬁcation to the level of % P expected in the Pd layer and request electroless nickel lay-

er(s) with % P content within 1% of the expected % P content for the samples which are to be measured.

5. It is also helpful if the standards are plated over a Cu laminated epoxy substrate containing Br if the samples to be tested

will also contain Br.

By requesting standards with these parameters, one can most closely approximate the inter- element matrix effects and spec-

tral effects discussed above, resulting in an optimized calibration which minimizes thickness measurement error. Use cali-

brations standards to check and if necessary correct for long term stability problems. Calibrations standards are used to check

that the calibration is still within tolerance from day-to-day, to assure that long term stability limitations of the instrument

do not compromise results.

Alternative XRF Instruments & Configurations: Advantages and Disadvantages In recent years, more capable XRF

instruments have been introduced for plating thickness measurement. The most signiﬁcant advance involves the use of solid

state detectors, typically silicon PIN diodes and most recently silicon drift detectors (SDD). These detectors offer a major

advantage over the common proportional counter used in XRF’s employed in the plating industry in terms of energy reso-

lution. It is possible to achieve energy resolutions of about 5X better than the proportional counter with these detectors. The

advantage instruments with these detectors offer is primarily in the Au layer measurement. In the case of Au, the Au L-β

peak and the Br K-α peaks can be better resolved, reducing overlap interference.

Furthermore, the Au L-α peak is fully resolved from any overlap with the Cu K peaks.

As a result, the Au L-α peak may be used without resorting to deconvolution techniques. While peak deconvolution routines

needed for proportional counters minimize error due to peak interference from other elements like Br, they include their own

inherent potential errors, smaller but nevertheless present. Peak deconvolution is especially difficult to employ well when

plating thickness becomes very thin. For Au layers less than 3 microinches, peak deconvolution methods can struggle to

achieve good accuracy when Br is present.

In addition, the nature of the peak deconvolution method requires excellent electronics stability to maintain measurement

accuracy without frequent recalibration. In some cases, instrument design does not offer the needed stability to avoid fre-

quent drift correction and recalibration.

Use of solid state detector based XRF systems eliminates the need for peak deconvolution methods with respect to Au thick-

ness measurements and permits Au measurements of layers less than 1 microinch. Therefore, solid state detector XRF sys-

tems tend to offer much better long term measurement stability and are much less reliant on operator know-how and vigi-

lance to achieve accurate Au results.

January 2013 IPC-4556

Solid state based XRF instruments offer some advantage for Pd measurement as well.

Although the varying background scatter issue is still present, the overall background level tends to be lower compared with

proportional counter systems, again allowing a clearer view of the Pd peak above the background. This translates into the

ability to measure lower thicknesses of Pd using solid state detectors relative to proportional counters. The disadvantage in

terms of Pd measurement is that solid state detectors are not as efficient as proportional counters in detecting the high energy

Pd K-line x-rays. As such, overall x-ray intensity for the same Pd thickness will be less when using a solid state detector than

a proportional counter, given all other factors being equal. This disadvantage is partially negated by the overall lower back-

ground levels observed with solid state detectors.

Other considerations noted for the proportional counter based XRF systems must be applied as well with solid state detector

based systems. These include corrections for phosphorous content in both the nickel and again, to a lesser extent, the Pd

layer. The advantages offered by the solid state detector when measuring ENEPIG come with other disadvantages, as well.

First, the cost for solid state detector based systems is higher than proportional counter based systems. Second, the size of the

detection area is smaller than proportional counters. As a result, less x-rays are detected per unit time under the same exci-

tation conditions. Generally, larger x-ray collimators must be used to compensate for the smaller detection area. This means

that measurement of sample areas less than 12 mils wide or in diameter can be problematic with solid state detector based

XRFs. Typically longer measurement times must be used to achieve the same level of measurement repeatability as a pro-

portional counter based XRF. However, if collimators that are 20 mils or larger can be used, then measurement time for both

types of XRF’s can be comparable to achieve similar repeatability.

XRF Instruments Using Cr Target X-ray Source Some proportional counter based XRF systems have been provided by at

least one manufacturer using a Cr target x-ray source. The principle advantage of the Cr target source is its ability to excite

the Pd L series peaks (~ 3 keV). Once these peaks are sufficiently excited, measurement precision is signiﬁcantly improved

for the Pd layer since the sensitivity of Pd L line x-ray intensity, in terms of unit thickness change, is excellent. This high

sensitivity to Pd thickness change is especially advantageous in the working range for immersion Pd thicknesses used in

typical ENEPIG applications. The improved measurement repeatability and shorter measurement times that may be used,

allow for better process control. Furthermore, by shifting the Pd analysis from the high energy K line typically used to the

low energy L line, the issue of background scatter variations from the substrate is eliminated. This is because low energy

scatter will not vary in this case.

Thus the measurement of Pd is less dependent again on operator know-how and vigilance. By using the Cr target source with

a solid state detector, one may have the same advantages the Cr target offers for the Pd layer and achieve the advantages for

the Au layer offered by the solid state detector. Again there is a down side to this strategy as well. Very small areas (<12

mils) are difficult, if not, impractical to measure (at least long measuring times are required). Secondly, Cr target sources

typically do not have as long an operating life as the common W or Mo target sources. Therefore, cost of ownership is

higher.

Vacuum Based XRF Systems Even higher cost XRF systems are available that use only solid state detectors and offer the

ability to evacuate the X-ray chamber of air. By evacuating the X-ray chamber, one gains the ability to directly detect phos-

phorous x-rays. Detection and counting phosphorous x-rays allows for the direct measurement of phosphorous content in

electroless nickel. This provides the user with direct, accurate composition measurement and allows the XRF to simultane-

ously calculate electroless nickel thickness based on the actual phosphorous content. This represents the most accurate way

to measure electroless nickel layers.

The same advantages and disadvantages described above for measurement of Au and Pd layers using solid state detectors

applies with one exception. If one chooses to conﬁgure the vacuum XRF with a Cr target source to analyze the Pd L peaks

intensity or alternatively uses x-ray optics to produce microbeams to measure very small sample areas, the use of an evacu-

ated environment reduces background substantially for the Pd L peak. When measuring Pd L intensity in atmosphere, argon

ﬂuorescence interferes with Pd L peaks (Ar is 1% of the atmosphere). By evacuating the sample chamber, this effect is elimi-

nated, making Pd L peak intensity determination more precise.

To summarize the various effects one must address when calibrating and measuring ENEPIG coatings by XRF, and the vari-

ous solutions offered by different XRF conﬁgurations, the following Table A4-1 is offered. In addition, the supporting section

to this article includes some example spectra that illustrate some of the effects discussed.

IPC-4556 January 2013