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HOME >> News Articles >> Advances in X-ray Thickness Measurement and Material Analysis
Advances in X-ray Thickness Measurement and Material Analysis
Thickness measurement is one of the most critical QA tools and requirements associated with PCB manufacture.
Example: A board with electroless nickel/Immersion gold finish should have 0.03-0.08µM of gold protecting the nickel from the atmosphere.
Too little and the gold will be porous, the nickel will pacify and the components might not stick to the board. Too much and the components might stick but the overenthusiastic gold content of the solder join may render it brittle.
You can accurately measure the nickel thickness through destructive micro-sectioning of a sample quantity or by weight gain on a test panel, this works as the required thickness for the layer of nickel (3-6µM), is much greater than that for the layer of gold. These methods will not work for the gold layer as it is to thin and to malleable, an XRF or similar is required.
Below is an article about the XRF we use at Quick Circuits, there is also a link to a further article which describes how an XRF works (the interesting bit).
Advances in X-ray Thickness Measurement and Material Analysis
The changes in European regulations have provided the impetus needed to dramatically improve the capabilities of X-ray software. Standard industrial XRF instruments will now not only detect very low levels of contaminants and banned substances demanded by RoHS, WEEE and ELV regulations, but they also have vastly improved and novel thickness measuring capabilities.
The latest techniques will allow the measurement of ultra thin coatings in the Nanometre range, complex multilayer coatings and plating solutions with Cat-ion concentrations approaching those acceptable in wastewater. These new techniques will even measure very light elements such as carbon and silicon, so that the thickness of a plastic component can be measured as well as its composition.
There are two fundamental interaction mechanisms of high-energy quantum radiation with matter: The Photo Effect and Scattering.
Whilst the photo effect leads to the emission of the fluorescence radiation, which is characteristic for each element and has been the basis of traditional XRF instruments, scattering only expresses itself in the so-called scattering background. Scattering was a nuisance before and was ignored, but it contains useful information, which, with the vastly increased power of modern computers, can be utilised to great effect. The photo effect dominates with heavy elements, such as the typical metals used in electroplating but scattering is predominant with light elements such as Carbon, Aluminium and Silicon.
The results tabulated in Example 1, below, show the measurement of a very thin platinum coating on a silicon wafer.
Several measurements were taken allowing a mean value and standard deviation to be calculated. The platinum is only 150 nanometres thick, yet the standard deviation is only .001 and surprisingly for those used to XRF measurements, the thickness of the silicon is also determined. The thickness of the silicon is obtained by modelling the scattered background spectrum, which previously would have been ignored.
Example 1Measurement of the thickness of a Si wafer coated with Platinum.
Fischerscope® XRAY XDAL
Application: 241 / Pt/Si Block : 5
n= 1 Pt 1 = 0.151 µm Si 2 = 365.8 µm
......
n= 30 Pt 1 = 0.153 µm Si 2 = 361.4 µm
n= 31 Pt 1 = 0.155 µm Si 2 = 366.2 µm
n= 32 Pt 1 = 0.153 µm Si 2 = 363.1 µm
Mean 0.152 365.1
Standard deviation 0.001 2.905
C.O.V. (%) 0.76 0.80
Range 0.005 11.80
Number of readings 32 32
A particular strength of these improved instruments is their capability for analysis of complex coatings. In practical applications, it is often necessary to examine components comprising several layers of differing alloys. This difficult task of analysing alloys buried beneath other coating layers, which themselves may be alloys, presents extremely difficult challenges. The new instruments can determine the thickness of many layers of different coatings, and also determine each layer's composition.
As mentioned above, the new regulations banning a range of substances in any products throughout the EU have been the driving force behind these improvements. Our EU politicians have decreed this ban or a limit on certain materials that pollute or harm the environment, generating these new EU wide regulations, RoHS and WEEE and the existing ELV (End of Life vehicle). All manufacturers, especially those of electrical and electronic equipment, PCBs and anything used in automobile construction, will find themselves particularly challenged as even one small component, coating or substrate will contaminate an entire vehicle or render a piece of equipment non-compliant.
The complete list of affected substances is lead (Pb), mercury (Hg), hexavalent chromium (Cr VI) and cadmium (Cd) as well as most polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBE), used as flame inhibitors in PCB manufacture. Deca-BDE is exempted, as it has been found not to be a risk to human health. After the EU regulations become effective in the UK on July 1, 2006, only 1000 ppm (parts per million) of the prohibited materials, and only 100 ppm for cadmium will be allowed in virtually all products manufactured or sold in the EU.
Spectra shows clear peak for Cadmium, one of the prohibited substances
There are exceptions for medical and the military (who incidentally face critical problems of ensuring that Pb is used on their assemblies - imagine tin whiskers, which can occur in pure tin solders, growing quietly in some ICBM silo), and there are other areas still under discussion or requiring clarification.
The onus is on manufacturers to demonstrate conformity - and continuing conformity with the EU regulations. The cost of a recall, if a failure were identified, would for most be catastrophic.
The improvements to industrial X-ray instruments make them especially economical for screening components as a first step of the inspection process. Personnel without special knowledge of analysis methods can easily identify specimens that are clear of harmful substances. Components that are near the critical borderline or above can then be analysed quantitatively or rejected. The supplier can then be required to provide proof of compliance with a detailed analysis or more simply, replacement parts.
The results tabulated in Example2 below, show the analysis of a plastic sample with contaminants of heavy metals. It can be seen that the banned substances are below the legal limit, but reliably measured with acceptable standard deviations and that the thickness of the plastic sample is also measured at approximately 1 mm.
Example 2 Determination of Heavy Metal Traces in Plastic.
Fischerscope® XRAY XAN-DPP
Application: 48 / RoHS Plastic 1
n d1[µm] Pb[ppm] Hg[ppm] Cd[ppm] Cr[ppm] Br[ppm] C [ppm] mq[ ]
1 1001 1.48 3.15 7.61 -3.00 999989 1.29
2 994.2 5.00 4.86 -2.57 4.47 999980 1.33
3 991.0 3.54 1.73 3.10 1.71 999984 1.33
4 996.5 2.22 3.13 -6.37 4.69 999993 1.33
5 1002 2.51 5.25 11.5 1.91 999975 1.37
...
10 1002 0.284 5.47 -3.22 0.293 999987 1.36
µm Pb Hg Cd Cr Br
Mean 998.3 2.119 3.157 0.130 5.519 1.239
Standard Deviation 4.183 1.985 1.891 4.749 3.364 2.461
Measuring time 100 sec
Typical X-Ray instrument used for RoHS measurements
The enhancements, both in software and hardware, on the new generations of x-ray instruments open up possibilities for ultra thin coatings in the nanometre range, for alloy coatings containing the same elements as undercoats or substrates, for low concentration solution analysis and for general analysis with resolutions in parts per million. The use of the scatter spectrum distinguish light elements, with possibilities of measurement of complex organic coatings; a completely new area for X-ray instrument measurement.
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