Discoloration / Staining
Stains can lead to materials/products failing.
Techniques such as XPS, SIMS, GC/MS and SEM/EDX can be utilised to identify stains as low as one atomic layer on a surface/ppb range and quantify elemental and molecular composition, whilst suspect materials can be fingerprinted in order to identify the source of the stain and eradicate it.
Analysis of a White Residue on a Circuit Board Connector
Populated circuit boards were supplied with SCSI connectors showing a white residue on most of the connector pins. It was suspected that this residue might be associated with solder flux or mould release from the connector housing itself. Surface analysis was carried out on the contaminated pins and compared to the pins of an unused connector using both XPS and ToF-SIMS.
Surface Chemical Compositions (Figures are in At%) from XPS
|Unused connector, good pin||41.8||29.3||19.2||4.8||4.9||-|
|Used connector, bad pin with white residue||79.5||17.6||1.0||0.3||0.4||1.1|
XPS showed that the levels of metallic species on the bad pins were quite low compared to the good pins. Concomitantly, the high carbon levels on the bad pins suggested an organic cause for the contamination. Examination of the carbon functionality by high resolution analysis showed that aromatic type species were present on the contaminated surface.
In agreement with XPS, ToF-SIMS analysis showed that the good pins presented a more metallic surface whereas the surface chemistry of the bad pins were dominated by organic species. In particular, aromatic hydrocarbon and carboxylate (abietate) signals were detected on these contaminated surfaces. Research into the composition of the solder flux indicated that it was based on a rosin containing mainly a mixture of carboxylic acids of the abietic type. This strongly suggested that the white contamination corresponded to solder flux residue where, in view of the sample's thermal history, the aromatic hydrocarbon reflected a partial degradation product of the original component based on a rosin.
Surface Staining on Device Wafers
This case study relates to the characterisation of a green surface stain, present on the reverse side of a failed device wafer using ToFSIMS and XPS.
- ToFSIMS spectra of stain and non-stain areas show the presence of silicon oxide, organic residues, sodium, aluminum, fluorine and chlorine.
- The relative levels of contaminants are similar on both areas.
- XPS analysis revealed similar levels of C, N and Al in both stain and non-stain areas.
- High resolution XPS of the Si2p spectral region revealed significantly higher levels of silicon oxide in the stain area.
Time of Flight SIMS and XPS combined to reveal higher levels of silicon oxide in the stain region. Identification of a faulty process step led to removal of the problem with subsequent yield improvement.
Degradation of Plastic Building Materials
Relevant testing of building materials in the environments where they will be used is essential to avoid short and long term defects occurring. One visual defect has plagued uPVC panels and extrusions applied to the outside of properties in the wetter parts of Northern Europe for several decades. This defect has become widely known in the UK as "Pinking".
In this example, part of the facia boarding has become pink while other parts of the uPVC structure have remained white, although they were only installed five years ago and have experienced the same weather conditions. This "pinking" problem has been comprehensively investigated and explained by methods developed at Lucideon, using a range of surface analysis techniques including XPS, ToF SIMS and Dynamic SIMS. These proprietary methods can be applied to any weatherable grade of uPVC which exhibits discolouration.
The value of undertaking this type of investigation is that:
- The cause of the problem can be determined and responsibilities established.
- Future materials designs can be developed to avoid this and other degradation problems in uPVC.
Discoloration of Steel Sheets used in Photo Film Canisters
The purpose of this analysis was to identify the cause of brown spots and a yellow discoloration on a defective steel sheet. Initial SEM-EDX analyses, conducted by the customer, identified the brown spots as iron oxide with an additional indication of a deficiency in the 10nm thick, electro-coated chromium overlayer used to prevent corrosion. Surface analysis was commissioned at Lucideon to further characterize the stains, identify the yellow coloration and a possible cause for the lower level of chromium on defective sheets.
Surface Chemical Compositions from XPS (Atomic%)
|Problem sample (yellow area)||4.5||3.6||38.1||50.0||3.8|
|Problem sample (brown stain)||10.1||trace||44.1||45.8||-|
The XPS data show that iron is not detected on the reference sample, indicating a chromium (and carbonaceous) overlayer of thickness greater than 5nm. By contrast, iron oxides are detected in both yellow colored and brown stain areas, indicating oxidation of the whole surface of the steel sheet.
High Resolution XPS - Chromium and Iron (2p) Oxidation States
|Sample||Cr (metal)||Cr (oxides) ||Fe (metal)||FeO||Fe2O3||Fe3O4 |
High resolution chromium XPS spectra show only oxidized chromium on the defect sample whereas a metallic state is present on the reference sample, indicating an intact, protective chromium overlayer. ToF-SIMS confirmed high levels of iron (and low levels of chromium) on the defect sample. In addition, for the defect sample only, a bisphenol-A containing species was detected. This contaminant is related to, or indeed the cause of, poor chromium deposition.