Background: A customer requested a stain on their PWB (Figure 1) be identified in order to establish what detriment it would be to the final assembly. The customer stated that the SMT step was being sub-contracted out and had actually taken place without issue.

Following visual observations, four analysis techniques were employed as a means of identifying the stain: Fourier transform infrared spectroscopy (FTIR) analysis of the stained an unstained areas, cleanliness testing through ion chromatography (IC) analysis of the board, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) analysis, and Sequential Electrochemical Reduction Analysis (SERA) of pads and or through-holes.

Test Methods:
FTIR analysis is a good tool for identifying organic unknowns and was performed first as this was a non-destructive test.

The IC testing was done in accordance with IPC TM-650 2.3.28 "Ionic Analysis of Circuit Boards, Ion Chromatography Method" and allows for ionic residues to be identified and quantified.

SERA is useful at identifying reducible species (e.g. metal oxides) and their thickness.

SEM/EDS analysis was the last analysis and provides high resolution images along with elemental analysis. In order to facilitate such testing the PWB was cut into three cross-sections, two from a stained area and one from an unstained area. One section each (stained and unstained) were individually mounted in either acrylic or epoxy mounting compound, which were then ground and polished. The mountings were sputter coated with gold and high magnification images taken with the SEM and elemental analysis was done at spot locations. A board section with the stain was sputter coated and analyzed.

Results:
The stain did not dissolve in either Isopropyl alcohol or Acetonitrile. FTIR analysis showed no significant difference between the stained area and an unstained adjacent area. The IC analysis indicated only chloride at 0.22 µg/inch2 and bromide at 7.91 µg/inch2. The bromide level was slightly above ACI's recommended level of 5 µg/inch2 for bare boards. SERA analysis of two available pads showed no significant reducible compound present.

Observations during polishing the surface of one area with a fine grit polish suggested the stain was not part of the underlying copper ground plane. The stain itself was composed of titanium along with other elements (Figures 2, 3, 4 and 5). Because of the location of the stain (at the last step or near to last step of the board fabrication process) SEM/EDS analysis of the silk screen printing material was performed. These results indicated the presence of titanium along with other elements (Figure 5).

Conclusions:
The stain was composed of carbon, oxygen, magnesium, silicone and titanium. The presence of the titanium appeared atypical and after discussion with a local board house the only source of titanium would be the plating basket used to hold the ingots of copper. The titanium plating basket would not be expected to be a source of titanium residue because of titanium's chemical stability.

Titanium was found in the silk screen printing and could be the source of the stain as titanium oxide is a common pigment. The IC analysis did not indicate any excessive levels of anionic residue. These analyses do not indicate that the residue would be detrimental. In addition, according to the customer the SMT process went well suggesting no solderability problems. Common standards like IPC 600 do not address such issues as stains.

The source of the stain was most likely from the screen printing step and a possible scenario may have been the board house made a mistake during printing and attempted to clean off the printed ink leaving some residue behind.

Recommendations:
If the scenario suggested is correct (printing pigment smeared under solder mask) because of the location of the stains and the fact that titanium could act as a conductive material either as titanium or titanium compounds, it was the EMPF’s recommendation to the customer that it would be beneficial to perform thermal or temperature/humidity excursions on assemblies to see what the long term effects would be.