RoHS Verification Using XRF

As the deadline for meeting the European Union's Reduction of Hazardous Substances (RoHS) requirements approaches, many companies have lead-free processes in place or are well on their way. With supply chains getting leaner and cycle times getting shorter, one of the practical challenges of compliance is ensuring that suppliers meet those requirements. One technology ready to help right now is X-ray fluorescence spectroscopy (XRF).

XRF is used in a variety of applications. Wafer fabs are starting to use it to perform non-destructive composition measurements on thin films, as well as to detect diffusion barrier breaches. It is also used to monitor sulfur content in the waste effluent from coal-fired power plants. Though chemical analysis should be used to verify RoHS compliance, XRF can be used as an inline tool to verify that incoming parts and materials meet specifications.

With supply chains getting leaner, a non-destructive inline test for all incoming materials can enable manufacturers to maintain lower inventory levels, since the risk of replacing non-compliant components is reduced. The penalties for non-compliance, and the opportunity loss that might also come, make an inline verification test attractive.

The materials restricted by RoHS are cadmium (<100 ppm), lead (<1000 ppm), mercury (<1000 ppm), hexavalent chromium (Cr⁶⁺, <1000 ppm), polybrominated biphenyl (PBB, <1000 ppm), and polybrominated diphenyl ether (PBDE, <1000 ppm). These levels are for homogeneous samples of materials. Despite the debate about what "homogeneous" means, it is understood that the 1000 ppm limit on lead, for example, is for the lead in solder, not the overall lead content of a circuit board.

XRF analysis can be a cost-effective component of a RoHS compliance screening scheme. (Source: Oxford Instruments)

For XRF, the strongest response is from electron transitions close to the nucleus, so atoms can be identified regardless of their chemical bonding situations. This makes XRF particularly useful for detecting cadmium, lead and mercury, which are restricted regardless of their chemical forms.

For Cr⁶⁺, XRF may be helpful with screening, depending on the sample. XRF can detect chromium, but it cannot discriminate between the hazardous hexavalent form and other forms. If the total chromium content of a sample is below the limit for the hexavalent form, it would follow that the hexavalent level is low enough. This would be the most likely scenario in an electronics manufacturing situation. When this is not the case, a destructive chemical analysis would be needed.

Screening for PBB and PBDE levels would follow the same kind of rule. XRF can determine the fraction of atoms in a sample that are bromine, and if the maximum possible PBB and PBDE levels for that bromine level are each below the limit, compliance could be verified.

There are more than a dozen companies that offer X-ray metrology equipment for the semiconductor and electronics industries. Of these, the companies currently offering XRF technology for the purpose of RoHS compliance testing of electronics include Hepco (Sunnyvale, Calif.), Horiba Jobin Yvon (Edison, N.J.), Innov-X Systems (Woburn, Mass.), Niton (Billerica, Mass.) and Oxford Instruments (Oxon, UK).

Many companies already have XRF equipment ready for RoHS compliance use, ranging from hand-held scanning equipment to workstations capable of providing composition profiles of films. The usefulness of an XRF analyzer for RoHS compliance testing seems to depend on three things: the spot size of the beam, intensity of the beam, and calibration.

The spot size of the beam is important because of the need to measure the composition of homogeneous samples. For bulk material, such as solder in a wave solder bath, a wide beam would be sufficient. Checking the lead content of a solder bump prior to BGA or flip-chip attach requires a spot size smaller than the bump. Checking the lead finish on discrete components is likely to be an important part of compliance verification, and to ensure accurate results, the beam needs to hit only the finished surface under test.

Beam intensity is also important, since that determines how deep into the material the beam goes. Again, to check the composition of the finish on a lead wire, the beam should only penetrate into the finish material to ensure that the measurement is of a homogenous material. Some of the machines available now can profile the composition of a film. This capability is more useful for engineering than for screening purposes, but it is an indicator of the control that currently exists.

As for any metrology tool, calibration is important. Some concerns have surfaced about the calibration and sensitivity of XRF analyzers for this application, and there are methods that are more accurate. But XRF analyzers can be a cost-effective component of an overall screening and testing scheme for RoHS compliance.

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