The Electroless Nickel / Immersion Gold (ENIG) deposit is one of the most widespread surface finishes in printed board manufacture today. The Institute for Printed Circuits (IPC) released their first specification for ENIG in 2002, followed by revision A in August 2017, with the intention of helping manufacturers create a more reproducible and reliable ENIG surface finish. Revision B was introduced in July 2021 which primarily focuses on corrosion with limited impact on XRF usage.
When the coating thicknesses are managed well, ENIG is an excellent surface finish for reliable solder joints and aluminium wire bonds, and has a relatively long shelf-life. The thin outer layer of immersion gold is very stable and prevents oxidation of the underlying nickel for the life of the component. However, the high performance of ENIG is very much dependent on the quality of the nickel and gold layers.
The July 2021 revision to the specification helps manufacturers understand what they need to do to meet printed board performance requirements, including the J-STD-003 printed board solderability specification. This revision (retaining XRF specifications introduced in IPC-4552A in August 2017) focuses on the thickness of the gold layer; the minimum allowable thickness has been reduced, and a new parameter for the maximum gold thickness has been introduced. If the gold thickness in finished part is too low, the deposit may not remain intact once in use. This would result in corrosion and cause weak solder joints and printed board failure. With the critical nature of the gold thickness in mind, the specification focuses on three critical factors:
In addition to specific information on thickness measurement, the IPC-4552B gives details on corrosion identification, with pass and fail criteria.
Revision A of the specification was the result of extensive testing by the IPC and associated organisations, and as such the specification is likely to result in a high quality finish. However, the stringent measurement controls imposed may be difficult to meet in a production environment.
One of the main issues is around the measurement equipment. XRF instruments are used to measure the thickness, and the IPC-4552B details the calibration methods required to meet the statistical requirement of the specification.
A specific issue is the measure of the gold thickness. In order to use an XRF, the instrument must meet specified performance requirements including stringent precision criteria. There are two major classes of XRF detectors in use for plating analysis: proportional counters and high-resolution semiconductor detectors such as a silicon drift detector (SDD). Both may be used to conform to IPC-4552B, so it is incumbent on the production facility to evaluate their existing equipment and decide which detector technology is most appropriate for their process, as this will have an impact on analysis time and control tolerance. High-resolution detectors can more easily differentiate gold peaks from copper and bromine which may improve results.
IPC-4552B goes into great detail on XRF setup and calibration, including: collimator size and measurement times, ENIG calibration standards and zero offset acceptability. You can obtain a full copy of the specification directly from IPC here.
Hitachi High-Tech Analytical Science are members of the IPC and we fully recommend following IPC guidelines for achieving quality and reliability for ENIG manufacture, specifically when related to XRF measurements. Our XRF instruments are developed to keep up with the rapid advances of PCB technology and are designed to help you achieve consistency and reliability in production.
Read more about conforming to the specification here:
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