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44 PCB007 MAGAZINE I OCTOBER 2025 surement systems are evaluated using a type 1 gage study. Low-capability systems (Cg ≤ 1.33) must either increase measurement counts, which is often impractical for production, or apply guard bands that further narrow the tolerance window. In practice, non-conforming XRF systems can reduce allowable gold tolerance by up to 20%, increasing non-confor- mances and forcing tighter process control. For fabricators, especially those with older XRF equipment, the combination of a tighter specifi- cation, increased SPC demand, and production cadence makes ENEPIG compliance under IPC- 4556A a significant manufacturing challenge. Deposit Thickness Measurement Capability in IPC-4556A IPC-4556A maintains the requirement for a type 1 gage study to confirm XRF measurement repeat- ability when monitoring ENEPIG deposit thickness. Repeatability (Cg/Cgk ≥ 1.33) ensures the same reading can be consistently obtained on the same part under identical conditions, leaving more of the specification window available for true process variation. As mentioned previously, older collimator-based XRF systems often fall short of this threshold, requir- ing guard bands that further tighten the tolerance window. Newer technologies, such as poly-capillary optic XRF, significantly improve repeatability, reduc- ing the need for guard bands and allowing full use of IPC 4556A's gold thickness range. In a comparative study, both systems measured a 1.93 µin ENEPIG gold standard over 30 identical readings. The collimator XRF produced Cg/Cgk val- ues of 0.84/0.76, triggering guard bands and cut- ting the usable tolerance to 76% of the standard. The poly-capillary system achieved 1.72/1.67, requir- ing no guard bands. Process capability analysis of production sam- ples showed both systems met IPC-4556A thick- ness limits, but the lower capability XRF reduced Cpk from 5.17 to 1.07. The results underline the importance of high-repeatability XRF systems to avoid unnecessary guard band penalties and main- tain stable ENEPIG gold thickness control. Electroless Nickel Phosphorous Content in ENEPIG The phosphorous content of electroless nickel (EN) directly affects corrosion resistance and, in turn, gold deposition. In ENEPIG, as in ENIG, immersion gold relies on a galvanic displacement reaction, making gold thickness sensitive to variations in the EN percentage of phosphorus. Fluctuations in the (phosphorous) content can cause gold thickness variation, challenging compliance with IPC-4556A's tight tolerances. Using an EN chemistry with minimal percentage phosphorus variation helps maintain consistent gold thickness. Hybrid or reduction-assisted gold systems can lessen, but not eliminate, the impact of EN percentage of phosphorus variation, as gold ini- tiation still depends on galvanic displacement. Electroless Palladium Challenges While adding a palladium layer between EN and Re g re s s i o n a n a l ys i s h i g h l i g ht i n g t h e i m p o r t a n c e of E N % P o n E N E P I G Au t h i c k n e s s . ▼ H y b r i d a n d i m m e rs i o n g o l d t h i c k n e s s c a p a b i l i t y w i t h 1 0 µ i n of Pd . ▼