Quality Control for PCB technology
Proving quality is becoming increasingly essential in industrial production. This also applies to PCB technology where Electroless Nickel Immersion Gold (ENIG) or Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) are important surface finishes. Thicknesses of individual layers are crucial for function and reliability. IPC-4552B (for ENIG) and IPC-4556A (for ENEPIG) define requirements for measurement equipment to check layer thickness. This is done by means of a measurement system analysis (MSA). The FISCHERSCOPE® X-RAY XDAL®-PCB 200 is developed for the needs in PCB industry and easily fulfills IPC requirements.
Surface finishes on copper
Due to its high conductivity and moderate costs, copper is the preferred material in PCB technology. Chemical reactions with the atmosphere or unprotected handling induce uncontrolled aging effects of the copper surface. For this reason, creating an electrical contact directly on copper e. g. by soldering does not deliver stable processes. As a solution surface finishes on top of the current carrying copper layer have been developed. Such Layer thicknesses have to lie within given tolerances to constantly keep quality on a high level. In order to guarantee this, not only the process needs supervision but also the measurement equipment. IPC-4552B (for ENIG) and IPC-4556A (for ENEPIG) require a demonstration of measurement system capability and the use of guard bands. A missing layer has to be measured as statical zero. This is also required by the IPC and needs to be verified.
Measurement system analysis
The result of a measurement system analysis are the gauge capability indexes. They are calculated from the given tolerance T and statistics of 30 (ENIG), 25 (ENEPIG) or more measurements on a traceable standard with the following formulas:
Where T is the tolerance, s the standard deviation and x̄ - xm the bias calculated as the deviation of the measured mean x̄ from the nominal value of the standard ₓm.
The standard value should be as close as possible to the nominal value of production. Cg and Cgk need to be bigger or equal to 1,33 to consider MSA as passed. For the gold layer of ENEPIG there is no upper limit and thus no gage capability is calculated. Instead, the MSA procedure is followed to calculate the lower guard band which should not be undercut.
Summary of gauge capability and guard bands for IPC-4552B
Table 1 and 2 sums up MSA results for the FISCHERSCOPE X-RAY XDAL-PCB 200 with a spot diameter of 0.3 mm, a measurement time of 60 s and a measurement distance of 5.5 mm. Especially thin layers of gold and palladium can be measured very precisely with the Silicon Drift Detector technology (SDD) used in the XDAL-PCB 200.
| IPC | 4552B | |||
|---|---|---|---|---|
| Standard | CAYGL | CAVZY | ||
| Layer | Au | NiP9 | Au | NiP9 |
| Thickness [nm] | 26 | 3200 | 47 | 2340 |
| Cg | 1,69 | 7,18 | 1,54 | 6,72 |
| Cgk | 1,49 | 7,08 | 1,45 | 6,61 |
| UGB [nm] | 96,4 | 5958 | 96,1 | 5955 |
| LGB [nm] | 43,6 | 3042 | 43,9 | 3045 |
Summary of gauge capability and guard bands for IPC-4556A
| IPC | 4556A | |||||
|---|---|---|---|---|---|---|
| Standard | CCHZU | CCIFP | ||||
| Layer | Au | Pd | NiP10 | Au | Pd | NiP10 |
| Thickness [nm] | 20 | 31 | 4700 | 49 | 105 | 4700 |
| Cg | - | 2,67 | 5,38 | - | 1,42 | 6,54 |
| Cgk | - | 2,54 | 5,33 | - | 1,39 | 6,13 |
| UGB [nm] | - | 287,5 | 5926 | - | 276,5 | 5939 |
| LGB [nm] | 33,5 | 62,5 | 3074 | 35,2 | 73,5 | 3061 |
IPC-4552B and IPC-4556A requirements
IPC-4552B requires missing layers to be measured (0 ±3) nm. Instead IPC-4556A requires no statistical significance. We chose k=2 as the acceptance criterium which requires absent layers to be measured within ±2 standard deviation from zero. The XDAL-PCB 200 passes IPC-4552B and IPC-4555A criteria with the above-mentioned measurement conditions.
Conclusion
The FISCHERSCOPE X-RAY XDAL-PCB 200 is suited to measure ENIG and ENEPIG according to IPC-4552B/IPC-4556A. It easily fulfills the MSA requirements and absent layers are measured statistically as zero as specified by IPC. As required by IPC we use the latest Silicon Drift Detector (SDD) technology which allows to measure ultrathin layers of gold and palladium. The XDAL-PCB 200 is specifically designed for the needs of the PCB industry.
