IPC-4562 stipulates the maximum resistivity for ED copper foils (see IPC-4562, point 220.127.116.11, table 3-2). However, these values are based on untreated copper foils, which is not representative for the final user. Our untreated foils are typically below the IPC limits, e.g. for a nominal thickness of 35 µm the resistivity is lower than 0.162 Ωg/m².
Treatment has a non-negligible impact on the foils electrical resistivity:
~4.7 to 5.4 µΩcm for Cu/Zn ratio ~85 % / ~15 % and
~6.3 µΩcm for Zn only.
The IPC-4562 indicates a value of 8.93 g/cm³ [5.16 oz/in³] for the ED-copper’s density (IPC-4562, table 1-1, footnotes).
The thermal expansion of our copper is in a range between 16 ppm and 17 ppm when tested in a temperature range from 20 – 250 °C.
According to the IPC-4562 the purity has to be determined on untreated copper foils. The purity of our copper is better than the IPC limit of 99.8 %.
Including the treatment, the purity is >99.7 %
The value of 35 µm is a nominal thickness value only. The nominal average area weight of 305 g/m² corresponds to a value of 34.15 µm. IPC-4562 stipulates for all copper foils a tolerance of ± 10 %. This means for 1 oz (35 µm) foil the area weight might vary between 274.5 g/m² and 335.5 g/m². By simple calculation using the copper density (8.93 g/cm³), the resulting theoretical thickness can vary between 30.74 µm and 37.57 µm.
For a typical average area weight for a 1 oz copper foil of 285 g/m² the calculated copper thickness is 31.9 µm.
Additionally, for all thickness measurements of copper foils in cross section the roughness of the treatment side has to be taken into account. The pyramidal and dendritic shaped matte side doesn’t allow a simple determination of the thickness. The graph below should explain this effect.
- corresponds to the total thickness measured with a micrometer device
- is the roughness as determined with a stylus profilometer
- corresponds to an absolute minimum thickness of the bulk foil without the pyramidal structure
- corresponds to the foil thickness using in addition ~? to ~½ of the treatment height