Heavy Cu Issues
Throughout the years, we have tested and perfected stackups and calculators that are instrumental in helping us to achieve the best of both worlds – keeping within the maximum thickness allowance while still pressing with enough resin to fill all voids.
Furthermore, our lamination presses use hot oil platens to ensure heat is applied uniformly across the entire production panel. This helps to guarantee and even cure across the panel, which is key to avoiding delamination.
Inner Layer Alignment
Generally, heavy copper PCBs are produced using thin core materials. Thin core materials are known to require varying degrees of scale factors to account for material shrinkage during lamination. Heavy copper PCBs are more prone to alignment issues that manifest themselves in the form of exposed copper in cutouts for windings and along the edges of the PCBs. This is due in part to the larger window of etch factor that is common for heavy copper PCBs.
We have avoided this issue in large part to studying varying core materials in combination with copper retention percentages. A key instrument in these studies is the use of the PerfecTest system and resulting knowledge database. We consult this database each time a new part number is produced.
As a result of our extensive experience with Heavy Copper PCBs, we have achieved UL certification of up to 6 ounce copper for inner and outer layers. For non-UL requirements, we have produced up to 20 ounce copper PCBs.
One part of the fluid dynamics of the etching process is called “puddling”. Since etching is a horizontal process, etching chemistry is sprayed on to the top and bottom of the panel. While the chemistry that is sprayed from the bottom falls back off the panel, the chemistry sprayed from the top remains on the panel, forming a puddle. This puddle then absorbs much of the force of the newly sprayed chemistry. Since etching is as much a function of the physical force of the chemistry hitting the copper as it is a chemical reaction, the etch rate on the top side of the panel is reduced. This requires the operator to slow down the line speed in order to properly etch the top surface. As a result, the bottom side is often over-etched.
Saturn has recently installed Hollmuller etching lines for both inner and outer layers. These lines have the latest anti-puddling spray technology incorporated within them. This technology uses a specific spray pattern and nozzle arrangement, along with an oscillating spray bar, to “sweep” away old etchant so that newly sprayed etchant is sprayed directly on to the copper surface. Furthermore, these lines automatically control temperature and chemistry concentrations to insure uniformity throughout the production process.