AN #: 05_005_16_001Market: PlatingSubcategory: ElectronicsProduct: HI8720, HI2004-1005

Application Note www.hannainst.com

Controlling ORP of a Copper Etch Bath

DescriptionMost people enjoy the ability to interact with electronics almost anywhere. This has led to a recent trend to make electronics smaller and more durable. In order to accomplish this, the manufacturing of printed circuit boards, one of the main components of portable electronic devices, had to change. Printed circuit boards, or PCBs, connect various components to each other electronically. The first PCBs were made from layered cardboard and brass wires that were bolted into place. Later, polymer resins were used to make PCBs for better durability. Eventually copper wire replaced brass wire because of its higher conductivity, lower price, and manufacturing ease.

Currently, printed circuit boards are made by layering several different materials and bonding them to each other. The combination of materials and processes has helped to improve durability, and decrease the size of PCBs. A common manufacturing practice uses a blend of fiberglass or epoxy as a base material, a copper foil layer, and a pattern mask. Holes are drilled into the epoxy boards in an orientation specific for the intended use. A photoresist material is pressed onto the copper foil layer; the photoresist material becomes soluble when exposed to UV light. A pattern mask, showing where copper will be etched is laid on top of the photoresist material, and then passed under a UV light. After the pattern is removed, an alkaline developer is used to dissolve the

areas of the photoresist that had been exposed to UV light. The pattern is left behind on the copper foil; this pattern will then be etched with copper to a thickness of about 0.04mm.

Cupric (II) chloride (CuCl₂) is a widely used etching solution in the manufacturing of PCBs. The exposed copper foil of the PCB attracts CuCl₂ in the etch solution and is etched, leaving cuprous (I) chloride (CuCl) in the used etch solution. In this reaction copper is reduced from Cu2+ to Cu+. By adding chlorine to the used solution manufactures are able to re-oxidize the copper to Cu2+. Replenishing the Cu2+allows manufacturers to reuse the bath for multiple batches, allowing them to refresh the bath less frequently and save on chemical costs. Manufacturers can monitor the oxidation-reduction potential (ORP) to determine when to make chlorine additions. When copper is predominantly present as Cu2+ the ORP of the bath will be approximately 650mV; as copper is reduced to Cu+ the ORP value will drop.

ApplicationA copper etching facility contacted Hanna to learn more about using ORP to regenerate their etch baths. After speaking with the customer, Hanna Instruments suggested an ORP controller. The controller would help to regenerate the etch solution automatically by activating a pump to dose chlorine to convert Cu+ to Cu2+ when the ORP of the bath dropped below a critical point. The HI8720 ORP Analog

Controller and the HI2004-1005 Industrial Flat Tip ORP electrode were recommended. The customer liked that HI8720 gave them the ability to choose a mV set point from ±1999 mV with 1 mV resolution. Based on the bath manufacturer’s recommendation, the customer determined that 600mV would be an ideal set point. The ability to set an alarm also was important to the customer. Since they were dosing chlorine, there was a risk of creating chlorine gas if the environment in the bath became strongly oxidizing. The alarm feature alerts operators if the ORP value rises more than 100mV from the set point of 600mV. The HI8720 ORP analog controller provided the customer an automated solution to help increase their runtime and productivity needs.