Electroless Copper Plating

Metallization is required on the wall of the drill holes to make them conductive. It can be done by chemical deposition or direct metallization. For double layer boards, chemical deposition is common. It is an electroless process because the deposition of copper is done in the absence of electrical current. The objective of the electroless copper plating process is to deposit a very thin layer of copper over the entire surface of the copper cladding and on bare hole walls. This layer should be thick enough to allow the electrical current during the subsequent electrolytic plating. Normally, a thickness of 0.001 Mil to 0.0025 Mil is acceptable for further electroplating at low start current. The thickness of the PTH will be increased to the required level after the electrolytic copper plating.

Several proprietary solutions are available for making electroless copper. Basically, a precious metal palladium is being used as an activator to provide a site for the adhesion of electroless copper. Electroless copper bath contains an alkaline chelated copper reducing solution that deposits thin layer of copper on activated surface.
The thickness of electroless copper needed can be observed from the following table which gives the thickness of copper foil on Aluminum pcb. For reliable operation, PTH wall thickness should be around 0.7 Mil to 1 Mil.

Electroless copper plating process is consists of 4 baths; acid predip bath, activator bath, postactivator bath and electroless copper bath. Most of the process can be done at room temperature. We'll use PTH process chemicals from Enthone-OMI as an example. Circuitprep 3023 is used to prepare the acid predip bath. It is a low acid solution to replace a bath of high concentration hydrochloride acid. An immersion for 1 - 2 minutes in Circuitprep 3023 is sufficient to attain balance in the activator bath. The preparation of this bath is simple, 100% concentration is needed. It should be noted that no water should be allowed from this bath onward until the PTH process is completed.

The next bath is the Circuitprep 3316 palladium catalystic process. Major ingredients are palladium chloride and hydrochloric acid. This is a catalyst for electroless deposition on insulators. This process should provide uniform void-free metal coverage in holes and good electroless copper to copper clad adhesion.

The next bath is the Circuitprep 4044 postactivator process. Working in conjunction with CP3316, CP4044 accelerates the initial deposition of copper onto an activated surface. Mechanical agitation is required to ensure the removal of air bubbles in the holes and a complete wetting of the PCB. The active ingredient is fluoboric acid. It remove the protective stannous from the CP3316 so that electroless copper can adhere to the activated surface easily.

Enplate CU-406 concentrates are used for electroless copper disposition. Heater is normally required in electroless copper bath. The solution itself is highly unstable and we made use of its thermal property to restrict the displacement of copper from the solution. Thus, depending on the formula, temperature can be used to control the process so that it won't precipitate all the copper when the solution is idling. In fact, a cooling coil can be installed in the bath to cool it down to around 12 ° C to save it for a longer period in time.