PTH process in PCB manufacturing

Electroless copper plating, also known as plated through hole (PTH), is a self-catalyzed redox reaction. The PTH process is performed after drilling two or more layers.

The function of PTH: On the drilled non-conductive cell wall substrate, a thin layer of electroless copper is electroless deposited as a substrate for subsequent electroplating copper.

Decomposition of PTH process: alkaline degreasing → 2 or 3 countercurrent rinses → roughening (micro-etching) → secondary countercurrent rinse → pre-dip → activation → secondary countercurrent rinse → debonding → secondary countercurrent rinse → sinking → two Level countercurrent flushing → pickling.

PTH detailed process description:

1. Alkaline degreasing:

Remove oil, fingerprints, oxides, dust in the holes of the board surface;

Adjust the charging of the pore wall from negative to positive to promote the adsorption of colloidal palladium in the subsequent process;

After degreasing, it should be cleaned strictly according to the guidelines and should be tested with copper backlight test

2. micro etching

Remove oxide from the surface of the board and roughen the surface to ensure good adhesion of subsequent copper layers to the copper on the bottom of the substrate.

The new copper surface has strong activity and can adsorb colloidal palladium well;

3. pre-impregnated

It mainly protects the palladium tank from the pollution of the pretreatment tank and prolongs the service life of the palladium tank. Except for palladium chloride, the main components are the same as the palladium tank, palladium chloride can effectively wet the pore wall, and promote the subsequent activation of the activation solution to form pores. sufficiently effective activation;

4. activation

Pretreatment After alkaline degreasing and polarity adjustment, the positively charged pore wall can effectively adsorb negatively charged colloidal palladium particles, ensuring the subsequent average, continuous and dense sinking of copper; activation is crucial to the quality of subsequent copper baths . Control points: specified time; standard stannous ion and chloride ion concentration; specific gravity, acidity and temperature are also important, and must be controlled strictly in accordance with the operating instructions.

5. Peptidation

The stannous ions of the colloidal palladium particles are removed, and the palladium nuclei in the colloidal particles are exposed to directly catalyze the initiation of the chemical copper precipitation reaction. Experience has shown that the use of fluoboric acid as a debonding agent is a better choice.

6. Electroless Copper Plating

The self-catalytic reaction of electroless copper plating is caused by the activation of the palladium nucleus, and both the new chemical copper and the reaction by-product hydrogen can be used as reaction catalysts for the catalytic reaction, allowing the copper precipitation reaction to continue. After this step, a layer of electroless copper can be deposited on the surface of the plate or on the walls of the holes. During this process, the bath should be kept under normal air agitation to convert more soluble divalent copper.

The quality of the copper plating process is directly related to the quality of the produced circuit boards. It is the main source process of vias and shorts. Visual inspection is inconvenient. Postprocessing can only be used for probabilistic screening by destructive experiments. Effectively analyze and monitor a single PCB board. Once a problem occurs, there will inevitably be a batch problem. Even if the test cannot be completed, the final product will cause great quality hazards and can only be scrapped in batches, so strictly follow the parameters of the work instructions.