Electroless plating copper, we also call Plated Through hole ( PTH), it is an autocatalytic redox reaction. The PTH process is performed after the two or more layers have been drilled.
The function of PTH: On the drilled non-conducting cell wall substrate, a thin layer of chemical copper is chemically deposited as a substrate for the subsequent electroplating of copper.
PTH process decomposition: alkaline degreasing→2 or 3 countercurrent rinsing→roughening (microetching)→secondary countercurrent rinsing→pre-dip→activation→secondary countercurrent rinsing→debonding→secondary countercurrent rinsing→sinking→two Stage countercurrent rinsing → pickling.
PTH detailed process explanation:
1. Alkaline degreasing:
Remove oil on the surface of the board, fingerprints, oxides, dust in the holes;
The pore wall is adjusted from a negative charge to a positive charge to facilitate the adsorption of colloidal palladium in the subsequent process;
After degreasing, the cleaning should be carried out in strict accordance with the guidelines, and the copper backlit test should be used for testing.
Remove the oxide on the surface of the board and roughen the surface to ensure good adhesion between the subsequent copper layer and the copper at the bottom of the substrate.
The new copper surface has strong activity and can adsorb colloidal palladium well;
It mainly protects the palladium tank from the contamination of the pretreatment tank and prolongs the service life of the palladium tank. The main component is identical to the palladium tank except for palladium chloride, which can effectively wet the pore wall and facilitate the subsequent activation of the activation liquid into the hole. Enough and effective activation;
After pre-treatment of alkaline degreasing polarity adjustment, the positively charged pore walls can effectively adsorb enough negatively charged colloidal palladium particles to ensure the average, continuity and compactness of subsequent copper sinking; Activation is critical to the quality of subsequent copper sinks. Control points: the specified time; standard stannous ion and chloride ion concentration; specific gravity, acidity and temperature are also important, and must be strictly controlled according to the operating instructions.
The stannous ion of the colloidal palladium particles is removed, and the palladium core in the colloidal particles is exposed to directly catalyze the initiation of the chemical copper precipitation reaction. Experience has shown that the use of fluoroboric acid as a debonding agent is a better choice.
6. Electroless plating copper
The auto-catalyzed reaction of the electroless copper is induced by the activation of the palladium nucleus, and both the new chemical copper and the reaction by-product hydrogen can be used as a reaction catalyst to catalyze the reaction, so that the copper precipitation reaction continues. After this step, a layer of chemical copper can be deposited on the surface of the board or the wall of the hole. During the 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 production circuit board. It is the main source process for the through hole and the short circuit. It is not convenient for visual inspection. The post-process can only be used for probabilistic screening through destructive experiments. Effective analysis and monitoring of a single PCB board, so once the problem occurs, it is bound to be a batch problem, even if the test can not be completed, the final product causes great quality hazards, only batch scrap, so strictly follow the parameters of the work instructions .