PCB Tech - A brief description of the circuit board copper sinking process

A brief description of the circuit board copper sinking process

The general process flow of PCB board includes: Cutting-drilling-sinking copper-pattern transfer-pattern plating-etching-solder mask-character-surface treatment-beer gong-final inspection- Packed and shipped. Cutting and drilling are not difficult for most PCB enthusiasts to understand, so this article focuses on the process of sinking copper! In printed circuit board manufacturing technology, this process is a more critical process. If the process parameters are not well controlled, many functional problems such as empty hole walls will occur.

1. Purpose and function of copper sinking:
On the non-conductive hole wall substrate that has been drilled, a thin layer of chemical copper is deposited chemically to serve as a base for electroplating copper;
2. Process flow:
Deburring-alkaline degreasing-two or three-stage countercurrent rinsing-roughening (micro-etching)-secondary countercurrent rinsing-pre-soaking-activation-secondary countercurrent rinsing-degumming-secondary countercurrent rinsing-copper deposition-secondary Countercurrent rinsing - pickling
3. Process description:
(1) Deburring:
Purpose: The PCB substrate passes through the drilling process before copper sinking. This process is the simplest to produce burrs, which is the most important hidden danger that constitutes the metallization of defective holes. It must be solved by deburring technology. Generally, mechanical methods are used to prevent barbs or plugging of the hole edge and inner hole wall.

(2) Alkaline degreasing:
Function and purpose: remove oil stains, fingerprints, oxides, and dust on the surface of the board; adjust the polarity of the hole wall substrate (adjust the hole wall from a negative charge to a positive charge) to facilitate the adsorption of colloidal palladium in the subsequent process;
Mostly alkaline degreasing system, but also acidic system, but the acid degreasing system is better than the alkaline degreasing system. Regardless of the degreasing effect, the charge adjustment effect is still poor. This is reflected in the production, that is, the poor backlight effect of the sinking copper and the combination of the hole wall The force is poor, the oil removal of the board surface is not clean, and the peeling and blistering phenomenon occurs simply.
Comparison of alkaline system degreasing and acid degreasing: the operating temperature is higher and cleaning is more difficult; therefore, when using the alkaline degreasing system, the cleaning requirements after degreasing are stricter.
The quality of the oil removal adjustment directly affects the backlight effect of the sinking copper;
(3) Micro-etching:
Function and purpose: remove oxides on the board surface, roughen the board surface, and ensure excellent bonding force between the subsequent copper immersion layer and the bottom copper of the substrate; the newly formed copper surface has strong activity and can well adsorb colloidal palladium ;
Coarsening agent: There are two main types of coarsening agents currently used in the market: sulfuric acid hydrogen peroxide system and persulfuric acid system, and sulfuric acid hydrogen peroxide system. Benefits: a large amount of copper dissolved (up to 50g/L), good washing performance, sewage treatment Simpler, lower cost, recyclable, disadvantages: uneven surface roughness, poor bath stability, hydrogen peroxide is easy to decompose, and air pollution is heavier. Persulfates include sodium persulfate and ammonium persulfate, and ammonium persulfate is better than persulfate. Sodium is expensive, water washability is slightly worse, and sewage treatment is more difficult. Compared with sulfuric acid hydrogen peroxide system, persulfate has the following advantages: better bath stability, uniform plate surface roughness, disadvantages: small amount of dissolved copper (25g/L ) In the persulfate system, copper sulfate is easy to crystallize and separate, and the washing property is slightly poor, and the cost is high;
Others include DuPont’s new micro-etching agent potassium monopersulfate. When used, the bath has good stability, uniform roughening of the board surface, stable roughening rate, and is not affected by copper content. The operation is simple, suitable for thin lines and small distances., High frequency board, etc.
(4) Pre-soaking/activation:
Purpose and function of prepreg: The first thing is to protect the palladium tank from the contamination of the pre-treatment tank, and to extend the service life of the palladium tank. The main components are palladium chloride and the palladium tank components together, which can moist the pore wall and facilitate subsequent activation The liquid enters the hole in time to activate it so that it can be satisfactorily and effectively activated;
The specific gravity of the prepreg is generally maintained at around 18 Baumes, so that the palladium tank can maintain a normal specific gravity of 20 Baumes or more;
Purpose and function of activation: After the pretreatment alkaline degreasing polarity is adjusted, the positively charged pore wall can effectively adsorb the negatively charged colloidal palladium particles to ensure the uniformity, continuity and fineness of the subsequent copper precipitation Performance; therefore, degreasing and activation play a very important role in the quality of subsequent copper deposits.
Special attention should be paid to the effect of activation in production, and the first thing to do is to ensure that the moment of satisfaction, concentration (or intensity)
The palladium chloride in the activation solution exists as a colloidal method. This negatively charged colloidal particles determines some of the key points in the maintenance of the palladium tank: ensuring a satisfactory amount of stannous ions and chloride ions to prevent colloidal palladium from disintegrating, (and insisting on satisfactory Specific gravity, generally above 18 Baume degrees) Sufficient acidity (appropriate amount of hydrochloric acid) to prevent the accumulation of stannous, the temperature should not be too high, otherwise colloidal palladium will accumulate, room temperature or below 35 degrees;
(5) Degumming:
Function and purpose: It can effectively remove the stannous ions enclosed by the colloidal palladium particles, exposing the palladium nuclei in the colloidal particles, and directly catalyze the electroless copper deposition reaction.
Principle: Because tin is an amphoteric element, its salt is soluble in both acid and alkali, so both acids and alkalis can be used as degelling agents, but alkalis are more active in water quality and are prone to accumulation or suspended solids, which can easily form copper sinks. Broken; Hydrochloric acid and sulfuric acid are strong acids, which are not only unlucky for multi-layer boards, because strong acids will invade the inner black oxide layer, but also simply constitute excessive debonding, dissociating colloidal palladium particles from the surface of the hole wall; generally used Fluoroboric acid is used as the primary debonding agent. Because of its weak acidity, it generally does not constitute excessive debonding. Tests have proved that when fluoroboric acid is used as a debonding agent, the binding force and backlight effect of the copper deposit and the fineness are significantly improved. ;
(6) Immersion copper:
Function and purpose: Through the activation of the palladium nucleus to induce the chemical copper deposition autocatalytic reaction, the newly generated chemical copper and the reaction by-product hydrogen can be used as the reaction catalyst to catalyze the reaction, so that the copper deposition reaction continues. After processing through this process, a layer of chemical copper can be deposited on the board surface or the hole wall.
Principle: Use the recoverability of formaldehyde under alkaline conditions to recover the complexed soluble copper salt.
Air agitation: the tank liquid must adhere to the normal air agitation. The purpose is to oxidize the cuprous ions in the tank liquid and the copper powder in the tank liquid to convert them into soluble divalent copper.