The general process flow of PCB board includes:      Cutting – Drilling – Copper sinking – graphic transfer – Graphic electroplating – etching – solder resist – characters – surface treatment – beer Gong – final inspection – packaging and shipment. Cutting and drilling are not difficult for most PCB lovers to understand, so this paper focuses on the copper sinking process!   In the manufacturing technology of printed circuit board, this process is a key process. If the process parameters are not well controlled, many functional problems such as hole wall cavity will occur.

       

   1、 Purpose and function of copper precipitation:

A thin layer of chemical copper is deposited on the drilled non-conductive hole wall substrate by chemical method as the substrate for copper electroplating;

2、 Process flow:

Deburring → alkaline degreasing → secondary or tertiary countercurrent rinsing → roughening (micro etching) → secondary countercurrent rinsing → prepreg → activation → secondary countercurrent rinsing → degumming → secondary countercurrent rinsing → copper precipitation → secondary countercurrent rinsing → acid leaching


Purpose, function and process flow analysis of copper deposition on PCB

3、 Process Description:

(I)    Deburring:

Purpose: before copper deposition, the PCB substrate goes through the drilling process, which is most likely to produce burrs, which is the most important hidden danger of poor hole metallization. It must be solved by deburring process. The mechanical method is usually adopted to make the hole edge and inner hole wall free of barb or hole blocking



        (II)   Alkaline degreasing:

Function and purpose: remove oil stain, fingerprint, oxide and dust in the hole; Adjust the polarity of the pore wall substrate (adjust the pore wall from negative charge to positive charge) to facilitate the adsorption of colloidal palladium in the subsequent process;

Most of them are alkaline oil removal systems, and there are also acidic systems, but the acidic oil removal system is worse than the alkaline oil removal system in terms of oil removal effect and charge adjustment effect. In production, that is, the effect of copper precipitation backlight is poor, the binding force of hole wall is poor, and the oil removal on the plate surface is not clean, which is prone to peeling and foaming.

Compared with acid oil removal, alkaline oil removal has higher operating temperature and difficult cleaning; Therefore, when using alkaline oil removal system, the cleaning requirements after oil removal are strict

Degreasing adjustment directly affects the effect of copper precipitation backlight;

(III)     Micro etching:

Function and purpose: remove the oxide on the plate surface, coarsen the plate surface, and ensure a good bonding force between the subsequent copper deposition layer and the bottom copper of the substrate; The newly formed copper surface has strong activity and can adsorb colloidal palladium well;

Coarsening agent:   At present, there are two main types of coarsening agents used in the market: sulfuric acid hydrogen peroxide system and persulfate system. Sulfuric acid hydrogen peroxide system has the advantages of large copper dissolution (up to 50g / L), good water washability, easy sewage treatment, low cost and recyclability. Disadvantages: uneven coarsening of plate surface, poor stability of tank liquid and easy decomposition of hydrogen peroxide, The air pollution is heavy, and persulfate includes sodium persulfate and ammonium persulfate. Ammonium persulfate is more expensive than sodium persulfate, with poor water washability and difficult sewage treatment. Compared with sulfuric acid hydrogen peroxide system, persulfate has the following advantages: better tank liquid stability and uniform plate surface coarsening. Disadvantages: the amount of dissolved copper is small (25g / L). Copper sulfate in persulfate system is easy to crystallize and precipitate, and the water washability is slightly poor, High cost;

In addition, there is DuPont’s new micro etchant potassium bisulfate. When used, the tank liquid has good stability, uniform plate surface coarsening, stable coarsening rate, is not affected by copper content, and the operation is simple. It is suitable for thin lines, small spacing, high-frequency plates, etc

(IV)   Prepreg / activation:

Purpose and function of pre leaching: it is mainly to protect the palladium tank from the pollution of pretreatment tank solution and prolong the service life of palladium tank. The main components are consistent with those of palladium tank except palladium chloride, which can effectively wet the pore wall and facilitate the subsequent activation solution to enter the pore in time for activation, so as to make it fully and effectively activated;

The specific gravity of the pre leaching solution is generally maintained at about 18 Baume degrees, so that the palladium tank can be maintained above the normal specific gravity of 20 Baume degrees;

Purpose and function of activation: after adjusting the polarity of alkaline degreasing after pretreatment, the positively charged pore wall can effectively adsorb enough negatively charged colloidal palladium particles to ensure the uniformity, continuity and compactness of subsequent copper deposition; Therefore, oil removal and activation play a very important role in the quality of subsequent copper precipitation,

Special attention shall be paid to the effect of activation in production, mainly to ensure sufficient time, concentration (or strength)

Palladium chloride in the activation solution exists in the form of colloid. This negatively charged colloidal particle determines some key points of palladium tank maintenance: ensure a sufficient number of stannous ions and chloride ions to prevent colloidal palladium from degumming (and maintain a sufficient specific gravity, generally above 18 Baume degrees), a sufficient amount of acidity (an appropriate amount of hydrochloric acid) to prevent stannous precipitation, and the temperature should not be too high, Otherwise, colloidal palladium will precipitate at room temperature or below 35 ℃;

(V)   Degumming:

Function and objective: it can effectively remove stannous ions surrounded by colloidal palladium particles, expose palladium nuclei in colloidal particles, and directly and effectively catalyze and start chemical copper precipitation reaction,?

Principle: because tin is an amphoteric element, its salt is soluble in both acid and alkali, so both acid and alkali can be used as degumming agent. However, alkali is more sensitive to water quality and is easy to produce precipitation or suspended solids, which is very easy to cause copper precipitation and hole breakage; Hydrochloric acid and sulfuric acid are strong acids, which are not only disadvantageous to multi-layer plates, because strong acids will attack the inner black oxide layer, but also easy to cause excessive degumming and dissociate colloidal palladium particles from the pore wall plate surface; Fluoroboric acid is generally used as the main degumming agent, which generally does not cause excessive degumming due to its weak acidity. Experiments show that when fluoroboric acid is used as the degumming agent, the adhesion, backlight effect and compactness of copper deposition layer are significantly improved;

(VI)   Copper deposit:

Function and objective: the autocatalytic reaction of chemical copper precipitation is induced by the activation of palladium core. The newly generated chemical copper and reaction by-product hydrogen can be used as reaction catalysts to catalyze the reaction, so that the copper precipitation reaction can continue. After this step, a layer of chemical copper can be deposited on the plate surface or hole wall.

Principle: the reducibility of formaldehyde under alkaline conditions is used to reduce the complexed soluble copper salt.

Air stirring: normal air stirring shall be maintained for the tank liquid to oxidize the cuprous ion and copper powder in the tank liquid and convert them into soluble divalent copper.

Leave a Reply

Your email address will not be published. Required fields are marked *