The causes of rough plating and burrs are as follows.
(1) The content of free sodium cyanide is too low: the precipitation of copper is easier with the reduction of free sodium cyanide, the content of copper in the coating increases, and when the content of free sodium cyanide in the solution is low to a certain limit, due to copper The deposition rate is too fast and the coating is rough. In severe cases, burrs occur.
The coating roughness and burr caused by the low content of free sodium cyanide are characterized by dark red coating (the lower the temperature, the lower the current, the more reddish), and the coating is covered with a brownish black film. Plated parts are rough or burr, regardless of the current size. The plating bath's ability to deep-deposit is reduced. The anode is light blue or black. The solution near the anode is light blue. The hydrogen bubbles in the cathode area are small. The coating cannot withstand polishing and is easily exposed. The sand marks cannot be covered after polishing.
(2) The content of copper is too high: As the content of metal copper ions in the solution increases, the precipitation of copper becomes easier. When the content of copper exceeds a certain limit, the same as when the amount of free NaOH is too small, the crystalline structure of the coating becomes coarse, and burrs occur in severe cases.
The characteristics of rough burrs caused by too high content of metallic copper in the solution are the same as when the amount of sodium cyanide is too small, the plating ability of the plating solution decreases, but the anodic dissolution is normal.
(3) The amount of free sodium hydroxide is too high or too low: When the content of free sodium hydroxide in the solution is too high, the dissociating concentration of stannate decreases, the precipitation of tin becomes more difficult, and the tin content in the coating decreases, relatively speaking , making the precipitation of copper easier. When the free sodium hydroxide content is too low, the stannate undergoes a hydrolysis reaction and a metastannic acid precipitate is formed. This precipitate is suspended in the solution, causing rough burrs in the upward portion of the plated part.
The feature of coating rough burrs caused by high free NaOH in the solution is that the coating is dark red (the lower the temperature, the smaller the current, the more dark red). The plating will be rough or have burrs regardless of the current size. The plating bath has a good deep-deposition capability, and the deposition speed is particularly fast. The coating is thick, and it is difficult to polish, and the sand marks cannot be covered. The rough coating and burr caused by excessive free sodium hydroxide are characterized by the roughness occurring on the surface of the plated part, the turbidity of the plating solution, the passivation of the alloy anode or the tin anode, and the black film covering.
(4) Excessive current density: As the current density increases, the tin content in the coating increases, and the crystals are fine and bright. However, when the current density exceeds a certain limit, a serious lack of metal ions near the cathode occurs at the cathode tip and convexity. The place will produce dendritic metal plating.
The roughness and burr caused by the excessive current density are distributed in different parts of the plated part. The amount of tin contained in the coating increases, the hydrogen bubbles in the cathode area increase, the anode plate turns black, and a large amount of oxygen is evolved.
(5) Excessive content of divalent tin: In the plating solution of tetravalent tin salt, a certain amount of divalent tin always exists inevitably. Since divalent tin is more easily precipitated at the cathode than tetravalent tin, when a certain amount of divalent tin is contained in the plating solution, the coating is roughened and burrs are caused by the rapid deposition of divalent tin at the cathode.
Because the tin is easily discharged at the cathode, the tin content of the coating is increased, and the coating is generally grayish. Plating parts will appear rough and burr, regardless of current size. The alloy anode or tin anode is gray and the hardness of the coating is increased, and the sand marks cannot be covered after polishing.
(6) Turbidity of plating solution: The plating solution is turbid due to the contamination of mechanical impurities and other suspended materials. At this time, if electroplating is performed, these fine-grained objects will fall on the plated part and be trapped in the plating layer, resulting in rough plating and burrs. .Due to the turbidity of the plating solution, the rough coating and burr are mostly on the plated part, but the anode does not passivate.