The following is the literature
Since the 820 brightener developed by Shanghai Institute of Light Industry came out, China's tin plating industry has developed rapidly. The basic components of domestic brightener are composed of the following three parts: surfactant (OP 2 1, OP 10, TX 10), benzylidene acetone and formaldehyde. Due to the different types and qualities of surfactants used, brighteners and additives (naphthol, nicotinic acid, cresol, hydroquinone, phenol sulfonic acid, diaminodiphenylmethane) have different formulas and properties. But as a stabilizer, it should have the following three characteristics:
(1) has a strong complexing ability to Sn4+;
(2) It has oxidation resistance to Sn2+;
(3) No other side effects.
However, fluoride should be used with caution, which not only has a strong corrosive effect on titanium tubes, but also may have an adverse effect on the performance of the solution. Although imported brighteners are superior to domestic brighteners in replacement capacity, oxidation resistance and dosage, so far, neither imported brighteners nor domestic brighteners have made a breakthrough, that is, the solderability of tin coating is unstable after aging at high temperature (185 C, 2h or155 C, 16h).
2 brightener characteristics
Take "820" brightener as an example. Brightener for tin plating is generally not marked with "opener" and "extender", and "820" and "82 1" are collectively called brighteners, which should be added at the same time in the process of cylinder preparation and daily electroplating, but the addition amount is different. Brightener should be added according to the dosage, such as 100A, 40 ml of "82 1" and 80 ~ 100 ml of "820" should be added every hour. On this basis, according to the specific conditions of the process (current fluctuation, solution temperature and amount of solution) and the brightness change of the plated parts, increase or decrease as appropriate.
Preparation of 3 solution
(1) Stannous sulfate with yellow color and poor solubility cannot be used; (2) sulfuric acid and potassium antimonate tartrate are reagent grade; (3) After stannous sulfate is completely dissolved, the solution must be cooled to below room temperature immediately to prevent Sn2+ from being hydrolyzed into Sn4+, which will make the performance of the solution worse. After adding brightener "820"15ml (it is forbidden to dilute brightener "820" with water), stir the solution evenly, electrolyze with corrugated plate at a low current of about 0.2A dm2 for more than 4 hours, and it is best to keep the solution stirred with a filter or a cooling circulating pump. This promotes the complete complexation of brightener in solution. After electrolysis, the solution can be electroplated at a suitable temperature. Before trial plating, the brightener "82 1" can be added for about 0.5 ml L, and the barrel plating is about 10min. If the part is not too bright, you can add brightener "82 1" about 0.5 ml L. After repeating these operations, the tin coating on the part should reach satisfactory brightness. After trial plating with the brightener of the above grade, the tin coating on the parts still can't reach satisfactory brightness or brightness. The following reasons should be checked: (1) Whether the rectifier power supply is out of phase; (2) Whether the contact between the conductive block and the wire is good; (3) Whether a large number of chloride ions enter the solution.
4 Control and influence of various process parameters
4. 1 stannous sulfate
The allowable content range of stannous sulfate is very wide, even if the content is only a few grams per liter, the coating with normal color and brightness can be plated. So its content is too high or too low, which mainly affects the current density, especially the hanging plating. The optimum content range is 20 ~ 35g·l, and the content of stannous sulfate can be controlled below 20 mL for the plated parts without welding performance requirements. Generally, as long as the anode is not passivated, the tin ions in the solution will remain in balance, but it must be: (1) the sulfuric acid content is normal; (2) The anode area is sufficient; (3) Adding a proper amount of stabilizer.
4.2 sulfuric acid
Although the allowable content of sulfuric acid is wide, it must be carefully controlled. Too high or too low sulfuric acid content is not conducive to anode dissolution, especially too low, which not only passivates the anode, but also hydrolyzes Sn2+ ions into Sn4+ ions, making the solution performance worse and the brightness of tin coating worse. If there is no chemical analysis condition, the content can be controlled by observing the change of voltage. When the tinplate is not passivated and washed, the voltage rises, indicating that the sulfuric acid content is too low, so it is necessary to add sulfuric acid, which can be added in a small amount for many times until the voltage returns to normal.
4.3 solution temperature
Practice has proved that lower solution temperature control is beneficial to the stability of solution performance and the denser crystallization of tin coating. The solution temperature is low, the dosage of brightener is small, and the brightness of the coating is good; The solution temperature is high, and vice versa. The optimum temperature of the solution is 10 ~ 20℃, the lower limit of barrel plating is 8 ~ 15℃, and the upper limit of hanging plating is 10 ~ 18℃ ..
4.4 Current density
During barrel plating, the cathode current density should be controlled at 0.4 ~ 0.7adm2 according to the actual area of the plated part. Or observe that there are not many bubbles on the liquid surface and try to turn on the current.
4.5 power supply
When designing the plating solution, it is advisable to control the charged amount of each liter of solution at 0.4~0.6A, which is not only beneficial to the stability of the solution, but also does not waste energy.
4.6 drum speed
According to the characteristics of the parts to be plated, choosing the appropriate drum speed is the key to successfully plating bright coatings. It is completely wrong to think that the brightness of tin coating can be improved by increasing the speed of drum. It is appropriate to control the rotating speed of tinning drum at 8 ~ 18r min.
5 Maintenance of electroplating solution
Compared with other metal plating solutions, acidic tin plating solution is very unstable. With the passage of time, Sn2+ in the solution is gradually oxidized to Sn4+. Even if the stabilizer is added and operated at a lower temperature, this oxidation reaction is inevitable, and it is even more impossible to reduce the generated Sn4+ to Sn2+. The performance of the solution depends first on the accumulation of Sn4+ ions, and then on the organic decomposition of brighteners and metal impurities.
5. 1 solid solution treatment
After electroplating for a period of time (the current must be greater than 100Ah L), the performance of tin plating solution becomes worse, the brightness of the coating decreases, the color becomes darker, the surface is prone to fog and spots, and the welding performance is not good. In addition, the anode is also easy to passivate. After treatment, the performance of aging tin plating solution has been improved, but it can't reach the performance of newly prepared solution. Therefore, for products with high welding performance requirements, try to use newly prepared solutions. Sn4+ ions can be treated with polyacrylamide (the smaller the molecular weight, the better). Organic decomposition products can be adsorbed by activated carbon. Potassium ferrocyanide solution can remove Cu2+.
5.2 Daily maintenance
When the (1) solution stops, corrugated plates are often used for low current density electrolysis. (2) After a large amount of water is flushed into the plating solution, sulfuric acid should be added immediately to the content specified in the formula, so as to avoid the hydrolysis of Sn2+ into Sn4+, which will worsen the performance of the solution. (3) Stannous sulfate can be directly dissolved in plating solution or dilute sulfuric acid, and it is forbidden to dissolve in water.
6 Pretreatment and Priming
(1) Red copper parts and phosphor copper parts can be degreased by rolling, and there is no need for triple acid pickling, but they must be activated by dilute sulfuric acid before entering the tank.
(2) Brass parts and iron parts should be plated with copper, and the thickness should be greater than 2μm as far as possible.
(3) Zinc-aluminum alloy is deplated with concentrated sulfuric acid, polished with mixed acids such as chromic acid, hydrofluoric acid and nitric acid, and then plated with copper after the mixed solution of hydrofluoric acid and sulfuric acid is activated. The thickness of copper layer must be greater than 3 microns. ..
(4) Although copper and nickel are excellent barrier layers, we should pay full attention to the fact that the nickel layer is easy to be passivated, left for a long time or insufficiently activated, and the tin coating is easy to bubble and even age at high temperature.
7 Post-treatment and anti-discoloration
Generally speaking, bright white tin coating will not change color after proper post-treatment. It is best not to do countercurrent rinsing for tin plating, otherwise the coating will be foggy, especially for barrel-plated parts. Immediately after pouring into the basin from the barrel, pour in clear water, stir it evenly, pour out the water, repeat this for three times, then soak it in the dilute solution of trisodium phosphate at about 50℃ (it feels greasy), and rinse it with clear water for three times. Passivation (10g LK2Cr2O7, 20LNA2CO3) can effectively prevent discoloration, but it must be cleaned. Soaking stearic acid solution to form a protective film on the surface of plated parts is an economical and effective method to prevent discoloration. For plated parts with long storage and transportation turnaround time, it is best to pack them in thick paper bags that meet the hygiene standards and cover them with plastic bags.
8 Weldability of coating
In recent ten years, some books and periodicals on electroplating in China introduced that inorganic salts containing metallic elements such as antimony, bismuth, indium, cerium and cadmium were added to tin plating solution in an attempt to form tin alloy, thus improving its welding performance. This not only proves that this statement and practice are meaningless in theory, but also in practice. It should be said that only by introducing lead into tin can it really form an alloy and fundamentally improve the weldability. Practice has proved that adding antimony potassium tartrate solution is beneficial to stabilize the brightness of the coating. In order to improve the solderability of tin coating, the following measures must be taken: (1) copper and nickel as the bottom layer; (2) Maintain good solution performance; (3) try to operate at low temperature; (4) the content of stannous sulfate is controlled within the range of 20-35g/L; (5) The coating thickness must be greater than 8μm, and the unidirectional coating thickness can be converted according to the relationship of 0.5A dm2-8μm-30min.
9 Re-plating and Deplating
For various reasons, the tin coating needs to be electroplated again. First soak in 5% dilute hydrochloric acid 10~20s ~ 20s, and then electroplate in the bath after cleaning. Remove the bad tin coating (without damaging the substrate)
(1) The matrix is iron or nickel.
Sodium hydroxide, Gl70 ~ 90
Sodium m-nitrobenzene sulfonate (anti-dye salt S), Gl70 ~ 90
t，C & gt80
(2) The matrix is copper and its alloy (smokeless pickling formula)
Sulfuric acid (industrial), mll400 ~ 500
Nitric acid (industrial), mll40 ~ 50
Hydrochloric acid (industrial), mll8 ~ 12
Sodium nitrate (industrial), Gl70 ~ 80
Mingjiao, Gl 1 ~ 1.5
Polyethylene glycol, g l 1 ~ 1.5
Test room temperature