PCB Blog - Defects of pure tin electroplating on PCB board

1. Introduction:In the production process of printed circuit boards, most manufacturers still use wet film technology for imaging due to cost factors, which will inevitably cause problems such as "bleeding, bright edge (thin tin)" and other undesirable problems when electroplating pure tin in graphics. In view of this, I will discuss with you the solutions to common problems of pure tin plating process that I have summarized over the years. Among them, the electroplating process of circuit boards can be roughly classified into: acid bright copper electroplating, nickel/gold electroplating, and tin electroplating. The article introduces the process of circuit board processing, electroplating process technology and process flow, as well as specific operation methods. Process flow: pickling-full board copper plating-pattern transfer-acid degreasing-secondary countercurrent rinsing-micro-etching-secondary countercurrent rinsing-acid soaking-tin plating-secondary countercurrent rinsing-countercurrent rinsing-acid soaking-pattern plating Copper - secondary countercurrent rinsing - nickel plating - secondary water washing - immersion in citric acid - gold plating - recovery - 2-3 grade pure water washing - drying.

2. Analysis of the reasons for "seepage plating" of wet film board (quality problem of non-pure tin syrup)
2.1 The copper surface brushed before screen printing must be clean to ensure good adhesion between the copper surface and the wet oil film.
2.2 When the exposure energy of the wet film is low, the light curing of the wet film will be incomplete, and the resistance to electroplating of pure tin will be poor.
2.3 The pre-baking parameters of the wet film are unreasonable, and the local temperature difference of the oven is large. Since the thermal curing process of the photosensitive material is sensitive to temperature, low temperature will lead to incomplete thermal curing, thereby reducing the resistance of the wet film to pure tin plating.
2.4 The lack of post-curing/curing treatment reduces the resistance to pure tin plating.
2.5 The boards from electroplating pure tin must be washed thoroughly. At the same time, each board must be inserted into a rack or dry board, and stacking of boards is not allowed.
2.6 Wet film quality issues.
2.7 The influence of production and storage environment and time. Poor storage environment or prolonged storage will cause the wet film to swell and reduce its resistance to pure tin plating.
2.8 The wet film is attacked and dissolved by pure tin light agent and other organic pollutants in the tin cylinder. When the anode area of the tin plating tank is insufficient, the current efficiency will inevitably decrease, and oxygen evolution during the electroplating process (electroplating principle: anode oxygen evolution, cathode hydrogen evolution ). If the current density is too high and the sulfuric acid content is high, the cathode will evolve hydrogen, which will attack the wet film and lead to the occurrence of tin penetration (that is, the so-called "seepage plating").
2.9 The high concentration of the stripping solution (sodium hydroxide solution), high temperature or long soaking time will produce tin flow or dissolved tin (that is, the so-called "seepage plating").
2.10 The current density of pure tin plating is too high. Generally, the mass current density of wet film is suitable for between 1.0 and 2.0A/dm2. Beyond this current density range, some wet film quality is prone to "seepage plating".

3. Causes and improvement countermeasures of "seepage plating" caused by the problem of potion
3.1 Reason: The problem of liquid medicine leads to the generation of "seepage plating" mainly depends on the formula of pure tin light agent. The light agent has a strong penetration ability and the attack on the wet film during the electroplating process produces "bleed-through". That is, when too much pure tin light agent is added or the current is slightly too large, "seepage plating" occurs. Under normal current operation, the resulting "seepage plating" is related to the uncontrolled operating conditions of the potion, such as too much pure tin light agent., high current, high stannous sulfate or sulfuric acid content, etc., these will accelerate the attack on the wet film.
3.2 Improvement countermeasures: The performance of most pure tin brighteners determines that they are more aggressive to wet film under the action of current. In order to avoid reducing the "bleeding" of wet film pure tin plating, it is recommended to produce wet film plating pure tin at ordinary times. The board must do three things:
1) When adding pure tin light agent, it must be monitored in a small amount of times, and the content of pure tin light agent in the plating solution is usually controlled at the lower limit;
2) The current density is controlled within the allowable range;
3) The control of the composition of the potion, such as stannous sulfate and sulfuric acid content control at the lower limit will also be beneficial to improve "seepage plating".

Fourth, the characteristics of market pure tin light agent
4.1 Some pure tin brighteners are limited to current density and have a relatively narrow operating range. Such pure tin brighteners are usually prone to wet film "bleeding". It controls the operating parameters of stannous sulfate, sulfuric acid and current density relatively. The allowable standard range is also narrow;
4.2 Some pure tin brighteners are suitable for a wide range of current density operation. This kind of pure tin brightener is usually not easy to produce wet film "bleeding". It is relatively the operating condition parameter control allowable standard for stannous sulfate, sulfuric acid and current density. wide range;
4.3 Some pure tin brighteners are prone to "missing plating, seepage plating, blackening" or even "glowing" at the edge of the wet film;
4.4 Some pure tin brighteners do not cause the problem of "brightness" of the edge of the wet film (no baking or UV curing), but there is still a "bleeding" problem, which can be cured by baking or UV curing. improve. Before the pure tin plating process on the wet film board, there is no problem of "brightening and seepage plating" of the line edge without baking or UV curing treatment. At present, there are really few pure tin brighteners on the market. The specific operation should be based on the characteristics of pure tin brighteners provided by different syrup suppliers, and the parameters such as the operating current density, temperature, anode area, stannous sulfate, sulfuric acid and tin brightener content should be strictly controlled.

5. The reason for the "brightness" of the line edge caused by pure tin plating on the wet film board
Because the formula of pure tin light agent generally contains organic solvent, and the wet oil film itself is composed of organic solvent and other materials, there is incompatibility between the two, especially the "brightness" of the edge of the line. Factors related to the "shiny" of the line edges:
5.1 Pure tin polish (under normal circumstances, the formula will contain organic solvents);
5.2 The current density is low (the lower the current density, the easier it is to produce "brightness" on the edge of the line);
5.3 The conditions of the baking board are inconsistent (the main purpose of the baking board is to volatilize the organic solvent of the wet oil film);
5.4 The thickness of the screen printing wet oil film is uneven (the thicker the oil film, the easier it is to "glow");
5.5 The quality of the wet oil film itself (choose the wet oil film to match the electroplating pure tin potion);
5.6 The quality of the acid degreaser in pretreatment (selecting a good acid degreaser not only enhances the water washability of the solution, but also greatly reduces the probability of remaining on the copper surface after degreasing);
5.7 Excessive tin light agent in the plating solution (too much tin light agent will cause organic pollution of the plating solution, in order to prevent the wet film tin plate from polluting the tin cylinder with the increase of production capacity, 8 hours of carbon core filtration is carried out every half month, at the same time electrolyzing for 5 hours, 2.5 hours and 0.5 hours with the current density of 5ASF, 10ASF and 15ASF every week);
5.8 It is related to temperature (the higher the temperature, the more uneven the position of the low-potential region, and the test proves that the higher the temperature, the easier it is to produce "brightness" of the line edge. In addition, the high temperature accelerates the oxidation of Sn2+ and the consumption of additives.);
5.9 Poor conduction (poor conduction directly causes the current density to be seriously low, when the current density is lower than 10ASF, the edge of the line is likely to "glow").
5.10 The wet film board should be stored for a long time (the wet film plated pure tin board should be stored in a workshop with a relatively good environment, and the storage time should not exceed 72 hours. The staff in the graphic electroplating process will take the board according to the production conditions, but the storage time in the electroplating workshop should not exceed 72 hours. 12 hours);
5.11 Insufficient anode area of pure tin plating bath (insufficient anode area of tin plating bath will inevitably lead to lower current efficiency and oxygen evolution during electroplating process. The ratio of anode to cathode area is generally 2~3:1, and the standard of pure tin bath anode spacing is about 5cm, whose purpose is to ensure sufficient anode area). Therefore, some bad problems are actually caused by the inconspicuous details of a certain process. As long as you think about it in multiple directions, you can find the key to the problem and solve it.

6. Master the advantages and disadvantages of wet film quality in the market
A good quality wet film is very beneficial to reduce the "brightness" of the line edge, but it cannot be completely eliminated. In addition, the oil film that is more suitable for pure tin plate is not necessarily a good oil film. The following briefly introduces the quality characteristics of wet film:
6.1 A good wet film is not easy to produce "seepage plating", the oil film is not easily broken down when the current density is high, and the film is relatively easy to remove;
6.2 Some wet films may indeed play a certain role in reducing the "brightness" problem of the line edge, but it is relatively difficult to remove the film. Such wet oil films are not suitable for potions with a wide range of current density operation, and a slightly higher current density is easy to produce. "Plating, film sandwich, blackening" and even breakdown of oil film and other problems.
Regarding electroplating tin, 1) purpose and function: the purpose of pure tin in graphic electroplating mainly uses pure tin as a metal anti-corrosion layer to protect circuit etching; 2) the bath is mainly composed of stannous sulfate, sulfuric acid and additives; the content of stannous sulfate is controlled At about 35 grams/liter, sulfuric acid is controlled at about 10%; the addition of tin plating additives is generally supplemented by the method of thousand ampere hours or according to the actual production board effect; the current calculation of electroplating tin is generally 1.5 ampere/square decimeter Multiply by the area that can be plated on the board; the temperature of the tin cylinder is maintained at room temperature, generally not more than 30 degrees, and more controlled at 22 degrees, so in summer because the temperature is too high, it is recommended to install a cooling temperature control system for the tin cylinder; 3) Process Maintenance: replenish tin-plating additives in time according to the thousand ampere hours every day; check whether the filter pump is working properly and whether there is air leakage; every 2-3 hours, use a clean wet cloth to clean the cathode conductive rod; every week Regularly analyze stannous sulfate (1 time/week) and sulfuric acid (1 time/week) in the tin cylinder, adjust the content of tin plating additives through the Hall cell test, and replenish relevant raw materials in time; clean the anode conductive rods and tanks every week Electrical connectors at both ends of the body; electrolyze with low current 0.2 to 0.5 ASD for 6 to 8 hours every week; check whether the anode bag is damaged every month, and replace the damaged one in time; and check whether there is anode sludge accumulated at the bottom of the anode bag, if necessary, it should be cleaned in time; continuous filtration with carbon core for 6 to 8 hours per month, and low-current electrolysis to remove impurities; every six months or so, it is determined whether large-scale treatment (activated carbon powder) is required according to the pollution status of the bath liquid; every two weeks Replace the filter element of the filter pump; 4) Major treatment procedure: A. Take out the anode, remove the anode bag, clean the surface of the anode with a copper brush, rinse and dry, put it into the anode bag, and put it into the acid tank for standby B. Put the anode The bag is soaked in 10% lye solution for 6 to 8 hours, rinsed with water, then soaked in 5% dilute sulfuric acid, rinsed with water and dried for later use; C. Transfer the tank liquid to the standby tank, and press 3 to 5 grams per liter. The activated carbon powder is slowly dissolved into the tank liquid. After the dissolution is complete, it is adsorbed for 4-6 hours. The tank liquid is filtered with a 10um PP filter element and filter aid powder into the cleaned working tank. According to 0. 2-0. 5ASD current density low current electrolysis for 6 to 8 hours, D. After laboratory analysis, adjust the sulfuric acid and stannous sulfate content in the tank to within the normal operating range; supplement tin plating additives according to the Hall cell test results ;E. After the color of the electrolytic plate is uniform, stop electrolysis; F. Trial plating is OK. When adding sulfuric acid, pay attention to safety. When the amount of sulfuric acid is large (more than 10 liters), it should be added slowly in several times; otherwise, the temperature of the bath will be too high, the stannous will be oxidized, and the aging of the bath will be accelerated on PCB board.