Analysis of defects in pure tin plating on PCB

1 Introduction

In the production process of circuit boards, most manufacturers still use the wet film process for imaging due to cost considerations, which will inevitably cause undesirable problems such as “bleeding, bright edges (tin thin)” when pattern plating pure tin. In view of this , I will discuss with you the solutions to common problems in the pure tin plating process that I have summarized over the years.

2. Analysis of the causes of “bleed-out” on wet film boards (quality issues with non-pure tin solutions)

(1). The polished copper surface must be clean before screen printing to ensure good adhesion between the copper surface and the wet oil film.
(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 pure tin plating will be poor.
(3). The wet film pre-baking parameters are unreasonable, and the local temperature difference in the oven is large. Since the thermal curing process of photosensitive materials is sensitive to temperature, low temperatures will lead to incomplete thermal curing, thereby reducing the wet film’s ability to resist electroplating pure tin.
(4). No post-processing/curing treatment reduces the resistance to pure tin plating.
(5). The boards produced by electroplating pure tin must be thoroughly washed with water. At the same time, each board must be separated by a rack or dry board, and stacking of boards is not allowed.
(6). Wet film quality issues.
(7). Influence of production and storage environment and time. Poor storage environment or long storage time will cause the wet film to expand and reduce its resistance to pure tin plating.
(8). The wet film is attacked and dissolved in the tin tank by pure tin brightener and other organic pollutants. When the anode area of the tin plating tank is insufficient, the current efficiency will inevitably decrease and oxygen will evolve during the electroplating process (electroplating principle: anode oxygen evolution) , hydrogen evolution at the cathode). If the current density is too high and the sulfuric acid content is high, hydrogen will evolve from the cathode, attacking the wet film and causing tin infiltration (the so-called “infiltration plating”).
(9). High concentration of the film stripping solution (sodium hydroxide solution), high temperature or long immersion time will produce tin flow or dissolved tin (the so-called “penetration plating”).
(10). The current density of pure tin plating is too high. Generally, the best current density for wet film quality is between 1.0~2.0A/dm2. Beyond this current density range, some wet film quality is prone to “bleed-through plating”.


3. Causes and improvement countermeasures for “penetration plating” caused by potion problems


The potion problem leading to “bleed-through plating” mainly depends on the pure tin brightener formula. The optical agent has strong penetrating ability and attacks the wet film during the electroplating process to cause “bleed-through”. That is, when too much pure tin brightener is added or the current is slightly too high, “penetrating plating” will occur. Under normal current operation, the resulting “penetrating plating” is related to the uncontrolled operating conditions of the potion. For example, if there is too much pure tin brightener, , high current, high stannous sulfate or sulfuric acid content, etc., all of which will accelerate the attack on the wet film.

(2) Improvement measures:

The performance of most pure tin brighteners determines that they are more aggressive to wet films under the action of electric current. In order to avoid reducing the occurrence of “bleed-through” of wet film-plated pure tin boards, it is recommended that the normal production of wet film-plated pure tin boards must be done

Three points:

①. When adding pure tin brightener, it must be monitored in small amounts and multiple times. The content of pure tin brightener in the plating bath usually needs to be controlled at the lower limit;
②. The current density is controlled within the allowable range;
③. Controlling the ingredients of the potion, such as controlling the content of stannous sulfate and sulfuric acid at the lower limit, will also be beneficial to improving “penetration plating”.

4. Characteristics of pure tin brighteners on the market

(1)Some pure tin brighteners are limited to current density and have a narrow operating range. This kind of pure tin brightener is usually prone to wet film “bleeding”. It operates relatively well with stannous sulfate, sulfuric acid and current density. The allowable standard range of conditional parameter control is also narrow;
(2)Some pure tin brighteners are suitable for a wide operating range of current density. This kind of pure tin brightener is usually not easy to produce wet film “bleed-plating”. Its operating conditions and parameters are relatively different for stannous sulfate, sulfuric acid and current density. The control allowable standard range is also wide;
(3).Some pure tin brighteners are easy to cause “missing plating, bleeding plating, blackening” and even “brightening” of line edges on wet films;
(4) Some pure tin brighteners do not cause the problem of “brightening” of the edges of the wet film (without baking the plate or without UV curing), but there are still problems with “plating bleeding” after baking the plate or UV curing. Curing treatment can be improved. Before the pure tin plating process of the wet film board, it does not undergo baking or UV curing treatment and will not cause problems such as “brightening or bleeding” of the line edges. There are indeed few pure tin brighteners on the market at present.
Specific operations should be based on the characteristics of pure tin brighteners provided by different potion suppliers, and parameters such as potion operating current density, temperature, anode area, stannous sulfate, sulfuric acid, and tin brightener content should be strictly controlled.


5. The reason why the edge of the line is “shiny” when plating pure tin on the wet film board

Because the formula of pure tin polishing agent generally contains organic solvents, and the wet oil film itself is composed of organic solvents and other materials, the two are incompatible, especially when the edge of the line is “shiny”.

 Relevant factors that produce “shiny” line edges:

(1). Pure tin polish agent (generally, the formula will contain organic solvents);
(2). The current density is low (the lower the current density, the easier it is to produce “brightening” of the line edges);
(3) The baking plate conditions do not match (the main purpose of the baking plate is to volatilize the organic solvent of the wet oil film);
(4). The thickness of the silk screen wet oil film is uneven (the thicker the oil film, the easier it is to “shine”);
(5). The quality problem of the wet oil film itself (select the wet oil film to match the pure tin solution for electroplating);
(6). Quality of pre-treatment acid degreaser (choosing a good acid degreaser will enhance the washability of the solution and greatly reduce the probability of residue on the copper surface after degreasing);
(7) Excessive tin polish agent in the plating solution (excessive tin polish agent will cause organic pollution of the plating solution. In order to prevent the wet film tin plating plate from contaminating the tin cylinder as the production capacity increases, conduct an 8-hour test every half month. Carbon core filtration, while electrolysis with current densities of 5ASF, 10ASF, and 15ASF for 5 hours, 2.5 hours, and 0.5 hours per week respectively);
(8). Temperature related (the higher the temperature, the more uneven the position of the low potential area. Experiments have proved that the higher the temperature, the easier it is to produce “brightening” of the line edges. In addition, high temperature accelerates the oxidation of Sn2+ and the consumption of additives.) ;
(9). Poor conductivity (poor conductivity directly causes the current density to be seriously low. When the current density is lower than 10ASF, it is most likely to cause “brightening” of the line edge).
(10). The wet film plate has a long storage time (wet film plated pure tin plate should be stored in a workshop with a relatively good environment, and the storage time cannot exceed 72 hours. Employees in the graphic electroplating process will take the board according to the production situation, but the storage in the electroplating workshop It is best not to exceed 12 hours);
(11). Insufficient anode area of pure tin plating bath (Insufficient anode area of tin plating bath will inevitably lead to reduced current efficiency and oxygen evolution during the electroplating process. The area ratio of anode to cathode is generally 2~3:1, and the anode spacing standard of pure tin bath is is about 5cm, the purpose is to ensure that the anode area is sufficient).
Therefore, some bad problems are actually caused by inconspicuous details of a certain process. As long as we consider it from all aspects, we can find the key to the problem and solve it.

6. Understand the advantages and disadvantages of wet film quality in the market

Good wet film quality is very beneficial to reducing the “shinyness” of line edges, but it cannot be completely eliminated. In addition, the oil film that is more suitable for making pure tin plates is not necessarily a good oil film. The following is a brief introduction to the wet film quality characteristics:

(1). A good wet film is not prone to “bleed-through”, the oil film is not easily broken down when the current density is high, and the film is relatively easy to remove;
(2) Some wet oil films may indeed play a certain role in reducing the “shiny” problem of line edges, but it is relatively difficult to remove the film. This type of wet oil film is not suitable for potions with a wide operating range of current density, and slightly higher current densities It is easy to produce “plating penetration, film sandwiching, blackening” or even breakdown of the oil film, etc.

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