PCB Plating Pure Tin Defect Analysis

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I. Introduction

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

2. Analysis of the cause of “seepage” of wet film board (quality problem of non-pure tin solution)

1. The copper surface brushed before the silk screen must be clean to ensure good adhesion between the copper surface and the wet oil film.

2. When the wet film exposure energy is low, the wet film photocuring will be incomplete, and the resistance to electroplating pure tin is poor.

3. The wet film pre-bake parameters are unreasonable, and the local temperature of the oven varies greatly. Since the thermal curing process of photosensitive materials is more sensitive to temperature, when the temperature is low, the thermal curing will be incomplete, thereby reducing the ability of the wet film to resist electroplating pure tin.

4. The absence of post/curing treatment reduces the resistance to electroplating pure tin.

5. The boards from electroplating pure tin must be thoroughly cleaned with water, and each board must be inserted into a rack or dry board. Stacking boards is not allowed.

6. Wet film quality problems.

7. Production and storage environment and time influence. Poor storage environment or long storage time will cause the wet film to expand and reduce its resistance to electroplating pure tin.

8. The wet film is attacked and dissolved in the tin tank by pure tin light agent and other organic pollution. When the anode area of ​​the tin plating tank is insufficient, it will inevitably lead to a decrease in current efficiency and oxygen evolution during the electroplating process (electroplating principle: anode oxygen evolution, cathode Hydrogen evolution). If the current density is too large and the sulfuric acid content is too high, hydrogen will evolve from the cathode, which will attack the wet film and cause tin permeation (the so-called “permeation”).

9. High concentration of the stripping solution (sodium hydroxide solution), high temperature or long immersion time will produce tin flow or dissolution (so-called “dialysis”).

10. The current density of pure tin plating is too large. Generally, the best current density of wet film quality is suitable for 1.0~2.0A/dm2. Beyond this current density range, some wet film quality is prone to “seepage”.

3. Causes of “diafiltration” caused by the syrup problem and improvement measures

1. Reason

The problem of potion causes “dipping” to occur mainly depends on the pure tin brightener formulation. The light agent has a strong penetrating ability and the attack on the wet film during the electroplating process produces “dialysis”. That is to say, when the pure tin brightener is added too much or the current is slightly too large, “dipping” occurs. Under normal current operation, the “dipping” produced is related to the uncontrolled operating conditions of the solution, such as 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 film under the action of electric current. In order to avoid reducing the “seepage” of wet film plated pure tin plate, it is recommended to produce wet film plated pure tin plate. Three points:

When adding pure tin brightener, it must be monitored in a small amount and multiple times, and the pure tin brightener content of the plating solution is usually controlled at the lower limit;

The current density is controlled within the allowable range;

The control of the composition of the syrup, such as stannous sulfate and sulfuric acid content at the lower limit, will also be beneficial to improve the “dialysis”.

4. the characteristics of pure tin light agent in the market

1. Some pure tin brighteners are limited to current density, and the operating range is relatively narrow. Such pure tin brighteners are usually prone to wet film “dipping”. It is relatively operating condition parameters for stannous sulfate, sulfuric acid and current density. The scope of control allowable standards is also narrow;

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 “dipping”. It allows for control of operating conditions and parameters relative to stannous sulfate, sulfuric acid and current density. The standard range is also wide;

3. Some pure tin brighteners are prone to “leak plating, penetration, blackening” and even “shiny” on the edge of the wet film;

4. Some pure tin brighteners do not cause the problem of “brightness” on the edge of the wet film (no baking sheet or UV curing treatment), but there are still problems of “penetration” after baking sheet or UV curing treatment can be improved. Before the pure tin plating process of the wet film board, there is no problem of “brightness and penetration” at the edge of the line without baking or UV curing. At present, there are indeed few pure tin brighteners on the market.

The specific operation should be based on the characteristics of pure tin brightener provided by different potion suppliers, and strict control of parameters such as the operating current density, temperature, anode area, stannous sulfate, sulfuric acid and tin brightener content of the potion should be strictly controlled.

5. Reasons for the “shiny” edge of the wet film board pure tin plating

Because the pure tin light agent formula generally contains organic solvents, and the wet oil film itself is composed of organic solvents and other materials, there is incompatibility between the two, especially the “shiny” at the edge of the line.

Relevant factors that produce the “brightness” of the edge:

1. Pure tin light agent (generally, the formula will contain organic solvent);

2. The current density is low (the lower the current density, the easier it is to produce “brightness” at the edge of the line);

3. The condition of the baking sheet is inconsistent (the main purpose of the baking sheet is to volatilize the wet oil film organic solvent);

4. The uneven thickness of the wet oil film of the silk screen (the thicker the oil film, the easier it is to “shiny”);

5. The quality of wet oil film itself (choose wet oil film to match electroplating pure tin potion);

6. The quality of the pre-treatment acid degreasing agent (choosing a good acid degreasing agent enhances the washability of the solution and greatly reduces the probability of residue on the copper surface after degreasing);

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 tank with the increase in production capacity, the carbonization is carried out for 8 hours every half month. Core filtration, and electrolysis with current density of 5ASF, 10ASF, 15ASF for 5 hours, 2.5 hours and 0.5 hours respectively);

8. Temperature-related (the higher the temperature, the more uneven the low-potential zone is. The test proves that the higher the temperature, the easier it is to “bright” the edge of the line. In addition, the high temperature accelerates the oxidation of Sn2+ and the consumption of additives.);

9. Poor conduction (poor conduction directly causes the current density to be seriously low, and when the current density is lower than 10ASF, the line edge “shiny” is most likely to occur).

10. Long storage time for wet-film boards (wet-film pure tin-plated boards should be stored in a workshop with a relatively good environment, and the storage time cannot exceed 72 hours. The graphics electroplating process employees take the boards according to the production status, but the storage time in the electroplating workshop is the longest. No more than 12 hours);

11. Insufficient area of ​​anode of pure tin plating tank (Insufficient area of ​​anode of tin plating tank will inevitably lead to lower current efficiency and oxygen evolution during electroplating. The ratio of anode to cathode area is generally 2~3:1, and the standard of pure tin tank anode spacing is 5cm The purpose is to ensure sufficient anode area).

Therefore, some undesirable problems are actually caused by the inconspicuous details of a certain process. As long as we consider all aspects, we can find the key to the problem and solve it.

6. Grasp the advantages and disadvantages of the market wet film quality

The good quality of the wet film is very beneficial to reduce the “shine” of the 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 the wet film:

1. A good wet film is not prone to “permeation”, when the current density is high, the oil film is not easily broken down and the film is relatively easy to retreat;

2. Some wet film may indeed play a certain role in reducing the problem of “shiny” on the edge, but it is relatively difficult to remove the film. This kind of wet oil film is not suitable for the potion with a wide current density operation range, and a higher current density is easy Problems such as “seepage, film clamping, blackening” and even breakdown of the oil film occur.

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