Discussion on Quality Improvement of Special Etching Solution in PCB Process(Part II)

- Dec 17, 2019-

Discussion on Quality Improvement of Special Etching Solution in PCB Process(Part II)


6. Methods to improve the technical performance of copper chloride etching solution:

1. Control the change of the specific gravity (baume degree) of the tank fluid

As the etching progresses, the copper content in the bath also increases. Studies have shown that the content of divalent copper ions in the bath is less than 82 g / L, which requires a long etching time. The etching rate is 82-120 g / L. Low, solution control is difficult; when 135 ~ 150g / L, the etching rate is high and the solution is stable; above 160g / L, the etching rate will not increase, but will decrease, and precipitation will begin to occur.

In order to ensure a constant etching rate, the Cu2 + content in the solution must be controlled within a regular range. The copper content of the single-liquid acid etching solution is controlled between 120-145 g / L, and the density of the bath solution is 1.180-1.20 g / cm3 (that is, 22-24 ° Be´). The copper content of the two-liquid acidic etching solution is controlled between 130-160 g / L, and the density of the bath solution is 1.270-1.345 g / cm3 (ie, 31-37 ° Be´). The copper content of the alkaline etching solution is controlled between 125 and 155 g / L, and the density of the bath liquid is 1.185 to 1.215 g / cm3 (that is, 22.5 to 25.5 ° Be´).

Note: The above data are the corresponding values of copper content and density (or Baume degree) at normal temperature (25 ° C). If you need to calculate the density and copper content of the bath at the operating temperature (50 ° C), the empirical values are: The density at 50 is 0.01 g / Cm3 lighter than that at 25 , and the copper content is about 10 g / L.

In actual production, the method of automatic specific gravity controller is used to control the density of the bath liquid (° Be´). When the specific gravity exceeds a certain value, the control system will start the automatic addition system to add the etching liquid to the etching tank. At the same time, part of the mother liquid is replaced from the etching tank, so that the specific gravity of the bath liquid is adjusted to a prescribed reasonable process range.

Influence of the concentration of divalent copper in the etching solution on the etching time.

 

, if the replacement volume of the bath is manually controlled, the most accurate method is:

Replacement amount (liter) = (analysis value-specified value) ÷ (analysis value × total volume)

In the formula: Analytical valuetested copper ion content in the bath (g / L)

Specified valuethe minimum copper ion content (g / L) of the specified bath is 130 g / L

Total volumethe total amount of etching solution in the etching tank (liters)

△△ Empirical methods

The displacement (liter) is 20 to 25% of the total volume.

2.Control the amount of monovalent copper

The etching process is essentially an oxidation-reduction process. There will be the production of monovalent copper, which will change the oxidation-reduction electron potential of the bath. As the monovalent copper in the etching solution continues to increase, its oxidation-reduction potential (E0) will decrease. Studies have shown that the presence of a small amount of monovalent copper can significantly reduce the etching rate. For example, in a bath with a copper content of 120g / L, if 4g is monovalent copper, the etching rate will be significantly reduced. Only if it is oxidized to divalent copper as soon as possible, the The monovalent copper content can stabilize the etching rate within a very low range (below 2g / L).

The relationship between the concentration of monovalent copper ions in the solution and the oxidation-reduction potential:

 

When E0 is 530mv, the concentration of (Cu1 +) is less than 0.4g / L, and the etching speed of the etching solution is fast and stable. This is the most ideal process parameter. The key management point of copper chloride acidic etching solution is to control the oxidation-reduction potential to control the monovalent copper ion concentration of the etching solution. Usually in the production process, the oxidation-reduction potential is controlled in the range of 530 ± 20mv. (That is, the monovalent copper ion concentration is in the range of 1.05 to 0.4 to 0.15 g / L).

Oxidation-reduction potential corresponding to monovalent copper content

1.2g / L, 510mv

0.7 g / L, 520 mv

0.4g / L 530 m v 530 530mv

0.3g / L, 540mv

0.15g / L, 550mv

, the relationship between oxidation-reduction potential and etching rate

Studies show that controlling the oxidation-reduction potential can also improve the etching coefficient. If the specific gravity of the etching solution is adjusted to 32 ° Be´, the acid equivalent is adjusted to 1.5mol / L, and the oxidation-reduction potential is controlled to 550mv, the etching coefficient will be in the range of 3 to 3.5; if the specific gravity of the etching solution is adjusted to 36 ° Be ´, the acid equivalent is adjusted to 1mol / L, the oxidation-reduction potential is controlled to 520mv, and the etching coefficient is basically constant between 4 and 4.5. This can fully ensure that the production requirements of fine lines are met.

In the actual production process, an oxidation-reduction potentiometer (ORP) automatic control system is used to control the oxidation-reduction potential of the etching bath to a precise range. It can ensure that the etching quality is stable and reliable. When the E0 of the two-liquid type acid etching bath is lower than 510mv, the electromagnetic cutting with the addition of hydrochloric acid is turned on, and the hydrochloric acid is added to the tank; meanwhile, the electromagnetic cutting with the addition of the oxidation solution is turned on, and the oxidation liquid is added to the tank . Generally speaking, the amount of added hydrochloric acid: the amount of added oxidizing solution = 2.3: 1 (volume ratio). At this time, the E0 of the bath fluid will gradually increase with the addition of the supplemental medicinal solution. When the E0 rises to 560mv, the replenishment stops. (Or according to production experience, control the supplementary time to determine the amount of each unit of hydrochloric acid and oxidizing solution.) At the same time, the same volume of waste liquid should be discharged outside the tank.

Method to eliminate monovalent copper

A. Supplement the supply of oxygen or oxidant. Such as:

a. Pass oxygen or compressed air into the bath

2Cu2Cl2 4HCl O2 ——> 4CuCl2 2H2O

b. Sodium hypochlorite regeneration

Cu2Cl2 2HCl NaOCl ——> 2CuCl2 NaCl H2O

c. Hydrogen peroxide regeneration

Cu2Cl2 2HCl H2O2 ——> 2CuCl2 2H2O

D. Add a small amount of oxo acid

B. Chlorine regeneration

An appropriate amount of chloride ion is supplemented to make monovalent cuprous chloride into divalent cupric chloride.

Cu2Cl2 2Cll-2CuCl2

The content of chloride ion in the bath should be controlled at 180 220g / L, (the optimal value is 190 210g / L). As the chloride ion content increases, the amount of copper etch will also increase, but if it is too high, it will also cause attacks on the resist metal layer.


Chlorine consumption corresponding to copper corrosion

 

Note: During the etching process, there is still 15 ~ 20% chlorine loss.

C. Use additives that can react with cuprous ions

Common additives: acidic etchant such as gelatin, ethylenediaminetetraacetic acid (code EDTA) and its sodium salt, alkaline etchant such as thiourea (code TU) and its derivatives, and other inorganic or organic complexes. Studies have shown that its complexing ability for monovalent copper ions is only average.

In order to find it: First, it can have a complex reaction with cuprous ions to promote the dissolution of copper. Second, it acts as an oxygen conducting agent. Third, it can improve the effect of copper removal and cleaning during etching. Additives suitable for acidic etching solution and alkaline etching solution. This research took nearly one year, conducted more than 100 screening tests, and finally found two kinds of additives with three functions, which are used as acid and alkali, respectively. Sexual etching solution. It has been proved in the production of Taihe Electronic Wire Chrome Road Slab Factory in Zhongkai Industrial Park of Huizhou that the occurrence of monovalent copper can be controlled within 1.2g / L without any changes to the operating conditions. During the process, any contamination on the copper surface will increase the etching speed by nearly 30%.


Discussion on nitric acid etching solution:

In the past three years, the company has conducted some discussions on the nitric acid type etching solution, and the preliminary results are: the nitric acid type etching solution has better etching speed and copper dissolution amount than the copper chloride type etching solution. Because copper reacts with dilute nitric acid, no monovalent copper is produced, so the etching factor is high and there is almost no side etching.

1.The reaction principle of nitric acid etching of copper

As is known to all, for the copper chloride type etching solution, the reaction between chlorine and copper is a substitution reaction, which is a chemical reaction. The reaction between nitric acid and copper is an oxidation-reduction reaction, which is an electrochemical reaction. That is: copper is oxidized by nitrate to Cu2 +, and hydrogen is reduced. Cu atom increased from 0 valence to +2 valence, that is, 6 electrons were lost; N decreased from +5 valence to +2 valence, and 6 electrons were obtained in total. The reaction of copper and nitric acid is divided into two cases:

, copper reacts with concentrated nitric acid

CU 4HNO3 Concentration) ——> Cu NO32 + NO2 4H2O

, copper and dilute nitric acid reaction

3CU 8HNO3 lean) ——> 3Cu NO32 2NO 4H2O

It can be known from the reaction equation that copper reacts with concentrated nitric acid to decompose red-brown carbon dioxide gas, and reacts with dilute nitric acid to decompose colorless carbon monoxide gas.

The boundary to distinguish between concentrated or diluted nitric acid is: when the nitric acid content in the solution is 6mol / L, it is called a concentrated nitric acid solution, and when the nitric acid content in the solution is <6mol / L, it is called a dilute nitric acid solution.

Studies have shown that, in order to ensure that the nitric acid type etching solution can have perfect technical characteristics, the nitric acid content in the solution should be controlled within the range of <6mol / L. Therefore, the nitric acid type etching solution should follow the reaction principle between copper and dilute nitric acid.

The chemical reaction equation is:

3CU 8HNO3 lean) ——> 3Cu NO3) + 2NO 4H2O

The ion reaction equation is:

3Cu 2No3- + 8H + ——> 3Cu2 ++ 2NO 4H2O

2. The main components of nitric acid etching solution and their functions

, the main agent: nitric acid

Performance of dilute nitric acid:

A. It is a strong oxidizing strong acid.It can react with most metals and non-metals and their compounds except Au and Pt.

B, unstable, easy to decompose under the action of light and heat

4HNO3 ——> 2H2O 4NO2 O2

Due to its properties, not only nitric acid itself directly undergoes a redox reaction with copper to produce copper nitrate, at the same time, the O2 produced by the decomposition reacts with copper to form CuO, and then reacts with a copper-dissolving accelerator to generate the corresponding copper salt.

, copper dissolving accelerator:

Add an appropriate amount of "copper dissolving accelerator YGX-TU" to the nitric acid solution, and it will be oxidized to produce a variety of products, which can accelerate the corrosion of copper.

Cathode depolarizer:

Most are oxides, such as sodium nitrate, potassium nitrate, sodium nitrite, etc. Its role is:

2Cu O2 4HNO3 ——> 2Cu NO32 2H2O

, hydrogen ion supplements:

The hydrogen ion supplement can promote the nitrogen oxides (gas) formed in the reaction process to reduce to dilute nitric acid and continue to react with copper.

3. Factors affecting the etching rate of nitric acid etching solution

The effect of nitric acid concentration:

High nitric acid concentration and fast etching speed.

The effect of copper content in the etching solution:

Because there is no production of monovalent copper when copper reacts with dilute nitric acid, there is no problem that the monovalent copper content affects the etching rate. The effect of the content of divalent copper on the etching rate is completely different. When the content of divalent copper in the bath reaches 40-50 g / L, the etching rate increases rapidly, and the etching rate does not increase until the Cu2 + content rises to 140-150 g / L. Will start to fall. However, the etching rate is still much higher than that of the copper chloride type etching solution.

The influence of operating temperature:

As the temperature increases, the etching rate increases, but the temperature should not be too high, and it is generally controlled within the range of 45 ± 5 ° C. If the temperature is too high, it will not only accelerate the decomposition of nitric acid, but also cause the proportion of solution components to be out of balance. At the same time, it is easy to cause attacks on certain resists, especially dry films on flexible boards. When used as a flexible plate for etching, the operating temperature should be controlled within the range of 40 ± 2 ° C.

The influence of load:

The reaction between copper and nitric acid is a relatively intense exothermic reaction.While ensuring the speed of the plate, the load of the plate should be controlled to ensure that the reaction heat release energy per unit time is consistent with the operation temperature control, so as not to cause tank fluid Excessive warming has adverse effects. (You must not take measures to reduce the speed of the board to control the load).

4. Final determination of formula:

The results obtained after orthogonal tests are:

The concentration of nitric acid should be controlled within the range of dilute nitric acid (<6mol / L). In order to maintain the ideal etching speed and the amount of dissolved copper, the concentration of nitric acid should be above 3.5mol / L.

The hydrogen ion supplement itself does not need to react directly with copper, but it is very important to reduce the NO2 decomposed by heating to nitric acid. The added amount should be determined based on the amount of NO2 generated during the etching process. Usually should be controlled at about 10%.

The copper dissolving accelerator "YGX-TU" is a copper ion complex suitable for the nitric acid system, and the addition amount is generally controlled within the range of 0.1 to 0.5%.

For electrochemical reactions with electron transfer, the effect of adding a small amount of cathode depolarizing agent is very obvious, and the amount is generally controlled within the range of 1 to 3%.

5. Comparative test of nitric acid etching solution and two-liquid copper chloride etching solution

Test conditions:

Using the nitric acid etching solution of the final formulation of this research, and the two-liquid copper chloride etching solution (acid equivalent of 2.94mol) produced by Kehui for comparison test.

The operating temperature is 45 ± 3 ,

The test board is 1 × Z double-sided bare copper plate 3 × 12 (Cm). The measured copper thickness was 32 μm.

All experiments were performed with fresh sub-liquid, and the initial copper content was 0 g / L.

6. Experimental results:

Experiments show that the nitric acid type does not attack screen printing resist materials at 45 ± 3 for 3 minutes.

Experiments show that the nitric acid type has a large etching coefficient and no side etch has been detected.

The experiment shows that the operating temperature is 45 ± 3 , when the copper content of the nitric acid solution reaches 40-50g / L, the immersion type (static), and the etching rate can reach 20μm / min (ie 0.8mil / min). The etching quality is ideal. When the copper content of the bath solution rises to more than 145g / L, the etching rate is reduced to 10 μm / min (that is, 0.4 mil / min), which is exactly the same as the maximum etching rate of the two-liquid copper chloride etching solution.

The amount of copper dissolved in the nitric acid etching solution can reach 165 ± 5g / L.

Although the production of NO and NO2 during the etching process, the amount of occurrence is much lower than the hypochlorous acid and hydrochloric acid mist produced by the two-liquid acid etching solution, and the operating environment can be greatly improved. To prevent the influence of NO gas on the working environment, an extraction system can be installed for processing.

The copper nitrate contained in the etching waste liquid can be recovered by direct cooling crystallization (the crystal particles are thick, gem blue, and the purity is 99.9%). After the filtrate is supplemented with effective ingredients, the waste liquid is regenerated and recycled.

7. Productive trial results

Productive trial was carried out in Dongguan Xinxiong Circuit Board Factory, the operating temperature was 45 ± 3 , all of the fresh sub-liquids were used, and the initial copper content was 0 g / L. When the copper content in the bath reaches 40 50g / L, the etching rate has reached the etching rate of the copper chloride solution type acid etching solution. As the copper content in the bath increases, the etching rate also increases rapidly. After the content is more than 100g / L, the etching speed has been increased to nearly 3 times of the etching rate of the liquid copper chloride etching solution.

Message

Since the etching speed of the nitric acid type etching solution is increased by three times or more than that of the hydrochloric acid type etching solution, it is impossible to use the existing etching machine for deeper productive research and development of the nitric acid type etching solution. A new type of etching machine is yet to be designed. , For more in-depth application tests.

In the laboratory, an immersion method using a nitric acid type etching solution has been used to conduct an etching test on ultra-fine wire diameter and fine-density flexible circuit boards with a copper thickness of 9 μm. (The temperature is controlled within 40 ° C, and the time 1 minute and 23 seconds), sent samples to the original company that produced the circuit board for testing. No quality problems were found, and it is impossible to believe that it was produced by manual immersion.

Therefore, we hope to find people of insight to cooperate and complete the scientific research project of this technological update so that it can be practically applied as soon as possible.

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