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

- Dec 17, 2019-

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

Summary of content: In the manufacture of printed circuit boards, the etching process is in the middle of the printed circuit board process, which accounts for more than half of the production cost. It directly determines the quality level and yield of the product. Especially with the rapid development of microelectronic technology and the widespread application of large-scale integrated circuits and ultra-large-scale integrated circuits, printed circuit boards are moving towards high precision (wire diameter 0.05-0.08mm) and high density (between two pads). The direction of wiring is 4 to 5), and higher and stricter requirements are imposed on the line width tolerance of the etching. Choosing a suitable image transfer method and selecting a good quality etching solution have become the keys to ensure the quality level and yield of the circuit.

I. Overview of the development of etching solution:

Since the development of the printed circuit board industry, the etching solution used in the etching process has been updated to the fourth generation. First generation product: hydrochloric acid-ferric chloride type etching solution, stable process, convenient operation, low cost, but serious pollution, difficult regeneration, difficult wastewater treatment, meanwhile the etching speed is not fast (20 30μm / min), copper dissolving The amount is not much (50 ~ 60g / L). Second-generation products: Sulfuric acid-ammonium persulfate type etching solution (or chromic acid): The etching speed and the amount of dissolved copper are low, easily decomposed, and the waste liquid contains the first class of harmful substances. These two generations are now obsolete. The third generation product: hydrochloric acid-hydrogen peroxide type etching solution: simple composition, hydrogen peroxide (35%) and hydrochloric acid (36%) are prepared in a volume ratio of 1: 2, and the etching speed can reach 25 30μm / min. It reaches 80 110g / L. So far, very few companies are using it. The fourth generation product: copper chloride (acid, alkaline) etchant: Since its invention in 1975, it has been widely used (performance characteristics will be discussed in the following).

It is predicted to be the fifth generation product: nitric acid-based etching solution, which is still under development, but few related reports have been seen.

 

Technical performance of etching solution:

1. The etching solution can meet the performance requirements of the corrosion protection layer (or anti-plating protection layer).

2. Wide range of process conditions, relatively good operating environment, and effective automatic control.

3. Stable efficacy, continuous operation and regeneration, and long service life.

4. Large etch coefficient and small side etch.

5. Large amount of dissolved copper.

6. Sewage treatment is easier. After the etching waste liquid is recovered by copper, it can be supplemented with effective ingredients, so that it can be recycled and recycled.

 

Performance characteristics of copper chloride type etching solution:

1.Type

According to the nature, it can be divided into two categories: acidic etching solution and alkaline etching solution. The acidic etching solution can be divided into: single-liquid type acidic etching solution and double-liquid type acidic etching solution. When an acid-resistant resist is used as a resist protective layer, an acidic etchant is used. When an alkali-resistant and anti-plating material is used as the plating protection layer (plating tin or tin / lead), an alkaline etchant is used.

2. The main chemical composition and technical performance of copper chloride type etching solution:

, single liquid acid etching solution: (fresh liquid)

Hydrochloric acid (31%) 360 400ml / L

Sodium chloride (industrial grade) 10 20g / L

Ammonium chloride (agricultural grade) 80 120g / L

Additives, the right amount

Water. The amount of money left over.

, two-liquid type acid etching solution: (fresh sub-liquid = oxidation solution + hydrochloric acid)

A. Oxidation solution (salt solution)

Sodium chlorate (industrial grade) 160 220g / L

Sodium chloride (industrial grade) 100 150g / L

Ammonium chloride (agricultural grade) 30 50g / L

Additives, the right amount, the right amount

Water, the amount of money left over, the amount of money left over

B. Hydrochloric acid (31%)

According to the requirements of the etching process, the molar concentration of hydrochloric acid is determined. Generally, the rigid plate is controlled at 2.0 to 3.0 mol / L, and the flexible plate is controlled at 1.5 to 2.0 mol / L.

Note: The oxidizing solution and hydrochloric acid should be stored separately. According to the requirements of the etching process, they should be transported to the etching cylinder of the etching machine through two pipes.

Alkaline etching solution: (fresh liquid)

Ammonia (21 25%) 400 600ml / L

Ammonium chloride (agricultural grade) 230 265g / L

Additives, the right amount, the right amount

Water, the amount of money left over, the amount of money left over

, technical performance summary table

4. Quality Evaluation of Copper Chloride Etching Solution:

The evaluation of the quality of the etching solution is usually measured from three aspects: the etching coefficient, the etching rate and the amount of dissolved copper.

1.Etching factor (also known as etching factor)

The etching coefficient is expressed in T / X. The etching coefficient is related to the characteristics of the etching solution, the etching method, and the etching temperature. The larger the etching coefficient, the better. The side etching occurs on the sidewall of the wire under the resist layer. .

T——thickness of copper foil (mil)

X——Side Etching (mil)

Amount of side erosion = [Width of bottom line after etching (ie, lower line width)-Upper line width after etching] ÷ 2

 

Calculation of etching coefficient:

Etching coefficient (T / X) = copper foil thickness T (mil) ÷ side etching (mil)

The total allowable side etch of copper foils of different thicknesses is as follows:

 

The etching coefficient of a single-fluid acid etching solution of good quality should reach 3, the etching coefficient of a high-quality two-fluid acid etching solution should reach 3.5 to 4.5, and the etching coefficient of a good quality alkaline etching solution should reach 3 to 3.5.

2. Etching rate (μm / min):

Etching rate It is often expressed as the depth of the copper foil dissolved by the etchant in a unit time, or the time required to dissolve a certain thickness of copper foil.

①①Etching rate of single liquid acid etching solution

Immersion: 0.20 0.28 mil / min (average about 6μm / min)

Spray type: 2.0 3.0 mil / min (average about 60μm / min)

②②Etching rate of two-liquid acid etching solution

Immersion: 0.28 0.36mil / min (average about 9μm / min)

Spray type: 3.2 4.0 mil / min (average about 90μm / min)

The etching rate of alkaline etching solution

Immersion: 0.24 0.32 mil / min (average about 7μm / min)

Spray type: 2.40 3.20 mil / min (average about 70μm / min)

Detection method and calculation formula of etching rate:

A. Detection method

Take a 5 × 5 (Cm2) square rectangular copper-clad plate, wash it with degreaser and water, place it in a blast drying oven, bake it at 103 ° C for 30 minutes, then place it in a dryer and cool to room temperature (Room temperature 25 ° C), weigh it (m1). Accurate to 0.0001g.

Take a certain amount of etching solution into a 250ml beaker, soak the weighed copper clad board in it for 2min, then take it out and wash it with water, put it in a blast drying box, and bake it at 103 ° C for 30min. Then put it in a desiccator, and after cooling to room temperature (room temperature 25 ° C), weigh it (m2). Accurate to 0.0001g.

B. Calculation of etching rate

V (μm / min) = (m1m2) / (ρ × S × T) × 10000

In the formula:

V (μm / min) —— Etching rate

m1 (g)-weight of copper clad laminate before etching

m2 (g)-weight of copper clad laminate after etching

ρ (g / Cm3) —— density of copper: 8.92

S (Cm2) —— CCL area: 5 × 5 = 25. If using a double-sided copper clad laminate, it should also be × 2

T (min) —— Soaking time: 2

10000——Conversion factor. (1Cm = 10mm = 10000μm)

3. Dissolved copper (g / L):

The amount of dissolved copper ---- the weight (g) of soluble copper per unit volume (liter) of the etching solution, expressed in g / L.

The amount of soluble copper in single-fluid acid etching solution with good quality is 140 ± 5g / L.

The copper content of the two-liquid acid etching solution with good quality is 160 ± 5g / L.

The amount of copper dissolved in the high-quality alkaline etching solution is 155 ± 5g / L.

 

V. Factors affecting the technical performance of copper chloride type etching solution:

1.Effect of fresh liquid on the technical performance of etching solution

, the effect of hydrochloric acid

The corrosion of copper by hydrochloric acid is electrochemical corrosion (electrolytic corrosion). It is impossible for high potential copper to be corroded by an acid solution with a low potential. When only oxygen or an oxide is present, the electrode potential is increased to be higher than that of copper. Copper is corroded by hydrochloric acid. Corrosion is an anodizing reaction (deelectronation).

The total reaction is: 2Cu + O2 + 4HCl ——> CuCl2 + 2H2O

As the concentration of hydrochloric acid increases, the speed of etching will increase, but if the concentration is too high, the volatilization of the acid is large, which not only easily causes corrosion of the equipment, but also the solubility of copper chloride is reduced. For a single-liquid acid etching solution, the amount of hydrochloric acid is generally controlled at about 4 mol / L (ie, 146 g / L), and the hydrochloric acid converted to a concentration of 31% is 470 g / L (ie, 408 ml / L). For a two-liquid type acid etching solution, the amount of hydrochloric acid must not exceed 3 mol / L, and the hydrochloric acid converted to a concentration of 31% is 353 g / L (ie, 305 ml / L). In fact, there are usually 2.0 to 2.5 mol / L for rigid boards, and 1.5 to 2.0 mol / L for flexible boards.

, the type of chloride and the effect of chloride ion

The corrosion effect of hydrochloric acid on copper is very limited, and it mainly depends on the corrosion of copper by copper. The corrosion of copper by chlorine is chemical (non-electrolyte corrosion). During the etching process, the generated water-insoluble cuprous chloride will form a cuprous film on the surface of the copper, preventing further etching. Only a sufficient amount of chloride ions will be provided to form a soluble complex with cuprous chloride (CuCl3) 2-, and dissolve from the surface of copper to increase the etching rate. Hydrochloric acid, ammonium chloride and sodium chloride are all providers of chloride ions.

In general, copper is a good complex-forming body because it has an incomplete d-orbital electron layer. In the presence of sufficient chloride ions in the solution, copper can form four coordination bonds in four forms:

The existence of divalent copper complex ions (CU2 ) can be evolved into H2CuCl4 as [Cu2 Cl4] 2

The monovalent copper complex ion exists (CU1 +) as [Cu2 Cl3] 1-, which can evolve into 2H2 [CuCl3].

Copper chloride (CuCl2), CuCl2 is soluble.

Cuprous chloride (CuCl). Cuprous chloride (CuCl) is insoluble.

It is known from "" that the amount of hydrochloric acid to be added needs to be controlled within a certain range. Therefore, only sufficient ammonium chloride and sodium chloride can be used to supplement a sufficient amount of chloride ions. Adding ammonium chloride is faster to increase the etching speed, but the amount of addition is also limited because the precipitation of copper ammonium chloride crystals is generated when the temperature of the bath is reduced. The addition amount of ammonium chloride in the sub-liquid of the acidic etching solution should only be controlled within the range of 30 to 120 g / L. The addition amount of ammonium chloride in the sub-liquid of the alkaline etching solution should only be controlled within the range of 230-265 g / L. The effect of sodium chloride on improving the etching speed is very close to that of hydrochloric acid. To increase the content of chloride ions in the etching solution, an appropriate amount of sodium chloride can be added.

The effect of ammonia

The pH value of alkaline sub-liquid changes with the content of ammonia water. Usually when preparing fresh sub-liquids, the pH should be controlled between 9.6 ± 0.2 to ensure that during the etching process, with the consumption of ammonia, the pH value of the bath continuously decreases and can still be maintained within the range of 8.1-8.8.

The effect of additives

The amount of additives in the sub-liquid is generally controlled within the range of 1 , the amount needs to be small, but the effect is huge.

The main effects of the additives of the etching solution are as follows: First, it can produce a complexation reaction with cuprous ions to promote the dissolution of copper. Second, it acts as an oxygen conducting agent. For example, sodium chlorate in acidic etching solution is a strong oxidant in the presence of an oxygen conducting agent. Third, it can improve the effect of copper removal and cleaning during etching.

Nowadays, no matter which supplier provides the etching solution, its main components and the amount of addition are generally the same, but the quality of the product is basically due to the different additives used. Zh

2. Influence of bath fluidization index on the technical performance of etching solution

, process conditions

A, temperature

The etching rate will increase about 1 time for every 10 ° C increase in temperature, but the temperature should not be too high. Generally, it should be controlled between 48 ± 2 ° C. If it is too high, it will cause HCl decomposition and NH3 volatilization, which will cause the composition of the etching solution to be out of balance. Especially when the concentration of acid is excessive and the temperature is too high, the corrosion (anti-plating) material is most likely to be damaged, resulting in etching waste. If the temperature is too low, the etching rate will also be significantly slowed down, and there will be some precipitation of copper ammonium complex crystals in the solution, which will cause blockage of the nozzle.

B. Nozzle pressure

The influence of the shape of the nozzle and the pressure of the nozzle on the etching quality must not be ignored. Compared with the inverted nozzle, the flat nozzle not only increases the spray angle, but also the spray pressure is more uniform, which improves the uniformity of etching Play a very obvious role. Generally speaking, for the control of the nozzle pressure, the upper nozzle is 2.0 2.5Kg / Cm3, and the lower nozzle is 1.5 2.0Kg / Cm3.

C, walking speed

At present, there are various specifications and models of etching machines, and there is no uniform standard for either manual or automatic production lines. As far as the length of the etching tank is concerned, there are two meters, three meters, four meters, five meters, six meters, and eight meters.

The running speed is determined by the etching completion time.

Etching completion time (min) = thickness of copper foil (μm) ÷ etching rate (μm / min).

Travel speed (m / min) = effective etching distance (m) ÷ etching completion time (min).

It is very obvious that the speed of the board is also restricted by the etching rate of the etching solution, the thickness of the copper foil of the circuit board, and the length of the etching tank of the etching machine.

Note: Effective etching distance (m) = ¾ length of etching cylinder. (For example: the etching tank is 2 meters, the effective etching distance is 2 × ¾ = 1.5 meters)

D. Load

To always maintain the etching solution in a stable quality state, there must be a relatively stable load, that is, the area of the processing board per unit time is relatively hidden. Do not flick more or less, flick more or less, stop and beat, do not punch through different areas and different thicknesses together, this will cause problems in etching quality.

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