CN101256134B - A method for measuring porosity of gold-plated layer of printed circuit - Google Patents

Abstract

The invention provides a method for testing the porosity of a gold-plated layer of a printed circuit board, which belongs to the technical field of chemical engineering and relates to the detection of the gold-plated layer on the surface of a printed circuit board, and is especially suitable for the detection of the porosity of the gold-plated layer of a printed circuit board conductor and a gold finger. The main components of the test solution are hydrochloric acid (HCl), sodium chloride (NaCl) and penetrant. Under certain temperature and time conditions, obvious corrosion products will be produced at the discontinuity of the gold plating layer. Volume-assisted observation, weighting the pore size, and calculating the porosity by observing under a microscope. The invention has the characteristics of quickness, simplicity, accuracy and safety, and can meet the surface quality detection requirements in the production process of printed circuit boards.

Description

A method for measuring porosity of gold-plated layer of printed circuit

technical field

The invention belongs to the technical field of chemical industry, relates to the detection of the gold-plated layer on the surface of a printed circuit board, and is especially suitable for the detection of the porosity of the gold-plated layer of the lead wire of the printed circuit board and the gold finger.

Background technique

The rise of surface mount technology (SMT) requires the metal coating on the surface of the printed circuit board pad to have the characteristics of anti-oxidation, high solderability, high conductivity and high heat dissipation. Organic Solderability Preservatives OSP, electroless nickel-gold or electroplated nickel-gold surface treatment can well meet the requirements of SMT soldering and are widely used in industrial production.

The disadvantage of the organic solderability protective coating (OSP) process is that the formed protective film is extremely thin and easy to be scratched (or scratched), so it must be handled and transported carefully. At the same time, discoloration or cracks will occur on the OSP film that has undergone multiple high-temperature soldering processes, affecting solderability and reliability. The thickness of the transparent OSP layer is not easy to measure, and the degree of coverage is not easy to see, which makes it difficult for upstream buyers to evaluate the quality stability of these aspects.

Electroless nickel-gold or electroplated nickel-gold surface treatment can make the surface of the printed circuit board (PCB) pad very flat and have good solderability. Secondly, due to the chemical inertness of gold in a normal environment, it can protect the surface of PCB pads from oxidation for a long period of time, and has resistance to environmental erosion and excellent electrical properties that other surface treatment processes do not have. At the same time, the wear resistance during plugging and unplugging is improved, the contact resistance is reduced, and the connection reliability is improved.

The nickel-gold (Ni/Au) surface coating is plated by electroplating or electroless plating after the printed circuit board is covered with a solder mask. The role of the nickel (Ni) layer is to act as a barrier layer to prevent the interdiffusion of copper and gold and improve the reliability of electronic interconnection. The role of the gold (Au) layer is to improve wear resistance, reduce contact resistance, prevent oxidation of copper/nickel and improve connection reliability. Porosity is an important index to characterize the continuity of nickel-gold (Ni/Au) surface coating.

At present, the detection methods for the surface porosity of gold coatings are mainly divided into three categories: ①gas exposure method; ②electrolytic imaging test method; ③salt spray test. In the national standard GB/T 17720-1999 (review of the porosity test of the metal coating), GB/T4677-2002 (test method for printed boards), GB/T19351-2003 (the porosity of the metal coating on the metal substrate of the metal coating) Determination of nitric acid vapor test) has made detailed regulations on the porosity test method of nickel-gold coating on copper substrate.

①Gas exposure method

The gases used in this method mainly include sulfur dioxide, hydrogen sulfide, chlorine, nitric acid vapor and the like. There are high requirements for test reagents, test equipment and test environment, and it takes too long, and the gases used are highly irritating and toxic, which can cause serious pollution to the atmosphere and pose safety hazards to test operators. The test result is to calculate the porosity by observing the number of rust spots and the degree of corrosion with a low-power optical microscope, and the accuracy is greatly affected by the operator. The usability of this test method and the reliability of the test results are not high.

② Electrolytic imaging test method

The method mainly includes acrylamide electrolysis test, dimethylglyoxime paper electrolysis phenomenon test, and cyclohexanedionedioxime paper electrolysis phenomenon test. This method is simple to operate, and the test results are less affected by the operator. The disadvantage is that special instruments are required, and the test paper and developer need to be specially made, and the detection operation involving gold, palladium, rhodium and other coatings on the nickel coating is a toxic operation, so only a few printed board manufacturers are using it.

③Salt spray test

This method mainly uses sodium chloride solution to spray continuously for a long time at a certain temperature. This method is simple to operate, but the disadvantage is that it requires special instruments, takes too long, and it is difficult to accurately calculate the porosity. At present, most printed board manufacturers use this method to qualitatively evaluate the coating quality.

Contents of the invention

The invention aims to provide a fast, simple, accurate and safe inspection method for the porosity of the nickel-plated gold layer of the printed board. The principle of the reaction is to use appropriate reagents to act on the exposed base metal at the discontinuity of the coating to form observable corrosion products. The test solution consists of hydrochloric acid, sodium chloride and penetrant. This method has not been reported at home and abroad.

Technical scheme of the present invention is:

A method for measuring the porosity of a printed circuit gold-plated layer, comprising the following steps:

Step 1. Prepare test solution

Prepare an aqueous solution mixed with HCl, NaCl, and penetrant. The mass percentage of each component is: HCl: 2%-2.7%, NaCl: 0.5-2.0%, penetrant: 0.02%-0.2%, and the rest is H ₂ O. The penetrating agent is a compounding agent containing one or more of the following surfactants: fatty alcohol polyoxyethylene (5-6) ether, fatty acid polyoxyethylene ether, sodium alkylnaphthalene sulfonate, dibutylnaphthalene sulfonate Sodium Naphthalene Sulfonate and Sodium Butyl Naphthalene Sulfonate. The function of the penetrant is to enhance the penetration of the solution and increase the contact degree between the test solution and the coating.

Step 2. Clean the possible oil stains on the surface of the sample to be tested. The sample to be tested must have exposed plating, such as gold fingers, ground copper, pads, etc. For specific cleaning, ethanol can be used as a cleaning agent.

Step 3. Put the sample to be tested after the surface degreasing treatment into the detection solution, control the temperature of the solution at 21°C-24°C, let it stand for 5 to 8 minutes, take out the sample to be tested and remove the moisture on the surface of the sample to be tested .

Step 4. Place the sample to be tested on a slope with an inclination angle of tanα between 0.15 and 0.25 (as shown in FIG. 1 ), and observe the pores on the surface of the sample with a low power optical microscope. The total observation area is not less than 50% of the total coating area of the sample.

Step 5. Calculate porosity. Count the number of pores in the area observed in step 4, and calculate the porosity (number of pores/cm ² ). The pore calculation is carried out according to Table-1.

Table-1 Calculation method of pore size

Pore size Calculate the number of pores Maximum diameter≤0.05mm 0 0.05mm＜maximum diameter＜0.1mm 1 0.1mm≤maximum diameter＜0.2mm 2  Maximum diameter ≥ 0.2mm 5

The specific method for removing the moisture on the surface of the sample to be tested described in the above step 3 can adopt the method of natural drying and drying, and also can use absorbent paper to absorb the moisture on the surface of the sample.

The inclined plane with an inclination angle tanα of 0.15-0.25 in the above step 4 can be realized by using an inclined plane with the same inclination angle tanα.

When using a low-power optical microscope to observe the pores on the surface of the sample in the above step 4, a transparent square grid material (as shown in Figure 2) with a line width of 0.1 mm and a spacing of 1 mm can be used to install it under the eyepiece of the low-power microscope. Improve the measurement accuracy of calculating the number of pores in the observation area. The transparent square grid material can be made of transparent film, transparent paper or transparent glass.

Test the reaction between the solution and the coating and the principle of the porosity test:

(1) Reaction mechanism

In order to increase the flex resistance of the products, flexible printed circuit board manufacturers use very thin nickel-plated gold layers, generally about 1.5 μm for the nickel layer and 0.02-0.1 μm for the gold layer. The gold-plated layer is extremely thin. In order to achieve effective coverage and protection of the gold-plated layer, an organic solderability protective coating (OSP) is used as a capping agent for the gold-plated layer in production. The protective coating is an extremely thin protective film that does not react with hydrochloric acid.

The reaction mechanism for measuring porosity is that in an acidic solution, hydrochloric acid penetrates into the nickel plating layer through the pores not covered by the gold plating layer under the action of the penetrating agent, and undergoes a redox reaction with the nickel plating layer, and the nickel is dissolved, leaving fine needles. The number of micropores can be observed and counted under a low-power microscope. Its reaction mechanism is as follows:

Ni+2HCl→NiCl ₂ +H ₂ ↑

Sodium chloride provides an electrolyte environment for the system, metal nickel is used as the negative electrode of the corroded primary battery, and gold is used as the positive electrode of the corroded primary battery, forming a corroded primary battery and accelerating the existing pinhole corrosion. The effect of pinhole corrosion is shown in Figure-3.

Corrosion of the primary battery electrode reaction is as follows:

Primary battery reaction negative electrode: Ni-2e→Ni ²⁺

Positive electrode of primary battery reaction: 2H ⁺ +2e→H ₂

(2) The reason why the sample is placed on the inclined plane

Because hydrochloric acid does not react with copper, when the nickel in the pinhole dissolves, the bottom layer is the copper layer. When light is irradiated, the gold surface reflects light, and the hole can also reflect light, thereby weakening the distinction between the hole and the gold surface. The slope of the pore cannot be resolved under the optical microscope (as shown in Figure 4). Therefore, the present invention uses an inclination angle to increase the reflection discrimination (as shown in FIG. 5 ).

(3) Check the effect of each component of the solution and the influence of temperature and time

The role of HCl: test the main components of the solution, provide an acidic environment, and react with exposed nickel.

The role of NaCl: provide ions to accelerate pinhole corrosion.

The role of the penetrant: as an acid-resistant penetrant, it can enhance the penetration of the solution, increase the contact degree between the test solution and the coating, and promote the reaction.

Temperature: Temperature has a significant impact on the reaction rate. If the temperature is too high, the reaction will be rapid, and the pores will be extremely enlarged, which will eventually lead to the dissolution of the nickel layer and the fall off of the gold layer. The appropriate temperature should be selected in combination with the production conditions, generally 21°C-24°C is appropriate.

Time: Soaking time has a greater impact on the pores. The longer the time, the more nickel will be dissolved and the pores will be larger. However, if the time is too long, the nickel layer will be completely dissolved and the gold layer will fall off. Generally, 5-8 minutes is appropriate.

Beneficial effects of the present invention:

1. The present invention provides a fast, simple, accurate and safe testing method for the porosity of the nickel-plated gold layer of printed boards, which meets the surface quality testing requirements of industrial production.

2. The main components of the test solution of the present invention are hydrochloric acid, sodium chloride and penetrant, which do not react with the organic solderability protective coating. The measured porosity is the porosity after capping, which truly characterizes the quality of the surface coating.

3. The present invention adopts the solution immersion method, and uses a low power optical microscope for observation, and can easily observe the tiny pores on the thin gold coating by using the inclined plane.

Description of drawings

Fig. 1 is a schematic diagram of a sloped body used in the present invention.

Fig. 2 is a schematic diagram of the present invention installed and the shape of the transparent grid material under the eyepiece of an optical microscope.

Fig. 3 is a schematic diagram of pinhole corrosion mainly reflecting the principle of the present invention.

Figure 4. Schematic diagram of reflection contrast for planar observation.

Figure 5 Schematic diagram of reflection contrast for oblique observation.

Detailed ways

The technical solution of the present invention has been described in detail in the content of the invention, and the actual test data of several different samples to be tested are only given below to further illustrate the operability and effect of the present invention. The experiment is carried out on a single printed board with gold fingers, ground copper skin and pads.

Embodiment one

Add 100ml of deionized water into a 250ml volumetric flask, measure 11.77ml of 36% concentrated HCl, mix well, add 1.25g of NaCl, then add 0.50ml of penetrant with a concentration of 10%, and add deionized water to tick marks. Both HCl and NaCl were of analytical grade.

Taking the printed board (sample 1) that has undergone the electroplating nickel/gold plating process, the thickness of the Ni layer is measured to be 3.324 μm, and the thickness of the Au layer is 0.125 μm. Put the sample in the above-mentioned test solution at a temperature of 22°C for 5 minutes after the oil is removed, blot it dry with absorbent paper, and observe it under a 20X optical microscope. When observing, use a transparent square mesh with a line width of 0.1 mm and a spacing of 1 mm. The Gefferin negative film is installed under the eyepiece of the low power microscope. The inclined plane with an inclination angle of tanα between 0.15 and 0.25 is a inclined plane made of rubber or wood with the same inclination angle of tanα. Finally, the porosity calculated according to Table-1 is 10.2/cm ² .

Embodiment two

According to the method of Example 1, prepare 250 ml of test solution with hydrochloric acid concentration of 2.3%, sodium chloride concentration of 1.0%, and penetrant concentration of 0.05%.

Taking the printed board (sample 2) after electroplating nickel/thick gold plating process, the thickness of the Ni layer was measured to be 6.570 μm, and the thickness of the Au layer was 0.520 μm. Put sample 2 into the above-mentioned test solution at 21°C for 8 minutes after removing the oil, blot it dry with absorbent paper, and observe it under a 20X optical microscope. When observing, use a transparent square mesh with a line width of 0.1 mm and a spacing of 1 mm. The Gefferin film is installed under the eyepiece of the low-power microscope. The inclined plane with an inclination angle of tanα between 0.15 and 0.25 is a inclined plane made of rubber or wood with the same inclination angle of tanα. The final porosity calculated according to Table-1 is 0.3/cm ² .

Embodiment three

According to the method of Example 1, 250 ml of test solutions with hydrochloric acid concentration of 2.5%, sodium chloride concentration of 1.5%, and penetrant concentration of 0.1% were prepared.

Taking the printed board (sample 3) subjected to the immersion soft nickel/immersion gold process, the thickness of the Ni layer was measured to be 1.248 μm, and the thickness of the Au layer was 0.070 μm. Put the sample 3 into the above-mentioned test solution at a temperature of 22°C for 5 minutes after removing the oil stains, blot it dry with absorbent paper, and observe it under a 20X optical microscope. When observing, use a transparent square mesh with a line width of 0.1 mm and a spacing of 1 mm. The Gefferin film is installed under the eyepiece of the low-power microscope. The inclined plane with the inclination angle tanα between 0.15 and 0.25 is a inclined plane with the same inclination angle tanα made of rubber or wood. The final porosity calculated according to Table-1 is 25.4/cm ² .

Embodiment four

According to the method of Example 1, 250 ml of test solutions with hydrochloric acid concentration of 2.7%, sodium chloride concentration of 2%, and penetrant concentration of 0.15% were prepared. Taking the printed board (sample 4) through the immersion hard nickel/immersion gold process, the thickness of the Ni layer was measured to be 1.562 μm, and the thickness of the Au layer was 0.02 μm. Put the sample 4 into the above-mentioned test solution at a temperature of 24°C for 5 minutes after removing the oil stains, blot it dry with absorbent paper, and observe it under a 20X optical microscope. When observing, use a transparent square mesh with a line width of 0.1 mm and a spacing of 1 mm. The Gefferin film is installed under the eyepiece of the low-power microscope. The inclined plane with an inclination angle of tanα between 0.15 and 0.25 is a inclined plane made of rubber or wood with the same inclination angle of tanα. The final porosity calculated according to Table-1 is 32.5 pieces/cm ² .

Embodiment five

According to the method of Example 1, 250ml of test solution with hydrochloric acid concentration of 2.5%, sodium chloride concentration of 2%, and penetrant concentration of 0.2% was prepared. Take the printed board (sample five) that has undergone the sinking soft nickel/electroless gold plating process, and the thickness of the Ni layer is measured to be 2.324 μm, and the thickness of the Au layer is 0.452 μm. Put the sample 5 into the above-mentioned test solution at a temperature of 22°C for 8 minutes after removing the oil, blot it dry with absorbent paper, and observe it under a 20X optical microscope. When observing, use a transparent square mesh with a line width of 0.1 mm and a spacing of 1 mm. The Gefferin negative film is installed under the eyepiece of the low power microscope. The inclined plane with the inclination angle tanα between 0.15 and 0.25 is a inclined plane with the same inclination angle tanα made of rubber or wood. The final porosity calculated according to Table-1 is 1.2 pieces/cm ² .

Claims (7)

1. A method for measuring the porosity of a printed circuit gold-plated layer is characterized in that it may further comprise the steps: Step 1. Prepare detection solution: Prepare an aqueous solution mixed with HCl, NaCl, and penetrant. The mass percentage of each component is: HCl: 2%-2.7%, NaCl: 0.5-2.0%, penetrant: 0.02%-0.2%, and the rest is H ₂ O; where the penetrating agent is a complex agent containing one or more of the following surfactants: fatty alcohol polyoxyethylene (5-6) ether, fatty acid polyoxyethylene ether, sodium alkylnaphthalene sulfonate, dibutyl Sodium naphthalene sulfonate and sodium butyl naphthalene sulfonate; Step 2. Clean the oil stains that may exist on the surface of the sample to be tested. The sample to be tested must have a bare coating, and the coating is a gold finger, a ground copper sheet or a pad; Step 3. Put the sample to be tested after the surface degreasing treatment into the detection solution, control the temperature of the solution at 21°C-24°C, let it stand for 5 to 8 minutes, take out the sample to be tested and remove the moisture on the surface of the sample to be tested ; Step 4. Place the sample to be tested on a slope with an inclination angle of tanα between 0.15 and 0.25, and use a low-magnification optical microscope to observe the pores on the surface of the rod; the total observation area is not less than 50% of the total coating area of the sample; Step 5. Calculate porosity: Table-1 Calculation method of pore size Pore size Calculate the number of pores Maximum diameter≤0.05mm 0 0.05mm＜maximum diameter＜0.1mm 1 0.1mm≤maximum diameter＜0.2mm 2 Maximum diameter ≥ 0.2mm 5 Count the number of pores in the area observed in step 4, and calculate the porosity, that is, the number of pores/ ^cm2 ; the calculation of pores is carried out according to Table-1. 2. The method for measuring the porosity of a gold-plated layer of a printed circuit according to claim 1, wherein ethanol is used as a cleaning agent when said step 2 cleans the possible oil stain on the surface of the sample to be tested. 3. the printed circuit gold-plated layer porosity assay method according to claim 1 is characterized in that, the concrete method of removing the moisture on the sample surface to be measured described in step 3 is to adopt the way of natural drying or drying. 4. the gold-plated layer porosity assay method of printed circuit according to claim 1, it is characterized in that, the specific method of removing the moisture on the surface of the bar product to be measured described in step 3 is to adopt the way of absorbent paper to absorb the moisture on the sample surface . 5. The method for measuring porosity of a gold-plated layer of a printed circuit according to claim 1, wherein the inclined plane having an inclination tanα of 0.15 to 0.25 in step 4 is realized by a slope with the same inclination tanα. 6. The method for measuring the porosity of the printed circuit gold-plated layer according to claim 1 is characterized in that, in step 4, when adopting a low power optical microscope to observe the pores on the sample surface, using a line width of 0.1 mm and a spacing of 1 mm The transparent square grid material is installed under the eyepiece of the low-magnification microscope to improve the measurement accuracy of calculating the number of pores in the observation area. 7. The method for measuring the porosity of a gold-plated layer of a printed circuit according to claim 6, wherein the transparent square grid material is made of transparent film, transparent paper or transparent glass.

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