TWI780602B - Solution and process for the activation of nonconductive area for electroless process - Google Patents

Abstract

The present invention discloses a novel activator system for electroless metallization deposition, particularly activators that may be free of tin and surfactants. Activators of the invention are preferably employed for electroless copper deposition.

Description

Activation solution and method for electroless plating treatment of non-conductive areas

The present invention generally relates to the field of activators for metal plating, and in particular to novel activator systems for electroless metal plating. The activator system of the present invention is preferably used in electroless copper plating.

In general, a Printed circuit board (PCB) is an assembly that integrates electrical leads so that various devices can be installed in it or electrically connected to each other. Due to the advancement of technology, PCBs with various forms and functions are constantly coming out. The demand for PCBs has been increasing with the growth of industries using PCBs, such as household appliances, communication equipment, semiconductor equipment, industrial machinery, vehicle electronic control and related industries.

Electroless metal plating has a wide range of applications, including the manufacture of printed circuit boards and non-conductor processes (such as decoration and processing of plastic substrates). Printed circuit boards include laminated non-conductive dielectric substrates and must rely on drilling and plating through-holes to make connections between two sides of the board or inner layers. As is well known, electroless plating has been used to prepare metal coatings on surfaces, and prior to metal plating, an activator must be pre-deposited on the dielectric surface. One of the common methods of catalyzing or activating non-conductive dielectric substrates is to treat the substrate with an aqueous tin/palladium colloid prior to electroless plating. The colloid consists of a metallic palladium core surrounded by a stabilizing layer of tin(II) ion complexes, such as SnCl ₃ ⁻ , as surface stabilizing groups to avoid aggregation of the colloid in suspension.

During the activation process, palladium colloids are adsorbed onto insulating substrates (such as epoxy or polyimide) to catalyze subsequent electroless copper plating. Theoretically, the activator particles function as carriers in the path of electron transfer from the reducing agent to the metal ions in the plating bath. Although the performance of electroless plating is also affected by other factors such as the composition of the electroplating bath solution and the choice of ligand (ligand), the activation step is still a key factor controlling the rate and mechanism of electroless plating. Tin/palladium colloids have been used commercially as activators for electroless metal plating for decades; however, there is still room for improvement due to the air sensitivity and high cost of palladium. In addition, the residual palladium adsorbed on the surface of the resin must be removed to prevent a possible short circuit between the two copper wires, which also increases the cost of the entire manufacturing process.

The search for new and better activators is ongoing. For example, due to the high cost of palladium, many efforts have focused on the development of non-noble metal activators, especially colloidal copper activators. However, such activators have not been shown to be sufficiently active or reliable for through-hole plating. In addition, these activators also typically become gradually inactive upon storage, making commercial use of such activators less reliable and impractical.

Accordingly, one aspect of the present invention relates to a composition for depositing an electroless plating activator on a substrate, the composition comprising one or more metal ions and one or more organic acids; wherein the organic acid has at least one carboxylic acid group (carboxylic group) and at least one hydroxyl group (hydroxyl group).

(I) ，其中R ₁選自直鏈型或分支型、經取代或未經取代的C ₁-C ₆醇類。 In some embodiments, the organic acid is dicarboxylic acid terminated and has the formula (I):

(I), wherein R ₁ is selected from linear or branched, substituted or unsubstituted C ₁ -C ₆ alcohols.

In some embodiments, the organic acid is selected from tartaric acid, citric acid, malic acid and 2,2-bis(hydroxymethyl)malonic acid (2,2-bis (hydroxymethyl) malonic acid).

In some preferred embodiments, the organic acid is tartaric acid or malic acid.

(II) ，其中R ₂選自直鏈型或分支型、經取代或未經取代的C ₁-C ₆醇類。 In some embodiments, the organic acid has the formula (II):

(II), wherein R ₂ is selected from linear or branched, substituted or unsubstituted C ₁ -C ₆ alcohols.

In some embodiments, the organic acid is selected from glyceric acid, glycolic acid and lactic acid.

In some preferred embodiments, the organic acid is glyceric acid.

In some embodiments, the metal ion is selected from palladium, copper, silver, gold, platinum, iridium, aluminum, cobalt and nickel ions.

In some preferred embodiments, the metal ion is copper ion.

In some embodiments, the pH of the composition is greater than 9.

Another aspect of the invention relates to a method of depositing an electroless plating activator on a substrate comprising: (a) applying the composition to the substrate; (b) applying a reducing agent to the substrate.

In some preferred embodiments, the reducing agent is Dimethylamine Borane (DMAB) or NaBH ₄ .

Yet another aspect of the present invention relates to a method for forming an electroless copper plating film on a substrate, comprising: (a) depositing the electroless plating activator on the substrate; (b) electroless copper plating on the substrate.

In some embodiments, the plating bath used in the electroless plating step includes: tartrate, copper ions, formaldehyde and 2,2-bipyridine.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict of meanings, this specification shall prevail.

The terms "plating" and "deposition" are used interchangeably in this specification. All amounts are percent by weight unless otherwise indicated. All numerical ranges are inclusive and combinable in any order unless such numerical ranges are constrained to logically add up to 100%.

The technical content, characteristics and effects of the present invention will be described below using specific embodiments, and implemented accordingly, but not intended to limit the scope of the present invention.

Usually, when the substrate to be metallized is the surface of a printed circuit board or a dielectric material on the wall of a through hole, the substrate will be degreased first, and then the wall of the through hole will be desmeared. ). Preferably, the substrate to be plated is a metal clad substrate having a dielectric material and a plurality of through holes. First, clean and degrease the base material, and then remove the scum on the through-hole wall. Typical via desmear or dielectric preparation and softening or desmearing utilizes a sweller solvent.

Conditioning can be carried out after desmearing. Examples of conditioning agents in the present disclosure include monoethanolamine, one or more quaternary amines, one or more nonionic surfactants, one or more conditioning agents Pore polymer and pH adjuster. In some embodiments, such conditioners comprise 5-20 g/L monoethanolamine, 0.1-15 g/L triethanolamine, 0.1-10 g/L triton X-100 (Dow Inc.) , 1-5 g/L of basotronic PVI (BASF) and sodium hydroxide for pH adjustment. Next, the substrate and the vias can be rinsed with water.

Microetching can be performed after the drilling. The purpose of micro-etching is to form microscopic roughness on the copper surface (such as inner layer copper and surface copper surface), so as to enhance the adhesion of subsequent electroless plating and electroplating layers. Microetch cleaning solution contains 50-150 g/L sodium persulfate and 10-30 ml/L sulfuric acid (98%). The substrate after microetching was rinsed with water for the following procedures.

The substrate and vias after microetching can be pre-dip, which helps to stabilize the pH of the activation bath and clean the metal surface. The advantage of using pre-dipping is that it helps to improve interconnection defects and increase reliability. The conventional pre-dipping solution is an inorganic or organic acid with a pH range of 3-5.

However, in some embodiments of the present invention, the activation is carried out under alkaline conditions, so the pH of the pre-dip solution can also be greater than 7. The pre-soak may be sodium hydroxide, sulfuric acid, boric acid or a combination thereof to adjust to the desired pH. In some embodiments of the invention, activation may then be performed under alkaline conditions with a composition comprising a chelating agent and a metal salt.

The activator composition comprises metal ions, organic acids with one or more carboxyl groups and one or more hydroxyl groups, and forms a stable complex aqueous solution, which can be used to catalyze the deposition of electroless metal plating. The activator compositions of the present invention are preferably basic and maintaining a substantially basic pH promotes complex formation of the composition components which also enhances the performance of the composition.

Organic acids with carboxyl and hydroxyl groups provide sufficient chelating ability for metal ions, especially copper ions, as chelating agents. Proposed chelation models for such chelators with metal ions are shown in Figure 1 (1-COOH, 1-OH) and Figure 2 (2-COOH, 1-OH), where R represents the linker . Tartaric acid in the present invention exemplifies the binding mode of an acid with 2 carboxyl groups and 2 hydroxyl groups (as shown in Figure 3).

The metal ions can be provided by conventional metal salts, usually the activator solution will contain such metal salts to provide 20 ppm to 5000 ppm, preferably 200 ppm to 1500 ppm of metal ions. Metal ions include, but are not limited to, silver, gold, platinum, palladium, copper, cobalt, and nickel ions. Preferably, the metal ions are selected from copper and palladium ions. Sources of metal ions can be provided using conventional water-soluble metal salts known in the art and found in the literature.

If the activator is an ionic activator in which the metal ions have not been reduced to a metallic state, then a reducing agent is applied to the substrate to reduce the metal ions of the activator to the metal. The reducing solution may be applied by immersing the substrate in the reducing solution or spraying the reducing solution on the substrate. A preferred reducing solution contains 1-25 g/L dimethylamine borane (Dimethylamine Borane, DMAB). However, in some embodiments of the invention, a reducing solution comprising NaBH4 is also used _. Next, the activated substrate and vias are optionally rinsed with water. It should be noted that in some embodiments of the present disclosure, no rinsing is required at this step.

Then use an electroless plating bath to plate metal, such as copper, copper alloy, nickel or nickel alloy, on the base material and the wall of the through hole. The preferred metal is copper plated on the via hole walls. Plating times and temperatures can be conventional, with the option of immersing the substrate in the electroless plating bath or spraying the electroless plating bath onto the substrate. Typically plating can be performed for 5 seconds to 30 minutes, however plating times may vary depending on the thickness of the metal on the substrate.

The performance of an activator system for catalyzed electroless plating can be evaluated by examining the coating coverage on the through-hole walls. Each substrate via was cut transversely to expose the copper-plated wall of the via, and the coating coverage was determined by observing how light passed through the hole wall under a microscope. If no light transmission is observed and the part is completely black, it is rated a 5 on the backlight test, indicating complete copper coverage of the via walls. If the light passes through the entire hole wall without any dark areas, this means that there is almost no metal plating on the wall, and that part is rated 0. If the hole walls have some dark and light areas, they are scored between 0 and 5.

When performing electroless plating on a non-conductive substrate using the aqueous activator solution of the present invention, the following methods can be used. The electroless copper plating is taken as an example below, and all the embodiments are not intended to limit the scope of the present invention, but to further illustrate the present invention.

Desmear/Deetch Process A The desmear/deetch process includes: Bath composition time temperature A1 leavening agent 5 points 60°C Sweller PC* NaOH 350 ml/L 3 g/L A2 rinse 1 point 20°C A3 permanganate etchant 10 points 75°C KMnO ₄ NaOH 60 g/L 45 g/L A4 rinse 1 point 20°C A5 reducing solution 5 points 40°C Neutralizer PFH* H ₂ SO ₄ (98%) 100ml/L 40ml/L A6 rinse 1 point 20°C *Supplied by Super Special International Co., Ltd.

The substrate is degreased by washing with an alkaline expansion agent solution. After washing with water, micro-etch the slag with an alkaline permanganate solution, then rinse with water, and finally immerse in a reducing solution containing a neutralizer and an acid.

The process after desmear/etch is further described as follows:

Example 1 Bath composition time temperature 1. Pore conditioner 10 points 60°C Monoethanolamine Triethanolamine Triton X-100 Basotronic PVI# 10 g/L 5 g/L 1 g/L 1 g/L 2. rinse 1 point 20°C 3. etch cleaner 1 point 25°C Sodium persulfate H ₂ SO ₄ (98%) 100g/L 20ml/L 4. rinse 1 point 20°C 5. Prepreg 5 points 25°C NaOH solution pH=9.0 6. activator 10 points 40°C palladium tartrate ion 1.44 g/L 0.2 g/L pH=12.1 (adjusted with NaOH solution) 7. rinse 1 point 20°C 8. reducing agent 2 minutes 40°C DMAB 6 g/L pH=9.5 (adjusted with NaOH solution) 9. rinse 1 point 20°C 10. Electroless Plating 8 points 33°C Electroless Cu MC* 11. rinse 1 point 20°C *Supplied by Chaote International Co., Ltd. #Supplied by BASF

In this example, the microetched substrate is immersed in a pre-dip solution containing NaOH for subsequent alkaline activation. The activation composition included 1.44 g/L of tartaric acid and 0.2 g/L of palladium ions to adjust the pH to 12.1. Activate at 40°C for 10 minutes, then rinse the activated substrate with water.

The reducing solution contained 6 g/L DMAB and the pH was adjusted to 9.5 using 1.0 N NaOH. Reduction was performed at 40 °C for 2 minutes, and then the activated substrate was rinsed with water.

Substrates were immersed in an Electroless Cu MC (Chaote International Co., Ltd.) plating bath at 33 °C for 8 min. The backlight test results are shown in Figure 4A.

Comparative example 1 Bath composition time temperature 1-4. as example 1 5. Prepreg 5 points 25°C _{H2SO4 solution} pH=2.3 6. activator 10 points 40°C palladium tartrate ion 1.44 g/L 0.2 g/L pH=1.3 (adjusted with NaOH solution) 7-11. as example 1

In this comparative example, the procedure was the same as in Example 1 except for the following conditions. The presoak solution contains 1.0 NH ₂ SO ₄ , pH=2.3. The activation composition included 1.44 g/L tartaric acid and 0.2 g/L palladium ions to adjust the pH to 1.3. The backlight test results are shown in Figure 4B.

According to Example 1 and its comparative example, it is proved that the activator composition containing tartaric acid is suitable for pH between 12.1 and 1.3. However, the backlight test showed that the activator system performed better under alkaline conditions than under acidic conditions, with scores of 4.75 and 4.25, respectively. Deprotonation of carboxylic acids promotes chelation, thereby stabilizing metal ions, allowing these acids to act as mediators during activation.

Example 2 Bath composition time temperature 1-4. as example 1 5. Prepreg 5 points 25°C H ₃ BO ₃ solution 5.0 g/L pH=9.0 (adjusted with NaOH solution) 6. activator 10 points 40°C Palladium malate ion 12.0 g/L 0.2 g/L pH=12.6 (adjusted with NaOH solution) 7-11. as example 1

In this example, the performance of the chelating agent malic acid in activation was tested. Dip the microetched substrate into a pre-dip solution containing 5.0 g/L boric acid, pH = 9.0, for 5 min at 25 °C. The activation composition included 12.0 g/L malic acid and 0.2 g/L palladium ions to adjust the pH to 12.6. Activate at 40°C for 10 minutes, then rinse the activated substrate with water.

The reducing solution contained 6 g/L DMAB and the pH was adjusted to 9.5 using 1.0 N NaOH. Reduction was performed at 40 °C for 2 minutes, and then the activated substrate was rinsed with water. Substrates were immersed in an Electroless Cu MC (Chaote International Co., Ltd.) plating bath at 33 °C for 8 minutes. The backlight test results are shown in Figure 5A.

Comparative example 2 Bath composition time temperature 1-4. as example 1 5. Prepreg 5 points 25°C H ₃ BO ₃ solution 5.0 g/L pH=2.3 (adjusted with NaOH solution) 6. activator 10 points 40°C Palladium malate ion 12.0 g/L 0.2 g/L pH=1.3 (adjusted with NaOH solution) 7-11. as example 1

In this comparative example, the procedure was the same as in Example 2 except for the following conditions. The presoak solution contains boric acid 5.0 g/L, pH=2.3. The activation composition contained 12 g/L malic acid and 0.2 g/L palladium ions, and the pH was adjusted to 1.3. The backlight test results are shown in Figure 5B.

Malic acid, another example of an organic acid having at least 1 carboxyl group and at least 1 hydroxyl group, can also be used in the activation process at a pH between 12.6 and 1.3. Backlight test scores under alkaline and acidic conditions were 4.75 and 3.25, respectively.

Example 3 Bath composition time temperature 1-5. as example 1 6. activator 10 points 40°C copper tartrate ion 5.0 g/L 1.5 g/L pH=12.0 (adjusted with NaOH solution) 7. rinse 1 point 20°C 8. reducing agent 2 minutes 40°C DMAB 12 g/L pH=9.5 (adjusted with NaOH solution) 9. Electroless Plating 15 marks 33°C Sodium Potassium Tartrate Copper Ion Nickel Ion NaOH Formaldehyde 2,2'-Bipyridine 30 g/L 2.5 g/L 0.5 g/L 10 g/L 4 g/L 60 mg/L 10. rinse 1 point 20°C

In this example, copper ions are used in the activator system instead of expensive palladium ions. Dip the microetched substrate into a pre-dip solution containing NaOH, pH = 9.0, for 5 min at 25 °C. The activation composition contained 5.0 g/L tartaric acid and 1.5 g/L copper ions, and the pH was adjusted to 12.0. Activate at 40°C for 10 minutes, then rinse the activated substrate with water.

Reduction also uses DMAB, and you can choose whether to rinse the reduced substrate and through holes with water. In some embodiments, however, electroless plating may be performed immediately after reduction without rinsing to avoid passivation of the as-deposited copper. The electroless plating bath contains 30 g/L potassium sodium tartrate, 2.5 g/L copper sulfate, 0.5 g/L nickel sulfate, 10 g/L NaOH, 4 g/L formaldehyde and 60 mg/L 2,2'-dipyridine . The substrate was immersed in an electroless plating bath and reacted at 33°C for 15 minutes. The backlight test results are shown in Figure 6A.

Comparative example 3 Bath composition time temperature 1-5. as example 1 6. activator 10 points 40°C copper tartrate ion 5.0 g/L 1.5 g/L pH=12.0 (adjusted with NaOH solution) 7. rinse 1 point 20°C 8. reducing agent 2 minutes 40°C DMAB H ₃ BO ₃ solution 12 g/L 20 g/L pH=3.0 (adjusted with NaOH solution) 9. Electroless Plating 12 points 33°C Sodium Potassium Tartrate Copper Ion Nickel Ion NaOH Formaldehyde 2,2'-Bipyridine 30 g/L 2.5 g/L 0.5 g/L 10 g/L 4 g/L 60 mg/L 10. rinse 1 point 20°C

In this comparative example, the procedure was the same as in Example 3 except for the following conditions. The reducing solution contained 12 g/L DMAB, 20 g/L boric acid, and the pH was adjusted to 3.0 using 1.0 N sulfuric acid. The backlight test results are shown in Figure 6B.

In Example 3 and its comparative example, it is proved that copper ions are compatible with the activator system, and the subsequent reduction step is also one of the factors affecting the activation performance. The backlight fraction of the reduction composition using alkaline or acidic conditions is 4.75 respectively and 3.

Example 4 Bath composition time temperature 1-5. as example 1 6. activator 5 points 40°C DL-copper glycerate 5.0 g/L 0.5 g/L pH=12.0 (adjusted with NaOH solution) 7. rinse 1 point 20°C 8. reducing agent 2 minutes 40°C NaBH ₄ 2 g/L pH=12.6 (adjusted with NaOH solution) 9. Electroless Plating 15 marks 33°C Sodium Potassium Tartrate Copper Ion Nickel Ion NaOH Formaldehyde 2,2'-Bipyridine 30 g/L 2.5 g/L 0.5 g/L 10 g/L 4 g/L 60 mg/L 10. rinse 1 point 20°C

The table above shows another example of using copper ions instead of palladium ions in the activator system. The activation composition contained 5.0 g/L glyceric acid and 0.5 g/L copper ions, and the pH was adjusted to 12.0. Activation at 40°C for 10 minutes. The reduction was carried out in a solution containing 2 g/L NaBH ₄ pH 12.6, and reacted at 40° C. for 2 minutes.

The electroless plating bath contains 30 g/L potassium sodium tartrate, 2.5 g/L copper sulfate, 0.5 g/L nickel sulfate, 10 g/L NaOH, 4 g/L formaldehyde and 60 mg/L 2,2'-dipyridine . The substrate was immersed in an electroless plating bath and reacted at 33°C for 15 minutes. The backlight test score was 5 (shown in Figure 7), showing that glyceric acid and NaBH ₄ can be used for the activation and subsequent reduction steps, respectively.

The above descriptions are only some preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Any changes and adjustments to the stereochemistry, concentration, temperature, pH, reaction time and spirit mentioned in the patent application scope should be included in the patent application scope of this work.

none

Figure 1 Depicts the proposed chelation model of a chelating agent with a metal ion having one -COOH and one -OH, where R represents the linker. Figure 2 Depicts the proposed chelation model of a chelating agent with a metal ion having two -COOH and one -OH, where R represents the linker. Figure 3 shows the proposed binding mode of tartaric acid to metal ions. Figure 4 shows (A) Example 1 and (B) Comparative Example 1 the through-hole coating coverage tested with backlight. Figure 5 shows (A) Example 2 and (B) Comparative Example 2 the through-hole coating coverage tested with backlight. Figure 6 shows (A) Example 3 and (B) Comparative Example 3 the through-hole coating coverage tested with backlight. Fig. 7 shows the through-hole coating coverage of embodiment 4 with backlight test.

none

Claims (14)

A composition for depositing an electroless plating activator on a substrate, comprising at least one metal ion and at least one organic acid; wherein the organic acid has at least one carboxylic group and at least one hydroxyl group. The composition as claimed in item 1, wherein the organic acid is dicarboxylic acid terminated (dicarboxyl acid terminated) and has the following formula (I):

(I), wherein R ₁ is selected from linear or branched, substituted or unsubstituted C _{1-6 alcohols.} The composition as claimed in claim 2, wherein the organic acid is selected from tartaric acid (tartaric acid), citric acid (citric acid), malic acid (malic acid) and 2,2-bis(hydroxymethyl)malonic acid (2,2-Bis(hydroxymethyl)malonic acid). The composition as claimed in item 3, wherein the organic acid is tartaric acid or malic acid. The composition as claimed in item 1, wherein the organic acid has the following formula (II):

(II), wherein R ₂ is selected from linear or branched, substituted or unsubstituted C _{1-6 alcohols.} The composition according to claim 5, wherein the organic acid is selected from glyceric acid, glycolic acid and lactic acid. The composition as claimed in item 6, wherein the organic acid is glyceric acid (glyceric acid). The composition as claimed in claim 1, wherein the metal ion is selected from palladium, copper, silver, gold, platinum, iridium, aluminum, cobalt and nickel ions. The composition as claimed in claim 8, wherein the metal ion is copper ion. Composition as described in claim item 1, its pH value is greater than 9. A method of depositing an electroless plating activator on a substrate, comprising: (a) applying the composition as described in claim 1 to a substrate; (b) applying a reducing agent to the substrate. The method according to claim 11, wherein the reducing agent comprises Dimethylamine Borane (DMAB) or NaBH ₄ . A method for forming an electroless copper plating film on a substrate, comprising: (a) depositing an electroless plating activator on a substrate according to the method described in claim 11; (b) electroless copper plating with a plating bath on the substrate. The method as described in claim 13, wherein the plating bath comprises: tartrate, copper ions, formaldehyde and 2,2'-bipyridine (2,2-bipyridine).

Images