JPS62149884A - Pretreatment for electroless copper plating - Google Patents

Abstract

PURPOSE:To prevent the defective deposition of plating when copper substrate is subjected to electroless copper plating, by bringing the substrate into contact with an aqueous soln. of a reducing agent selected among formaldehyde, hypophosphorous acid, hypophosphite and hydrazine immediately before the copper plating. CONSTITUTION:When a copper substrate is subjected to electroless copper plating, the substrate is brought into contact with an aqueous soln. of a reducing agent selected among formaldehyde, hypophosphorous acid, hypophosphite and hydrazine immediately before the copper plating. The concn. of the soln. is 10-100%. The contact time is regulated to 1min-5sec according to the concn. The preferred contact method is immersion.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pretreatment method for electroless copper plating of a copper base material, and in particular to electroless plating of copper used in manufacturing printed wiring boards from copper-clad substrates. This invention relates to a pretreatment method in which a copper substrate is immersed in a specific reducing agent aqueous solution bath immediately before electroless plating in order to prevent poor plating adhesion. The present invention can be effectively applied as a pretreatment method for electroless copper plating, particularly for substrates having independent pattern parts.

The production of printed wiring boards basically consists of forming through holes in a copper-clad laminate, forming circuits by etching, and connecting the circuits by electroless plating the through holes. Based on this, many methods have been advocated and implemented with the aim of simplifying the process, reducing cost, improving accuracy, etc. For example, a predetermined circuit is formed by selectively etching a copper foil, a through hole is formed at a predetermined position of the circuit, an electroless copper plating pretreatment layer is formed on the through hole wall and the circuit, and the circuit is formed by selectively etching a predetermined circuit on a copper foil. A method in which the pretreatment layer above is mechanically removed, a solder resist film is formed on areas other than the through holes and the circuits connected to them, and then electroless steel plating is applied to the through holes and the necessary circuits; A method in which formation is performed after forming a solder resist film; After forming a through hole and plating the through hole, copper plating is performed on the through hole wall and copper foil surface by electrolytic method, and ink is filled in the through hole. A method of forming an etching resist film on the necessary circuit, and then etching away unnecessary parts of the steel foil;
A through-hole is formed in a copper-clad laminate, the inside of the through-hole is activated with a catalyst, a predetermined portion of the copper foil is etched away, a solder resist film is formed on the desired portion other than the land area and the through-hole, and electroless copper is formed. There are too many ways to perform plating. Many changes have been made to the order of through-hole formation, pattern formation, catalyst deposition, resist formation, etching, etc. from various viewpoints.

Under these circumstances, recently, ICs, resistors, and other mounted components have been made into chips, and the parts of the board that are connected to the through-holes that have been treated with a catalyst and the parts that are not connected to the through-holes, that is, independent patterns. It has become necessary to have a department. Figure 1 shows an example of such a situation. Copper foil portions A and B are connected to catalyst-equipped through holes (2) and (2), respectively, but independent patterns mc, D, E, and F are not connected to any through holes. In such a case, the steel foil portions A and B connected to the through-hole with catalyst are equivalent in potential to the through-hole and the catalyst, but the independent pattern portion has a potential only to Cu. That is, the independent pattern portion has a higher potential than the portion connected to the through hole.

For this reason, when performing electroless copper plating on through-holes, a situation often occurred in which the plating was difficult to adhere to the independent pattern portions. This not only deteriorates the appearance of the substrate, but also causes a height difference in the plating thickness between the land portion and the independent pattern portion as shown in FIG. 2 after plating. In the past, this problem has been largely ignored, but as parts are now mounted on printed wiring boards in a flat package format, the problem has become impossible to ignore.

Summary of the invention The deposition potential of electroless copper on copper foil is -α7v (V, S,
Ag/AgC1) or less, and the cause of the force plating failure as described above is that the electrode potential of the independent pattern portion does not fall below -17V when the substrate is immersed in the electroless plating solution. For example, when we actually measure the electrode potential when plating defects occur, -(L2~-0.5 V (V, S, Ag/
AgC1).

Therefore, in order to avoid plating defects, it is necessary to lower the overall electrode potential sufficiently to below -0.7V.

Based on this idea, we investigated ways to lower the electrode potential sufficiently and found that immediately before electroless plating, an aqueous solution of a reducing agent selected from K, formalin, hypophosphorous acid or hypophosphite, and hydrazi. It has been found to be effective to perform a pretreatment in which the substrate is brought into contact with the dipping solution (dip solution).

Thus, the present invention provides that when performing electroless plating of copper on a copper substrate, immediately before electroless plating, an agent selected from the group of formalin, hypophosphorous acid, hypophosphite, and hydrazine is used. Provided is a pretreatment method for electroless copper plating that prevents poor plating deposition by bringing an aqueous solution of a reducing agent into contact with a copper base material.

The present invention relates to a method of electroless plating copper onto a copper substrate. Here, the copper base material includes copper or copper alloy foil, copper paste, and copper plating material. Industrially, printed wiring boards are the main target example. As shown in Figure 1, in the production of printed wiring boards, ■ through-holes that have been treated with a catalyst ■ ■ copper foil parts connected to ■ ■ independent pattern steel foil parts that are not connected are electrolessly plated. Therefore, it is necessary to attach copper. Colloidal particles of Pd adhere to the through-hole walls during the catalyst application process, activating and promoting the through-hole walls. Therefore, the copper foil part connected to the through-holes has a potential difference with the through-holes. It is equivalent to , and copper adheres well during electroless plating. However, due to the manufacturing process, the independent pattern part is covered with resist during the process of applying catalyst to the through hole, so the catalyst is not attached and the potential is higher than the part connected to the through hole. Therefore, copper does not adhere during electroless plating. Therefore, there is a need for measures to prevent the occurrence of copper-adhered areas and non-copper-adhered areas during electroless plating, that is, to prevent poor plating deposition.

Conventionally, as a pretreatment method for electroless copper plating, the steps of soft etching using ammonium persulfate, water washing, pickling, and water washing were performed, and then electroless steel plating was performed. Accordingly, contact treatment with a specific reducing agent aqueous solution is performed immediately before electroless copper plating. The contact is
It can be carried out by dipping, coating, spraying, etc., but dipping or pre-dipping is preferred.

Suitable reducing agents include formalin, hypophosphorous acid, hypophosphite and hydrazine. These reducing agents are diluted or dissolved with pure water to a set degree of m. The concentration is determined by reducing solution (
It is effective in the range of 2 to 100 in the ratio to K (undiluted solution), but if the concentration is low, the immersion time in the pre-dip solution becomes long, so 10 to 100 is preferable.

Room temperature is sufficient for pre-dipping. The immersion time for pre-dip depends on the concentration of the reducing solution and cannot be determined uniquely, but for example, when the concentration of the reducing solution is high (1oo
%) in 5 seconds and when the concentration of reducing solution is low (10%
) is effective for more than 1 minute.

When electroless plating is performed on the copper base material pretreated in this manner, electroless plating is reliably deposited on the copper base material.

Effects of the Invention Therefore, according to the present invention, when electroless copper plating is performed on through-holes, there is no situation where the plating does not adhere to the independent pattern portion, so that after plating, there is no difference in the height of the plating thickness between the land portion and the independent pattern portion. This eliminates the problem of automatic component mounting machines malfunctioning when mounting components such as ICs and resistors.
Improved reliability.

It should be noted that the present invention is not limited to the field of manufacturing printed wiring boards, but is also applicable to the production of copper substrates, including copper foils, copper plating films, etc., without the use of metals such as copper and nickel without using catalysts. It is equally applicable where deposition by electrolytic plating is required.

Example 1 and Comparative Example 1 An experiment was conducted using a substrate having an independent pattern portion to examine the state of plating and precipitation thereon.

1 Conditions ■Reducing chemicals used...Formalin ■Formalin concentration of Bredip solution (ratio to 37HCHO stock solution) (Ch)...10
0% ■Temperature of Predates liquid ('C)...10.25.
35 ■ Immersion time in pre-dip solution (see)...5
．． 10.50.60.120 ■Electroless plating solution composition CuSO4-5H20109/I EDTA 30 i/IHCH03
7% aqueous solution 5 ml/IPH (NaO
H) 12.8 Organic additives
Small amount ■Plating temperature: 70°C λ Method After completing the steps of conditioner treatment, water washing, pickling, and water washing, the substrates were immersed in the pre-dip solution within 30 seconds, and 10 substrates each were processed under predetermined conditions. After the treatment, it was immersed in an electroless plating solution within 30 seconds to perform plating with a thickness of 30 μm. After plating, the presence or absence of plating precipitation on the independent pattern portions was observed. In order to investigate the effect of immersion in the pre-dip solution, a substrate slightly immersed in the pre-dip solution was electrolessly plated at the same time, and the presence or absence of plating precipitation on the independent pattern portions was observed.

5. As a result, all (10) substrates that were not immersed in the pre-dip liquid were defective because plating did not precipitate on a part of the independent pattern part, but all of the substrates that were immersed in the pre-dip liquid were defective under each condition. For (10 sheets), plating was deposited on the independent pattern part, which was good.

Example 2 and Comparative Example 2 1 Conditions ■Reducing chemicals used...Formalin ■Formalin concentration of pre-dip solution (ratio to 37HCHO stock solution) (%)...2,
5.10 ■ Temperature of pre-dip liquid ('C)...10.25.3
5 ■ Immersion time in brad dip solution (sec)...30
, 60, 120 ■ Plating solution and plating temperature Same as Example 1 Z Method After completing the process of conditioner treatment → water washing → pickling → water washing, the substrate is immersed in the pre-dip solution within 30 seconds, and then heated under the specified conditions. Ten sheets each were processed. After the treatment, it was immersed in an electroless plating solution within 30 seconds to perform plating to a thickness of 30 μm. After plating, the presence or absence of plating precipitation on the independent pattern portions was observed. In order to examine the effect of immersion in the pre-dip solution, a substrate that was not immersed in the pre-dip solution was electrolessly plated, and the presence or absence of plating precipitation on the independent pattern portions was observed.

5. As a result, all (10) substrates that were not immersed in the pre-dip solution were defective because the plating did not precipitate on a part of the independent pattern part, but for the substrates that were immersed in the pre-dip solution, all of the substrates (10) were defective under each condition. Regarding 10 sheets), plating was deposited on the independent pattern part, which was good.

Reference Example Confirmation tests were conducted on some samples of Examples 1 and 2 using an electrochemical method.

The copper deposition potential bar a of the material to be plated in the electroless copper plating solution is 7 V (V, S, Ag/AgC1) or less. Furthermore, the following relationship exists between the change in electrode potential at the initial stage of electroless plating when copper foil is immersed in an electroless plating solution, and the state of precipitation of electroless copper plating.

(1) After dipping the copper foil, the electrode potential drops to -0.7V within 10 seconds.
The following results... Electroless copper plating is deposited.

(2) After immersing the copper foil, it takes 10 seconds for the electrode potential to drop to -[L7V or lower...electroless copper plating may or may not precipitate. It is in an unstable state.

(3) After immersing the copper foil, the electrode potential does not drop to -0.7V...electroless copper plating does not precipitate.

In this example, based on the above findings, the effect of immersion in the pre-dip liquid was confirmed by electrode potential measurement.

t Pre-dip conditions 2 Electroless copper plating solution used Experimental method After completing the soft etching process → water washing → pickling → water washing, the copper foil was immersed in the pre-dip solution, and immediately immersed in the electroless copper plating solution. Changes in electrode potential at the initial stage of electroless plating were measured.

As a blank, changes in electrode potential at the initial stage of electroless plating of copper foil without pre-dip treatment were measured.

4 Experimental results Table 3 0...The electrode potential drops below 1α7V within 10 seconds Δ--It takes more than 10 seconds for the electrode potential to drop below -t17V ×...The electrode potential drops below 1α7V ■The electrode potential of copper foil without pre-dip treatment is -α
On the other hand, the electrode potential of the copper foil subjected to the pre-dip treatment decreased to -17 V or less within 10 seconds, whereas it either did not drop to 7 V or took 10 seconds or more even if it did. The effect of Bledip was clearly recognized.

Example 3 and Comparative Example 3 t Condition ■Reducing chemical used...Hydrazine hydrate■
Hydrazine concentration in pre-dip solution (hydrazine hydrate/ratio to stock solution) (ch)...
2.5.10.20 ■ Temperature of pre-dip liquid (°C)...10.25.35
■Immersion time (see) in the brad dip solution...5.1
0.30.60.120 ■Plating solution and plating temperature Same as Example 1 Ten sheets each were processed under the following conditions. After the treatment, it was immersed in an electroless plating solution within 30 seconds to perform plating with a thickness of 30 μm. After plating, the presence or absence of plating precipitation on the independent pattern portions was observed. In order to examine the effect of immersion in the pre-dip solution, a base material that was not immersed in the pre-dip solution was electrolessly plated, and the presence or absence of plating precipitation on the independent pattern portions was observed.

5. As a result, all (10) substrates that were not immersed in the pre-dip solution were defective because the plating did not precipitate on a part of the independent pattern part, but for the substrates that were immersed in the pre-dip solution, all of the substrates (10) were defective under each condition. For 10 sheets), plating was deposited on the independent turn portions, and the results were good.

t Condition ■ Reducing chemicals used ... Hypophosphorous acid solution ■ Hypophosphorous acid concentration of pre-dip solution (ratio to 60% hypophosphorous acid stock solution) (%) ... 2
, 5, 10, 20 ■Temperature of the bread dip liquid (°C)...10.25.35
■Immersion time in pre-dip solution (8eC)...5.1
0.30.60.120 ■Plating solution and plating @ degree Same as Example 1 2. Method After completing the process of conditioner treatment → water washing → pickling → water washing, the substrate is immersed in the pre-dip solution within 30 seconds, Ten sheets each were processed under predetermined conditions. After the treatment, it was immersed in an electroless plating solution within 30 seconds to perform plating with a thickness of 30 μm. After plating, the presence or absence of plating precipitation on the independent pattern portions was observed. In order to examine the effect of immersion in the pre-dip solution, a substrate that was not immersed in the pre-dip solution was electrolessly plated, and the presence or absence of plating precipitation on the independent pattern portions was observed.

5. As a result, all (10) substrates that were not immersed in the pre-dip solution were defective because the plating did not precipitate on a part of the independent pattern part, but for the substrates that were immersed in the pre-dip solution, all of the substrates (10) were defective under each condition. Regarding 10 sheets), plating was deposited on the independent pattern part, which was good.

FIG. 1 is a top view illustrating the existence of an independent pattern portion in a part of a printed wiring board, and FIG. 2 is a sectional view thereof.

i, n: Through hole

Claims (1)

[Claims] 1) When performing electroless plating of copper onto a copper substrate, immediately before electroless plating, an aqueous solution of a reducing agent selected from the group of formalin, hypophosphorous acid, hypophosphite, and hydrazine and a copper base A pretreatment method for electroless copper plating that prevents defects in plating precipitation due to contact with the metal.