HK1168132A

HK1168132A - Method for depositing a palladium layer suitable for wire bonding on conductors of a printed circuit board and palladium bath for use in said method

Info

Publication number: HK1168132A
Application number: HK12108801.8A
Authority: HK
Inventors: J．艾伯; E．马卡; W．马舒特; S．奥尔舒兰格
Original assignee: 多杜科有限公司
Priority date: 2009-11-10
Filing date: 2010-11-06
Publication date: 2012-12-21

Description

Method for depositing palladium layers suitable for bonding wires to conductors of printed circuit boards and palladium bath for use in said method

Technical Field

The invention relates to a method having the features described in the preamble of claim 1. Such a process is known from WO 2006/074902 a 2.

Background

According to a known method, firstly, nickel is deposited by chemical reduction from a chemical nickel plating bath (i.e. without applying an external current) on a conductor made of copper, which is located on an organic or ceramic printed circuit board. It is also feasible to deposit nickel alloys instead of nickel from the corresponding nickel alloy baths without applying an external current. A thin 10 nm-40 nm thick palladium layer is then deposited on the nickel or nickel alloy layer by charge exchange from a palladium exchange bath. The palladium exchange bath is generally composed of a palladium salt of an inorganic or organic acid with said acid and an inorganic compound of at least one element, copper, thallium, selenium and tellurium, preferably copper sulfate. The pH of the palladium exchange bath is less than 1, i.e. the bath is a strong acid. The palladium exchange bath is also referred to as a palladium immersion bath or a palladium activation bath and the deposit from this bath is referred to as an immersion or activated deposit.

Finally, a coating made of gold having a thickness of less than 0.1 μm is deposited on the palladium layer. The gold is preferably deposited from a gold exchange bath. The gold layer can be increased to a greater thickness from an electroless gold bath by autocatalytic deposition, i.e. by reductive deposition.

First, palladium is the basis for a wire made of gold or aluminum that is subsequently attached by adhesion. In addition, the palladium layer serves to prevent nickel from diffusing to the gold surface to maintain its bonding ability. The purpose of using a gold layer is to protect the palladium surface from chemical changes due to its high catalytic activity, which may lead, for example, to the so-called "brown powder" effect.

The disclosure in WO 2006/074902 a2 indicates that the existing requirements relating to various soldering and bonding capabilities are only met when inorganic compounds of at least one element (copper, thallium, selenium and tellurium), in particular copper sulfate, in addition to palladium salts, are added to the palladium bath. Without the addition of an inorganic compound of at least one of the elements to the palladium exchange bath, the palladium layer cannot obtain sufficient adhesion and shielding effects.

Various methods are known on the market for applying copper conductors to printed circuit boards, according to which palladium is first deposited onto the copper from a customary palladium exchange bath. More palladium is then deposited from a chemical (reduced) palladium bath onto the first layer and finally a gold layer is deposited as a coating. According to the manufacturer's opinion, the known method results in the creation of a multilayer which is suitable for soldering but not for the bonding of wires made of gold and aluminum.

A method is also known on the market which makes it possible to form an adhesive surface on the copper conductors of a printed circuit board. The method is to deposit palladium onto the copper conductor from a common palladium activation bath. A nickel layer is then deposited thereon from a chemical (reducing) nickel bath. Another palladium layer is deposited onto the nickel layer from a common palladium activation bath and the thickness of the palladium layer is increased by deposition of palladium from a chemical (activation) palladium bath. Finally, a gold layer is prepared by dip coating to protect the palladium surface. This known method involves extraordinary efforts.

Disclosure of Invention

The object of the invention is to provide an alternative method according to which an adhesive-capable palladium layer can be deposited on conductors of a printed circuit board, in particular on conductors made of copper, with little effort.

This object is achieved by a method having the features of claim 1. The subject matter of claim 10 is a palladium bath which is particularly suitable for carrying out the process. Advantageous developments of the invention are the subject matter of the dependent claims.

According to the invention, a palladium layer is deposited by charge exchange on a conductor of a printed circuit board, in particular a conductor made of copper, from a palladium exchange bath comprising an organic brightener.

In addition to the method known from WO 2006/074902 a2, this provides the person skilled in the art with another method for depositing an adhesive-capable palladium layer on the conductors of a printed circuit board. This method has the following additional advantages:

the palladium layer can be deposited not only on a conductor previously coated with nickel, but also directly on a conductor made of copper.

The intermediate nickel layers known in the prior art are therefore optional.

The palladium coating is also suitable for high-frequency applications, since an intermediate layer made of nickel may be present, whereas nickel is less suitable for high-frequency applications due to its magnetic properties.

The palladium layer deposited from the palladium exchange bath containing the organic brightener proved to be very dense, fine in particles and homogeneous. Thus, using palladium from a reduced palladium bath, a suitable foundation can be formed to further increase the thickness of the palladium layer compared to conventional palladium activation baths. This type of palladium is preferably reductively deposited from a chemical palladium bath to give a thickness of 0.05 mm to 0.5 mm, in particular 0.1 mm to 0.3 mm. This is greater than the thickness of the lower palladium layer, which is deposited from a palladium exchange bath according to the invention and which is particularly suitable for connection by adhesion from gold or aluminum, and is advantageous as an improvement of the invention.

A suitable pH for the bath of the invention is maintained at less than 4. It is particularly preferred to carry out the bath according to the invention at a pH of 2.

The palladium exchange bath of the present invention is sufficiently stable.

Preferably, suitable organic brighteners include compounds from the group consisting of the compounds according to claim 2. 3- (1-pyridine) -1-propanesulfonic acid is particularly preferred as brightener and gives substances in which the residue R in the formula₁-R₅Is hydrogen, a residue R₆is-CH₂-CH₂-CH₂-, and wherein X is SO₂. In the absence of this compound, the palladium deposited from the exchange bath appears dark to black, especially in and at the pores of the printed circuit board, and the deposit thus obtained is porous, like a sponge. However, in the presence of the organic compound, a significantly better, brighter, more uniform and optically aesthetic palladium layer is formed in the palladium exchange bath.

Suitably, the bath comprises an organic brightener in a concentration of from 0.01 to 50 g/l, preferably from 1 to 10 g/l. Particularly good results are obtained when the concentration of 3- (1-pyridine) -1-propanesulfonic acid in the palladium exchange bath is from 3 to 6 g/l. At low concentrations, the effect of brighteners in the palladium exchange bath is reduced. As the brightener concentration increases, the deposition rate decreases, which means that the concentration of organic brightener should not exceed 50 g/l.

Suitably, the palladium is present in the palladium exchange bath of the invention in an amount of from 150 mg/l to 250 mg/l, preferably in the form of a palladium chloride solution. Lower concentrations can result in increased or inappropriately prolonged residence times in the bath to achieve the desired layer thickness. Therefore, the palladium concentration should not be less than 150 mg/l. With increasing palladium concentration, the resulting deposition rate increases and layers over 50 nm thick can be obtained. However, the porous palladium layer is also increased in this case, and the uniformity and homogeneity of the palladium layer are reduced, while the conductor under the palladium layer is likely to be strongly corroded, and the less noble metal constituting the conductor is replaced by noble metal palladium. Therefore, it is preferred to limit the concentration of palladium in the exchange bath to 250 mg/l.

Another suitable brightener comprises benzaldehyde, in particular benzaldehyde-2-sulfonic acid, in a concentration of from 0.1 to 50 g/l, preferably about 1 g/l.

The palladium is preferably present in the palladium exchange bath as palladium chloride and dissolved in hydrochloric acid. However, it is also possible to form the palladium exchange bath with other salts, for example palladium sulfate dissolved in sulfuric acid, or palladium phosphate dissolved in phosphoric acid, or palladium acetate dissolved in acetic acid. However, it is preferred to use palladium chloride dissolved in hydrochloric acid, since the bath has proved to be particularly stable.

For stabilization, the palladium exchange bath of the present invention preferably also comprises an inorganic complexing agent in an amount of up to 150 g/l, preferably 30-80 g/l. Although the deposition rate decreases with increasing complexing agent concentration, the bath is not very sensitive to changes in the inorganic complexing agent concentration and therefore its concentration should not exceed 150 g/l, preferably 80 g/l.

Ammonium salts are particularly suitable as inorganic complexing agents, the anion of which should preferably be the same as the anion of the palladium salt. Thus, if palladium chloride is used in the palladium exchange bath (as is preferred), the inorganic complexing agent should likewise be a chloride, particularly ammonium chloride. If palladium sulfate is used as palladium salt, it is recommended to use the same sulfate as the inorganic complexing agent, in particular ammonium sulfate. If palladium acetate is used as the palladium salt, the inorganic complexing agent should likewise be an acetate salt, in particular ammonium acetate, etc. However, it is also feasible to use a cation other than ammonium in the inorganic complexing agent, such as sodium or potassium, but the ammonium salt gives a somewhat better stability of the resulting bath.

It has been found that the addition of an organic complexing agent can improve the deposition of palladium even further from the palladium exchange bath. In particular, the tendency of the palladium bath to form spots or darker deposit spots on a large number of continuous surfaces of the base of a less noble metal can be reduced. This beneficial effect of the organic complexing agent contributes to the beneficial effect of the organic brightener used in the present invention to achieve uniform bright, dense and fine particle palladium deposition. Furthermore, organic complexing agents have been found to significantly improve the stability of the bath, especially when the flow of printed circuit boards through the bath is large.

Very suitable as organic complexing agents are carbonic acid, amines, EDTA and EDTA derivatives. Diethylenetriamine has proven particularly suitable, in particular at concentrations of between 0.01 and 5 ml/l. The deposition rate decreases with increasing concentration of organic complexing agent in the palladium exchange bath.

The temperature of the palladium exchange bath during the deposition process is suitably maintained between room temperature and 60 c, preferably in the temperature range 35-50 c. This range has proven particularly suitable. Using the temperature and bath of preferred composition, layers of desired thickness can be deposited within 5 minutes, the layer thickness preferably being between 25 nm and 35 nm and should not exceed 50 nm. Preferably, the deposition process is limited to 2-3 minutes.

As mentioned above, the palladium exchange bath of the present invention should preferably be adjusted to a pH of 2. In the hydrochloric acid bath containing palladium as inorganic complexing agent in the form of palladium chloride and ammonium chloride, the pH can be adjusted by adding hydrochloric acid or ammonia. At pH values above pH =2, the complexation of palladium is more complete, i.e. less free palladium is available for deposition. Thus, the deposition rate decreases with increasing pH. At pH above 4 there is no longer any significant deposition.

The thickness of the palladium layer deposited from the palladium exchange bath may be increased by more palladium being deposited from the chemical palladium bath by the reducing agent. According to the invention, the palladium layer deposited from the palladium exchange bath is particularly suitable as a substrate for increasing the thickness of said palladium layer, which is particularly dense, fine and homogeneous in particles due to the use of organic brighteners. In particular, there is no need to use a nickel layer as a diffusion barrier to prevent copper from diffusing into the palladium layer underlying the palladium layer formed by the reductive deposition of palladium. If the thickness of the layer is no longer increased by depositing more palladium from the reduced palladium bath, it is recommended to provide a nickel layer below the palladium layer deposited from the palladium exchange bath. Increasing the thickness can form a palladium layer with a thickness of several mm. However, a palladium layer of several mm thickness is not economical for the bonding process. The preferred range of palladium layer thickness for bonding is 50 nm to 500 nm.

Detailed Description

Suitable chemical (reduction) baths for depositing palladium are known. A very suitable bath has the following composition:

0.5-3 g/l of palladium in the form of dissolved chloride, sulphate, acetate, phosphate or similar salts, or in combination with palladium complexes, such as chloro complexes, amino complexes, nitrite complexes or similar complexes.

Reducing agent, 5-50 g/l sodium hypophosphite or 1-50 g/l sodium formate or 1-50 g/l formic acid.

Inorganic complexing agents, for example sodium citrate, malonates, succinates, potassium phosphate, monopotassium phosphate, ammonium sulphate.

Organic complexing agents, such as EDTA, EDTA derivatives, amines, such as triethanolamine, tris- (2-aminoethyl) -amine, diethylenetriamine, triethylenetetramine, 1, 3-diaminopropane, each of which can be used alone or in combination.

Furthermore, the electroless palladium bath may comprise further components, mainly stabilizers and promoters known to the person skilled in the art from EP 0698130B 1.

Electroless palladium baths of the type in question are operated at temperatures of from 40 ℃ to 90 ℃ and in particular at temperatures of about 70 ℃. The coating time can be from 30 to 60 minutes, depending on the desired layer thickness. Known electroless palladium baths typically operate in the weak acid to weak base range. In a chemical palladium bath of the type described above, it is possible to operate at a pH of 8.

The most important steps in an exemplary method of palladium deposition are described below, wherein the above-described chemical palladium bath can be used and a chemical palladium bath comprising 150-250 mg palladium (as palladium chloride) per liter, 50g ammonium chloride per liter, 3g 3- (1-pyridine) -1-propanesulfonic acid per liter and 1-5 ml diethylenetriamine per liter is used, which can be operated at 35-50 ℃ and pH = 2.

Treating a printed circuit board having a copper conductor using the bath comprises the steps of:

1. removing oil in weak acid degreasing bath;

2. washing with water;

3. immersing the copper substrate into an etching solution to activate the surface of the copper;

4. flushing;

5. coating palladium in the palladium exchange bath of the present invention;

6. immersing in a dilute hydrochloric acid solution to remove foreign substances (foreign seeds) on the palladium surface;

7. washing with water;

8. depositing palladium in a chemical (reduced) palladium bath;

9. a static rinse soak to collect residual palladium bath material attached to the printed circuit board and to prevent immersion of the residue into subsequent baths;

10. washing with water;

11. depositing gold plating;

12. rinsing and soaking statically;

13. washing with water;

14. and (5) drying.

The gold plating may have a thickness of, for example, 20 nm to 30 nm, which is commonly used to protect palladium surfaces and is known to those skilled in the art. Gold is preferably deposited from an exchange bath.

Claims

1. A method for depositing a palladium layer suitable for bonding to conductors of printed circuit boards, in particular made of copper, by depositing palladium from a palladium exchange bath, characterized in that: a palladium exchange bath containing an organic brightener was used.

2. The method of claim 1, wherein: the organic brightener is selected from the group consisting of:

（a）

wherein

R₁-R₅Each is a hydrogen atom or a halogen atom or a formyl, carbamoyl, C_1-4Alkyl, amino, phenyl or benzyl, wherein the alkyl, phenyl and benzyl moieties may optionally be substituted by one or more hydroxy or amino groups or by halogen atoms,

R₆is a group having 1 to 6C atoms, which is obtained from a saturated or unsaturated aliphatic hydrocarbon, wherein the group can be substituted, and

x is SO₂A group or a CO group, or a combination thereof,

(b) benzaldehyde.

3. The method of claim 2, wherein: 3- (1-pyridine) -1-propanesulfonic acid or benzaldehyde-2-sulfonic acid is used as brightener.

4. The method according to any of the preceding claims, characterized in that: the concentration of the organic brightener in the palladium exchange bath is adjusted and maintained at a value between 0.01 g/l and 50 g/l, preferably between 1 g/l and 10 g/l.

5. The method according to claim 4, characterized in that: 3- (1-pyridine) -1-propanesulfonic acid is used in a concentration of 3 g/l to 6 g/l or benzaldehyde-2-sulfonic acid is used in a concentration of 1 g/l.

6. The method according to any of the preceding claims, characterized in that: the palladium exchange bath is maintained at a temperature between room temperature and 60 c, preferably between 35 c and 50 c, during deposition.

7. The method according to any of the preceding claims, characterized in that: the palladium exchange bath is operated at a pH of less than 4, preferably at a pH of 2.

8. The method according to any of the preceding claims, characterized in that: the residence time of the printed circuit board in the palladium exchange bath does not exceed 5 minutes, preferably 2 to 3 minutes.

9. The method according to any of the preceding claims, characterized in that: after palladium deposition from the palladium exchange bath, palladium is deposited from the chemical (reduced) palladium bath.

10. A palladium exchange bath for use in the method according to any one of the preceding claims, comprising palladium in the form of a palladium salt dissolved in an acidic aqueous solution, characterized in that: which comprises a brightener from the group of the following compounds:

（a）

wherein

R₆is a radical having 1 to 6C atoms, which is derived from a saturated or unsaturated aliphatic hydrocarbon, wherein the radical can be substituted, and

x is SO₂A group or a CO group, or a combination thereof,

(b) benzaldehyde.

11. The bath according to claim 10, characterized in that: the brightener is 3- (1-pyridine) -1-propane sulfonic acid or benzaldehyde-2-sulfonic acid.

12. The bath according to claim 10 or 11, characterized in that: it contains palladium in an amount of 150 mg/l to 250 mg/l.

13. The bath according to claim 10, 11 or 12, characterized in that: which comprises palladium in the form of palladium chloride.

14. The bath according to any one of claims 10 to 13, characterized in that: it contains an organic brightener in a concentration of 0.01 to 50 g/l, preferably 1 to 10 g/l.

15. The bath according to any one of claims 10 to 13, characterized in that: it comprises 3- (1-pyridine) -1-propanesulfonic acid in a concentration of from 3 g/l to 6 g/l or benzaldehyde-2-sulfonic acid in a concentration of 1 g/l.

16. The bath according to any one of claims 10 to 15, characterized in that: it also contains an inorganic complexing agent in an amount up to 150 g/l.

17. The bath of claim 16, wherein: it contains 30-80 g/l of inorganic complexing agent.

18. The bath according to claim 16 or 17, characterized in that: the inorganic complexing agent is ammonium salt.

19. The bath according to claims 13 and 18, characterized in that: the ammonium salt is ammonium chloride.

20. The bath according to any one of claims 10 to 19, wherein: it comprises diethylenetriamine as inorganic complexing agent, preferably in a concentration of between 0.01 and 5 ml/l.