JP2003304051A - Circuit board and method of improving its solder wettability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit board that can remarkably reduce solder voids. <P>SOLUTION: In a method of improving solder wettability of circuit board, the surface of a plated-Ni film formed on a metallic circuit is cleaned with electrolytic water of ≥10 in pH, ≤-800 mV in equilibrium potential, and ≥10.0 mS/m in electrical conductivity. It is preferable to heat-treat the surface of the plated-Ni film at a temperature of 280±10°C in a vacuum of ≤1 Pa before cleaning. The circuit board is cleaned by this method. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board and a method for improving solder wettability of the circuit board. More specifically, the present invention relates to a technique capable of drastically reducing defects called solder voids when soldering a semiconductor element such as a silicon chip to a Ni-plated metal circuit.

[0002]

2. Description of the Related Art Today's problems to be solved by a module equipped with a semiconductor element are caused by the semiconductor element as well as the durability of a circuit board, while the output is being increased with the recent development of electronic technology. Efficiently and quickly releasing heat to the outside of the system, for example, reducing solder voids that hinder heat conduction.

The basic structure of a circuit board is such that a metal circuit is formed on the front surface of a ceramic substrate and a metal heat dissipation plate is formed on the back surface, and the metal circuit and the metal heat dissipation plate are plated with Ni. Then, when the module is assembled, the semiconductor element is mounted on the metal circuit, and the base plate for mounting the heat sink is soldered to the surface of the metal heat dissipation plate.

Materials for the ceramic substrate include alumina, aluminum nitride, silicon nitride, metal circuits,
Copper, aluminum, alloys thereof, and the like are used as materials for the metal heat dissipation plate and the base plate. In addition, the bonding of the ceramic substrate, the metal circuit, and the metal heat sink is made of Ag, C
The active metal brazing method using a brazing material containing an u or Ag-Cu alloy and an active metal component such as Ti, Zr, or Hf is predominant.

When a thermal load such as a heat cycle is applied to the circuit board, thermal stress is generated due to a difference in thermal expansion between the ceramic substrate and the metal, and the ceramic substrate, the metal circuit, and the metal radiator plate (hereinafter, both are referred to as “metal circuit”). Etc.)), a crack is generated in the ceramic substrate at the joint end face. This crack progresses with an increase in the number of cycles of heat load, and in an extreme case, it causes dielectric breakdown. like this,
In order to suppress the generation of cracks, Al having a thermal stress smaller than that of Cu is used as a material for the metal circuit or the like.

Since Pb-Sn solder is used for joining the Al circuit and the semiconductor element and the Al heat sink and the base plate, it is necessary to apply Ni plating to the surface of the metal (Al) circuit or the like. Even when the metal circuit or the like is made of Cu, Ni plating is generally applied in order to prevent oxidation and decrease in reliability due to reaction with solder. For soldering, a method of reflowing in air or nitrogen using a flux, a method of reflowing in a hydrogen atmosphere without using a flux, and the like are adopted.

One of the factors that cause solder voids is the wettability of solder to the Ni plating film on the surface of a metal circuit or the like. If the wettability is insufficient, it will cause solder voids. Conversely, if it is too strong, it will cause solder flow to areas other than the soldered part. There is value. This wettability is deteriorated by the presence of a minute oxide film or organic matter that cannot be visually determined, but there is no method for evaluating it appropriately.

Therefore, it is considered that the semiconductor element is conventionally soldered after relying on experience and washing with washing water having an appropriate composition. However, the composition of the cleaning water has not been clarified, and the solder voids have not been drastically reduced,
There was room for further improvement because the solder void ratio was several percent, and in many cases several tens percent.

[0009]

SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a circuit board in which solder voids are drastically reduced. The object of the present invention can be achieved by cleaning the surface of the Ni-plated film applied to the metal circuit or the like with electrolyzed water having a specific composition.

[0010]

That is, the present invention provides p
Method for improving solder wettability of a circuit board, which comprises cleaning the Ni plating film surface applied to a metal circuit with electrolyzed water having H10 or more, oxidation-reduction potential of -800 mV or less and electric conductivity of 10.0 mS / m or more Is. In this case, it is preferable that the Ni-plated film surface before cleaning is heat-treated at a temperature of 280 ± 10 ° C. in a vacuum of 1 Pa or less. Further, the present invention is a circuit board cleaned by such a method.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below.

The circuit board to be cleaned by the present invention is
A metal circuit or the like is formed on a ceramics substrate and is plated with Ni. There are many commercial products and prior art documents for such a circuit board, which can be used in the present invention, but will be outlined below.

The material of the ceramic substrate is preferably aluminum nitride or silicon nitride from the viewpoint of high reliability and high insulation. The thickness of the ceramic substrate can be freely changed depending on the purpose. Usually 0.635 mm, but 0.5
A thin material of about 0.3 mm may be used. When it is desired to remarkably increase the withstand voltage under a high voltage, a material having a thickness of 1 to 3 mm is used.

The material of the metal circuit or the like is preferably Al, Cu or Al-Cu alloy. These are used in the form of a single body or a laminated body such as a clad including the single layer. Al has a smaller yield stress than Cu, is rich in plastic deformation, and can significantly reduce the thermal stress applied to the ceramic substrate during thermal stress load such as heat cycle. Therefore, it suppresses cracks generated in the ceramic substrate more than Cu. It becomes possible to obtain a highly reliable circuit board.

The thickness of the metal circuit is preferably 0.4 to 0.5 mm in the case of the Al circuit and 0.3 to 0.5 mm in the case of the Cu circuit in terms of electrical and thermal characteristics. On the other hand, the thickness of the metal heat dissipation plate is determined so as not to cause warpage during soldering. Specifically, in the case of the Al circuit, 0.1 to 0.1
0.4 mm, and the Cu circuit is preferably 0.15 to 0.4 mm.

In order to form a metal circuit or the like on the ceramic substrate, a method of bonding the metal plate and the ceramic substrate and then etching, a method of bonding the circuit punched from the metal plate and the pattern of the heat dissipation plate to the ceramic substrate, etc. Can be done by The active metal brazing method is suitable for joining, but the DBC method or the like may be used.

The surface of the metal circuit or the like before being plated with Ni is preferably smoothed by grinding, physical polishing, chemical polishing or the like, and the surface roughness is preferably Ra ≦ 0.2 μm. . The Ni plating is preferably an electroless method, which makes it possible to deal with fine patterns.
The Ni plating film thickness is preferably 2 to 8 μm.

In order to increase the reactivity between the Ni plating film and the Sn component of the solder, the Ni plating film should be heat-treated at a temperature of 280 ± 10 ° C. in a vacuum of 1 Pa or less to enhance the crystallinity of Ni. preferable. When the degree of vacuum exceeds 1 Pa, the Ni plating film is significantly oxidized, and the reactivity with the Sn component is deteriorated. On the other hand, if the heat treatment temperature is lower than 270 ° C., the crystallinity of Ni is not sufficiently enhanced, and if it exceeds 290 ° C., the Ni plating film is hardened, which may damage the circuit board. The heat treatment time is preferably 10 to 30 minutes.

A major feature of the present invention is that the surface of the Ni plating film is washed with electrolyzed water having a pH of 10 or more, an oxidation-reduction potential of -800 mV or less and an electric conductivity of 10.0 mS / m or more. As the cleaning mode, a shower ring, rocking immersion, etc. are possible, but a shower ring is preferable.
The cleaning may be performed after Ni plating or immediately before soldering the semiconductor element. The former has the process advantage that a cleaning tank can be provided after the plating process, and the latter has the advantage that soldering can be avoided because new oxide films, organic substances, etc. are formed and adhere to the Ni-plated surface. There is. Moreover, you may wash by both.

If the pH of the electrolyzed water is less than 10, the generation of OH groups effective for solder wettability will be insufficient. When the redox potential is higher than -800 mV, it is difficult to suppress the generation of oxide film. When the electric conductivity is less than 10.0 mS / m, it is difficult to remove the fine particles containing metal ions. It is preferable that the amount of impurities in the electrolyzed water is as small as possible.
The concentration of negative ions such as l ⁻ and SO ₄ ²⁻ and positive ions such as Cu ²⁺ and Na ⁺ is preferably 0.1 mg / L or less.

Such electrolyzed water can be produced by adding salt or dilute hydrochloric acid in an electrolysis tank, electrolyzing until the desired characteristics are obtained, and irradiating with ultrasonic waves. Further, commercially available products such as Nippon Aqua Co., Morinaga Milk Industry Co., Ltd., Asahi Glass Engineering Co., Ltd. (see Ultra Clean Technology Vol. 11 No. 1 p19) can also be used.

When the circuit board of the present invention washed in this way is used, even if the semiconductor element is soldered to the Ni-plated film surface, the solder void ratio becomes smaller than 1.0%, and the module The heat radiation property of is significantly improved even though the Ni plating method is an electroless method. The solder void fraction can be automatically measured using a soft X-ray flaw detector or an ultrasonic flaw detector. An example of the measuring device is a soft X-ray flaw detector (“PROX” manufactured by Softex).
-TEST 100 ") and an ultrasonic flaw detector (" HA-701 "manufactured by Honda Electronics Co., Ltd.).

To solder the semiconductor element on the metal circuit surface of the circuit board and the base plate on the metal heat sink surface, Pb-Sn solder is used. For soldering, there are a method of reflowing in air or nitrogen using a flux, and a method of reflowing in a hydrogen atmosphere without using a flux, but the latter is preferable from the viewpoint of process simplification and environmental problems.

As described above, the heat dissipation characteristics of the module are better as the solder void rate is smaller, and in particular, the module greatly changes at the solder void rate of 2%. Therefore, when the circuit board is mass-produced, it is desirable to perform the following simple test and perform manufacturing control so that the average value (+ 4σ) of the solder void ratio of the test number 10 is 2% or less.

That is, Pb (9
0%)-Sn (10%) solder pieces are sandwiched between the silicon chips. The solder piece has a bottom area of 5 to 225 mm ^{2 and a} thickness of 0.1 to 0.5 mm, and the silicon chip has a bottom area of 5 to 225 mm ² and a thickness of 0.4 to 1.0 mm. Then, in a hydrogen atmosphere, the temperature up to 150 ° C is increased to 15 to 20 ° C
/ Min, and then the temperature is raised at a rate of 2.3 to 2.5 ° C / min to raise the temperature to 350 ° C ± 5 ° C, and then quickly cooled naturally at room temperature for soldering. . Where 150
The temperature rising rate to ℃ is slower than 15 ℃ / min, or 35
If the heating rate up to 0 ° C is slower than 2.3 ° C / min, Ni
If the plating film surface is oxidized and the rate of temperature increase up to 350 ° C. is higher than 2.5 ° C./min, the melting of the solder becomes insufficient, and in any case, the solder wettability cannot be correctly evaluated. On the other hand, the rate of temperature increase up to 150 ° C is 120
The equipment becomes large-scale to make it faster than ℃ / min.

[0026]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Examples 1 to 5 Comparative Examples 1 to 5 Commercially available aluminum nitride substrates were prepared as ceramic substrates. The size is 2 inches square x thickness 0.635m
m, thermal conductivity of 175 W / by laser flash method
mK, three-point bending strength is 420 MPa. As the Al plate, one having a size of 2 inches square and a thickness of 0.4 mm for forming a circuit and one having a size of 2 inches square and a thickness of 0.1 mm for forming a heat sink were used.

On the front and back surfaces of the ceramics substrate, Al plates were stacked with a bonding material (Al 96.0%, Cu 3.5%, Mg 0.5% alloy foil having a thickness of 20 μm) interposed therebetween, and carbon plates were screwed into the substrate. A pressing jig was used to uniformly press the ceramic substrate in the vertical direction. Joining is
It was performed under pressure in a vacuum or nitrogen atmosphere at a temperature of 550 to 635 ° C.

After joining, an etching resist was screen-printed and etched with a FeCl ₃ solution. The pattern is
Both the circuit and the heat dissipation plate were square (corner R was 2 mm) and formed in the central portion of the ceramic substrate (creeping distance 1 mm). Then, after removing the etching resist, a commercially available alkaline etching agent (main component: caustic soda) was used to remove oxides on the Al surface.

A of the circuit board manufactured as described above
Electroless Ni-P plating ("Nimden SX" manufactured by Okuno Chemical Industries Co., Ltd.) was performed on the l circuit surface to form a Ni plating film having a thickness of 5 μm. For some of the circuit boards, the Ni plating film was heat-treated under a vacuum of 1 Pa or less at a temperature of 280 ° C. for 20 minutes. After that, various kinds of electrolyzed water shown in Table 1 were used for cleaning by showering or immersion for 10 minutes.

Using these circuit boards, the following soldering was performed assuming actual module production, and the solder void ratio was measured. The results are shown in Table 1. That

Measurement of Solder Void Ratio A silicon chip (bottom area 169 mm ² × thickness) with Pb (90%)-Sn (10%) solder pieces (bottom area 169 mm ² × thickness 0.1 mm) sandwiched between Al circuit surfaces. 0.4 mm) and put it in a hydrogen atmosphere up to a temperature of 150 ° C at 15 ° C /
After increasing the temperature to 350 ° C by raising the temperature at a rate of 1 minute and then at a rate of 2.5 ° C / minute, promptly perform natural cooling at room temperature to perform soldering to determine the solder void rate with soft X-ray flaw detection. It measured using the apparatus ("PRO-TEST 100" by Softex Co., Ltd.), and calculated | required the average value of the test number 10.

[0033]

[Table 1]

From the comparison between the example and the comparative example in Table 1, it is understood that the cleaning method of the present invention drastically reduces the solder voids to 1% or less. Further, from the comparison between Example 1 and Example 5,
If the Ni plating film before cleaning is subjected to heat treatment,
It can be seen that the solder voids are further reduced.

[0035]

According to the present invention, a circuit board in which solder voids are drastically reduced is provided.

Continued front page    F term (reference) 4K022 AA02 AA42 BA14 DA01 EA01                       EA02                 4K053 PA07 PA13 QA04 RA07 RA21                       SA04 SA06 TA04 TA06 TA18                       YA02 YA03 YA04                 5E319 AA03 AB06 AC04 AC17 BB02                       BB07 CC33 CD01 GG03                 5E343 AA23 BB16 BB24 BB28 BB44                       BB67 CC38 CC43 CC62 DD51                       EE04 EE06 EE15 EE52 ER13                       ER33 GG18

Claims (3)

[Claims] 1. A pH of 10 or more and a redox potential of −800 m.
V or less, electrolyzed water with an electric conductivity of 10.0 mS / m or more,
A method for improving solder wettability of a circuit board, which comprises cleaning a surface of a Ni plating film applied to a metal circuit. 2. The Ni plating film surface before cleaning is 1 Pa.
The method according to claim 1, which is heat-treated at a temperature of 280 ± 10 ° C. in the following vacuum. 3. A circuit board cleaned by the method according to claim 1.