JP2006024659A - Wiring board manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a flip-chip wiring board, wherein electronic components can be surely and firmly mounted on the surface of the body of the board via solder bumps having few bubbles. <P>SOLUTION: The method of manufacturing the wiring board 1 comprises a process of forming conductor pads 6 on the surface of the body 2 of the board, a process of forming, on the pads 6, a conductive paste 8 containing alloy powder having a melting point lower than that of the conductor constituting the pads 6, and a reflow process of heating the conductive paste 8 into the solder bumps 9. The reflow process comprises preheating steps S1 and S2, wherein the pads 6 and the conductive paste 8 are heated at a temperate increase rate of 0.5°C/sec or less, and are preheated in a temperature zone of the melting point of the low melting point alloy ±10°C; and heating steps S3 and S4, wherein, after the preheating steps, the conductive paste 8 is heated up to a temperature higher by 10 to 30°C than the melting point of the low-melting point alloy to be melted and formed into the solder bumps 9. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a flip-chip type wiring board in which an electronic component such as an IC chip is mounted on the surface of a substrate body.

  In a flip chip type wiring board, a solder bump is formed on a pad made of a conductor formed on the surface of the substrate body by reflow, an external electrode in an electronic component such as an IC chip is placed on the solder bump, and again. The electronic component is mounted by reflowing. The solder bump is formed by printing a conductive paste containing an alloy powder having a melting point lower than the conductor of the pad and a flux on the pad, and once heated and held in a temperature range of about 150 ° C., The reflow is performed by heating to a temperature higher than the melting point of the alloy powder.

By the way, if air bubbles (voids) are included in the solder bump after reflow, the electrical connection between the internal wiring of the wiring board and the mounted electronic component is reduced, and the mounting strength of the electronic component is reduced. There has been a problem that a situation occurs in which an electronic component is detached due to a prepared external force.
In order to suppress air bubbles in the solder bumps, a thermosetting resin having an active action and a heating temperature higher than the solder melting point temperature is included when electronic components are sequentially mounted on the first surface and the second surface of the substrate. There has been proposed a method for manufacturing a mounting substrate in which a solder bonding paste is used only on a second surface which will be a mounting target surface later (see, for example, Patent Document 1).
However, since the above manufacturing method is directed to a substrate on which electronic components are sequentially mounted on the first surface and the second surface, a general flip chip type wiring substrate on which electronic components are mounted only on the first surface (front surface) Has a problem that it cannot be applied.

JP 2004-47774 A (pages 1 to 8, FIG. 3)

  The present invention solves the above-described problems in the background art, and a method of manufacturing a flip-chip type wiring substrate capable of securely and firmly mounting electronic components such as IC chips on the surface of the substrate body via solder bumps with few bubbles It is an issue to provide.

In order to solve the above-mentioned problems, the present invention is based on the inventors' diligent research and investigation. As a result, when the conductive paste formed on the pad located on the surface of the substrate body is reflowed to form a solder bump, the reflow process is performed. This was made by paying attention to optimizing the heating temperature history.
That is, the method of manufacturing a wiring board according to the present invention includes a step of forming a pad made of a conductor on the surface of a substrate body, and a conductive material containing an alloy powder having a melting point lower than the melting point of the conductor constituting the pad on the pad. Forming a conductive paste, and a reflow process for heating the conductive paste to form solder bumps. The reflow process is a step of increasing the pad and the conductive paste by 0.5 deg ° C./second or less. A preheating step of heating at a temperature rate and preheating in a temperature range of the melting point ± 10 ° C. of the low melting alloy, and after the preheating step, the melting point of the low melting alloy is 10 A heating step of forming the conductive paste on a solder bump by heating to a temperature higher by 30 ° C. to 30 ° C.

In other words, according to the present invention, the low-melting-point alloy is a Sn—Ag—Cu-based alloy, and the preheating temperature (S2) is in the range of 200 ° C. to 220 ° C. It is also possible.
In addition, the wiring board is manufactured at an initial heating rate (S1) of 0.05 ° C./second to 0.5 ° C./second, preferably 0.1 ° C./second to 0.2 ° C./second. It is also possible to use a method.

According to the method for manufacturing a wiring board of the present invention, in the reflow process for forming solder bumps, first, the pad and the conductive paste are gradually heated at a temperature rising rate of 0.5 deg ° C./second or less, and A preheating step of preheating in the temperature range of the melting point of the low melting point alloy ± 10 ° C. is performed. For this reason, since the flux contained in the said electrically conductive paste can fully be removed, the bubble in the solder bump formed can be reduced reliably. In addition, by controlling the preheating time, the metal of the conductor constituting the pad or the plating metal coated on the surface of the pad and the metal element of the low melting point alloy diffuse to each other, and the metal between the two It becomes possible to suppress the situation where the compound is generated near the joint between the pad and the solder bump.
Therefore, when the external electrode of the electronic component such as an IC chip or a crystal resonator is placed on the solder paste and reflowed, splashing due to air bubbles is prevented, and the electronic component and the internal wiring of the wiring board are prevented. Sufficient conduction can be obtained between the two, and it is possible to prevent a situation in which the electronic component is inadvertently detached from the solder bump due to an external force.

In addition, while making the low melting-point alloy contained in the said electrically conductive paste into a Sn-Ag-Cu type-alloy, and making the said preheating temperature into the range of 200 to 220 degreeC, this invention can be implemented concretely and this The above-described effects of the invention can be obtained more reliably.
Further, it is formed by setting the temperature rising rate at the beginning of the preheating (S1) to 0.05 ° C./second to 0.5 ° C./second, preferably 0.1 ° C./second to 0.2 ° C./second. It is possible to reliably reduce bubbles in the solder bumps. Moreover, the conductor of the pad or the plating metal coated on the surface of the pad and the metal element of the low-melting-point alloy can be interdiffused to suppress the formation of an intermetallic compound between them.

In the following, the best mode for carrying out the present invention will be described.
FIG. 1 shows a cross section in the vicinity of the surface 3 of a substrate body 2 in which a plurality of ceramic layers mainly composed of alumina, for example, are laminated. Such a substrate body 2 has a square or rectangular shape in plan view, has an overall thickness of about 2 mm, and a plurality of green sheets mainly composed of alumina or the like are laminated, and a not-shown internal pattern with a predetermined pattern therebetween. Wiring is formed in advance and the whole is fired.
As shown in FIG. 1, a plurality of pads 6 are formed on the surface 3 of the substrate body 2 so as to be electrically connected to the internal wiring via the via conductors 4. The pad 6 is made of a conductive paste containing W or Mo powder, screen-printed on the surface 3 where the end face of each via conductor 4 is exposed, formed into a disk shape, and then sintered at the same time as the firing. It is a conductor.

Next, using each via conductor 4 as a conductor with a plating electrode, electrolytic Ni plating and electrolytic Au plating are sequentially applied to the surface of each pad 6 to form a Ni plating layer having a thickness of about 4 μm and a thickness of about 0.1 to 0. 3 μm Au plating (both not shown) are respectively coated.
Furthermore, as shown in FIG. 2, the conductive paste 8 is individually formed in a substantially cylindrical shape (average diameter 200 μm × height 50 μm) on the plating layer of each pad 6 by screen printing. The conductive paste 8 contains a powder of Sn—Ag—Cu alloy (low melting point alloy) having a particle size of about 6 to 12 μm and a flux. The melting point of the Sn—Ag—Cu alloy is 216 ° C., which is lower than the melting points of W and Mo constituting the conductor of the pad 6.

Next, the substrate body 2 on which the conductive paste 8 is formed on each pad 6 is inserted into a batch-type heating furnace or a tunnel-type heating furnace (both not shown), and the conductive paste 8 is described below. A reflow process of heating along each step is performed.
As shown in FIG. 4, the reflow process includes preheating steps S1 and S2, heating steps S3 and S4, and a temperature lowering step S5.

In the preheating steps S1 and S2, the pad 6 and the conductive paste 8 near room temperature are gradually heated (S1) at a temperature increase rate of 0.5 deg ° C./second or less, and the Sn—Ag—Cu based alloy is formed. Preheating (S2) is performed for about 80 to 100 seconds (t2) at a temperature around the melting point of 216 ° C, for example, in a temperature range of 200 ° C to 220 ° C.
According to this, since heating is performed at a slow temperature increase rate (200 to 220 ° C./t1) and preheating is performed in the above temperature range, the flux contained in the conductive paste 8 can be considerably removed. As a result, bubbles in the solder bumps to be formed later can be reduced. In addition, since the preheating time t2 is set to 200 seconds or less, the plating metal coated on the surface of the pad 6: Ni and the metal element: Sn of the Sn—Ag—Cu alloy are interdiffused to form a space between the two metals. Formation of compound: SnNi ₃ , Sn ₂ Ni ₃ , or Sn ₄ Ni ₃ can also be suppressed.

As shown in FIG. 4, the heating steps S3 and S4 performed after the preheating step are performed at a temperature, for example, by 24 ° C. higher than the melting point 216 ° C. of the Sn—Ag—Cu alloy. , Heated to 240 ° C. for 20 seconds (t4). As a result, the Sn—Ag—Cu-based alloy powder in the vicinity of the surface layer of the conductive paste 8 is completely melted, so that a solder bump 9 having a substantially hemispherical shape is formed as shown in FIG.
Finally, a temperature lowering step S5 is performed for lowering the temperature of the substrate body 2 including the pads 6 and the solder bumps 9 to near room temperature.
As a result of the above reflow process, the conductive paste 8 becomes a substantially hemispherical solder bump 9 as shown in FIG. 3, and the remaining flux is removed by washing, whereby a wiring having a plurality of the solder bumps 9 on the surface 3 is obtained. The substrate 1 can be obtained.

In FIG. 5, external electrodes 14 provided on the bottom surface 13 of the main body 12 of the crystal resonator 10 are placed on each solder bump 9 in order to mount the crystal resonator (electronic component) 10 on the surface 3 of the wiring board 1. Shows the state. In such a state, the wiring substrate 1 and the crystal unit 10 are inserted into a batch type heating furnace or a tunnel type heating furnace, and re-flow is performed to heat to about 210 ° C. to 240 ° C. The external electrode 14 is made of, for example, a Cu-based alloy.
As a result, when reflowing at a temperature higher than the melting point 216 ° C. of the Sn—Ag—Cu alloy, the external electrode 14 of the crystal unit 10 is welded to the top of each melted solder bump 9 as shown in FIG. The When reflowing is performed at a temperature near the melting point of the alloy, the bump 9 and the external electrode 14 are joined.

  By using the wiring substrate 1 manufactured through the reflow process, since there are few bubbles in each solder bump 9 at the time of the reflow, the welding or bonding with the external electrode 14 of the crystal resonator 10 can be performed firmly. . Moreover, since it is possible to prevent splashing of the alloy that occurs when bubbles blow out with re-reflow heating, metal pieces adhere to the bottom surface 13 of the crystal resonator 10 or between the wirings exposed to the bottom surface 13. Short circuit can be surely prevented.

Here, specific examples of the present invention will be described together with comparative examples.
A plurality of substrate main bodies 2 having alumina as a main component and having an outer shape of length × width: 100 mm × 100 mm and thickness: 2 mm were prepared under the same manufacturing conditions. On the surface 3 of these substrate bodies 2, a total of 6 pieces each having a diameter: about 100 μm × thickness: about 20 μm and W as a main component and pads 6 in length × width: 2 × 3 are arranged in a grid pattern. Yes.
The surface of each pad 6 in each substrate body 2 was subjected to electrolytic Ni plating under the same conditions to individually coat a Ni plating layer having a thickness of 3.5 μm. Furthermore, the surface of the Ni plating layer was subjected to electrolytic Au plating under the same conditions, and individually coated with an Au plating layer having a thickness of 0.3 μm.

Next, a substantially cylindrical conductive paste 8 (average diameter 200 μm × height 50 μm) was individually formed on the plated layer of each pad 6 in each substrate body 2 by the same screen printing. The conductive paste 8 contains 90 wt% of Sn—Ag—Cu alloy powder and 10 wt% of flux.
Each substrate body 2 on which the conductive paste 8 is formed on each pad 6 is inserted into a tunnel-type heating furnace, individually heated and heated (S1) at different heating rates shown in Table 1, and 216 ° C. ± Preheating steps S1 and S2 in the reflow process of holding at 10 ° C. for 100 seconds (S2) were performed.
Subsequently, heating steps S3 and S4 in the reflow process of heating and holding each substrate body 2 at 240 ° C. for 20 seconds are performed to obtain a plurality of wiring boards (1) in which the conductive paste 8 is formed on the solder bumps 9. It was.
Each wiring board (1) was divided into Examples 1-4 and Comparative Examples 1 and 2 according to the temperature rising rate shown in Table 1, and 20 wiring boards (1) were manufactured for each example.

In order to mount the crystal resonator (electronic component) 10 on the surface 3 of the wiring board (1) of each example, the bottom surface 13 of the main body 12 of the crystal resonator 10 is placed on each solder bump 9 as shown in FIG. The external electrode 14 provided on the substrate was placed. In this state, the wiring board (1) of each example and the crystal resonator 10 were inserted into a tunnel-type heating furnace, and re-flowing was performed by heating to 240 ° C. for 20 seconds.
As shown in FIG. 6, a rod 18 was erected vertically with an adhesive 16 in the center of the upper surface of the main body 12 of the crystal resonator 10 mounted on the surface 3 of the wiring board (1) of each example.

As shown by the arrows in FIG. 6, a destructive test is performed by fixing the wiring board (1) of each example and pulling the rod 18 by applying a load along the axial direction as indicated by an arrow in FIG. 6. It was. And the fracture mode and fracture strength at the time of fracture (break) at any position were measured, and these are shown in Table 1.
The breaking strength is the same for each wiring board (1) when the joints of the six pads 6, solder bumps 9, and external electrodes 14 in one wiring board (1) are all destroyed. The average value for each of the six pads 6 was calculated.
Further, as shown in the left side of FIG. 7, the destruction mode is an A mode in which the destruction between the pad 6 and the solder bump 9 is performed, and as shown near the right center of FIG. Those broken in between were classified as C mode, and those broken in the middle of the solder bumps 9 were classified as D mode as shown on the right side of FIG. As shown in Table 1, the B mode was defined as a mixture of the A mode and the C mode.

For each wiring board (1) in each example before destruction, the ratio (%) of bubbles generated in the solder bumps 9 was measured by X-rays. Among these, the ratio of bubbles exceeding 30 μm in diameter (%, The average value was calculated individually. They are also shown in Table 1.
According to Table 1, in the wiring boards 1 of Examples 1 to 4, the ratio of voids having a diameter exceeding 30 μm (the number of bumps in which voids exceeding 30 μm were confirmed / the total number of bumps inspected) was 8% or less. It was. This is because the heating rate in the preheating step S1 of the reflow process was lowered to 0.4 deg.degree. C./second or less and preheating (S2) was performed in the temperature range, so that the flux component in the conductive paste 8 could be sufficiently removed. It is because. In particular, in Example 4 where the temperature increase rate in the preheating step S1 was 0.04 deg ° C./second, which was the slowest temperature increase, the bubble generation rate was minimized.

On the other hand, as shown in Table 1, in the wiring boards of Comparative Examples 1 and 2, the bubble generation ratio in the solder bumps 9 was 10% or more. This is because the heating rate in the preheating step S1 was increased to 0.6 deg.degree. C./second or more, and thus the flux component in the conductive paste 8 could not be sufficiently removed despite the preheating (S2) in the temperature range. It is because of that.
Further, according to Table 1, in the wiring boards 1 of Examples 1 to 4, all the broken positions were between the solder bumps 9 and the external electrodes (chips) 14 (C mode). This is because the formation of an intermetallic compound in the vicinity of the joint portion between the pad 6 and the solder bump 9 is small, and therefore, the joint is broken at the position where the joint strength is lower than this position.

From the results of the wiring boards 1 of Examples 1 to 4 as described above, the operation of the present invention was understood and the effect was supported.
As shown in Table 1, the breaking strengths of Examples 1 and 3 were minimized. This is because the rate of temperature increase in the preheating step S1 of the reflow process is set to 0.4 deg ° C./second or less, so that the plating metal: Ni on the surface of each pad 6 and the Sn—Ag—Cu system in the conductive paste 8 It is presumed that the metal element of the alloy: Sn is interdiffused, and the intermetallic compound: SnNi ₃ is formed in the vicinity of the joint between the pad 6 and the solder bump 9 and the strength of the joint is slightly reduced. Is done. Therefore, it is important to adjust the temperature increase rate and the preheating time in the preheating steps S1 and S2 so as to break in the A mode rather than the C mode in the destructive test.

The present invention is not limited to the embodiments and examples described above.
The substrate body 2 may include a resin or metal core substrate, a plurality of resin layers formed on at least one of the front surface and the back surface, and internal wiring provided therebetween.
The pad 6 is not limited to the front surface 3 of the substrate body 2 and may be provided on the back surface opposite to the front surface 3. The solder bump 9 is applied to the pad 6 on the back surface by performing the above steps. It may be formed. That is, the “front surface” of the present invention includes the back surface of the substrate body. In the case of the substrate body 2 having the pads 3 on the front surface 3 and the back surface, the above steps may be performed simultaneously for all the pads 6 on the front and back surfaces or may be performed separately for the pads 6 on the front surface 3 and the back surface. good.
Furthermore, the pad 6 may be formed on the bottom surface of a cavity that is recessed in the surface 3 of the substrate body 2 and has a square or rectangular shape in plan view.
In addition to the Sn-Ag-Cu alloy, the Sn-Ag alloy, Pb-Sn alloy, Sn-Cu alloy, Sn-Zn alloy and the like are included in the low melting point alloy contained as a powder in the conductive paste. It may be used.

The schematic sectional drawing which shows 1 process in the manufacturing method of this invention. FIG. 2 is a schematic sectional view showing a step following FIG. 1. FIG. 3 is a schematic sectional view showing a step following FIG. 2. Schematic which shows the temperature history of the reflow process of this invention. Schematic which shows the process of mounting an electronic component on the surface of the wiring board obtained by this invention. Schematic which shows the said wiring board which mounted the electronic component on the surface, and its destructive test. Schematic which shows the wiring board of the Example after the said destructive test, and a comparative example.

Explanation of symbols

1 …………… Wiring board 2 …………… Board body 3 …………… Surface 6 …………… Pad 8 ……………… Conductive paste 9 …………… Solder bump S1, S2… Preheating step S3, S4 ... Heating step

Claims (1)

Forming a conductor pad on the surface of the substrate body;
Forming a conductive paste containing an alloy powder having a melting point lower than the melting point of the conductor constituting the pad on the pad;
A reflow step of forming the solder bump by heating the conductive paste,
The reflow step is a preheating step in which the pad and the conductive paste are heated at a temperature rising rate of 0.5 degC / second or less and preheated in a temperature range of the melting point of the low melting point alloy ± 10 ° C.
After the preheating step, in order to melt the conductive paste, a heating step of forming the conductive paste on a solder bump by heating to a temperature higher by 10 ° C. to 30 ° C. than the melting point of the low melting point alloy. Have
A method for manufacturing a wiring board.

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