JP2002009425A - How to mount electronic components without using underfill - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mounting method for an electronic component which has high connection reliability where the electronic component and a motherboard sealed by package technology without using underfill. SOLUTION: A conductor pad is coated by a precoating process 5 for a mounted component joining pad of a motherboard 1 and sufficient strength is secured between a solder ball 7 used for joining to an electronic component 8 and the conductor pad 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for mounting an electronic component, and more particularly, to a high connection reliability without using an underfill at a joint between an electronic component and a mother board sealed by a packaging technique. The present invention relates to a mounting method of an electronic component having the following.

[0002]

2. Description of the Related Art Heretofore, electronic components such as ICs and LSIs have been sealed by various packaging techniques, and have been mounted on a printed wiring board after a stable electrical conductivity has been established. In the field of technology, active research has been actively conducted in recent years, and mounting on printed wiring boards has shifted from QFP to BGA or CSP as an alternative to the multi-pin package, which has been limited in mounting with fine pitch QFP. Is expected to change to a bare chip mounting method with the aim of a higher density mounting method.

On the other hand, in a component mounting method of higher density, there is a need for a technology for reliability of a joint between a printed wiring board and a package component following the component mounting method. Among the technologies that have penetrated so far, a technique of injecting mainly a thermosetting liquid resin into a joint between a motherboard and a package component to reinforce the joint has been studied. This means of injecting resin into the joint and curing it to reinforce the joint between the pad and the solder ball is often used as an underfill as a simple and optimal method, and has been actively studied in recent years. Has filed a patent application.

[0004] Recently, with the miniaturization and thinning of printed wiring boards, the miniaturization of mounting components, and the miniaturization of pads of BGA and CSP, impact and thermal fatigue at joints between motherboards and electronic components have been reduced. For the purpose of uniformly dispersing the accompanying stress, an underfill is often used between a component and a motherboard.

[0005] Behind the various industrial advantages of using the underfill, however, several problems in its use have been pointed out. For example, QF
Comparing the difficulty of the process between P and BGA, it is considered that the mounting of BGA itself is not difficult compared with QFP. However, the mounting structure of the BGA on the motherboard has an electrode bonding portion between the BGA substrate and the motherboard. For this reason, it is difficult to visually confirm the joint of the BGA as compared with the QFP. Furthermore, in order to secure stable electrical conductivity, the joints are bonded and fixed using the above-mentioned underfill, and it is difficult to repair the portions once bonded and fixed by thermosetting for the purpose of repair. Also, the means for peeling the package substrate from the motherboard may cause damage to the motherboard side. That is, it has been considered that the repair property of the BGA with respect to the QFP due to the occurrence of the defect at the joint is difficult from such a background. Therefore, the technology of reinforcing the joints using underfill has few advantages from the viewpoint of inspection and repairability after reinforcement, and considering the mounting components that will be miniaturized in the future, the It is estimated that inspection and repair work after filling will be more difficult.

[0006] Under such a background, in consideration of the cost and process of the underfill itself, the time and quality spent on the underfill itself, and the management such as the underfill, the motherboard and the electronic parts where the underfill is not used are considered. There is a need for a technique for improving the bonding strength.

[0007] Thus, there is a great industrial advantage that a condition that an underfill for strengthening the joint between a component to be mounted typified by BGA or CSP and a motherboard is not required is obtained. First, simplicity due to the elimination of the underfill step in the manufacturing process and the accompanying improvement in productivity can be expected. It is also considered effective in reducing the cost associated with the underfill material and other costs. In addition, it is considered that a cleaning step of a residue at a joint portion between the BGA and the motherboard becomes unnecessary in the process.
Further, from a mechanical point of view, it is considered that the underfill does not exist between the mounted component and the motherboard, so that the repair characteristics are effectively improved, and the inspectability of the joint between the bump and the solder is excellent. From this point of view, the effect of the necessity of underfill when mounting components on a motherboard is directly reflected in cost reduction and improvement in functionality on an industrial level.

Patents disclosed so far for such technical problems include, for example, JP-A-2000-0
Japanese Patent No. 49254 discloses a technique in which an underfill is not required at the time of mounting by a method by reducing a difference in thermal expansion coefficient between a motherboard and a chip size package.

However, Japanese Patent Application Laid-Open No. 2000-04925
The method disclosed in Japanese Patent Application Laid-Open No. H10-29139 is effective for a specific motherboard and chip size package whose coefficient of thermal expansion is controlled in advance, but is difficult to use for a conventional general-purpose motherboard and package substrate.
Therefore, a technology relating to a mounting method that does not use underfill at an industrial mass production level has not been disclosed yet. In addition, on general-purpose motherboards and package boards,
In fact, there is no technology that can be mounted without using an underfill simply by applying a surface treatment by solder pre-coating.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the problems encountered when using an underfill as described above, and its main purpose is to provide a built-up multilayer wiring board. By applying solder pre-coating, the bonding strength and reliability of the motherboard and the electronic components mounted on it are improved, so that it has sufficiently high bonding strength and bonding reliability without using an underfill. An object of the present invention is to provide a mounting method of an electronic component.

[0011]

Means for Solving the Problems The present inventor has conducted various studies in order to achieve the above object. As a result, the bonding strength and reliability of the motherboard and the electronic components mounted thereon are improved, and as a method capable of responding to the mounting method of an electronic component having a sufficiently high bonding strength without using an underfill, The present inventors have found that the solder precoat treatment is useful, and have completed the present invention.

That is, the present invention provides an underfill wherein an electronic component is mounted on the motherboard after a solder precoat layer is formed on a bonding pad portion for mounting components of the motherboard, and then the solder is deposited. The above object has been achieved by a mounting method of an electronic component that does not use.

Further, the present invention provides a method for forming a solder pre-coat layer on a bonding pad portion for a mounting component of a package component,
Next, the above object is achieved by a method of mounting an electronic component without using an underfill, characterized by mounting the package component on a motherboard after depositing solder.

Further, according to the present invention, a solder pre-coat layer is formed on a bonding pad portion for mounting components of both a mother board and a package component, and then, after solder is deposited,
The above object has been achieved by a method of mounting an electronic component without using an underfill, characterized by mounting the package component on the motherboard.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, it is desirable to perform plating on bonding pads for mounted components before forming a solder precoat layer from the viewpoint of preventing oxidation and corrosion of connection pads. For example, electroless Ni /
Au plating is preferred.

The solder precoat layer according to the present invention uses a solder paste containing organic acid lead and tin as main components, particularly a solder paste containing 15 to 35% by weight of organic acid lead and 25 to 45% by weight of tin. It is particularly advantageous to form them.
The solder paste composed of the organic acid lead and tin causes a chemical reaction to proceed in a reflow process. That is, in the solder paste heated to about 200 to 250 ° C. in the reflow step, first, the lead ions of the organic acid lead are ionized by heat. Next, a part of the lead ions undergoes a substitution reaction with nickel on the underlying nickel surface, and a thin layer of lead is formed on the nickel surface. In addition, most of the lead ions undergo a substitution reaction with tin and precipitate as a single element of lead, and the simple substance mixes with tin and precipitates as solder.

There are two characteristics in this reaction. One is where a thin layer of lead forms on the nickel surface. The effect of this function is to suppress the diffusion of nickel into tin because the base nickel does not come into contact with tin, thereby suppressing the segregation of phosphorus and improving the bonding strength at the interface between the solder ball and the bonding pad. have. The other point is that lead precipitates as a simple substance, and the simple substance mixes with tin to precipitate as a solder. Due to this phenomenon, the solder is characterized in that it is gradually deposited in the reflow process and is selectively deposited only on the pad surface of the metal portion. Therefore, it can be sufficiently used even in a printed wiring board having a high density, and it becomes possible to fill a blind via hole (BVH) used for connection between the L1 layer and the L2 layer with solder by voidless.

When a general solder paste made of tin and lead is used, the solder inside the paste is rapidly melted in the reflow process and comes into contact with the BVH portion.
Inner bubbles generated by the dent of H are difficult to escape from the paste,
Voids easily occur in BVH.

The solder precoat layer according to the present invention uses a metal mask having a thickness of about 100 to 300 μm, which is formed by opening the above-mentioned solder paste at, for example, a BGA or CSP location on a motherboard which requires high bonding strength. , Formed by screen printing.

The thickness of the metal mask is proportional to the amount of solder deposited in the next reflow process, and the thickness of the metal mask is adjusted by the amount of solder to be deposited on the pad. For example, in a solder paste printed by using a 150 μm metal mask, solder having a height of about 20 to 50 μm is deposited on a flat pad, and about 0 to 30 μm in a pad portion having a BVH structure. Solder deposits.
Therefore, the amount of solder deposited on the pad can be controlled by the thickness of the metal mask, and a desired amount of solder can be deposited.

In addition, the printed solder paste is
The solder is deposited by the reflow process as described above.
That is, the motherboard on which the solder paste is printed enters a reflow process and receives heat for about 2 minutes at a temperature of 200 to 250 ° C. The heat is absorbed by the solder paste, and the solder is deposited by the above-described solder deposition mechanism. Further, since the deposited solder is deposited only on the pad of the metal portion, sufficient soldering can be easily performed even on the fine pitch pad portion. After the reflow step, ultrasonic cleaning with warm water or the like is performed.

In the present invention, the mounting of the electronic components on the motherboard after the solder has been deposited is performed via solder balls having a general composition.

[0023]

The present invention will be further described with reference to the following examples.

Embodiment 1 FIG. 1 is a schematic process drawing showing a first embodiment of the present invention, which will be described below with reference to FIG. First step B in motherboard 1 having solder resist 4
An electroless Ni is applied to the pad part 2 for mounting components having no VH.
/ Au plating 3 was applied (FIG. 1A). Second step The pad portion 2 plated with 3 in the first step is
Using a metal mask having a thickness of 0 to 300 μm, a solder paste containing organic acid lead and tin (“Super Solder Paste SP50” manufactured by Harima Chemicals, Inc.)
1 ") was printed by a screen printing method to form a solder precoat layer 5 (FIG. 1B). Third Step The motherboard 1 on which the solder precoat layer 5 was formed in the second step was heated at 220 ° C. for about 2 minutes in a reflow furnace to deposit solder 6 (FIG. 1C). Fourth Step After the motherboard 1 obtained in the third step was washed with an organic solvent, it was further washed with hot water and water. Fifth step Pad portion 2 of motherboard 1 cleaned in the fourth step
An electronic component (package) 8 was mounted on the substrate via a solder ball 7 made of tin and lead (FIG. 1D).

Embodiment 2 FIG. 2 is a schematic process diagram showing a second embodiment of the present invention, which will be described below with reference to FIG. First Step An electrolytic Ni / Au plating 10 is applied to the mounting component pad portion 2 of the package substrate 8 having the solder resist 4.
(FIG. 2A). Second step 1 is applied to the pad portion 2 on which the plating 10 is applied in the first step.
Using a metal mask having a thickness of 00 to 300 μm, a solder paste containing organic acid lead and tin (“Super Solder Paste SP501” manufactured by Harima Chemicals, Inc.)
Was printed by a screen printing method to form a solder precoat layer 5 (FIG. 2B). Third Step The package substrate 8 on which the solder precoat layer 5 was formed in the second step was heated at 220 ° C. for about 2 minutes in a reflow furnace to deposit solder 6 (FIG. 2C). Fourth Step After the package substrate 8 obtained in the third step was washed with an organic solvent, it was further washed with hot water and water. Fifth Step A package was mounted on the mother board 1 via the solder balls 7 made of tin and lead on the pad portions 2 of the package substrate 8 cleaned in the fourth step (FIG. 2D).

Embodiment 3 FIG. 3 is a schematic process diagram showing a third embodiment of the present invention, which will be described below with reference to FIG. First Step The electroless Ni / Au plating 3 was applied to the mounting component pad portion 2 of the motherboard 1 having the solder resist 4. Further, electrolytic Ni / Au plating 10 was applied to the bonding copper pad portion 2 for mounting components on the package substrate 8 (FIG. 3A). Second Step Solder paste containing organic acid lead and tin (Harima Chemical Co., Ltd.) is applied to each pad portion 2 on which platings 3 and 10 are applied in the first step, using a metal mask having a thickness of 100 to 300 μm. "Super Solder Paste SP"
"501" was printed by a screen printing method to form a solder precoat layer 5 (FIG. 3B). Third Step The motherboard 1 and the package substrate 8 on which the solder pre-coat layer 5 has been formed in the second step are subjected to 220 reflow furnace.
At about 2 ° C. to deposit solder 6 (FIG. 3).
(C)). Fourth Step After the motherboard 1 and the package substrate 8 obtained in the third step were washed with an organic solvent, they were further washed with hot water and water. Fifth Step The electronic components (package substrate) 8 cleaned in the fourth step are mounted on the pads 2 of the mother board 1 cleaned in the fourth step via solder balls 7 made of tin and lead (FIG. 3). (D)).

COMPARATIVE EXAMPLE 1 The same procedure as in Example 1 was carried out except that the solder precoat process was not performed.
Electronic components (package substrate) were mounted.

Comparative Example 2 The same procedure as in Example 1 was carried out except that the solder pre-coating treatment was not performed.
Electronic components (package substrate) were mounted. Further, after mounting, an underfill was processed between the motherboard and the package substrate (FIG. 4).

Test Example 1 In order to confirm the effects of the present invention in a mounted state, a package component was mounted on a motherboard via solder balls, and a cooling / heating cycle test was performed on the assumption that the product was used as a product. The thermal cycle test is -65 ° C (30 minutes) ⇔1
The number of cycles until the conduction resistance value abruptly increases due to cracks at the joint between the pad and the solder, comparing the conduction resistance value in each test state with 25 ° C. (30 minutes) as one cycle number. Were compared. The test condition is Product 1
As a product 2, solder pre-coating is applied to both the motherboard and the package as a product 2. As a comparative product 1, only plating is applied without applying a solder pre-coating to both the motherboard and the package. It went in the state of. Comparative Example 2 was performed in a state in which both the motherboard and the package were not subjected to the solder precoating process and only the plating process was performed, and an underfill was processed between the motherboard and the package substrate. Comparative Example 3 was performed in a state where the motherboard was subjected to a solder precoating process and an underfill was processed between the motherboard and the package substrate.
Table 1 shows the results.

[0030]

[Table 1]

As is apparent from Table 1, the number of cycles required for the product 1 in which the mother board was subjected to the solder pre-coating process to rapidly increase the conduction resistance value was 450 cycles. From this result, it was confirmed that the substrate was in a bonded state having high reliability.

The product 3 in which both the mother board and the package substrate were subjected to the solder pre-coating process had 550 cycles until the conduction resistance sharply increased, and particularly in a bonded state having excellent and high reliability. It was confirmed that there was.

It was confirmed that the comparative product 1 in which both the mother board and the package substrate were not subjected to the solder pre-coating process had the lowest reliability during the test.

The comparative product 2 using the underfill has a cycle number of 600 cycles before the conduction resistance value sharply increases, and is in a bonded state having high reliability.
The numerical values were almost the same as those of Examples 1 and 3.

As described above, it was confirmed that the technique for improving and stabilizing the bonding strength by the solder precoating process according to the present invention and the remarkable effectiveness of maintaining a stable mounting state without using an underfill were confirmed.

[0036]

According to the present invention, since the connection between the motherboard and the electronic components is strong by the solder precoating process and the mounted components do not fall off the motherboard, the connection reliability is extremely excellent without using an underfill. A component mounting board can be obtained.

In particular, if a solder precoat layer is formed using a solder paste containing organic acid lead and tin as main components,
In the reflow furnace process, since solder is gradually deposited on the pad portion of the metal surface, it is possible to solder in a flat state without generating a void even in a place where a dent exists, such as a BVH structure, Fine pitch solder deposition is also possible, and no bridge is generated even for narrow pitch designs.
A substrate with particularly excellent SP / BGA connection reliability can be provided.

Further, if Ni / Au plating is performed before the formation of the solder precoat layer, lead ions are generated during the solder deposition step. Therefore, during the solder deposition reaction, the lead layer is formed by ion exchange with nickel. Is formed, the lead layer suppresses the diffusion of nickel into the solder layer during hot sensation, and as a result, the segregation of phosphorus can be suppressed.
The connection between the connection portion of the solder ball and the conductive pad is particularly strengthened.

[Brief description of the drawings]

FIG. 1 is a schematic process diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic process diagram showing a second embodiment of the present invention.

FIG. 3 is a schematic process diagram showing a third embodiment of the present invention.

FIG. 4 is a schematic process diagram showing a second comparative example of the present invention.

[Explanation of symbols]

 1: motherboard 2: pad portion 3: electroless Ni / Au plating 4: solder resist 5: solder precoat treatment 6: solder 7: solder ball 8: electronic component (package substrate) 9: BVH pad portion 10: electrolytic Ni / Au Plating 11: Underfill

Claims (5)

[Claims] An electronic device without using an underfill, comprising: forming a solder precoat layer on a bonding pad portion for a mounting component of a motherboard, depositing a solder, and then mounting an electronic component on the motherboard. Component mounting method. 2. An underfill, which is characterized in that a solder precoat layer is formed on a bonding pad portion for a mounting component of a package component and then solder is deposited, and then the package component is mounted on a motherboard. How to mount electronic components. 3. A solder pre-coat layer is formed on bonding pad portions for mounting components of both the mother board and the package components, and after the solder is deposited, the package components are mounted on the mother board. An electronic component mounting method that does not use underfill. 4. The mounting of an electronic component without using an underfill according to any one of claims 1 to 3, wherein a plating process is applied to a bonding pad portion for a mounting component before forming the solder precoat layer. Method. 5. The electronic component without using an underfill according to claim 1, wherein the electronic component is mounted on a mother board via a solder ball after the solder is deposited. Implementation method.