JP2795535B2 - Electronic component mounting method on circuit board - Google Patents

Description

The present invention relates to a method for mounting an electronic component on a circuit board.

[Prior art] Conventionally, when electronic components are surface-mounted on a circuit board,
First, a thin preliminary solder is formed on the circuit board pad by hot gas leveler method, etc. Then, a predetermined amount of cream solder is applied to each pad on which the preliminary solder is formed by a printing method, and electronic components are placed thereon. After that, the cream solder is melted by heating the whole in a reflow oven or the like, and the pads (including through holes for mounting electronic components; the same applies hereinafter) and the leads of the electronic components (including electrodes; the same applies hereinafter) are soldered. The method of attaching is taken.

〔Task〕

Recently, however, the lead pitch of electronic components has been reduced to 0.8 mm, 0.65 mm, and 0.5 mm due to the demand for lighter, thinner and smaller electronic devices.Further studies on 0.36 mm, 0.3 mm, and 0.15 mm have been advanced. ing. The conventional mounting method, in which cream solder is printed for each pad, can be used for lead pitches of up to about 0.5 mm, but below that, solder bridges tend to occur between adjacent pads, making it possible to mount components. Disappears. For this reason, there is a demand for the development of a mounting method that can handle a fine pitch.

In the conventional mounting method, after soldering is completed,
A process of cleaning cream solder residue with chlorofluorocarbon is necessary, but since the use of chlorofluorocarbon has become an environmental problem, we have developed a soldering method that can be cleaned without using chlorofluorocarbon or that does not require cleaning. Is desired.

[Means for solving the problem and its operation]

The present invention provides a mounting method of an electronic component which has solved the above-mentioned problems.

The present inventors previously applied a paste-like composition containing an organic acid Pb and Sn powder to a pad array portion of a circuit board, and by heating, selectively soldered a Sn-Pb alloy on a pad. A method for depositing a layer was proposed (Japanese Patent Laid-Open No. 1-157796). The present invention utilizes this method.

One electronic component mounting method provided by the present invention is:
On a pad array portion of a circuit board, a paste-like composition containing organic acid Pb and Sn powder is applied and heated, thereby selectively forming a preliminary solder of Sn-Pb alloy on the pad, Further applying a paste-like composition containing an organic acid Pb and Sn powder, a step of placing an electronic component thereon, and a pad and a lead of the electronic component by solder deposited from the paste-like composition by heating. Is soldered.

Conventional soldering is based on re-melting of solder,
The remelting of the solder occurs almost instantly as the temperature is increased, and the time required for the entire solder to melt is shorter than the time required for the solder to melt and spread on the pad. Is raised by the surface tension, causing the sag to exceed the limit of the surface tension, and as a result, the bridging is easily generated in the fine pattern.

The flux component in the cream solder liquefies when heated and flows between the pads. At this time, the solder particles flowing together melt instantly between the pads, causing a bridge.

On the other hand, the present invention relates to Sn caused by a difference in ionization tendency between Sn and Pb in a high-temperature solution containing Sn powder and organic acid Pb.
Precipitation of Pb into the system by substitution of Pb, and
It is based on the principle of Sn-Pb alloying by melting at the atomic level with Sn powder. Since the substitution reaction of Sn and Pb and the alloying of each Sn powder take time, it is compared with the prior art. Solder deposition occurs much more slowly. Furthermore, since the reaction is performed at a high temperature, on the pad surface, the Sn-Pb alloy solder deposited thereon immediately reacts with Cu, which is the material of the pad, to form intermetallic compounds such as Cu ₃ Sn and Cu ₆ Sn _5. Form.
Following this, Sn- deposited on this inter-alloy compound layer
The Pb alloy solder forms a solder layer while having a chemical bonding force with this compound layer.

The biggest difference from the conventional method is that, when forming the solder layer, in the conventional cream solder, as described above, each of the solder particles is melted instantaneously and all in a uniform time to form the solder layer. On the other hand, in the present invention, many fine solder particles having a variety of times until the solder is deposited on the pad are deposited on the pad, and then these solder particles in the vicinity are diffused with each other. Then, a solder layer is formed on the pad. That is, the solder layer is formed more slowly than in the case of the conventional method.

In addition, in the case of the present invention, even if solder particles are deposited between the pads, such particles are instantaneously melted and difficult to be integrated like conventional cream solder because the particles have a variety of structures.

From the above points, according to the method of the present invention, a bridge is hardly generated between pads.

Also collided with Sn powder near or between pads
The Pb forms a Sn-Pb alloy at the Sn powder level, where the Pb is attracted to the solder portion deposited on the pad and absorbed on the pad. In this way, even at a fine pitch of 0.5 mm or less, soldering can be performed without causing a bridge.

When the solder is deposited from the paste-like composition, when the paste-like composition becomes liquid by heating, a liquid pool in which the Sn powder has settled on the pad array portion is generated, and the Sn powder is formed by the liquid. With it covered
It is desirable that the solder deposition reaction proceed. This is because, when the paste-like composition is heated, the viscosity is significantly reduced, and when the viscosity is too low, the liquid greatly spreads in the vicinity of the pad arrangement portion, and as a result, the amount of the organic acid Pb with respect to the Sn powder is reduced. This is because there is a shortage and a sufficient amount of solder cannot be obtained. Although a little spread is acceptable, a lot of Sn powder settles on the pad array part, and keeping the state that the liquid melted over it forms a relatively thick solder layer with a uniform thickness Is the point. After forming the preliminary solder layer in this way, an organic acid Pb is further added on the preliminary solder layer.
Then, a paste-like composition containing and a Sn powder is applied, and then the electronic component is placed thereon, and the pad is soldered to the lead of the electronic component by heating.

Another electronic component mounting method provided by the present invention is to apply a paste-like composition containing an organic acid Pb and Sn powder to a pad array portion of a circuit board, and heat the paste-like composition to selectively form on a pad. Forming a solder layer of an Sn-Pb alloy in an amount necessary for mounting the electronic component, applying flux to at least one of the solder layer and the lead surface of the electronic component, and placing the electronic component thereon. And melting the solder layer by heating, and soldering the pads and the leads of the electronic component.

In this method, the method of forming the solder layer is the same as the method described above, but first, an amount of solder necessary for soldering is placed on the pad, and the lead of the electronic component is soldered with the solder. Therefore, the process can be shortened as compared with the case where the preliminary solder layer is formed. In this case, if a non-halogen type flux is used as the flux, cleaning after soldering becomes unnecessary, or at least it can be easily cleaned with a cleaning agent other than Freon. Therefore, it can contribute to the prevention of ozone layer depletion due to chlorofluorocarbon which is a problem today.

Further, when a pad arrangement portion having a fine pitch of 0.5 mm or less and a pad arrangement portion having a coarser pitch are mixed on the same circuit board, the following mounting method can be adopted.

First, a solder layer having a thickness of, for example, several tens of μm required for soldering is formed on the pad arrangement portion having a fine pitch, and about 5 to 10 μm as a preliminary solder is formed on the pad arrangement portion having a coarse pitch. Is formed. Cream solder is printed on the pad array portion to which the preliminary solder has been applied, and the electronic component is temporarily fixed by the adhesive force. Thereafter, the solder is melted by heating in a reflow furnace or the like, and soldering is performed. on the other hand,
A flux is applied to the pad array portion on which the solder having a thickness of several tens of μm required for soldering is applied, and the electronic component is temporarily fixed by the adhesive force, and soldering is performed through a reflow furnace or the like.

Also, since the area and arrangement pitch of the pads are different on the same circuit board, the required thickness of the solder layer may differ greatly depending on the individual pad arrangement portions. In this case,
If a different amount of the paste composition according to the required thickness is applied to each pad array portion and heated, a solder layer having a different thickness can be formed on each pad array portion.

On the other hand, the solder layer formed to have a thickness of several tens of μm may have a bump shape as shown in FIG. In such a case, even if the lead of the electronic component is placed on the solder layer 3, it cannot be temporarily temporarily fixed only by the adhesive force of the flux. That is, there is a case where the displacement between the pad and the lead cannot be completely prevented. In FIG. 1, 1
Is an insulating substrate, and 2 is a pad.

To avoid this problem, after depositing a thick solder layer, for example, a solder layer having a thickness of several tens of μm, on a pad having a small area or a small pitch, the solder layer is pressed from the upper surface to flatten the upper surface of the solder layer. It is good to take measures to do. At this time, a method of applying pressure while heating is also effective depending on the thickness and width of the solder layer. The heating temperature at this time varies depending on the composition of the deposited solder, but is preferably in a temperature range lower by 20 to 0 ° C. than the deformation temperature of the deposited solder. The pressing force and the holding time are appropriately selected depending on the thickness and the area of the solder layer. With such a simple process, it is possible to prevent the displacement of the components.

〔Example〕

 Hereinafter, embodiments of the present invention will be described.

(1) Example of the invention of claim 1 Organic acid Pb 46%, activator 18%, Sn powder 25%, viscosity modifier 11
% (% By weight) to obtain a paste composition having a viscosity of 280,000 to 290,000 cps (centipoise). This paste-like composition, pad size 0.26 × 2mm, pad arrangement pitch 0.3
A 6 mm, 284-pin QFP mounting pad array was solid-coated to a thickness of 0.5 mm. Thereafter, the circuit board was heated at 215 ° C. for 2 minutes. As a result, the preliminary solder formed on the pad had an average of 9 to 10 μm, the variation σ = 1 to 2 μm, the solder composition was Sn 64 to 69% by weight, and no bridge was generated.

Next, an organic acid is placed on the pad
A paste composition composed of 40% Pb, 33% activator, 18.4% Sn powder, and 8.6% viscosity modifier was solid-coated, and a lead pitch 0.36 mm, 284-pin QFP was temporarily fixed thereon. This was passed through a reflow furnace and heated under the conditions of 215 ° C. × 30 seconds and 220 ° C. × 2 minutes to deposit solder and perform soldering.

As a result, it was possible to obtain a good state of soldering with the lead of the pad without a bridge.

(2) Embodiment of Claim 2 The following three types of components are mounted.

QFP: Lead pitch 0.65mm 100 pins Overall dimensions 20 × 13mm Chip parts: Overall dimensions 4.2 × 7mm SOP: Lead pitch 0.8mm 20 pins Overall dimensions 9 × 8mm The circuit board used is a 200 × 250mm glass epoxy board Pads corresponding to the above components are formed. Pad dimensions for QFP are 0.5 x 2 mm, pad dimensions for chip components are 1.5 x 1.5 mm, and pad dimensions for SOP are 1.2 x 0.8 m
m.

The paste composition used is organic acid Pb 40%, activator
It is a mixture of 33%, Sn powder 18.4% and viscosity modifier 8.6%.

This paste-like composition is applied to one pad pattern.
Solid painting at a rate of 0.5 to 0.9 g, and then passing through a reflow furnace using a far-infrared heater as a heating source, 215 ° C x 30 seconds, 220
Heating was performed under the conditions of ° C. × 2 minutes to deposit solder. As a result, a solder layer of 74% by weight of Sn and a thickness of 30 μm was formed.

Next, a weakly active flux having an aqueous solution resistance of 100,000 Ω · cm or more (micro solder: F-40 Harima Chemicals, Inc.)
Was applied, and the components were placed thereon, heated in a reflow furnace, and the solder was melted. The leads of each component were soldered to pads.

As a result, a soldered state free of bridges and solder balls was obtained. Also, after soldering, without washing, the presence or absence of ionic residue on the circuit board was measured using an omega meter (model 600, manufactured by KENKO, USA). The result was 0.1 to 0.3 μg NaCl / sq.in. Very good.

In this embodiment, unlike the conventional cream solder, the amount of solder deposited depends on the amount of Sn powder settled on the pad and its vicinity. Therefore, there is a relationship between the thickness H (maximum value) of the solder layer deposited on the pad and the width W of the pad (see FIG. 1).

As a result of various studies, the present inventors have found a relationship of H = aW between the two, as shown in FIG.
It has been found that when 05 <a <0.5, preferably 0.1 ≦ a ≦ 0.3, an optimum solder layer for component mounting can be formed.

Here, if 0.05> a, constriction (part where the amount of solder is extremely small) occurs in a part or a plurality of places of the deposited solder layer, and it becomes difficult to form a fillet necessary for component mounting.

On the other hand, when a> 0.5, projections are formed on a part or a plurality of portions of the deposited solder layer, and when mounting the components, the leads of the components are easily disturbed by the projections, which may cause misalignment. There is a problem that it is difficult to do.

An example in which a solder layer that satisfies the condition of 0.1 ≦ a ≦ 0.3 in the above relational expression is formed and a component is mounted by applying a flux on the solder layer will be described below.

First, a QFP having a lead pitch of 0.5 mm and 288 pins as a component to be mounted, and a circuit board having pads having a width of W = 270 μm and a length of 1 mm were prepared for mounting the QFP. Next, the thickness H is formed on this pad by using the method of claim 2 described above.
A 56 μm solder layer was formed. The composition of this solder layer is Sn 68
~ 71% by weight.

Subsequently, a flux having the physical properties shown in Table 1 was applied to the solder layer in a thickness of about 50 μm, and the 288-pin QFP leads were fixed thereon. The coating thickness of the flux is preferably 10 μm or more and 150 μm or less. In particular, 50 to 100 μm is optimal.

The reason is that if it is less than 10 μm, it is difficult to apply a uniform thickness, and if it is more than 150 μm, the mounted electronic component is easy to move. That is, the thickly applied flux is easy to move, and accordingly, the mounted electronic component is also easy to move.

In addition, a thermoplastic flux with a glass transition point of 60 to 65 ° C is used for this flux in consideration of the repairability of electronic components (easiness of re-mounting if mounting fails). did.

Thereafter, this was heated under the heating conditions shown in FIG.
The component leads were soldered to the pads. The width of the lead of the mounted component is 180 μm.

The joint strength of the solder portion was examined for the thus soldered product. Table 2 shows the results. As shown in this table, very good joining results were obtained.

By the way, when the electronic component is mounted by the method of the first aspect of the present invention, the temperature for forming the preliminary solder layer can be lower than that in the case of using the conventional cream solder. This is particularly significant when the preliminary solder layer is formed using a Pb-rich high melting point solder.

For example, a PGA packaged in plastic
In a (pin grid array) or multi-chip module, when an IC chip mounted in a package is bonded to a circuit in the package with bumps formed by Pb-rich solder, the circuit is formed rather than forming bumps on the IC chip side Forming the bumps on the part side enables manufacturing without affecting the yield of the IC chip, so that an advantage that manufacturing cost is reduced can be expected. However, the method of forming bumps on the circuit board side requires a high heat-resistant board material because the circuit board must be able to withstand the formation of a Pb-rich solder layer with a high melting point, and the total cost is high. Become.

In the conventional method using cream solder, when soldering, it is necessary to raise the temperature by 30 to 60 ° C. higher than the melting point of the solder. This is because diffusion of the solder and copper of the pad must be performed in a short time at the same time as melting the solder. On the other hand, in the method of the present invention, by appropriately selecting the composition ratio of the organic acid Pb and the Sn powder in the paste-like composition, almost the same solder deposition is performed regardless of the Sn / Pb ratio of the completed solder layer. It can be done at temperature. For example, in the case of a Pb-rich high melting point solder, the solder layer can be formed at a temperature considerably lower than the melting point of the deposited solder. Also, at the time of forming the solder layer, since the intermetallic compound has already been formed by diffusion between the solder and copper which is the material of the pad, after that, when soldering using the solder layer Does not require energy for forming an intermetallic compound by post-heating, such as fusion, which is required in a conventional electroplating method or the like. Therefore, the process can be shortened, which contributes to cost reduction.

Hereinafter, an example in which a Pb-rich solder layer is formed will be described.

(1) A paste composition composed of 67% of organic acid Pb, 10% of activator, 13% of Sn powder, and 10% of viscosity modifier is placed on a copper pad with 400%.
It was applied to a thickness of about 500 μm and heated at 220 ° C. for 2 minutes to form a solder layer. The thickness of this solder layer was 8 to 10 μm, the composition was Sn / Pb = 2/8, and the melting point was 293 ° C. On the other hand, when the solder layer is formed using a conventional cream solder,
It is necessary to expose the substrate in molten solder (Sn / Pb = 2/8) maintained at 293 ° C. or higher, which is much higher than 220 ° C. of the present invention. As a result, the reliability of the substrate is greatly impaired.

(2) A paste composition composed of 55% of organic acid Pb, 15% of activator, 13% of Sn powder, and 17% of viscosity modifier is placed on a copper pad with a thickness of 300%.
It was applied to a thickness of about 400 μm and heated at 220 ° C. for 2 minutes to form a solder layer. The thickness of this solder layer was 4 to 8 μm, the composition was Sn / Pb = 3/7, and the melting point was 278 ° C. In the conventional method using a cream solder, it is necessary to heat to about 320 ° C. in the case of this solder composition.

As the organic acid Pb of the paste-like composition, naphthenic acid Pb, rosin acid Pb, octylic acid Pb, oleic acid Pb, stearic acid Pb, and the like are used.
Alkanolamine, amine acid and the like can be used, and as a viscosity modifier, caster wax, cellulose powder, butyl carbitol, hexyl carbitol, squalene and the like can be used. The Sn powder is a spherical powder made by an atomizing method with a particle size of 80 μm or less, preferably 25 μm or less, and has an oxygen content of 1500 ppm or less, preferably 100 ppm or less.
It is good to use the thing of 0 ppm or less.

〔The invention's effect〕

As described above, according to the first aspect of the present invention, it is possible to mount electronic components without generating a bridge in a pad arrangement portion having a fine pitch of 0.5 or less. Can contribute.

According to the second aspect of the present invention, the process can be shortened as compared with the first aspect of the present invention, and further, as an application example, the electronic component can be mounted with a non-halogen type flux. Effective in solving.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a state in which a thick solder layer is formed on a pad of a circuit board, FIG. 2 is a graph showing a relationship between a pad width and a thickness of a solder layer, and FIG. It is a graph shown. 1: Insulating board, 2: Pad, 3: Solder layer

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hisao Irie 423 Kamizume, Yoneda-cho, Takasago City, Hyogo Prefecture (58) Field surveyed (Int.Cl. ⁶ , DB name) H05K 3/34 B23K 35 / 22,35 / 34

Claims (3)

(57) [Claims] An organic acid Pb and Sn are provided on a pad array portion of a circuit board.
A step of selectively forming a preliminary solder of a Sn—Pb alloy on a pad by applying and heating a paste-like composition containing powder and a lead (including an electrode on the preliminary solder or an electronic component. The same) a step of applying a paste-like composition further containing an organic acid Pb and Sn powder to at least one of the surfaces, a step of placing the electronic component thereon, and a solder deposited from the paste-like composition by heating. A method for mounting an electronic component on a circuit board, comprising a step of soldering a pad and a lead of the electronic component. 2. An organic acid Pb and Sn are provided on a pad array portion of a circuit board.
By applying a paste-like composition containing powder and heating, the amount of Sn required for electronic component mounting can be selectively selected on the pad.
A step of forming a solder layer of a Pb alloy, a step of applying a flux to at least one of the solder layer and the lead surface of the electronic component, a step of placing the electronic component thereon, and heating the solder layer by heating. A method for mounting an electronic component on a circuit board, comprising a step of melting and soldering a pad and a lead of the electronic component. 3. The method according to claim 2, wherein the width of the pad is W,
When the thickness of the solder layer formed on the pad is H,
The relationship between W and H is H = aW, where 0.05 <a <0.
5. A method for mounting an electronic component on a circuit board, which preferably satisfies 0.1 ≦ a ≦ 0.3.