JP2008277360A - Method for mounting electronic part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To conduct a junction without a defective soldered joint in a soldered joint on a wiring board using cream solder. <P>SOLUTION: Dripping heated flux when solder melts is made to escape to holes 9 and its spread is prevented by forming the holes 9 in a region held by adjacent joining terminal sections 2a and 2b. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electronic component mounting method for obtaining an electronic circuit board by mounting electronic parts such as an IC chip, a chip capacitor, and a piezoelectric element on a wiring board.

  2. Description of the Related Art Conventionally, an electronic component having a lead terminal that spreads in a gull wing shape around a package is known as a bonding electrode for bonding to an electrode of a wiring board. For example, QFP (Quad Flat Pack). In this type of electronic component, in order to increase the number of lead terminals (number of I / Os) while maintaining the strength of the lead terminals spreading around the package to some extent, the package size must be increased.

  Therefore, in recent years, electronic components of the lower surface electrode type in which a plurality of bonding electrodes are arranged in an array on the lower surface of the package are becoming mainstream. For example, LGA (Land Grid Array), BGA (Ball Grid Array), CSP (Chip Size Package), and the like. In these electronic components, the number of I / Os can be increased without increasing the size of the package by arranging flat bonding electrodes in an array on the lower surface of the package.

  Such a bottom electrode type electronic component is generally mounted on a wiring board using cream solder. Specifically, first, cream solder is printed on each of the plurality of substrate electrodes in the wiring pattern of the wiring substrate by a printing method using a printing mask. Next, the electronic component is placed on the cream solder printed on the wiring board. In the reflow process, the wiring board is put into a reflow furnace together with the electronic components and heated to melt the solder grains as the conductive bonding material in the cream solder. Thereafter, the molten solder is solidified by cooling, thereby joining the substrate electrode on the wiring board and the joining electrode of the electronic component.

  In such an electronic component mounting method, if the coefficient of thermal expansion differs between the material of the electronic component and the material of the wiring board, the relative position of the electrodes in the reflow process accompanied by heating shifts, causing a misalignment of the bonding position. There is a risk that. In recent years, where finer pitch bonding tends to be required as electronic devices become smaller, a slight difference in coefficient of thermal expansion is expected to appear as a large misalignment in the bonding position. The

As another conventional electronic component mounting method, an electronic component mounting method is known in which only cream solder is melted by irradiating a laser beam to perform soldering. According to this electronic component mounting method, the electronic component and the wiring board are not heated as a whole, but only the electrode and the surrounding area are heated locally. Can be suppressed.
JP 2006-303356 A

  However, even in an electronic component mounting method in which soldering is performed by irradiating a laser beam to melt only cream solder, there is a risk of causing a bonding failure between the electronic component and the wiring board or causing a solder failure.

  In general, cream solder is composed of fine particles of solder balls and flux, and the solder balls are dispersed in the flux. The purpose of the flux is to improve applicability and remove the oxide film on the joint surface. Flux has a high viscosity at room temperature, and when cream solder is applied, it has a viscosity that can maintain the shape when applied.

  As shown in FIG. 8A, when the cream solder 3 is attached to the joining terminal portions 2a and 2b provided adjacent to the wiring board 1 and this cream solder 3 is heated with the laser beam 4, first, the cream solder 3 The viscosity of the flux inside drops rapidly, and the flux spreads and spreads from the joining terminal portions 2a and 2b to which the cream solder 3 is attached.

  At this time, the spread flux 5 may spread to the adjacent joining terminal portions 2a and 2b as shown in FIG. 8B. In this case, when the flux 5 spreads, the solder particles 6 in the cream solder 3 are also attracted by the flow of the flux 5 and spread to the adjacent junction terminal portions 2a and 2b, as shown in FIG. As shown, the solder particles 6 are melted between the joining terminal portion 2a and the joining terminal portion 2b, and a portion that should originally be opened is short-circuited by the solder 7 and becomes defective.

  Even if it is not short-circuited, as shown in FIG. 8 (c), when the solder 7 is melted, the temperature of the laser irradiation portion rapidly increases, and the cream solder that has spread once becomes a high temperature region with melting. If there is unmelted cream solder in the adjacent joint terminal portion 2a or joint terminal portion 2b, the solder paste is drawn so as to be attracted by the aggregation of cream solder once spread. In such a case, as seen in the junction terminal portion 2a of FIG. 8D, a junction terminal portion without (or extremely few) solder 7 is generated and an electronic component (not shown) is placed. Will cause poor mounting.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an electronic component mounting method and a wiring board that can suppress the occurrence of defective soldering and poor bonding between the electronic component and the wiring board.

  In the electronic component mounting method of the present invention, when the electronic component is mounted on the substrate by heating and melting the cream solder attached to the joining terminal portion provided adjacent to the wiring substrate, the electronic component is adjacent to the wiring substrate. At least one or more holes are formed in advance in the region sandwiched between the joint terminal portions provided as described above, and the cream solder is heated by heating and flows from the application region of the joint terminal portion. It is prevented from flowing into the hole and flowing out toward the opposite junction terminal portion.

  The wiring board according to the present invention is characterized in that a hole is formed in a region sandwiched between adjacent joint terminal portions to release the flux that has spread from the application region of the joint terminal portion.

  According to this configuration, the flux that has begun to spread due to heat dripping due to heating reaches the hole provided between the joining terminal portions before entering the adjacent joining terminal portions. Since the flux that has reached this hole is in a high temperature state, its viscosity is reduced. Therefore, it does not collect and spread in the hole due to capillary action.

The electronic component mounting method of the present invention will be described below based on specific embodiments.
(Embodiment 1)
1 and 2 show Embodiment 1 of the present invention.

1A and 1B show a wiring board used for carrying out the electronic component mounting method of the present invention.
When an electronic component is to be mounted using the cream solder 3 between the junction terminal portion 2a and the junction terminal portion 2b of the wiring board 1, the wiring terminal 1 has the junction terminal portions 2a, A penetrating hole 9 is formed on a line 8 that bisects between adjacent joint terminal portions 2a and 2b in a region sandwiched by 2b. The number of holes 9 may be at least one.

  Specifically, the joining terminal portions 2a and 2b have a size of about 0.5 mm square and are provided adjacent to each other with an interval of about 0.5 mm. Eight holes 9 having a diameter of 0.2 mm are provided.

  When mounting an electronic component, as shown in FIG. 2A, about 1 mg of cream solder 3 is applied to one of the joint terminal portions 2a and 2b. Then, heating for melting the cream solder is performed. At this time, since the soldering is performed using cream solder, a non-contact heating method is desirable. In this embodiment, the cream solder 3 was melted by heating using the laser beam 4. At this time, since rapid heating causes bumping of the cream solder 3, stepwise heating is desirable. In this embodiment, preheating and main heating are provided. In the preheating, laser light 4 having a power of 1 W is irradiated for about 3 seconds, and then laser light 4 having a power of 4 W is irradiated for about 3 seconds as main heating. ing.

  As shown in FIG. 2 (b), the cream solder 3 drips and spreads from the applied region at the time of preheating, but when the flux 5 reaches the hole 9, the flux 5 becomes a capillary tube of the hole 9. By filling the hole 9 due to the phenomenon, the flux does not flow out to the adjacent joining terminal portions 2a and 2b, and at the time of the main heating, the solder is melted and the electronic component can be mounted by soldering.

  Further, since a flux having a boiling point lower than the melting point of the solder can be accumulated in the vicinity of the junction terminal portions 2a and 2b, the heat capacity is locally increased. By doing so, since the tolerance with respect to heat histories, such as reheating in another process, improves, it can prevent solder joint failure.

  In this way, it is possible to create a state in which the solder 7 remains evenly on the joining terminal portions 2a and 2b as shown in FIG. Therefore, the electronic component can be favorably mounted between the joining terminal portions 2a and 2b by pressing the electronic component against the molten solder 7 on the joining terminal portions 2a and 2b.

  In addition, although the diameter of the hole 9 was 50-250 micrometers, and the favorable effect was confirmed, since the flux permeates to the depth of a hole by making it 0.2 mm, the quantity of the flux 52 which can be removed is reduced. Can be maximized. Moreover, the total amount of the flux which can be removed is controlled by making the number of the holes 9 into a plurality, and the total opening area of the holes 9 with respect to the total area on the joining terminal portion 2a and the joining terminal portion 2b. By setting it to about 30%, it is possible to remove only the flux that reaches the adjacent joint terminal portion.

The reason why 0.2 mm is the best will be described with reference to FIGS.
As shown in FIG. 3, when the cream solder 3 is applied to one of the joint terminal portions 2b and the applied cream solder 3 is irradiated with the laser beam 4, the joint terminal portion 2b to the joint terminal portion 2a are indicated by 5a. In order to prevent the flux from flowing and spreading, a part of the hole 9 flows into the hole 9 to leave the flux on the wiring board 1 as shown by 5b. When the hole 9 has a diameter of 50 to 200 μm, the hole diameter and the hole When the relationship of the penetration depth of the flux which penetrates into No. 9 was measured, the result shown in FIG. 4 was obtained. As the diameter of the hole 9 increases, the penetration depth also increases. When it reaches a peak at 200 μm and has a diameter larger than 200 μm, the penetration depth decreases.

  Next, the result shown in FIG. 5 was obtained for the relationship between the hole diameter and the amount of flux filled in the hole 9. The amount of flux filled corresponds to the amount of flux removed. When the diameter is larger than 200 μm, there is no significant difference in the amount of flux to be filled. However, when the hole diameter is increased, the strength of the substrate is lowered, so that the hole diameter of 200 μm is the most optimal.

The grounds for setting the total opening area of the holes 9 to about 30% of the total area on the joining terminal portion 2a and the joining terminal portion 2b are as follows.
When 0.1 mg of paste solder is applied to the 0.5 mm square joint terminal portions 2a and 2b, the metal contained in the paste solder is about 0.85 mg and the volume is about 15 × 10 ⁻³ mm ^3. . When the volume ratio of the flux contained in the paste solder is 50%, the flux is also included in the order of about 15 × 10 ⁻³ mm ³ . When this flux spreads in all directions so that it reaches the adjoining junction terminal part due to heat dripping, the hole 9 provided in the middle of the adjoining junction terminal part must be filled with about half of it. When considered, a hole that can be filled with a flux of about 15 × 10 ⁻³ mm ³ must be provided. When the holes 9 having a diameter of 200 μm are provided, it is necessary to provide eight holes having a diameter of 200 μm, assuming that one hole 9 is filled with the flux to a depth of 2 mm. At this time, the surface area of the hole 9 is about 30% with respect to the total area of two adjacent joining terminal portions.

(Embodiment 2)
6 (a) and 6 (b) show a second embodiment of the present invention.
In FIG. 6, the junction terminal portions 2 a and 2 b are formed adjacent to each other on the wiring board 1. The holes 9 are arranged in two rows on the lines 8a and 8b that divide the region sandwiched between these joint terminal portions into three equal parts. Since the heating method for melting the cream solder 3 is the same as that in the first embodiment, a description thereof will be omitted.

  In this embodiment, the joining terminal portions 2a and 2b are provided so as to be adjacent to each other with a size of about 0.5 mm square and an interval of about 0.5 mm. Further, a total of eight holes of 0.2 mm in diameter penetrating the wiring board 1 are provided in two rows in a region sandwiched between the junction terminal portions 2.

  When either one of the adjacent joining terminal portions 2a and 2b is heated to melt the cream solder 3 and then the other joining terminal portion is heated to melt the cream solder, the joining terminal portion is as in this embodiment. It is better to arrange the through holes in two rows between them.

This will be described in detail.
By heating one of the joining terminal portions, the flux that has caused the heat dripping is filled in the holes 9, and the heating is stopped, the temperature of the flux is lowered, and the viscosity is increased. After that, even when the flux spreading due to the heat reaches the position of the through hole when the other joint terminal portion is heated, if the holes 9 are in one line, the flux with increased viscosity is filled. In addition, the flux does not flow into the hole 9 due to the capillary phenomenon, and the flux remains excessively, resulting in poor solder joints.

  In the case where the holes 9 are arranged in two rows, the flux that has been heated by heating one of the joint terminal portions is preferentially filled into the holes 9 arranged on the side close to the heated joint terminal portion. The flux 9 does not enter the holes 9 arranged on the far side. Thereafter, the flux that has been heated by heating the other joint terminal portion is filled in the through-hole that is not filled with the flux, so that excess flux can be removed and solder joint failure can be prevented.

  Further, since a flux pool having a boiling point lower than the melting point of the solder can be formed in the vicinity of the joint, the heat capacity is locally increased. By doing so, since the tolerance with respect to heat histories, such as reheating in another process, improves, it can prevent solder joint failure.

  In each of the embodiments described above, the laser beam 4 is irradiated from the surface side of the wiring board 1. However, when the wiring board 1 is formed of a laser light transmissive material, as shown in FIG. The same applies to the case where the solder solder 3 is melted by irradiating the joining terminal portions 2a and 2b with the laser beam 4 from the back surface side of the wiring substrate 1 through the wiring substrate 1.

  The present invention can contribute to improving the reliability of various electronic circuit boards in which electronic components such as IC chips, chip capacitors, and piezoelectric elements are mounted on a wiring board.

The front view and side view of the wiring board in Embodiment 1 of this invention Explanatory drawing of the effect of the hole of the embodiment Explanatory drawing of experimental device of hole diameter, flux penetration amount and flux filling amount of the same embodiment Measurement result diagram of hole diameter and flux penetration of the same embodiment Measurement result diagram of hole diameter and flux filling amount of the same embodiment Front view and side view of wiring board according to embodiment 2 of the present invention Front view of a wiring board for explaining another embodiment Mounting process diagram explaining the problem

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wiring board 2a, 2b Joint terminal part 3 Cream solder 4 Laser beam 9 Hole

Claims (7)

When mounting the electronic component on the board by heating and melting the cream solder attached to the joint terminal portion provided adjacent to the wiring board,
At least one or more holes are formed in advance in a region sandwiched between joint terminal portions provided adjacent to the wiring board,
An electronic component mounting method that prevents the cream solder from dripping due to heating and causing the flux that has flowed and spread from the application region of the joint terminal portion to flow into the hole and to flow toward the opposite joint terminal portion. The electronic component mounting method according to claim 1, wherein the hole is arranged on a line that bisects between adjacent joint terminal portions. The electronic component mounting method according to claim 1, wherein the holes are arranged in two rows on a line that equally divides between adjacent joint terminal portions. The electronic component mounting method according to claim 1, wherein the total opening area of the holes is set to 30% or more with respect to the total area of the adjacent joining terminal portions. The electronic component mounting method according to claim 1, wherein the hole has a diameter of 50 to 250 μm. A wiring board in which a hole is formed in a region sandwiched between adjacent joint terminal portions provided to release the flux that has spread from the application region of the joint terminal portion. The wiring board according to claim 6, wherein an interval between adjacent joining terminal portions is about 0.5 mm, and a diameter of the hole is 50 to 250 μm.

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