JPH0555733A - Mounting method for electronic component on printed wiring board - Google Patents

Abstract

PURPOSE:To obtain a highly accurate printed wiring board having a sufficiently high holding force by removing a thickener or bonding agent from the flux of cream solder and using an aqueous emulsion bonding agent. CONSTITUTION:Cream solder (CH) 41 is put on a screen 37 with a prescribed pattern. A CH layer 35 is printed on wiring conductors 32 at places where electronic components 47 are mounted on the printed wiring board 31 by means of a squeegee 39. The CH 41 is adjusted to a low viscosity so that precise printing can be performed without causing cobwebbing by removing a viscosity adding bonding agent from the flux of the CH 41. A bonding agent 43 is fed between facing solder layers 35 from a dispenser nozzle 1 as liquid droplets. The components 47 are stuck to the substrate 31 so that the components 47 can bridge the solder layers 35. At the time of sticking the components 47, the external terminals 49 of the components 47 are stuck to the layers 35. The layers 35 have no adhesive strength, but the bonding agent 43 has a sufficiently high adhesive strength. After sticking the components 47, the entire body of the board 31 is heated including the electronic components 47.

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 electronic parts on a printed circuit board and soldering them.

[0002]

2. Description of the Related Art Methods for mounting a small electronic component on a printed circuit board are roughly classified into a flow (flow) solder method and a cream (reflow) solder method. In the flow soldering method, at a predetermined position on a printed circuit board with printed wiring, an adhesive is applied to the middle of the opposing wiring conductors, electronic components are temporarily attached or adhered on it, and then the entire board is melted to flow By making contact with the solder, electrical connection is made between the external terminal of the electronic component and the wiring conductor. On the other hand, in the cream solder method, a creamy solder containing an adhesive adhesive is applied in advance to a wiring conductor facing each other at a predetermined position on a printed wiring board by a printing method such as a screen printing method, and then the solder is applied. An electronic component is mounted and temporarily attached on the cream solder, and the entire printed circuit board is placed in a heating furnace in an air or N ₂ atmosphere to melt the cream solder and complete the soldering. In some cases, an adhesive may be applied between the opposing wiring conductors to compensate for the insufficient adhesive force.

[0003]

The cream solder method has been widely used because of its excellent workability and other well-known advantages. However, it also has its own drawbacks. In order to give tackiness for temporarily attaching electronic parts to the flux of cream solder, solid adhesives such as rosin, dammer, copal, rosin ester, indene, coumarone, hercoline, buton, xylene resin, epoxy resin It contains a flowable adhesive. However, as electronic components have become smaller and the spacing between wiring conductors has become narrower as in recent years, cream solder printing needs to be refined in order to prevent short circuits. Causes stringing at the time of printing and cannot perform accurate printing. On the contrary, if the amount of the adhesive is reduced, the holding force for the electronic component cannot be secured sufficiently.

As long as a cream solder containing a tacky adhesive is used, this problem does not change even if the adhesive is applied midway between the opposing conductors after printing the cream solder. Furthermore, as for the adhesive used in the conventional method, a method of applying an epoxy thermosetting adhesive or a solution adhesive is known, but the former requires a thermosetting step because it has no adhesive force immediately after application. However, this impairs the characteristics of the cream solder and is not desirable. The latter causes stringing at the time of application and adheres to the conductor portion, which is an obstacle to high precision.

Therefore, an object of the present invention is to achieve both precision printing of cream solder on a printed wiring conductor of a printed circuit board and provisional attachment or adhesion of electronic components with sufficient holding power and application of an adhesive with a high degree of precision. Another object is to provide a method for mounting an electronic component on a printed wiring board.

[0006]

According to the present invention, an adhesive-free cream solder is applied to opposing portions of a wiring conductor of a printed circuit board, and an aqueous emulsion adhesive is applied in the middle of the opposing conductor portions to form droplets. The electronic component is mounted on a printed wiring board, which is characterized in that the electronic component is adhered to the adhesive portion, and then the cream solder is melted by heating to be bonded to an external terminal of the electronic component. Is the way. According to the present invention, (1) printability can be improved by removing the tackifying component from the flux component of the cream solder. (2) The problem of stringing can be solved by separately applying an adhesive agent using an emulsion type adhesive agent. (3) The emulsion can be easily dried to naturally hold the electronic component. Examples of the present invention will be described below.

[0007]

1 to 3 illustrate the method of the present invention. 31
Is an insulating printed wiring board, and 32 is a wiring conductor formed in a predetermined pattern. First, FIG. 1 shows a solder printing process by a screen printing method. Cream solder 41
Is applied to a screen 37 having a predetermined pattern, and a squeegee 39 prints a cream solder layer 35 on the wiring conductor of the printed wiring board where the electronic component should be mounted.
The cream solder used has the tackifying adhesive removed from its flux and is adjusted to have a low viscosity so that precise printing can be performed without causing stringing.

FIG. 2 shows the next adhesive applying step. This is because stringing cannot be avoided with conventional adhesives. The adhesive is dispensed from the dispenser nozzle 1 as droplets 43 between the opposing solder layers 35. The adhesive used here must be an aqueous emulsion. Since the aqueous emulsion has a low viscosity, it can be precisely positioned at a predetermined position without stringing. The adhesive must have sufficient adhesive force to prevent the electronic parts from falling off or shifting even if the printed circuit board is inverted or heated, while it needs fluidity to correct the posture even if it shifts. .. Also, it is necessary to supply an efficient adhesive. There are various factors as the requirements for this, but the main ones are that the tackiness of the adhesive is sufficiently high, that the adhesive can be supplied in a sufficient amount that can be adjusted at one time and with good reproducibility, Good cutting (no stringing from the dispenser nozzle), etc.

As an adhesive satisfying these requirements, Japanese Patent Application Nos. 2-11925 and 2-2344, which are the prior applications of the present applicant, are available.
There are thermoplastic or thermosetting acrylic modified emulsion adhesives described in No. 21 and the like. If the adhesive is dried after being dispensed, the adhesive easily loses its water content and has a sufficient adhesive force. According to the same application, such an adhesive can be dispensed efficiently by using an adhesive supply device as shown in FIGS. The supply tank 2 of the adhesive 15 is maintained at a predetermined constant pressure so that the droplet of the adhesive is ejected from the injection nozzle 5 by opening the solenoid valve 4, and preferably the viscosity of the adhesive passing through the injection nozzle 5 is constant. In order to keep the temperature constant, the temperature is kept constant including the other parts. The adhesive 11 flows from the conduit 3 to the solenoid valve 4
Be led to. By controlling the opening time in the solenoid valve 4, a fixed amount of adhesive measured is discharged from the injection nozzle 5. The tip of the injection nozzle 5 when not in use is always wet with water to prevent it from drying. The tip of the injection nozzle is preferably coated with a water-repellent material such as a fluororesin in order to improve liquid drainage. The adhesive passes from the conduit 3 to the internal passage 17 of the housing 25.
After that, the valve chamber 19 is entered. The upper end of the injection nozzle 5 is formed as an outlet valve seat 26, and since the valve body 20 is always biased downward by the compression spring 21, the outlet valve seat 26 is normally closed. When the valve body 20 is pulled up by the electromagnetic coil 24, the valve body 20 allows the adhesive to flow to the injection nozzle 5, and the adhesive under pressure is made into droplets to eject the injection nozzle 5.
To release from. The valve body 20 is made of a soft magnetic material,
Similarly, the magnetic core 22 is screwed above the nut 2
It is fixed by 3. The electromagnetic coil 24 is a lead 27
Powered by. The power supply time is set to an arbitrary predetermined value by an adjuster (not shown), and the supply amount of one drop of adhesive required to temporarily fix the electronic component is adjusted arbitrarily.

The next step is the electronic component mounting step shown in FIG. 3. In this step, the electronic component 47 is mounted by an automatic mounting machine so as to bridge the opposing solder layers 35 and is bonded with an adhesive. At this time, the external terminals 49 of the electronic component come into contact with the solder layer. The solder layer does not have adhesive force, but the acrylic modified adhesive 43 shows sufficient adhesive force.

After that, the entire substrate and components shown in FIG. 3 are placed in a heating furnace (not shown) such as a known inert atmosphere (N ₂ ) and heated at a predetermined temperature for a predetermined time.

[0012]

As described above, according to the method of the present invention, since the thickener or the adhesive is removed from the flux of the cream solder, the solder layer pattern can be precisely printed, while the low viscosity aqueous emulsion adhesive is used. Since the electronic parts are attached to the substrate by using, the sufficient adhesive force can be obtained and there is no problem with stringing.

[Brief description of drawings]

FIG. 1 is a diagram showing a screen printing process of the method of the present invention.

FIG. 2 is a diagram showing an adhesive applying step of the method of the present invention.

FIG. 3 is a diagram showing an electronic component mounting process of the present invention.

FIG. 4 is a front view showing an adhesive supply device used in the adhesive applying step of the present invention.

5 is a sectional view showing details of a solenoid valve of the adhesive supply device of FIG.

[Explanation of symbols]

 31 printed wiring board 33 wiring conductor layer 35 cream solder layer 37 screen 43 adhesive 47 electronic component

Claims (3)

[Claims] 1. A solder paste containing no adhesive is applied to opposing portions of a wiring conductor of a printed circuit board, and an aqueous emulsion adhesive is applied in the form of droplets in the middle of the opposing conductor portions to form an electronic component. A method of mounting an electronic component on a printed wiring board, which comprises bonding to the adhesive portion and then heating and melting the cream solder to bond to an external terminal of the electronic component. 2. The method according to claim 1, wherein the aqueous emulsion adhesive is dried after coating, and the heating and melting of the cream solder are performed in a heating furnace in an inert atmosphere. 3. The method according to claim 1, wherein the aqueous emulsion adhesive is an acrylic modified thermoplastic adhesive or an aqueous emulsion of an acrylic modified thermosetting adhesive.