Cleaning-free activated resinous composition and method for surface mounting using the same   

BACKGROUND OF THE INVENTION

The present invention relates to an activated resinous composition useful for mounting of flip-chip or the like and a method for surface mounting using this activated resinous composition.

Conventionally, surface mounting technologies for various parts such as BGA parts have been carried out by a method comprising the steps of fluxing a printed wiring board; mounting BGA parts on the printed wiring board; reflow soldering; and cleaning and removing flux; filling space between the printed wiring board and the BGA parts with an underfill resin and hardening the underfill resin. The flux containing a compound having a carboxylic acid such as a rosin resin as an activator is well known (PATENT DOCUMENT 1, claim 2).

Recently, the number of chips mounted on the BGA part tends to increase for functional advancement and a body size of the BGA part correspondingly tends to grow in size.

However, having grown in size, the BAG part itself may interface with the desired cleaning effect and leave unremoved flux (flux residue) behind in the step of cleaning and removing flux. Consequently, there has been possibility that the activator ingredient contained in flux residue might cause corrosion reaction in the subsequent step of heat hardening of the underfill resin.

To overcome such problem, cleaning-free flux (flux free from requirement of cleaning) characterized in that the activator has active force sufficiently low to restrict possibility of corrosion has already been proposed (PATENT DOCUMENT 2). However, when such cleaning-free flux is used, this cleaning-free flux itself may generate cracked gas in the step of heat hardening the underfill resin and, in consequence, destroy the BGA part.

With the BGA part having grown in size, joints in the BGA part may interfere with a desired filling effect in the step of filling the underfill resin. Particularly when there are surface irregularities (e.g., circuit irregularities and/or solder mask irregularities) on the surface of the printed wiring board, it is often impossible to fill every hole and corner of the irregularities completely with the underfill resin and, as a consequence, voids and/or unfilled space may leave behind, significantly deteriorating quality and reliability of the product. Furthermore, if deficiency such as void is overlooked and the subsequent step of hardening the underfill resin, it will be no more possible to repair the product and such faulty product must be scrapped. This leads directly to reduction of yielding percentage.

SUMMARY

In view of the problem as has been described above, it is an object of the present invention to provide an activated resinous composition improved so as to produce effects as will be described below.

1) In a method for surface mounting, a step of flux cleaning can be eliminated not only to reduce a manufacturing cost but also at to improve the productivity.

2) Neither gas bubble nor void space is present in a coated resinous layer after hardened and thus reliability of the product can be improved.

3) The coated resinous layer after hardened exhibits sufficiently high thermal stability to eliminate an apprehension that the coated resinous layer might cause corrosion reaction and/or generate cracked gas on heating (e.g., in the step of heat hardening the underfill resin).

4) Filling of the underfill resin can be facilitated. Consequentially, even when a BGA part having a large body size is mounted, there is no possibility that gas bubble, void or the other unfilled space might be left in regions filled with the underfill resin and hardened. In this way, reliable joint (adhesion) can be assured and reliability of the product can be improved.

The object set forth above is achieved by the present invention developed by the inventor on the basis of experimental findings.

The present invention on a first aspect thereof provides an activated resinous composition containing, on the basis of an epoxy resin being solid at a room temperature of 100 parts by weight, a carboxylic acid compound of 1 to 10 parts by weight, a hardening agent of 1 to 30 parts by weight, a hardening reaction initiation temperature of the hardening agent being 150° C. or higher, and a solvent of 10 to 300 parts by weight.

The present invention on a second aspect thereof provides a method for surface mounting comprising the steps of coating at least soldered surface of a printed wiring board with the activated resinous composition defined by the first aspect of the present invention, loading the printed wiring board with a part to be surface mounted, subjecting this to reflow soldering and heat hardening the coated resinous layer.

The present invention on a third aspect thereof provides a method for surface mounting further including a step of drying and/or heating the coated resinous layer at a temperature corresponding to a softening point or higher but lower than the hardening reaction initiation temperature before the printed wiring board is loaded with the part to be surface mounted.

The present invention on a fourth aspect thereof provides a method for surface mounting further including a step of filling and hardening the underfill resin after the coated resinous layer has been heat hardened.

The activated resinous composition according to the present invention may be used to obtain the effects as will be described below.

1) In the method for surface mounting, a step of flux cleaning can be eliminated not only to reduce a manufacturing cost but also at to improve the productivity.

2) Neither gas bubble nor void space is present in the coated resinous layer after hardened and thus reliability of the product can be improved.

3) The coated resinous layer after hardened exhibits sufficiently high thermal stability to eliminate an apprehension that the coated resinous layer might cause corrosion reaction and/or generate cracked gas on heating (e.g., in the step of heat hardening the underfill resin).

4) Filling of the underfill resin can be facilitated. Consequentially, even when the BGA part having a large body size is mounted, there is no possibility that gas bubble, void or the other unfilled space might be left in regions filled with the underfill resin and hardened. In this way, reliable joint (adhesion) can be assured and reliability of the product can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a printed wiring board employed in an embodiment of the present invention (A) and a sectional view taken along the line a-a′ (B) in FIG. 1A;

FIG. 2 is a bottom plan view of a BGA part employed in the embodiment of the present invention (A) and a sectional view taken along the line a-a′ (B) in FIG. 2A and

FIG. 3 is a plurality of sectional views illustrating respective steps of a surface mounting process according to the present invention.

DETAILED DESCRIPTION

Details of the present invention will be described on the basis of the preferred embodiments.

An activated resinous composition according to the present invention contains an epoxy resin which is solid at room temperature. The epoxy resin functions as a matrix resin. In addition, the epoxy resin also to react with an activator which will be described later in more detail in the course of hardening reaction and thereby to deactivate the activator. Thereby the coated resinous layer after hardened exhibits sufficiently high thermal stability to eliminate an apprehension that the coated resinous layer might cause corrosion reaction and/or generate cracked gas on heating (e.g., in the step of heat hardening the underfill resin). Softening point of the epoxy resin is preferably in a range of 70 to 150 (More preferably in a range of 80 to 100) ° C. More specifically, the epoxy resin may be selected from the group consisting of various cresol novorac-type epoxy resins, dicyclopentadiene-based epoxy resins, bisphenol-A-type solid epoxy resins and solid alicyclic epoxy resins.

The activated resinous composition according to the present invention contains a carboxylic compound functioning as an activator. Specifically, the carboxylic compound may be selected from the group consisting of p-hydroxybenzoic acid, dihydroxybenzoic acid, phenylacetic acid, abietic acid, copolymer such as styrene-maleic acid resin and acrylic acid copolymer.

The activated resinous composition according to the present invention contains a hardening agent. A hardening reaction initiation temperature is 150° C. or higher (preferably in a range of 160 to 200° C.). The hardening agent having such high reaction onset temperature may be used to assure that heating for a short time causes no hardening reaction occurs by brief heating and the activated resinous composition can be prevented from being hardened even in the step of reflow. Specifically, dicyandiamide maybe used as the hardening agent.

The activated resinous composition according to the present invention contains a solvent. A boiling point of the solvent is preferably lower than the hardening reaction initiation temperature and more preferably in a range of 150 to 200° C. Specifically, the solvent may be selected from the group consisting of glycol ether, ethylene glycol ether/ester, propylene glycol ether/ester, and N-methylpyrrolidone.

The activated resinous composition according to the present invention may further contain the other additive agents such as polydimethylsiloxane as defoamant, silane coupling agent and aerosol.

The activated resinous composition according to the present invention contains, on the basis of the solid epoxy resin of 100 parts by weight, the ingredients as follows: a carboxylic compound in a range of 1 to 10 (preferably in a range of 2 to 5) parts by weight, a hardening agent in a range of 1 to 30 (preferably in a range of 2 to 7) parts by weight and a solvent in a range of 10 to 300 (preferably in a range of 30 to 100) parts by weight.

The method for surface mounting according to the present invention will be described with reference to the accompanying drawings. According to this method, in a first step, at least surface of a solder pad (FIG. 3A, 2) on the printed wiring board (FIG. 3A, 1) is coated with the activated resinous composition (FIG. 3B, 3) according to the present invention. Preferably, the entire surface of the printed wiring board or only the solder pad surface only may be coated with the activated resinous composition. It is also possible to coat at least the solder pad surface (FIG. 3C, 9) of the part to be surface mounted (FIG. 3C, 4) with the activated resinous composition. In other words, the entire surface of the part to be surface mounted or the solder bump only may be coated with the activated resinous composition. Thickness of the coated resinous layer formed by such coating process is usually in a range of 10 to 50 μm.

Then, the coated resinous layer (FIG. 3B, 3) is dried and the solvent is removed. The coated resinous layer after having been dried normally forms a tack-free coated film. Drying condition may be, for example, 10 to 30 minutes at a temperature of 80 to 120° C.

Then, the coated resinous layer is preferably heated at a temperature higher than the softening point but lower than the hardening reaction initiation temperature of the epoxy resin. With such controlled heating, the coated resinous layer normally develops tackiness and facilitates the parts to be surface mounted. Heating condition may be, for example, 1 to 10 minutes at a temperature in a range of 80 to 130° C.

It should be noted that one or both of the step of drying the coated resinous layer (FIG. 3B, 3) and the step of heating the coated resinous layer at the temperature corresponding to the softening point or higher may be carried out or eliminated. If carried out, these two steps may be carried out successively or at once.

Now the part to be surface mounted (FIG. 3B, 3) is loaded on the printed wiring board. The present invention allows a relatively large part to be surface mounted, for example, apart of 50 mm×50 mm or larger to be handled. Examples of the part to be surface mounted include package parts (e.g., BGA parts, CSP parts, MCM parts, IPM parts and IGBT parts) and semiconductor chips.

The step of loading the part is followed by a step of reflow soldering (FIG. 3D). The step of reflow soldering may be carried out, for example, for a time period in a range of 1 to 10 minutes at a temperature in a range of 240 to 300° C. In this step, no hardening reaction occurs as has previously been described. In addition, in the course of this reflow soldering, gas bubble, void or moisture in the coated resinous layer is evacuated in the form of vapor from the coated resinous layer. Consequently, none of such gas bubble, void and moisture is present in the coated resinous layer after hardened.

The step of reflow soldering is followed by a step of heat hardening the coated resinous layer (FIG. 3E). This step of heat hardening may be carried out, for example, for a time period in a range of 1 to 4 hours at a temperature in a range of 150 to 200° C. In this step, the carboxylic acid compound serving as the activator reacts with the epoxy resin to be deactivated. Consequently, there is no more apprehension that the reliability of the product might be deteriorated due to corrosion reaction or the like.

The hardened film (coated resinous layer after hardened) (FIG. 3E, 10) formed in this manner absorbs (planarizes) irregularities on the printed wiring board in some measure and facilitates the subsequent step of filling the underfill resin.

Then, packaging and, if desired, filling and hardening of underfill resin (FIG. 3F, 11) are carried out. Specifically, void spaces left between the printed wiring board and the part to be surface mounted are filled with the underfill resin and this resin is hardened.

Details of the present invention will be further specifically described on the basis of example.

Homogeneous paste of the activated resinous composition composed of the following ingredients was prepared.

Composition: cresol novolac-type epoxy resin (softening point: 94° C. of 100 parts by weight; p-hydroxybenzoic acid of 4 parts by weight; dicyandiamide of 5 parts by weight; and propylene glycol methyl ether acetate of 50 parts by weight.

Surface of 100 mm×100 mm printed wiring board (pad pitch: 0.6 mm, pad diameter: 0.3 mm) (FIG. 1) was coated with the paste of activated resinous composition by screen printing. This printed wiring board was heated at a temperature of 100° C. for 20 minutes to dry the coated resinous layer. The coated resinous layer on the printed wiring board was cooled to a room temperature to obtain the solid resinous layer having no tackiness and surface\'s pencil hardness of HB.

Then the printed wiring board was heated to 120° C. to soften the coated resinous layer and develop tackiness. BGA part of 70 mm×70 mm (bump pitch: 0.6 mm; bump diameter: 0.3 mm) was loaded on the printed wiring board. The printed wiring board having BGA part loaded thereon was guided through a reflow apparatus set to a temperature of 260° C. and thereby soldered.

As has previously been described, the printed wiring board having the BGA part soldered thereto was cooled to obtain solid resinous layer having the surface pencil hardness of HB. This solid resinous layer was heated again at a temperature of 120° C. and thereby the resinous layer softened again and the tackiness was developed again.

Then the printed wiring board having the BGA part soldered thereto was heated at a temperature of 190° C. for 2 hours to harden the coated resinous layer. It was confirmed on the basis of measurement after the step of hardening that the coated resinous layer was completely hardened to the surface pencil hardness of 8 H.

The BGA part was physically peeled off from a part of the printed wiring board intermediately obtained through the above-described steps and the hardened coated resinous layer was observed using a lens of 20 magnifications. None of voids due to gas babble and/or moisture was observed.

Remaining printed wiring board was filled with the underfill resin and heat at a temperature of 150° C. for 60 minutes to harden the underfill resin. In this manner, the BGA part mounted product was finished. X-ray observation of the finished product indicated that the printed wiring board was completely filled with the underfill resin without the presence of gas babble and/or void.