JP2773087B2 - Electronic component mounting method - Google Patents

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, in particular, a QFP (Quad Flat Package) and an SOP (Single).
(Outline Package) and the like.

[0002]

2. Description of the Related Art In recent years, as electronic devices have become lighter, thinner and smaller, surface mounting technology has become widespread, and surface mounting electronic components such as various IC chip components have been used.

[0003] As a method of mounting IC chip parts, especially the above-mentioned IC package, on a printed circuit board, reflow soldering, dip soldering, iron soldering, and the like are used. Recently, reflow soldering capable of high-density mounting is used. Is the mainstream.

[0004] By the way, among the surface mount electronic components, semiconductor I
The number of leads (pins) of C electronic components is increasing due to higher integration, and lead pitches are becoming narrower.

[0005]

As a method of mounting these semiconductor IC electronic components on a printed circuit board, a reflow soldering method using paste solder is used, but the pitch between the leads is narrow. In the soldering process, defects such as solder bridges are easily generated between the leads.

FIG. 5 is a process sectional view for explaining a solder bridge which is this defect. First, as shown in FIGS. 5A1 and 5A2, a surface mount electronic component such as QF
Copper (C) formed on the printed circuit board 10 on which P is mounted
As shown in FIGS. 5 (b1) and (b2), a paste solder 12 is printed on a Cu bonding pad (hereinafter, referred to as Cu pad) 11 made of u).

[0007] Next, as shown in FIGS. 5 (c 1) and (c 2), the leads 13 of the QFP are mounted on the printed circuit board 10. In mounting the QFP, the lead 13 of the QFP is connected to the paste solder 1 on the Cu pad 11 of the printed circuit board 10.
Set to 2. Next, the printed circuit board 10 on which the QFP is mounted is transported into a reflow furnace (not shown), and heated in a high-temperature atmosphere (183 ° C. or higher) in the reflow furnace to melt the paste solder (see FIG. 5 (d 1)). (D2)) to form the molten solder 14, and thereafter, the printed board 10 is cooled by unloading the printed board 10 from the furnace. By this cooling, the solder melted in the reflow step is solidified, and the leads 13 of the QFP are joined to the Cu pads 11 of the printed circuit board 10.

However, if the temperature distribution in the reflow furnace is not uniform, and the surface-mounted electronic components passing through the reflow furnace also generate a temperature difference at each part and the molten state of the solder varies, FIG. ) And (e2),
A so-called solder bridge 15, in which solder is joined between adjacent leads 13, is generated. Reference numeral 16 denotes a fixed solder. Conventionally, the same material, eg, Sn-Pb (7: 3) solder eutectic alloy, has been used as the material of the paste solder on the printed circuit board to be soldered and the coating material such as lead plating.

As a measure against the above-mentioned solder bridge, (1) The supply amount of the paste solder is reduced. (2) Improve printing accuracy by making the solder particle size uniform and fine. However, it was impossible to wipe out the solder bridge.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of mounting an electronic component capable of preventing a solder bridge which is one of defects generated between leads which are connection terminals of the electronic component in the reflow soldering step. I do.

[0011]

According to the present invention, a paste solder is supplied onto a substrate, and when the electronic component is bonded and mounted by a reflow soldering method, the paste solder is used as the paste solder. The problem is solved by a method for mounting an electronic component, characterized in that solder having a melting point higher than the melting point of the surface material of the joint is used.

According to the present invention, the surface material of the lead of the electronic component mounted on the printed board is, for example, Sn-P
b-Bi solder (solidus temperature (ST): 120 to 13)
0 ° C., liquidus temperature (LT): 152-168 ° C.). Sn-Pb is a paste solder having a melting point higher than the melting point of the lead surface material provided on the printed circuit board.
Eutectic solder (melting point 183 ° C.) is preferred. In the present invention, it is preferable that the melting point of the paste solder to be applied to the printed circuit board is higher by about 10 to 30 ° C. than the melting point of the surface material of the lead in order to alleviate the force at which the eutectic solder particles melt at once.

[0013]

According to the present invention, the melting point of the paste solder 3 formed at the solder joint of the printed circuit board 1 is selected to be higher than the melting point of the surface material of the lead 4 of the electronic component to be mounted. Therefore, in the preheating temperature raising step (about 100 to 170 ° C.) of the reflow step, the eutectic solder particles are gradually diffused into the molten portion of the low melting solder on the surface of the lead 4, and during the main heating (183 ° C. or higher) The force at which the eutectic solder particles are melted and aggregated at once is reduced. The relaxation of the melt cohesion prevents the bridging phenomenon caused by the concentration of solder on the high-temperature lead portion due to the partial temperature nonuniformity due to the high or low mounting density, which has been a problem in the past.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing an embodiment of an electronic component mounting method according to the present invention. FIG. 2 is a perspective view showing an example of an electronic component QFP (Quad Flat Package) used in the embodiment according to the present invention. The number of leads of the QFP 7 shown in FIG. 2 is 16 for easy understanding. But,
Actually, 48 pieces were used.

In this embodiment, first, as shown in FIG. 1A, a Cu bonding pad (hereinafter, referred to as a Cu pad) 2 formed as a land on a printed circuit board 1 is, for example, Sn having a melting point of 183 ° C. A Pb (7: 3) alloy solder paste is printed and applied in advance, and the QFP 7 is bonded and mounted on the printed circuit board 1 in the same manner as the process described in the related art.

That is, it is provided in the ceramic IC package 6 of the QFP 7 and is made of 42 alloy and has
FIG. 1 shows a lead 4 plated with a Sn—Pb—Bi alloy having a melting point lower than that of a Pb alloy solder to a thickness of about 10 μm.
(A) was mounted on a Sn-Pb based solder paste on the Cu pad 2 and charged into a reflow furnace.

S for the lead surface material used in this embodiment
As shown in Table 1, the n-Pb-Bi ternary alloy composition is A, B,
C and D were selected. Table 1 also shows the solidus temperature (ST) and liquidus temperature (LT) of each alloy.

[0019]

[Table 1]

[0020]

FIG. 4 shows the relationship between the eutectic point of the solder paste used and the ST and LT temperatures of A, B, C and D of the lead surface material.

The QFP 7 used in this embodiment is 0.5 mm
In the soldering process by the reflow process with 48 leads (pins) of pitch, the above A,
In addition to B, C, and D, the same Sn—Pb (7: 3) alloy as the paste solder was used as a conventional example.

FIG. 3 shows a reflow temperature profile in the reflow soldering process using the present embodiment and the conventional example.

As shown in FIG. 3, 100-170 ° C. is a preheat zone, 210-240 ° C. is a reflow zone,
Reflow peak temperature was 230-240 ° C.

The results of the reflow soldering process are shown in Table 2 with respect to the QFP reflow solder bridge occurrence rate and (A) the reflow solder bridge occurrence rate per number of QFPs and (B) the read gap per QFP.

The number of QFP samples is 200,
Reflow solder bridge occurrence rate per number of FPs (A)
And the reflow solder bridge occurrence rate (B) per read gap of the QFP is given by the following equations, respectively. (A) Occurrence rate of reflow solder bridges per number of QFPs (%) = (Number of QFPs with solder bridges /
(200 pieces) × 100% (However, even if a plurality of bridges occur in the same QFP, they are counted as one.) (B) Reflow solder bridge occurrence rate per read gap of QFP (%) = (number of bridges) Occurrence location / 20
0 × 44) × 100% FIG. 1B shows a solder bonding state between the lead 4 and the Cu pad where no solder bridge is caused. Reference numeral 5 denotes the fixing solder.

[0027]

[Table 2]

[0028]

As shown in Table 2, the lead surface materials A,
In the case of the present invention using B, C, and D alloys, the ordinary Sn-
The number of reflow bridges generated was much lower than in the conventional example using a Pb (3: 1) alloy.

[0030]

As described above, according to the present invention,
In the reflow soldering, the uneven distribution of the solder to the leads is prevented, and the occurrence of a solder bridge between the leads is prevented.

[Brief description of the drawings]

FIG. 1 is a view showing an embodiment of a method for mounting an electronic component according to the present invention.

FIG. 2 shows a QFP (Qua) used in the embodiment according to the present invention.
FIG. 4 is a perspective view showing an example of (Flat Package).

FIG. 3 is a diagram showing a reflow temperature profile in a reflow soldering step used in the example.

FIG. 4 is a diagram showing the relationship between the melting point of the surface material (plating material) of the lead used and the melting point of the solder paste used.

FIG. 5 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 1,10 Printed circuit board 2,11 Cu bonding pad 3 Paste solder (Sn-Pb-based) 4 Lead (Sn-Pb-Bi-based alloy plating on the surface) 5,16 Fixed solder 6 Ceramic IC package 7 QFP (Quad Flat Package) ) 12 Paste solder (Sn-Pb) 14 Melting solder 15 Solder bridge (defect)

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H05K 3/34 505 H05K 3/34 507

Claims (1)

(57) [Claims] When supplying paste solder to a substrate and bonding and mounting an electronic component by a reflow soldering method, a solder having a melting point higher than a melting point of a bonding surface material of the electronic component is used as the paste solder. A method for mounting an electronic component, comprising: