JPH0786482A - Lead terminal structure of surface mount component - Google Patents

Abstract

PURPOSE:To eliminate the occurrence of defective soldering at the time of soldering surface mount components. CONSTITUTION:A pattern is formed in advance on a wiring board and lead terminals are soldered to a plurality of pads composed of metallic thin films by positioning the lead terminals by making them abutting on the pads after printing paste on the pads and passing the wiring board through a reflow furnace. In such a lead frame structure for surface mount components, the front ends of the lead terminals are folded up in parallel with gaps for generating capillary phenomena in between.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lead terminal structure which suppresses defective soldering. At present, surface mounting of electronic components is generally performed in information processing devices and communication devices. That is, a solder paste is applied to a bonding pad position formed on a printed wiring board or a ceramic wiring board by a method such as screen printing, and a flat package type IC is applied to this position.
The terminals such as chip type capacitors and resistors are positioned and soldered by reflow soldering through an electric furnace in which the heating temperature is set according to the melting point of the solder to form an electronic circuit. ing.

[0002]

[Prior Art] Surface Mounting Techn
ology abbreviation SMT) is an L that constitutes the main body of the information processing device.
This is a mounting method in which semiconductor integrated circuits such as SI and VLSI are mounted on a substrate with high density, but it is necessary to form a large number of wiring patterns on the substrate on which this is mounted.

That is, at least wirings such as signal lines, power supply lines, and ground lines are required to form semiconductor integrated circuits such as ICs and LSIs. The number of these wirings increases as the degree of integration increases. Further, since the semiconductor integrated circuits are densely mounted on the substrate, the circuit board on which the semiconductor integrated circuits are mounted inevitably has a multilayer structure, and the uppermost layer is formed by a thin film forming technique and photolithography technique (photolithography). A large number of different bonding pads are formed, and the electronic components are surface-mounted using the bonding pads.

However, the number of bonding pads formed on the substrate is extremely large, while the size thereof is very small. Now, as an example, the number of lead terminals that make up a flat package of 32 mm in width, 28 mm in height, and 4.4 mm in height is 104, and 44 mm in width, 40 mm in height, and 40 mm in height.
The number of lead terminals that make up a 4.4 mm flat package is 168, and the thickness of the lead terminals is 0.2 to 0.3 mm.
Of copper alloy, which are arranged with a terminal pitch of about 0.8 mm.

Further, in a flat package mounting an LSI having a higher degree of integration, the number of lead terminals has been remarkably increased, and the terminal pitch has been reduced to about 0.3 mm. Therefore, the bonding pads are patterned on the substrate corresponding to the pitch of the lead terminals.

Here, the number of surface mount components mounted on the substrate is not limited to one type, and there are many types, and the size and pitch of the bonding pads are peculiar to each electronic component. Each pattern is formed on the substrate.

A large number of bonding pads having different sizes and pitches are formed on the substrate in this way. For surface mounting, first, a solder paste is printed on the bonding pads using a metal mask, and then the bonding pads are printed. The lead terminals of the surface mount components are brought into contact with the bonding pads to be aligned with each other, and in this state, a reflow furnace whose temperature is set according to the melting point of the solder is passed through to perform soldering.

However, when the amount of the solder paste printed on the bonding pad is excessive, the solder overflows from the bonding pad and comes into contact with an adjacent bonding pad to form a so-called solder bridge, resulting in a short circuit state. If it is too small, a connection failure will occur and a wire breakage will result, reducing reliability.

[0009]

Although a bonding pad having a small area is patterned at a fine pitch on a multi-layered circuit board for surface mounting, solder bridges, disconnections, or other solder may occur after reflow soldering. Poor attachment is likely to occur. Therefore, this solution is an issue.

[0010]

Means for Solving the Problems The above-mentioned problem is that a pattern is formed on a wiring board, and after solder paste is printed in advance on a plurality of pads made of a metal thin film, the lead terminals of electronic parts are attached to the pads. In the lead terminal of the electronic component to be soldered by contacting and positioning and passing through the reflow oven, the tip of the lead terminal is folded in parallel with a gap that causes a capillary phenomenon. This can be solved by configuring the lead terminal structure of.

[0011]

In the surface mount reflow soldering process, short circuits and disconnections occur because the amount of solder paste applied to each bonding pad is not appropriate.
Here, the solder paste is a mixture of solder powder and flux vehicle in a weight ratio of about 9: 1. The constituent metal of the solder, the particle size and shape of the powder, the constituent material of the flux vehicle, etc. Although various solder pastes have been put into practical use, the most common solder constituent metal is a eutectic solder consisting of 63 wt% tin (Sn) -37 wt% lead (Pb) and has a melting point of 183 ° C. On the other hand, the flux vehicle is composed mainly of rosin (pine resin Rosin), a thickener, a solvent, an activator, etc. By changing the types and concentrations of these, flux vehicles with various versatile properties are made. There is.

The solder paste is applied to the bonding pads formed on the wiring board by using a metal mask. However, the applied thickness of the solder paste is naturally determined by the thickness of the metal mask.
Appropriate application thicknesses are based on experience and achievements.The inventors have found that solder paste thicknesses of 200 μm and 0.5 mm pitch are 150 mm for bonding pad pitches of 0.8 mm.
The thickness of 100 μm is adopted for the one with μm and 0.3 mm pitch.

However, since many kinds of bonding pads having different sizes and pitches are patterned on an actual wiring board, the inventors have adopted a typical solder paste thickness of 200 μm. In this case, a package with lead terminals with a narrower pitch is more likely to cause a short-circuit fault, while a package or component with a lead terminal with a wider pitch will lack the amount of solder paste. A lack of solder paste is not fatal, unless it causes defects.

Therefore, the present invention absorbs the solder paste excessively applied on the bonding pad,
As a method of preventing the generation of solder bridges, the tip of the lead terminal is bent to cause a capillary phenomenon to absorb excess solder paste. That is, FIG. 2 shows a conventional lead terminal structure of a flat package. The tip of the lead terminal 1 is bent by a forming process, and the tip portion 2 is brought into contact with a bonding pad for soldering. The invention is shown in FIGS. 1A and 1B.
As shown in FIG. 3, the tip portions 3 and 4 are bent so that they are superposed upward or downward.

Here, the capillarity phenomenon is a phenomenon that the liquid level rises or falls when a capillary is erected in the liquid, and the liquid level rises when the wettability of the liquid to the pipe is large (adhesiveness is large). On the other hand, when the wettability is small, it is a phenomenon of dropping, but the wettability of the solder paste to the lead terminals is extremely good.

That is, in the reflow process, the solder paste melts in a state where the solvent evaporates and the viscosity rises as the temperature of the solder paste rises, the rosin coating blocks the outside air, and the activator activates the metal surface. Although the soldering is carried out by wetting the solder with the metal, the wettability is improved at the stage where the solvent is evaporated by heating and good soldering is carried out. The present invention utilizes the improvement of wettability at the time of reflow soldering, and absorbs excess solder paste by utilizing a capillary phenomenon.

[0017]

Example: The melting point of the solder that constitutes the solder paste is
It is a eutectic solder (grain size 20-45 μm) consisting of Sn 63 wt% -Pb 37 wt% at 183 ° C and its melting point is 183 ° C. On the other hand, the flux vehicle is composed mainly of rosin (pine resin Rosin), a thickener, a solvent, an activator, etc., and the viscosity of the solder paste is 300,00 cp.

On the wiring board, the pitch is 900 μm.
Bonding pads for mounting two types of flat packages of 150 μm were patterned. Ie one is width
900 bond pads with 500 μm and 2.41 mm length
The pattern is formed by arranging 100 pieces at a pitch of μm and the other 300 pieces at a pitch of 150 μm.

A solder mask was printed on this wiring board using a metal mask having a thickness of 400 μm, and then lead terminals of each flat package were brought into contact with each other. This is a flat package lead terminal with a pitch of 900 μm (width 30
0 μm, thickness 170 μm), paste thickness 400 μm
Since m is appropriate, the tip of the lead terminal was not bent and the conventional shape was used, but the lead terminal (width 100 µm, thickness 170 µm) of the flat package with a pitch of 150 µm is shown in Fig. 1 (A). For comparison, a flat package which is bent and abutted as shown, and which is not bent as in the conventional case is also mounted. And the maximum temperature is 250 ℃
Soldering was performed through the reflow furnace set to.

As a result, no soldering failure was found in the package of 900 μm pitch and the package of 150 μm pitch in which the lead terminals were bent, and in the package of 150 μm pitch without bending, soldering was performed. The incidence of bridges was 23%.

[0021]

EFFECTS OF THE INVENTION By implementing the present invention in which excess solder paste is absorbed by utilizing the capillary phenomenon, it is possible to prevent the occurrence of solder bridges, and thus the yield is improved even when various bonding pads with different pitches are arranged. Good bonding can be performed.

[Brief description of drawings]

FIG. 1 is a lead terminal structure of a flat package embodying the present invention.

FIG. 2 is a lead terminal structure of a conventional flat package.

[Explanation of symbols]

 1 Lead terminal 2, 3, 4 Tip

Claims (1)

[Claims] 1. A solder paste is printed in advance on a bonding pad which is patterned on a wiring board, and a lead terminal is brought into contact with the pad for positioning.
In a lead terminal of a surface mount component to be soldered by passing through a reflow oven, the lead of the surface mount component is characterized in that the tip of the lead terminal is folded in parallel with a gap that causes a capillary phenomenon. Terminal structure.