JPH07202395A - Printed wiring board - Google Patents

Abstract

PURPOSE:To avoid solder balls and solder bridges and improve the reliability of a printed wiring board applied to any type of electronic apparatus. CONSTITUTION:Electrodes 2 and solder resist layers 3 are formed on a board 1 with spacings between each other. A semicylindrical silk screen 4 is formed on the resist layers 3. By supporting a component with the silk screen 4, the protrusion of solder cream can be minimized. Further, even if the protruding solder cream is melted, as the melted solder cream flows down along the silk screen 4 and collects in the spacings and is solidified in that state, solder balls and solder bridges are not produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printed wiring board for use in various electronic devices such as computers and word processors, for mounting components such as semiconductors.

[0002]

2. Description of the Related Art In recent years, mounting of components on a printed wiring board has been required to have a higher density as electronic devices have become smaller.

A conventional printed wiring board will be described below with reference to the drawings. FIG. 4 is a cross-sectional view of a conventional printed wiring board after IC mounting. Reference numeral 11 denotes a substrate made of glass epoxy or the like, and an electrode 12 made of copper foil is formed on the surface of the substrate 11, and an electronic component such as an IC is connected to the electrode 12. A solder resist 16 is coated on the surface of the substrate 11 other than the place where the electrode 12 is formed in order to protect the substrate 11.

A method of mounting electronic components on the conventional printed wiring board having the above-described structure will be described. First, the solder cream 13 is printed on the electrode 12 by a metal plate, and the IC 14 mounted on the solder cream 13 is printed.
Is mounted so that the lead 15 provided on the lower surface of the IC 14 faces the electrode 12 via the solder cream 13. Then, in this state, the printed wiring board is placed in a reflow oven and heated to melt the solder cream 13 and to melt the electrode 1.
The electronic component is mounted by joining the lead 2 and the lead 15.

[0005]

However, the conventional printed wiring board has a problem that a short circuit due to a solder ball or a solder bridge is apt to occur as the density of mounted components increases. The generation of solder balls and solder bridges will be described below.

In FIG. 4, when the amount of the solder cream 13 printed on the electrode 12 is large, when the position of the electrode 12 and the lead 15 is deviated when mounting the IC 14, or when the IC 14 is soldered with the solder cream 13 If a large pressure is applied from above when mounting via, the solder cream 13 protrudes from between the electrode 12 and the lead 15, and when the printed wiring board in this state is put into the reflow oven, the protruding solder cream melts, What remains in a ball shape with the adjacent electrode becomes a solder ball, and what is connected to the adjacent electrode becomes a solder bridge.

With the recent miniaturization of electronic equipment, the density of mounted parts has increased, and the pitch between leads of ICs has become 0.6 mm.
To 0.4 mm, and it is being studied to make it 0.3 mm. Therefore, when a solder ball is generated between the electrodes, a short pitch is likely to occur, and a solder bridge is likely to occur.

Conventionally, the solder balls and solder bridges could be easily removed by chlorofluorocarbon cleaning, but it has become difficult to remove the solder balls and solder bridges due to the recent abolition of fluorocarbon cleaning.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide a printed wiring board having improved reliability by preventing the generation of solder balls and solder bridges.

[0010]

To achieve the above object, a printed wiring board according to the present invention has a gap between a substrate, an electrode provided on the substrate for connecting components, and the electrode. The resist provided on the substrate so as to have, and provided by printing on the resist, has a shape such that the thickness of the cross section increases from the peripheral portion to the central portion, and the height combined with the resist is It is made of silk that is higher than the electrode.

[0011]

Since the printed wiring board of the present invention is formed so that the combined height of the resist and the silk is higher than that of the electrodes, even if a large pressure is applied from the top when mounting the parts, the parts can be formed by the resist and the silk. Supported. Therefore, even if the solder cream is crushed by the component, the amount of the solder cream protruding from between the electrode and the lead can be minimized. Furthermore, since silk has a shape in which the thickness of the cross section increases from the peripheral portion to the central portion,
When the protruding solder cream is melted by the reflow oven, the melted solder flows on the silk, flows into the gap between the electrode and the resist, and solidifies as it is, so that solder balls or solder bridges do not occur.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1 is a cross-sectional view of a printed wiring board according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view of a solder cream after the IC mounting in the present embodiment is displaced.
FIG. 3 is a sectional view showing a state after reflow in the present embodiment.

In FIG. 1, reference numeral 1 is a substrate made of glass epoxy or the like. An electrode 2 made of copper foil is formed on the surface of the substrate 1, and an electronic component such as an IC is connected to the electrode 2. A solder resist 3 is formed on the surface of the substrate 1 so as to form a gap with the electrode 2, and the solder resist 3 plays a role of protecting the substrate 1. If the gap between the electrode 2 and the solder resist 3 is too wide, it does not play a role of protecting the substrate 1. On the contrary, if it is too narrow, the melting and flowing solder overflows, so the gap is 10 to 30 μm.
m is desirable. Then, a kamaboko-shaped silk 4 is formed on the upper surface of the solder resist 3 by printing, and the combined height of the solder resist 3 and the silk 4 is higher than that of the electrode 2. Even if a large pressure is applied when mounting the component The parts are supported by the solder resist 3 and the silk 4. If the total height of the solder resist 3 and the silk 4 is too high, the distance between the electrode 2 and the component is widened, making soldering difficult. Conversely, if the height is too low, the distance between the electrode 2 and the component is narrowed. The thickness of the solder resist 3 is about 50 μm, and the maximum thickness of the silk 4 is 20 to 40 μm because the generation of boards and solder bridges cannot be sufficiently prevented.
Is desirable.

A method of mounting components in the printed wiring board of this embodiment constructed as described above will be described. Figure 2
In, the solder cream 5 is printed on the electrode 2 by a metal plate, and the IC 6 mounted on the solder cream 5 is
The lead 7 provided on the lower surface of the IC 6 is mounted so as to face the electrode 2 via the solder cream 5. At this time, as shown in FIG. 2, the solder cream 5 may squeeze out between the electrode 2 and the lead 7, but the solder cream 5 is I
Even if it is crushed by C6, since the component is supported by the solder resist 3 and the silk 4, the amount of the solder cream 5 protruding from between the electrode 2 and the lead 7 can be minimized.

Then, as shown in FIG. 3, when the printed wiring board is placed in a reflow oven in this state and heated, the protruding solder cream 5 is melted, but the silk 4 is in the shape of a kamaboko. The melted solder flows down on the semi-cylindrical silk 4 and collects in the gap between the electrode 2 and the solder resist 3 and solidifies as it is, so that no solder ball or solder bridge is generated and the electrode 2 and the lead 7 are soldered. It

As described above, in the printed wiring board of this embodiment, the solder resist 3 and the semi-cylindrical silk 4 are formed so that their height is higher than that of the electrode 2, thereby preventing the generation of solder balls or solder bridges. It improves reliability and does not require CFC cleaning.

[0017]

As described above, in the printed wiring board of the present invention, by forming the resist and the silk, the generation of solder balls and solder bridges is prevented, the reliability is improved, and the fluorocarbon cleaning is not required.

[Brief description of drawings]

FIG. 1 is a sectional view of a printed wiring board according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a state where the solder cream after the IC mount is displaced in the present embodiment.

FIG. 3 is a cross-sectional view of a state after reflow in this embodiment.

FIG. 4 is a cross-sectional view of a conventional printed wiring board after IC mounting.

[Explanation of symbols]

 1 substrate 2 electrode 3 solder resist 4 silk 5 solder cream 6 IC 7 lead

Claims (4)

[Claims] 1. A substrate, an electrode formed on the substrate for connecting components, a resist formed on the substrate so that a gap is formed between the electrode, and printing on the resist. A printed wiring board which is formed and has a shape such that the thickness of the cross section increases from the peripheral portion to the central portion, and which is made of silk so that the height of the resist and the resist is higher than the electrode. 2. The printed wiring board according to claim 1, wherein the silk has a semicylindrical cross section. 3. The printed wiring board according to claim 1, wherein the silk has a maximum thickness of 20 to 40 μm. 4. The printed wiring board according to claim 1, wherein the gap between the electrode and the resistor is 10 to 30 μm.