JPS6337689A - Printed wiring board - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a printed wiring board for mounting electronic components used in various electronic devices, particularly micro-sized electronic devices.

[Prior Art] In hybrid integrated circuits, etc., as the circuit components (ICs, active or passive elements, etc.) used in these circuits become smaller, the pitch of lead terminals in components with lead terminals (including ICs) has become smaller. is very narrow.

Therefore, in the printed circuit boards used in such hybrid integrated circuits, the distance between adjacent conductive layers that are completely patterned and formed is currently very narrow. In particular, the conductive layer (
Conductor lands) should be at least 0.5 mm apart, which is the minimum spacing specified in the Japanese Industrial Standards.

Small intervals may be required.

FIG. 4 shows an example of a printed wiring board on which such narrow pitch conductor lands are formed.

In the figure, 10 indicates this printed wiring board as a whole, and on the surface of the printed wiring board 1 having a predetermined thickness,
The conductive layer 2 is etched to form a predetermined pattern.

The figure shows a conductor land 2a that has been formed particularly in the portion of the through hole 3.
* 2 b * . . . is mainly shown.

A plurality of through holes 3 formed in the printed circuit board 1 are provided with lead terminals 4a of a circuit component 4 as shown in the figure.

4b, . . . are mechanically and electrically connected to the conductive layer 2 by the solder layer 5 in the fully inserted state.

By the way, in such a printed wiring board 10,
With the miniaturization of mounted components, conductor lands 2a, 2b,
. . . The interval W has also been narrowed down.

For this reason, when the component 4 is attached and soldered by dipping the conductive layer side into a molten solder tank or a jet solder tank, the conductor lands 2a, 2b adjacent to each other,
. . . may adhere to the solder layer provided on the other conductor land, so-called bridging may occur.

When solder bridging occurs, not only does extra work such as cutting the bridge in that area become necessary, but it also requires various work such as inspecting the presence or absence of solder bridging and quality inspection after removing the bridging. It had drawbacks such as:

In order to improve these drawbacks, the following printed wiring boards are known.

The printed wiring board 10 shown in FIG.
This printed wiring board 10 has conductor lands 2a and 2b where bridging is likely to occur, although the detailed explanation will be omitted since it is basically the same as that disclosed in Publication No. 41102.
, . . . The bridge prevention layer 20 is completely formed between the conductor lands 2a.

2b, . . . The purpose is to prevent bridging of the solder layer 5 by the bridging prevention layer 20 projecting upward.

In this figure, 25 indicates a solder resist layer.

[Problems to be Solved by the Invention] Incidentally, even when a bridging prevention layer 20 as shown in FIG. 5 is formed, it is not sufficient to prevent solder bridging. The reason for this will be explained below.

That is, in the printed wiring board 10 shown in FIG. 5, after the solder resist layer 25 is formed, the bridging prevention layer 20 is formed at a predetermined location.

In this case, marks such as part numbers of mounted components such as resistors and capacitors are printed (symbol mark printing) on the printed circuit board 1 to make it easy to understand the relationship between the mounting position and the mounted components.

Normally, the anti-bridging layer 20 can be formed at the same time as this mark printing. This is because by performing simultaneous printing, only one printing process can be omitted.

Here, the conductive layer, therefore in this case the conductor land 2a
, 2b, . . . is usually about 35 μm, whereas the solder resist layer 25 has a thickness of 10 μm.
It is about 1. In this case, conductor lands 2a, 2b,
..., the thickness of the solder resist layer 25 is 10 um or more because the interval is very narrow.

In addition, although the thickness of the mark printing layer used as the bridging prevention layer 20 is about 15 to 20 μm, the thickness of the mark printing layer used as the bridging prevention layer 20 is
Even if the bridging prevention layer 20 is formed on the mark printing layer 25'', the height difference between the conductor lands 2a, 2b, . . . is only about 5 to 15 μI11.

Therefore, this degree of level difference is not sufficient to prevent bridging, and in some cases, the solder layer may extend beyond this bridging prevention layer 2o to the adjacent conductor lands 2a, 2b.

There is a risk of it adhering to...

Therefore, the present invention solves the above-mentioned problems and proposes a printed wiring board that can reliably prevent bridging caused by solder.

[Technical means for solving the problems] In order to solve the above-mentioned problems, the present invention provides a patterned conductive layer on the surface of a printed circuit board.

It is characterized in that a bridging prevention layer containing a foaming agent is provided between the conductive layers formed at a narrow pitch.

[Function] Since the bridging prevention layer contains a foaming agent, if this bridging prevention layer is formed at the structure prevention position and then foaming treatment is performed, the bridging prevention layer will not touch the conductive layer forming surface of the printed circuit board. Since the conductive layer expands in the vertical direction, a sufficient barrier is formed between the conductive layers.

It can be formed with a height 2 to 3 times higher than that of the conventional method.

As a result, even if the spacing between the conductor lands is very narrow, solder bridging between the conductor lands will not occur.

[Example] Next, an example of a printed wiring board according to the present invention will be described in detail with reference to FIG. 1 and subsequent figures.

In this invention, as shown in FIG. 1, a bridging prevention layer 30 containing a foaming agent is applied to areas where the pitch of conductor lands 2a, 2b, . . . is very narrow.
is formed.

When mounting chip components such as ICs, the intervals between the IC bins are narrowly spaced, so even in such cases, as shown in FIG.
The anti-bridging layer 30 can be formed at the necessary locations of 2b, . . . .

The bridging prevention layer 30 can be made of an ultraviolet curable resin (acrylic resin, etc.) impregnated with a predetermined weight of a foaming agent, such as microcapsules.

In this case, if the weight of the blowing agent to be impregnated is large compared to the resin, the printability on the printed circuit board will deteriorate.On the other hand, if the amount of impregnated is small, the foaming effect will be reduced. It will fade and no longer function as a solder barrier.

For these reasons, it has been found through various experiments that, for example, a material in which 100 parts by weight of an ultraviolet curable resin is impregnated with 10 to 20 parts by weight of a foaming agent is most suitable.

Note that as an alternative to the ultraviolet curable resin, a thermosetting resin such as an epoxy resin can be used.

The anti-bridging layer 30 is formed at the same time as the mark is printed, but in this case, it is necessary to print the solder resist layer 25 between the conductor lands where the anti-bridging layer 30 is to be printed, as in the past. There isn't.

FIG. 3 shows an example of a process for manufacturing the printed wiring board of the embodiment shown in FIG.

First, as shown in FIG. 1A, a printed circuit board 1 on which a conductive layer 2 has been formed is prepared. A solder resist layer 25 is formed on this printed circuit board 1, leaving a predetermined area.
can be screen printed (Figure B).

Subsequently, predetermined conductor lands 2a with narrow pitches,
2 b + . . . , a bridging prevention layer 30 containing a foaming agent as described above is formed using a method such as screen printing (FIG. C). In this case, if mark printing is required, the bridging prevention layer 30 is simultaneously printed with the mark.
is printed. Conductor lands 2a, 2b, ...
When it is necessary to print a mark in between, the anti-bridging layer 30 is patterned and printed so that it also functions as a mark.

Since the thickness of the mark printing layer is usually 15 to 20 μm, the thickness of the anti-bridging layer 30 is also the same thickness as this mark printing layer.

After printing the anti-bridging layer 30, the temperature of the internal atmosphere in which the printed circuit board is placed is maintained at about 100 to 140°C.
A foaming process of the foaming agent is performed.

Since the anti-bridging layer 30 is sandwiched between the conductor lands 2a, 2b, . Expands only to the side; two things happen. Therefore, due to this expansion, the thickness of the anti-bridging layer +=3O becomes several times higher than before foaming (D in the same figure).

According to experiments, when the anti-bridging layer 30 was printed with the above-mentioned thickness under the above-mentioned component ratio, the height of the anti-bridging layer 30 was approximately BOura.

When the conductor lands 2a, 2 have a thickness of this level,
It was confirmed that bridging of the solder layer did not occur even when the distance between b, . . . was 0.5 mm or less.

After foaming with the foaming agent, a predetermined curing process is performed.

The curing treatment after the foaming treatment differs depending on the basic material of the first grade prevention layer 30. When using an acrylic resin, the anti-bridging layer 30 is cured by irradiation with ultraviolet rays. On the other hand, if the epoxy resin is used up, the anti-bridging layer 30 will be cured by applying a predetermined amount of heat.

By the above curing treatment, a printed wiring board 10 having a bridging prevention layer having a predetermined thickness is obtained.

[Effect of the invention] As explained above, according to this clarification, since the bridging prevention layer containing the foaming agent is formed, the printed wiring board has sufficient height during use. Even if the conductor land is very narrow, bridging of the solder layer can be reliably prevented.

When using this as a bridging prevention layer by printing a mark printing layer on the top surface of the solder resist layer as in the conventional method, it is necessary to print the solder resist layer and the bridging prevention layer twice. need to be executed.

Therefore, depending on the printing accuracy, it is not possible to obtain the designed height even by printing a bridging prevention layer between conductor layers with a narrow pitch, where solder structures are likely to occur when the misalignment caused by two printings overlaps. Sometimes there isn't.

In contrast, in this invention, by printing the bridging prevention layer once,
Since a sufficient thickness can be obtained, a predetermined barrier can be formed without requiring much printing precision.

Therefore, as described above, the present invention is extremely suitable for application to printed wiring boards used in hybrid integrated circuits incorporated in microelectronic devices; q, etc.

[Brief explanation of drawings]

1 and 2 are cross-sectional views of essential parts showing an example of the printed wiring board according to the present invention, FIG. 3 is a process diagram showing an example of the method for manufacturing the printed wiring board according to the invention, and FIGS. FIG. 5 is a sectional view of a conventional printed wiring board for explanation. 10... Printed wiring board 1... Printed circuit board 2... Conductive layers 2a, 2b... Conductor land 5... Solder layer 25... Solder resist layer 30... Bridging prevention layer

Claims (4)

[Claims] (1) A printed wiring board characterized in that a patterned conductive layer is formed on the surface of the printed circuit board, and a bridging prevention layer containing a foaming agent is provided between the narrow pitch conductive layers. (2) The printed wiring board according to claim 1, wherein the anti-bridging layer is made of an ultraviolet curable resin. (3) The printed wiring board according to claim 1, wherein a thermosetting resin is used as the bridging prevention layer. (4) Claims 1 to 3, characterized in that the foaming agent is a microcapsule.
Printed wiring board as described in section.