GB2324753A

GB2324753A - Manufacturing printed circuit and printed wiring boards

Info

Publication number: GB2324753A
Application number: GB9809281A
Authority: GB
Inventors: Kazuyuki Kawashima
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1997-05-02
Filing date: 1998-04-30
Publication date: 1998-11-04
Anticipated expiration: 2018-04-30
Also published as: GB2324753B; JPH10308573A; JP3173423B2; GB9809281D0

Abstract

A printed circuit or wiring board has a substrate 10 with at least one electronic part 4, a solder pad 3 arranged at a predetermined position on the substrate, and at least one solder resist layer 21. The resist layer has a predetermined thickness such that the electronic part 4 is maintained with the distance between an electrode 5 and the solder pad 3 opposite thereto determined by the thickness of the solder resist layer. The electrode of the electronic part then being soldered to the solder pad.

Description

PRINTED CIRCUIT AND PRINTED WIRING BOARDS AND METHODS OF MANUFACTURE The present invention relates to printed circuit and printed wiring boards.

A particular printed circuit board, that is suitable for high density printed wiring and that has an electronic part, will be described below, by way of example, in illustration of the present invention. The particular board provides improved reliability of the solder connections to the electronic parts at a comparatively low cost.

A printed circuit or wiring board is described in a report in the "SHM Conference Journal", 1997, Vol. 13, No. 1? at page 29 by Sanki Mori et. al. January 1, 1997, of the Material Device Laboratory, of Toshiba Corp. This previous proposal shows that when the front surface of a solder pad, which is disposed on a printed circuit or wiring board, is spaced apart from the rear surface of an electrode of a part mounted thereon by 60 Am or more, even if the size of the solder pad is equal to the size of the electrode of the part to be mounted, a sufficient reliability of solder connections is obtained.

To improve the reliability of solder connections, it has previously been proposed that an appropriate amount of a filler should be incorporated in the solder, a part to be mounted on the front surface of a substrate being spaced by the filler in such an arrangement, and the part being soldered by a solder re-flow process.

Alternatively, it has been proposed that a spacer with a particular thickness should be disposed at a relevant position on the front surface of a printed wiring board. While a surface mounted part is being held by the spacer, the part is soldered.

However, such previously proposed methods have the following drawbacks.

As a first drawback, when solder containing a filler and/or a spacer is/are used, the fabrication cost is increased. In the case of solder containing a filler, the filler should be of an equal size to the solder and should be equally disposed in the solder. Thus, the cost of the resultant solder is increased.

In the case of the use of a spacer, in addition to the cost of the spacer, it is necessary to arrange the spacer in a particular position on the printed circuit or wiring board.

As a second drawback, in the case of solder containing a filler, it is very difficult, after a solder re-flow process has been carried out, to mpdify a circuit and to replace a part that has been soldered. When a part has been manually soldered on to a printed circuit or wiring board, where other parts have been soldered by the solder re-flow process, the strength and the reliability of the joint of the portion which has been soldered by the manual soldering process is different from that which has been soldered by the solder re-flow process. This is because, in the case of solder which contains a filler, unless fillers are disposed between all of the electrodes of the surface mounted parts and the solder pads, high reliability of the solder connections cannot be obtained, and it is almost impossible to perform such work by a manual soldering process.

Features of a printed circuit or wiring board to be described below by way of example in illustration of the present invention are that it has a comparatively high reliability of solder connections, and comparatively low cost, that it enables the size of a solder pad to be reduced, the density of mounted parts to be increased, and the solder quality in a re-flow soldering process to be improved.

An improved method of making a printed circuit or wiring board will also be described below, by way of example, in illustration of the present invention.

A particular printed circuit or wiring board to be described below by way of example in illustration of the present invention includes a substrate having at least one electronic part, at least one solder pad arranged at a predetermined position on the substrate, and at least one solder resist layer, having a predetermined thickness, on the substrate, a portion of the substrate corresponding to the solder pad being left free from the solder resist layer, wherein the position of the electrode part is maintained by the solder resist layer such that the distance between an electrode of the electrode part and a solder pad disposed opposite thereto is equal to the predetermined thickness of the solder resist layer, and wherein the electrode of the electronic part is soldered to the solder pad.

Another printed circuit or wiring board to be described below, by way of example, in illustration of the present invention includes a substrate having at least one electronic part, a conductor pattern layer on the substrate, a first insulation layer arranged on the substrate, a first insulation layer arranged on the substrate in such a manner that the first insulation layer covers the conductor pattern, a second insulation layer, having a predetermined thickness arranged on the first insulation layer in such a manner that the second insulation layer exposes the first insulation layer in a predetermined pattern, and a solder pad which includes a conductor and which is disposed on a part of the surface of the first insulation layer which is free from the second insulation layer, wherein the electronic part is maintained by the solder resist layer at a predetermined height or distance, of which the thickness of the solder pad is subtracted from the predetermined thickness of the second insulation layer, and wherein the electrode of the electronic part is soldered to the solder pad.

The predetermined height or distance is at least around 60 Cun. Thus, in one arrangement to be described, since an electronic part is soldered while it is being maintained in a spaced position by a solder resist on a printed circuit or wiring board, a comparatively high degree of reliability of the solder connections is achieved at a relatively low cost. Thus, the size of a solder pad can be decreased, and a higher density of components and circuits on the printed circuit wiring board can be achieved. In addition, with a decrease in the size of the solder pad, the amount of solder required decreases. Thus, since the pushing force resulting from the surface tension of solder in the solder re-flow process is decreased, the displacement or rising of the chip parts can be minimised. The "rising of chip parts" is a phenomenon that takes place in the situation in which, when two electronic parts are mounted, if the surface tension and cooling velocity of one part are different from those of the other part, a solder layer disposed therebetween acts upon the two electrodes of the parts with different forces and thereby the electronic parts are caused to bend or rise. This phenomenon is sometimes referred to as the "Manhattan Phenomenon".

Ideally, an insulation layer is coated on a conductor pattern layer having a solder pad so that the thickness of the insulation layer with respect to the front or upper surface of the solder pad is preferably around 60 Am or more. With a selectively formed opening for a solder pad, the rear or lower surface of the electrode of an electronic part to be mounted and the front or upper surface of the solder pad are soldered within a distance of around 60 Am or more, regardless of the solder re-flow process or manual solder process.

Thus, a comparatively high reliability of solder connections can be accomplished.

Since, the size of a solder pad in the proposed arrangement is comparatively small, the amount of solder is decreased and thereby the surface tension is decreased. Thus, chip parts can be restrained from being displaced and caused to rise in the solder re-flow process.

A method for fabricating a substrate having at least one electronic part, will also be described below by way of example in illustration of the invention, which method includes the steps of arranging a solder pad on a conductor pattern layer on a surface of the substrate, providing an insulation layer with a predetermined thickness on the surface of the substrate in such a manner that the insulation layer covers the conductor pattern layer and the solder pad, selectively removing the insulation layer from a portion corresponding to the splder pad so as to form an opening or a window that exposes the front or upper surface of the solder pad and to form a solder resist layer having a predetermined height with respect to the solder pad, positioning the electronic part with respect to the solder resist layer in such a manner that the electrode of the electronic part is opposite to the solder pad, and soldering the electrode and the solder pad together.

The thickness of the solder resist layer may be adjusted by machining the front surface of the insulation layer so that the solder resist layer is at a predetermined height with respect to the surface of the solder pad.

There will also be described below, by way of example in illustration of the invention, a method for fabricating a substrate having at least one electronic part, which method includes the steps of coating a photosensitive insulation resin on a conductor pattern layer arranged on a surface of the substrate so as to form a first insulation layer, forming a hole through the first insulation layer, providing a second insulation layer on the first insulation layer so that the height of the second insulation layer with respect to a surface of a-conducting layer on the first insulation layer is at a predetermined distance, exposing a predetermined pattern on the first insulation layer through the second insulation layer, the second insulation layer forming a solder resist layer, providing a solder pad on the surface of the exposed first insulation layer, positioning the electronic part with respect to the solder resist in such a manner that an electrode of the electronic part is opposite to the solder pad, and soldering the electrode and the solder pad together.

Thus, in the printed circuit or wiring board to be described below by way of example in illustration of the present invention, a spacer that spaces an electronic part to be mounted may be provided that substantially uses elements of a previously proposed fabrication process for a printed circuit or wiring board. Thus, solder connections of high reliability can be achieved at low cost. With the small size of the solder pad required, a high density of elements on the printed wiring board can be obtained.

Arrangements illustrative of the invention will now be described by way of example with reference to the accompanying drawings, in which Figs. 1A to 1D are schematic diagrams for use in illustrating steps in the fabrication of one printed wiring board, and Figs. 2A to 2E are schematic diagrams for use in illustrating steps in the fabrication of another printed wiring board.

Referring to Figs. 1A to lD, the respective steps will be referred to as steps (a), (b), (c) and (d).

In the steps (a), (b), and (c) of Figs. 1A to lC respectively there is shown the fabrication of a printed wiring board and at the step (d) of Fig. 1D there is shown an electronic part being mounted on the printed wiring board.

At step (a), a solder pad 3 is arranged on a conductor pattern 1 on the upper surface (as shown) of a substrate 10. The conductor pattern layer 1 is made, for example, of copper.

At step (b), an insulation layer 2 is arranged on the upper surface of the substrate 10 in such a manner that the insulation layer 2 covers the conductor pattern layer 1 and the solder pad 3. The thickness of the insulation layer 2 with respect to the upper surface of the solder pad 3 is preferably around 60 Am. The insulation layer 2 can be coated by a overlap coating method, such as curtain coating method, or by a spin coating method.

In the curtain coating method, a resin material is sprayed like a curtain and the substrate is conveyed in the curtain flow of the resin. Normally, in a process the substrate is conveyed on a belt conveyer, and the speed of the conveyer is controlled so as to control the film thickness of the resin. This method is suitable for forming a relatively thick film.

On the other hand, in the spin coating method, a liquid resin material is first placed at the centre of a substrate. By rotating the substrate at high speed around the centre, the resin material becomes evenly coated over the surface of the substrate due to centrifugal force. In this method, the film thickness of the resin material that can be evenly coated is up to around 20 Am. Thus, when a film with a thickness of 20 Rm or more is required, after the coated film hardens, the same process is performed repeatedly.

At step (c), the insulation layer 2 is selectively removed by an etching process so that a portion corresponding to the solder pad 3 is exposed. Thus, an opening or window is formed in the insulation layer 2 so that the upper surface of the solder pad 3 is exposed.

Consequently, a solder resist layer 21 of insulation resin that covers the upper surface of the substrate 10 and of the conductor pattern layer 1 is formed. Before or after the window has been formed, if necessary, the front surface of the solder resist layer 21 may be buffed, or etched with chemicals, so as to adjust the thickness of the solder resist layer 21.

At step (d), an electronic part 4 is mounted on the substrate 10 by soldering together the respective electrodes 5 and the solder pads 3 by means of a solder layers 6, while the surface of the electronic part 4 nearest to the substrate is maintained spaced from the substrate by the solder resist layer 21.

At step (d), the electronic part 4 is arranged at a predetermined position with respect to the substrate 10 and is held with respect to the substrate so that the distance between the electrodes of the electronic part 4 and the solder pads 3 is a predetermined height H, corresponding to the thickness of the solder resist layer 21 (namely, preferably, around 60 Am or more). Thus, regardless of whether the electrodes 5 and the solder pads 3 are soldered by the solder re-flow process or the manual soldering process, the upper surfaces of the solder pads 3 and the lower surfaces of the electrodes 5 of the electronic part 4 can be soldered so that they are separated by a distance of around 60 Am or more.

Thus, a printed circuit or wiring board made as described with reference to Fig. 1 has a substrate 10 (with an electronic part 4), at least one solder pad 3 that is arranged at a predetermined position with respect to the substrate 10 and that is soldered to an electrode of the electronic part 4, and at least one solder resist layer 21 on the substrate 10, except for a portion corresponding to the solder pad 3, the electrode 5 of the electronic part 4 and the solder pad 3 being soldered together and the part 4 and the substrate 10 being spaced by a predetermined height H (preferably, around 60 Am or more).

In Figs. 2A to 2E, which show steps (a) to (e) respectively in making a printed circuit board, at step (a), a photosensitive insulation resin which provides a first insulation layer 2a, is shown coated on a conductive pattern layer 1 which is made, for example, of a conductor such as copper. At step (b), a hole 22 is formed through the insulation layer 2a by a mask exposing process or by a laser beam machining process.

The mask exposing process is a process that is used to form holes through an insulation layer. A W beam is radiated to a photosensitive agent layer formed on the upper surface of a substrate through a polyimide film, or a glass dry plate, that has holes corresponding to a desired pattern. The exposed portion is removed by means of chemicals. This process is suitable for forming a plurality of holes at a time through to a substrate.

On the other hand, in the laser beam machining process, laser light is radiated from, for example, a carbon dioxide gas laser to a desired position of the substrate so as to form a hole. With the laser beam machining process, a more fine hole can be formed than with the mask exposing process.

At step (c), a second insulation layer 2b is coated on the first insulation layer 2a so that the thickness of the second insulation layer 2b is a predetermined thickness (preferably, around 60 Am or more) with respect to a solder pad 3 that is provided, for example, by a plating process, and that is a layer of a metal such as copper. Next, by means of a known process such as a mask exposing process, or an etching process, the second insulation layer 2b is given a particular pattern, as a result of which the first insulation layer 2a is exposed in a predetermined pattern.

At step (d), a metal such as copper is provided, for example by plating, on the exposed surface of the first insulation layer 2a. Thus, a solder pad 31 and a circuit or wiring pattern are formed. As a result, an insulation layer (composed of the first and second insulation layers 2a and 2b) and a solder resist layer (composed of the insulation layer 2b) are formed. If necessary, the front or upper surface of the solder resist layer may be lightly buffed or etched so as to adjust the thickness of the solder resist layer.

Thereafter, the same step as step (d) shown in Fig.

1 is carried out. Thus, an electronic part is mounted on the substrate. At step (e) showr, in Fig. 2, a printed wiring board is obtained. When the electronic part is mounted, regardless of whether the solder re-flow process or the manual solder process is carried out, as with the first embodiment shown in Fig. 1, the front or upper surface of the solder pad 3 and the rear or lower surface of the electrode 5 of the electronic part 4 are soldered by the solder layer 6 in such a manner that the front or upper surface of the solder pad 3 is spaced apart from the rear or lower surface of the electrode 5 of the electronic part 4 by around 60 Wm or more.

The resin material of the first insulation layer 2a and the resin material of the second insulation layer 2b may be the same or different.

The resultant printed wiring board made as described with reference to Fig. 2 includes a substrate 10 (having at least one electronic part 5), a conductor pattern layer 1 (arranged on the substrate 10), a first insulation layer 2a (that is arranged on the substrate 10 and that covers the conductor pattern layer 1), a second insulation layer 2b (that is arranged above the first insulation layer 2a, that allows the first insulation layer 2a to be exposed in a predetermined pattern, and that has a predetermined thickness), and solder pads 31 (that are arranged on the first insulation layer 2a so that they are free from the second insulation layer 2b and that are made of a material that is a conductor).

The electrodes 5 of the electronic part 4 and the solder pads 31 are soldered by means of the solder layer 6 in such a manner that the electrodes 5 of the electronic part 4 and the solder pads 31 are spaced by a predetermined distance equal to a value in which the thickness of the solder pads 31 is subtracted from the predetermined thickness of the second insulation layer 2b.

The distance between the electrodes of the electronic part and the solder pads is preferably around 60 Am or more.

As described above, the arrangements described have the following effects.

One effect is that, using known techniques for the fabrication of printed circuit or wiring boards, a solder resist layer is used as a spacer for an electronic part and is formed on a printed circuit or wiring board. As the distance between a solder pad on the printed wiring board and the rear or lower surface of an electrode of the electronic part is comparatively large, the reliability of the solder connections is improved.

Furthermore, when an electronic part is soldered on a printed wiring board, it is maintained in a spaced relationship by a solder resist layer. Thus, regardless of whether a solder re-flow process or a manual soldering process is carried out, a printed circuit or wiring board with a high degree of reliability in the solder connections can be obtained at a comparatively low cost.

In addition, since the reliability of the solder connections is improved, even if the size of a solder fillet, that largely contributes to improving the reliability of the solder connections, is decreased (namely, the size of a solder pad is decreased), a sufficient reliability of the solder connections can be accomplished. As a result, the size of the solder pads can be reduced.

Thus, since the amount of solder to be used can be decreased, chip parts can be restrained from displacing and rising. In addition, when the solder re-flow process is employed, the surface tension of solder decreases and thereby the force of the solder applied to the chip parts decreases. Consequently, the reliability of the printed wiring board can be improved.

Although particular arrangements illustrative of the present invention have been described by way of example with reference to the accompanying drawings, it will be understood that variations and modifications thereof, as well as other arrangements, may be conceived within the scope of the protection sought by the appended claims.

Claims

l. A printed circuit or wiring board including a substrate having at least one electronic part, at least one solder pad arranged at a predetermined position on the substrate, and at least one solder resist layer having a predetermined thickness, and being so arranged on the substrate that a portion of the substrate corresponding to the solder pad is left free, wherein the electronic part is maintained by the solder resist layer in such a manner that the distance between an electrode of the electronic part and the solder pad arranged opposite thereto is equal to the predetermined thickness of the solder resist layer, and wherein the electrode of the electronic part is soldered to the solder pad.
2. A printed circuit or wiring board as claimed in claim 1, wherein the predetermined height is at least around 60 Am.
3. A printed circuit or wiring board including a substrate having at least one electronic part, a conductor pattern layer arranged on the substrate, a first insulation layer arranged on the substrate in such a manner that the first insulation layer covers the conductor pattern, a second insulation layer having a predetermined thickness, and being arranged on the first insulation layer in such a manner that the first insulation layer is exposed in a predetermined pattern by the second insulation layer, and a solder pad made of a conducting material arranged on a part of the surface of the first insulation layer which is free from the second insulation layer, wherein the electronic part is maintained by the solder resist layer at a predetermined height of which the thickness of the solder pad is subtracted from the predetermined thickness of the second insulation layer, and wherein the electrode of the electronic part is soldered to the solder pad.
4. A printed circuit or wiring board as claimed in claim 3, wherein the predetermined height is at least around 60 Am.
5. A method of making a printed circuit or wiring board including a substrate having at least one electronic part, the method including the steps of arranging a solder pad on a conductor pattern layer on a surface of the substrate, arranging an insulation layer with a predetermined thicknes on the surface of the substrate in such a manner that the insulation layer covers the conductor pattern layer and the solder pad, selectively removing the insulation layer from a portion of the insulation layer corresponding to the solder pad, so as to form an opening or a window through which the surface of the solder pad is exposed and forming a solder resist layer having a predetermined height with respect to the solder pad, the electronic part being maintained by the solder resist layer in such a manner that the electrode of the electronic part is opposite to the solder pad, and soldering the electrode and the solder pad.
6. A method as claimed in claim 5, wherein the surface of the insulation layer is machined and thereby the thickness thereof is adjusted so that the height of the solder resist layer with respect to the surface of the solder pad is predetermined.
7. A method as claimed in claim 5, wherein the predetermined height is around at least 60 Wm.
8. A method of making a printed circuit or wiring board including a substrate having at least one electronic part, the method including the steps of coating a photosensitive insulation resin on a conductor pattern layer disposed on a surface of the substrate so as to form a first insulation layer, forming a hole through t first insulation layer, arranging a second insulation layer on the first insulation layer so that the height of the second insulation layer with respect to the surface of a metal layer provided on the first insulation layer is at a predetermined height, the second insulation layer being arranged so as to expose the first insulation layer in a predetermined pattern, and to form a solder resist layer, a part of the second insulation layer being removed so as to enable a solder pad to be formed on the surface of the exposed first insulation layer, and the electronic part being arranged with respect to the solder resist layer in such a manner that the electrode of the electronic part is opposite to the solder pad, and soldering the electrode and the solder pad.
9. A method as claimed in claim 8 wherein the surface of the second insulation layer is machined to adjust the thickness thereof so that the height of the solder resist layer with respect to the surface of the solder pad is at the predetermined height.
10. A method as claimed in claim 8 wherein the predetermined height is around at least 60 Zm.
11. A printed circuit or wiring board as claimed in claim 1 substantially as described herein with reference to Figs. 1A to 1D or Figs. 2A to 2E of the accompanying drawings.
12. A method of making a printed circuit or wiring board as claimed in either claim 5 or claim 8 substantially as described herein with reference to either Figs. 1A to 1D or Figs. 2A to 2E of the accompanying drawings respectively.