GB2041663A

GB2041663A - Printed circuit board and method of attaching component leads thereto

Info

Publication number: GB2041663A
Application number: GB7904489A
Authority: GB
Original assignee: CHUO MEIBAN Mfg CO Ltd
Current assignee: CHUO MEIBAN Mfg CO Ltd
Priority date: 1979-02-08
Filing date: 1979-02-08
Publication date: 1980-09-10
Also published as: GB2041663B

Abstract

A printed circuit base board comprises an insulating substrate having on one surface thereof a thin conductive layer. Where through-holes would be provided for lead attachment, the board is drilled only through the insulating portion. The conductive layer is left intact. Component leads are inserted through the holes to break the conductive layer to which they are then soldered. Slits or small holes may be made in the conductive layer to facilitate break-through.

Description

SPECIFICATION Printed circuit board and method of attaching component leads thereto The present invention relates to an improvement in the method for attaching leads of electronic components to a printed circuit base board and to a printed circuit base board that can be used in conjunction with the method.

In general, a printed circuit base board is formed by attaching electrically conductive thin strips for connecting components of an electronic applicance to one surface of an electrically insulating substrate, and printed circuit base boards of this type are industrially manufactured by the copper laminate etching process (for example, the photoresist method, the silk screen method, the offset method, the rubber plate method and the electrophotographic method). Resistors, condensors, transistors, diodes and other electronic components of electronic appliances are attached to such printed circuit base boards according to an electric circuit pattern formed on one surface of the base board.

A conventional method of attachment of component leads to a printed circuit board will now be described with reference to Figures 1 to 4 of the accompanying drawings, of which: Figure 1 is a cross-sectional view of part of a printed circuit board; Figure 2 is a similar view of the board with a component lead attached; Figure 3 is a cross-sectional view of a component lead attached with a fault in the soldered joint; and Figure 4 is a cross-sectional view of a base board with a solder mask.

Referring to Figures 1 to 4 through holes 4 are formed on a printed circuit base 1 comprising an electrically insulating substrate 2 and a cooper foil 3 laid out on one surface of the substrate 2 according to a desired electronic circuit pattern, at component attachment positions A so that leads 5 of components of an electronic appliance (referred to as "component leads" in the instant specification and appended claims) will be inserted into these holes 4 and soldered to the printed circuit base board 1 in the thus inserted state. Component attaching lands 3a are formed on the peripheral edges of one opening 4b of these through holes 4.Then, the component leads 5 are inserted into the through holes 4 from the side of the back surface of the printed circuit base board 1, namely from the other openings 4a of the through holes 4, and these component leads 5 are projected on the front surface of the printed circuit base board 1. Then, the projected portions 5a of the component leads 5 are soldered to the component-attaching lands 3a formed of the copper foil 3 around the openings 4b of the through holes 4 on the front surface of the printed circuit base board 1.

The above-mentioned through holes 4 are ordi narilyformed by press processing or drilling. These through holes are generally formed with certain dimension allowance for errors in the drilling operation, shrinkage or printed circuit base boards and deviations in the thickness of component leads owing to differences of the kinds and makes thereof and in due consideration of facilitation of the operation of inserting component leads into the through holes.

Accordingly, as conceptually shown in Figure 2, slight clearances or gaps are ordinarily formed between the side walls of the through holes 4 and the component leads 5 when the leads 5 are inserted into the holes 4. Because of the presence of such clearances, in the conventional attachment method, the following disadvantages are brought about during or after the attachment operation.

In the practical attachment process line, the above mentioned-operations of insertion and soldering of component leads into through holes are performed by an automatic soldering machine. In this soldering process, because of the presence of such clearances or gaps and/or because of a gas generated from the insulating substrate by the action of heat for the soldering operation, the solder is caused to fall out partially. Namely, a so-called tunnel phenomenon (formation of such defects as holes 6a shown in Figure 3) is often caused to occur.

This tunnel phenomenon results in insufficient electric contact between the component leads and the component lead-attaching lands. Therefore, these defects should be eliminated by the correcting operation after the soldering step. Accordingly, in the above-mentioned automatic soldering operation, it is indispensable to check formation of the defects by the tunnel phenomenon and perform the correcting operation for eliminating these defects, and these troublesome additional operations require much labour and time and hinder attainment of the labour-saving effect in the soldering process.

As means for elminating the foregoing disadvantage, there have been proposed and worked a one-surface through hole method and a through hole plating method. These methods, however, are not suitable for the manufacture of commercial circuits because the productivity is very low, and they are not advantageous from the economical viewpoint.

As pointed out hereinbefore, the presence of the gaps between the side walls of the through holes and the component leads results in the abovementioned insufficient electric contact between the component lead-attaching lands and the component leads attached thereto by the automatic soldering operation or the like. Still further, if the abovementioned gaps are present, only by simple soldering, no complete atachment is attained and breakages of the leads are readily caused by external forces such as vibrations. Accordingly no sufficient electric or functional reliability can be attained and the durability, for example, the vibration resistance, is poor.

Moreover, in the conventional attachment method, in order to attain a soldering effect assuredly, a solder mask (resist) 7 (Figure 4) should be formed on the land 3a for attachment of the component lead 5 while the entire surface of the land 3a is kept in the uncoated exposed state. Accordingly, the circuit density cannot be enhanced and it is impossible to obtain a compact printed circuit.

it is a primary object of the present invention to provide an improved method for attaching component leads to a printed circuit base board.

In accordance with one aspect of the present invention, there is provided a method for attaching component leads to printed circuit base boards, which comprises drilling a printed circuit base board comprising an insulating substrate having on one surface thereof a thin electric circuit layer composed of an electrically conductive material as parts of the insulating substrate continguous to component lead attaching land-forming portions of said electric circuit layer from the opposite surface of the printed circuit base board, while leaving said land-forming portions undrilled, thereby to form lead-inserting holes having a size sufficient to insert component leads exactly therein and having one end closed on the side of said land-forming portions of the electric circuit layer and the other end opened, inserting component leads into said inserting holes from the open ends thereof, extending the top ends of the component leads through the land-forming portions to project the top ends of the component leads on the printed circuit base board, and soldering said projected top ends of the component leads to said land-forming portions of the electric circuit layer.

In accordance with another aspect of the invention there is provided a printed circuit base board comprising an insulating substrate having on one surface thereof a thin electric circuit layer composed of an electrically conductive material, wherein said printed circuit base board is drilled at parts of the insulating substrate contiguous to component lead attaching landforming portions of said electric circuit layer from the opposite surface of the printed circuit base board, while leaving said land-forming portions undrilled, thereby to form composite lead-inserting holes having a size sufficient to insert component leads exactly therein and having one end closed on the side of said land-forming portions of the electric circuit layer and the other end opened.

The invention will further be described with referpence to Figures 5 to 12 of the accompanying drawings, of which: Figure 5 is an enlarged sectional view illustrating component lead-attaching portions of the printed circuit base board of the present invention; Figures 6-A and 6-B are plan and front view of a micro-drill to be used for formation of lead-inserting holes in the present invention; Figure 7 is an enlarged sectional view illustrating the state where component leads are inserted according to the present invention and they are hold by fragments of holding plates which are formed by piercing of the component leads through the holding plates.

Figure 8 is an enlarged sectional view illustrating the state where component leads are soldered to lead-attaching lands according to the present invention; Figure 9 is an enlarged sectional view showing the state where a solder mask is formed on a printed circuit base board of the present invention; Figures 10-A, 10-B and 10are enlarged sectional views showing another embodiment of component lead-attaching portions in the printed circuit base board of the present invention; Figures 11-A, 11-Band 11-Care plan views showing embodiments of the printed circuit base board of the present invention where line slits or cross line slits are formed on holding plates; and Figure 12 is an enlarged section view showing another embodiment of the component leadinserting holes in the printed circuit base board of the present invention.

While according to the conventional technique, as shown in Figure 1,through holes 4 piercing not only an insulating substrate 2 but also an electrically conductive material 3 applied on the surface of the substrate 2 are formed at component lead-attaching portions A (component lead attaching land-forming portions) of a printed circuit base board 1 for attachment of component leads. However, in the described embodiments of the present invention, as shown in Figure 5, the conductive layer is not drilled at component lead attaching land-forming portions A' but is left undrilled and predetermined holes 14 are selectively formed by drilling only the insulating substrate 12. Thus, blind component lead-inserting holes 14 are formed, being closed by so-called plate portions 13b of the conductive layer.

The drilling method employed for drilling only the substrate may use a micro-drill as shown in Figures 6-A and 6-B.

As shown in Figures 6-A and 6-B, the micro-drill that is used for formation of inserting holes has a top end configuration which is different to some extent from that of an ordinary piercing drill. As pointed out hereinbefore, in the present invention, the purpose of the drilling operation is not to form through holes but the conductive material layer should be left undrilled. In order to attain this feature, the microdrill that is used in the present invention has a specific top end configuration. Namely, the blade edge A is relatively flattened so as not to form an excessively sharp blade end and the micro-drill has another blade has another blade portion B capable of forming a winecup-like shape which is effective for guiding a component lead into the resulting hole.

The drilling operation may be accomplished according to a computer-controlled automatic drilling system. Contrary to the conventional punching technique, drilling is effected from the back surface of the substrate where the conductive material is not present.

In the present invention, according to the abovementioned drilling operation, there are formed leadinserting holes 14 each having one end closed by holding plate portions 13b. Referring to Figure 5, the top end of a component lead 15 is inserted into such lead-inserting hole 14 from the open end 14a thereof in a direction indicated by an arrow, and the top end of the component lead 15 is pushed and thrust along the inserting hole 14. Thrusting is continued even when the top end of the component lead 15 bears against the holding plate 13b of the attaching land-forming portion, so that the holding plate 13b is broken by the thrusting force, and the top end of the component lead 15 finally projects through the front surface of the printed circuit base board 11.

By this inserting and thrusting operation, as shown in Figure 7, the holding plate 13b of the attaching land-forming portion is broken and the resulting fragments 1 3b' of the holding plate 13b are bulged and bent toward the front surface of the printed circuit b base board 11 to encircle and hold the projected portion 15a of the component lead 15.

Then, as shown in Figure 8, the projected portion 1 5a of the component lead 15 is soldered to the attaching land 13a so that the holding. plate fragments 13b' encircling and holding the projected portion 1 5a of the component lead 15 are completely covered with solder 16.

Thus, the component leads 15 can be attached to the printed circuit base board 11 while electrically connecting the peripheral parts of the projected portions 15a of the component leads 15 of the attaching lands 13a.

As will be apparent from the foregoing illustration, according to the method of the present invention for attaching component leads to a printed circuit base board, holes for insertion of component leads are formed only at parts of the insulating plate contiguousto a land-forming conductive layer while leaving said land-forming conductive layer undrilled and the component leads are inserted while piercing holding plates of the undrilled land-forming conductive portion at the inserting step. Accordingly, fragments of the holding plates broken by the inserted component leads are bulged and bent toward the front surface of the printed circuit base board while encircling the projected portions of the component leads in the tightly contacted state. Therefore, these encircling fragments of the holding plates always hold tightly the projected portions of the lead components.In this tightly held state, the projected portions are soldered and hence, soldering can be effected assuredly. Namely, fragments of the holding plates encircling the projected portions of the component leads facilitate spreading of the solder and the projected portions can be completely soldered.

It is found that with the method described above in accordance with the invention component leads can be attached completely and tightly even by a customary automatic soldering operation and occurrence of the tunnel phenomenon can be prevented.

Moreover, complete soldering becomes possible and the better joints allow improved electric characteristics in the printed circuit. Furthermore, breakage of leads by vibrations or the like can-be reduced, thereby improving the electric strength of the circuit.

Since the reliability of the automatic soldering operation can be thus improved, the productivity in automatic insertion of component leads into inserting holes formed on the printed circuit base board can be substantially enhanced and automation of this operation can be remarkably promoted.

Additionally, since soldering is conducted on component leads tightly held by fragments of holding plates of the land-forming conductive portions, in the solder masking operation conducted for preventing bridging (formation of short circuits) at the soldering step, such large escape areas as adopted in the conventional methods (areas having a width a in Figure 4), for example, escape areas of a width of 0.8 to 251l or more to the substantial diameter of inserting holes, need to be formed, and in some cases, it is possible to form a solder mask 17 including a masking resist formed throughout all the areas except the circular portions having a diameter corresponding to that of the inserting holes 14, namely a complete overcoat 17a (complete overcoat on areas of a high component density in IC, LSI or the like), as shown in Figure 9.Thus, the solder masking effect can be remarkably improved, and the distance between two adjacent inserting holes can be narrowed. Accordingly, the circuit density can be improved by designing printed circuits.

Synthetic resin insulating plates, especially laminated plates comprising a phenol-formaldehyde resin plate and a paper substrate, are ordinarily used as insulating substrates of printed circuit base boards.

In addition, laminates comprising an epoxy resin, melamine resin, silicone resin or fluorine resin sheet and a glass cloth or flexible thermoplastic resin plates can be used according to need. An electrically conductive material, typically a copper foil, is applied in a thickness of 0.01 to 0.2 mm, especially 0.035 to 0.1 mm, on the surface of such insulating substrate.

The copper foil constituting the attaching landforming portions is very thin, and component leads are ordinarily composed of a copper wire having a diameter of about 0.5 and about 0.7 mm and they are relatively strong and stiff. Accordingly, when the component leads are inserted and thrust into the inserting holes, the copper foil constituting the holding plates of the attaching land-forming portions are readily broken by the thrusting force and the top ends of the component leads readily pierce through the land-forming portions. According to need and preferably, in order to rupture the holding plates more easily and more assuredly and/or to rupture the holding plates into fragments as uniform in the shape and size as possible, various contrivances as shown in Figures 10-A to 10-C may be made of the holding plates of the attaching landforming portions.

For example, as shown in Figure 10-A, a guide hole 18 having a diameter smaller than the inserting hole 14 may be formed at the centre of the holding plate 13b of the attaching land-forming portion closing one end of the inserting hole 14. The diameter of this guide hole 18 is not particularly critical and may be changed in a broad range depending on the method of inserting component leads into inserting holes, the diameter of combo nent leads, the thickness of holding plates and other factors. In general, however, the diameter of the guide hole 18 is adjusted to 2/10 to 8/10 of the diameter of the inserting hole 14 and preferably to 3/10 to 7/10 of the diameter of the inserting hole 14.

Further, as shown in Figure 10-B, a concave face 19 may be formed on the surface of the holding plate 13b on the side of the inserting hole 14 so that the thickness is gradually decreased toward the centre from the periphery of the hole 14. The maximum depth (d) of the concave face is preferably about 1/30 to about 1/20, especially 1/30 to 2/30, of the thickness of the copper foil constituting the holding plate.

Further, a guide hole 20 may be formed at the centre of such concave face 19 as shown in Figure 10-C. The diameter of this guide hole 20 may be 2/10 to 8/10, especially advantageously 3/10 to 7/10, of the diameter of the inserting hole 14, though the preferred diameter of the guide hole 20 varies depending on the depth of the concave face 19 and other factors.

When a guide hole is disposed on the holding plate and/or a concave face is formed on the holding plate as described above, rupture of the holding plate portion by the component lead can be remarkably facilitated. This guide hole 20 may be formed at the circuit pattern etching step.

In accordance with one preferred embodiment of the present invention, it has been found that when at least one line slit or cross line slit extending in the radial diretion from the centre of the holding plate is formed on the holding plate portion, the fragmentation of the conductive layer becomes uniform and consistent.

The number of such line slits or cross line slits to be formed on the holding plate is not particularly critical, but it can be changed in a broad range depending on the thickness of the holding plate and other factors. In general, however, good effects are obtained when 2 to 6 slits are formed. These slits may be arranged, for example, as shown in Figures 11 -A to 11-C. Namely, a line slit or cross line slit 21 may be formed as one line passing through the centre of the holding plate 13b as shown in Figure 11-A. Further, three line slits or cross line slits 21 a, 21 b and 21 c may be formed so that they extend from the centre of the holding plate 13b in three directions, respectively, as shown in Figure 11-B.Still further, four slits 21 a, 21 b, 21 c and 21 d extending from the centre in four directions, respectively, in a cruciform pattern may be formed as shown in Figure 11-C.

It is preferred that the length of such line slits or cross line slits be adjusted so that the top end thereof arrives at the edge of the inserting hole 14 or is extended up to just before the edge of the inserting hole 14.

Such line slits or cross line slits can easily be formed by mechanical processing or chemical processing such as photo-etching. For example, they may be formed simultaneously with formation of a circuit pattern.

When line slits or cross line slits are formed on the holding plates as described above, at the step of inserting the component leads 15 into inserting holes 14, the holding plates 1 3b can be rupture into fragments having predetermined size and shape, and the peripheries of the projected portions 15a of the component leads 15 can be held in the homegeneous (free of deviations) and stable state by these fragments. Accordingly, the intended effects can be attained stably and assuredly.

In accordance with another preferred embodiment, a tapered guide portion 22 is formed on the open end 14a of the inserting hole 14 as shown in Figure 12, whereby the operation of inserting the component lead 15 into the inserting hole 14 can be performed very easily and promptly and especially, the automatic inserting operation can be accomplished very assuredly.

As will be apparent from the foregoing illustration, according to the present invention, the abovementioned troubles and disadvantages caused in the conventional methods for attaching component leads to printed circuit base boards by the presence of gaps or clearances between component leads and inserting holes or printed circuit base boards can be eliminated or reduced, and the electric reliability and operation efficiency in the attaching method of this type can be improved. Automation of the operation of attaching component leads to printed circuit base boards can be promoted. Still further, the pull strength (resistance against pulling) or component leads after soldering can be improved. For example pull strengths of 7 to 8 kg/cm2 have been measured with use of the present invention, whereas a pull strength of 3 to 4 kg/cm2 is usual in the conventional methods.

Claims

1. A method for attaching component leads to printed circuit base boards, which comprises drilling a printed circuit base board comprising an insulating substrate having on one surface thereof a thin electric circuit layer composed of an electrically conductive material at parts of the insulating substrate contiguous to component lead attaching landforming portions of said electric circuit layer from the opposite surface of the printed circuit base board, while leaving said land-forming portions undrilled, thereby to form lead-inserting holes having a size sufficient to insert component leads exactly therein and having one end closed on the side of said land-forming portions of the electric circuit layer and the other end opened, inserting component leads into said inserting holes from the open ends thereof, extending the top ends of the component leads through the land-forming portions to project the top ends of the component leads on the printed circuit base board, and soldering said projected top ends of the component leads to said land-forming portions of the electric circuit layer.

2. A printed circuit base board comprising an insulating substrate having on one surface thereof a thin electric circuit layer composed of an electrically conductive material, wherein said printed circuit base board is drilled at parts of the insulating substrate contiguous to component lead attaching land-forming portions of said electric circuit layer from the opposite surface of the printed circuit base board, while leaving said land-forming portions undrilled, thereby to form composite lead-inserting holes having a size sufficient to insert component leads exactly therein and having one end closed on the side of said land-forming portions of the electric circuit layer and the other end opened.

3. A method or board according to Claim 1 or Claim 2 wherein the thickness of the component lead attaching lead-forming portions is in the range of from 341l to36cm.

4. A method or board according to any of the preceding claims wherein guide holes having a diameter smaller than the diameter of the component lead-inserting holes are formed on the landforming portions closing one end of said inserting holes.

5. A method or board according to Claim 4 wherein the diameter of said guide holes is 2/10 to 8/10 of the diameter of said component leadinserting holes.

6. A method or board according to Claim 1, Claim 2 or Claim 3 wherein concave faces are formed on the land-forming portions closing one end of the component lead-inserting holes so that the thickness of the land-forming portions are gradually decreased from the peripheries toward the centres thereof.

7. A method or board according to Claim 6 wherein the maximum depth of the concave faces is 1/30 to 1/20 of the thickness of the land-forming portions.

8. A method according to Claim 6 or Claim 7 wherein a guide hole is formed at the centre of each of said concave faces.

9. A method according to Claim 8 wherein the diameter of the guide hole is 2/10 to 8/10 of the diameter of the component lead-inserting holes.

10. A method according to Claim 1, Claim 2 or Claim 3 wherein at least one line slit or cross line slit is formed on each of the land-forming portions closing one end of the component lead-inserting holes so that said line slit or cross line slit extends from the centre of the land-forming portion in the radial direction thereof.

11. A method of attaching component leads to printed circuit base boards substantially as hereinbefore described with reference to any of Figures 5 to 12 of the accompanying drawings.

12. A printed circuit base board substantially as hereinbefore described with reference to any of Figures 5 or 9 to 12 of the accompanying drawings.