GB2398937A

GB2398937A - Connecting method and connecting structure of printed circuit boards

Info

Publication number: GB2398937A
Application number: GB0404460A
Authority: GB
Inventors: Makoto Totani; Toshihiro Miyake
Original assignee: Denso Corp
Current assignee: Denso Corp
Priority date: 2000-08-04
Filing date: 2001-08-01
Publication date: 2004-09-01
Anticipated expiration: 2021-08-01
Also published as: GB2398937B; GB0404459D0; GB2398936A; GB2398936B; GB0404460D0

Abstract

Lands 13a formed on a printed circuit board 5 are electrically connected with lands 11a formed on a printed circuit board 2 through solder 14. At this point, solder resist 20 is formed between neighboring two lands on the printed circuit board 2, and is terminated with an end portion that is interposed between the printed circuit board 2 and the printed circuit board 5. Accordingly, even when surplus solder is extruded onto the rigid printed circuit board 2, the solder resist 20 can prevent solder bridges from being formed between the lands. The two boards are fused together by subjecting them to pressure and temperature sufficient to allow soldering. Board 5 may be flexible whilst board 2 may be rigid.

Description

CONNECTING METHOD AND CONNECTING STRUCTURE OF PRINTED CIRCUIT

BOARDS

Description

This invention relates to a connecting method and a connecting structure of printed circuit boards.

Conventionally, solder is used to connect terminals of a flexible printed circuit board and a rigid printed circuit board.

For instance, "Guidance to High Density Flexible Board" (written by Kenji Numakura, printed by Nikkan Kogyo Co. Ltd.) discloses on page 100 a connecting structure of a rigid (hard) printed board and a flexible board using thermal fusion with solder According to this structure, as shown in FIG. 1, a land 60a of a rigid printed board 60 is connected with a land 61a of a flexible board 61 by solder 62. The flexible board 61 is adhered to the rigid printed board 60 by adhesive 63.

However, when connection of terminals (lands) of the two boards is achieved by using the solder 62 as shown in FIG. 1, if the amount of solder is excessive, solder bridges are occasionally formed between two adjacent lands. That is, when the lands, which are overlapped with each other after the solder is supplied, are heated and pressurized, the solder is heated and becomes molten. At that time, despite very low Nettability of an insulating substrate material to solder, if the amount of solder is excessive, the pressurized solder can flow out from the land to the insulating substrate and to a neighbouring land so as to form solder bridges.

An aim of preferred embodiments of the present invention is to provide a connecting method and connecting structure for printed circuit boards, capable of reducing the occurrence of solder bridges According to a first aspect of the present invention there is provided a method of connecting a first printed circuit board and a second printed circuit board respectively having a first plurality of lands and a second plurality of lands, comprising forming a solder resist portion on the second printed circuit board between two neighboring lands of the second plurality of lands, and the first and second printed circuit boards being overlapped with each other so that the solder resist portion is arranged to have an end portion that is interposed between the first and second printed circuit boards.

The method may comprise fomming a conductive pattern having a first plurality of lands on a first printed circuit board and forming a conductive pattern having a second plurality of lands on a second printed circuit board. The first and/or second printed circuit board may comprise a board made of thermoplastic resin.

The method suitably further comprises supplying solder to at least one side of the first plurality of lands and the second plurality of lands.

The method suitable further comprises pressurising and heating a connecting portion of the first printed circuit board and the second printed circuit board at a predetermined temperature to electronically connect the first plurality of lands and the second plurality of lands through the solder. Suitably, the pressurising and heating step adheres the first printed circuit board to a surface of the second printed circuit board by thermoplastic resin of the first printed circuit board that is softened and deformed at the predetermined temperature.

Suitably, the predetermined temperature is equal to or higher than a glass transition temperature of the themmoplastic resin and is equal to or higher than a fusing temperature of the solder.

Suitably, the solder resist portion is terminated to have an end portion that is interposed between the first printed circuit board and the second printed circuit board at the connecting portion.

Thus, because the solder resist portion is fommed between two neighboring lands of the second plurality of lands, solder bridges can be prevented from being fommed between the lands even when surplus sower is extruded onto the surface of the second printed circuit board when the two boards are pressurized. In the structure in which the end portion of the solder resist portion is interposed between the first and second printed circuit boards the region where the first and second printed circuit boards are adhered to each other can be provided sufficiently, and the solder resist portion causes no decrease in adhesive strength.

According to a second aspect of the present invention, there is provided a method of connecting printed circuit boards comprising, forming a protective film on a second conductive pattern of a second printed circuit board, and overlapping the first printed circuit board with the second printed circuit board to form a gap exposing the second conductive pattern, between an edge face of the first printed circuit board and an edge face of the protective film. The gap works as an escape space into which surplus solder can be extruded from the connecting portion between the first and second printed circuit boards when the connecting portion is pressurized.

A method according to the second aspect of the invention, wherein the protective film is formed on the second printed circuit board except on a connecting portion where the second conductive pattern is to be connected to the first conductive pattern of the first printed circuit board.

Suitably, the method of the second aspect of the invention further comprises applying solder to at least one side of the first plurality of lands and the second plurality of lands.

Suitably, when the first printed circuit board is overlapped with the second printed circuit board in the method according to the second aspect of the invention, the solder is interposed between the first plurality of lands and the second plurality of lands.

According to a third aspect of the invention, when a connection portion of first and second printed circuit boards is pressurized, a pressurizing member is used to apply larger pressure to a middle portion of a first conductive pattern on the first printed circuK board than that to an end portion of the first conductive pattern. The pressurizing member preferably has an edge portion that is tapered, recessed or stepped. Therefore, surplus solder movers from the middle portion receiving larger pressure toward the end portion receiving smaller pressure, resuming in decreased amount of extruder solder.

In a fourth aspect of the present invention there is provided a connecting structure of printed circuit boards comprising: a solder interposed between a first plurality of lands on a first printed circuK board and a second plurality of lands on a second printed circuK board, which solder electrically connects the first pluralKy of lands and the second pluralKy of lands, and a solder resist portion disposed on the second printed circuK board between two neighbouring lands of the second pluralKy of lands, extending to a connecting portion of the first and second circuK boards, and being terminated with an end portion that is interposed between the first printed circuK board and the second printed circuK board.

Any feature of any aspect of the present invention may be combined with any feature of any other aspect of the present invention.

Other aims and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below, provided by way of example only, with reference to the following drawings, in which: FIG. 1 is a cross-sectional view showing a prior art connecting structure of two printed circuK boards; FIG. 2 is a perspective view showing an electronic equipment in a first preferred embodiment; FIG. 3 is a perspective view showing a connecting portion of two boards according to a first embodiment; FIG. 4 is a cross-sectional view for explaining a method for connecting the two boards according to the fiat embodiment; FIG. 5 is a plan view schematically showing a bonding region according to the first embodiment; FIGS. 6A and 6B are cross-sectional views showing a connecting structure according to a second preferred embodiment; FIG. 7 is a plan view schematically showing a flexible printed circuit board according to the second embodiment; and FIGS. 8A and 8B are cross-sectional views showing a connecting structure according to a third preferred embodiment.

(First Embodiment) A first preferred embodiment of the present invention is described below with reference to appended drawings.

FIG. 2 shows a part of an electronic equipment accordingtoafirst embodiment of the present invention. A rigid printed circuit board 1 and a rigid printed circuit board 2 are supported inside the electronic equipment.

Various types of electronic components are mounted on the rigid printed circuit board 1, and FIG. 2 shows a state where an IC Sofa DIP package is mounted by using pins3a. Similarly,various types of electronic components 4 are mounted on the rigid printed circuit board 2. Each of the rigid printed circuit boards 1 and 2isconstructedofaninsulatingsubstratelOmadeof,forinstance, glass clothbased epoxy resin.

A flexible printed circuit board5is electrically connected with the edge portions of the rigid printed circuit board 1 and the rigid printed circuit board 2 that are placed horizontally on the upper and the lower sides. An insulating substrate 12 (see figure 3) of the flexible printed circuit board 5 is made of thermoplastic resin (@PEEK) containing 65 to 35 % by weight of polyether ether ketone (PEEK) and 35 to 65 % by weight of polyether imide (PET).

@PEEK is thermoplastic resin that softens at a temperature not lower than the glass transition temperature.

FIG.3shows an enlarged view of the portion where the rigid printed circuit board 2 is connected with the flexible printed circuit boards. InFIG.3, it should tee nosed that theinsulating substrate 12 and a solder resist film 16 are illustrated as being transparent for making the connecting structure understood easier for convenience.

Several conductive patterns 11 are formed on the upper surface of the rigid primed circuit boards, end are respectively terminated at positions located at a predetermined distance from the edge portion of the rigid printed circuit board 2. Further, lands lla are respectively formed on the end portions of the conductive patterns 11. Solder 14 is supplied onto the lands lla as a connection material.

Likewise, several conductive patterns 13 are formed on the surface of the flexible primed circuit boards in correspondence totheconductivepatternsllprovidedontherigidprintedcircuit board 2, and lands 13a are respectively formed as connection terminals on the end portions of the conductive patterns 13. The conductive patterns 11 and 13 are mace of copper. The conductive patterns 13 include several wiring members extending in a longitudinal direction of the flexible printed circuit board 5, and are covered with solder resist 16 as a protective film except for the lands 13a.

At the connecting portion of the rigid printed circuit board 2 and the flexible printed circuit board 5, the lands lla of the conductive patterns 11 are respectively connected with the lands 13a of the conductive patterns 13 by the solder 14. The glass epoxy resin (insulating substrate) 10 constituting the rigid printed circuit board 2 is adhered to the @PEEK (insulating substrate) 12 constituting the flexible printed circuit board by the SPEED 12 that is deformed at the portions exposed between the conductive patterns 11 and 13.

On the rigid printed circuit board 2, solder resist 20 is disposed between adjacent two conductive patterns 11 (including thelandslla). The solder resist20 provides a specific gap(30pm or more) with each of the conductive patterns 11, and extends along the conductive patterns 11. An end portion of the solder resist 20 intrudes into (is interposed between) the adhering surfaces of the rigid printed circuit board 2 and the flexible printed circuit board 5.

Next,themethodforconnectingtheflexibleprintedcircuit board 5 as a first printed circuit board and the rigid printed circuit board 2 as a second printed circuit board is explained below with reference to FIGS. 4 and 5.

First, the conductive patterns 11 are formed on the insulating substrate 10 of the rigid printed circuit board 2.

At this point, the conductive patterns 11 are not formed on the front edge portion of the rigid printed circuit board 2, so that the glass epoxy resin 10 is exposed at the front edge portion of the rigid printed circuit board 2. Then, the solder resist is formed to cover the conductive patterns 11 on the rigid printed circuit board 2. The region where the solder resist 20 isformedisshowninFIG.5. AsshowninFIG. 5, the solder resist is formed to cover almost the entire surface of the insulating substrate 10 having the conductive patterns 11, apart from the edge oftheinsulatingsubstratel2Of the flexible printed circuit board 5. Further, the solder resist 20 has several protruding portions each of which extends between adjacent two conductive patternsllto have en end portionintervening between the adhering surfaces of the two insulating substrates 10, 12 when the two boards 2, 5 are overlapped with each other.

Next, solder paste is applied to the lands lla of the conductivepatternsllasthesolderl4thereon. Inthisinstance, the solder 14 may be formed on the lands lla by solder plating or solder coaling. In the present case, tin-lead eutectic solder is used as the solder 14, which has a melting point (fusing temperature) of 183 C.

A flux or a hydrocarbon compound such as an alkane is applied to or mixed with the solder 14 to secure nettability of the solder 14. Particularly,incase of coating using a hydrocarbon compound such es analkane, it is preferred that not only the solder 14, but also the entire overlapping surfaces of the both substrates are coated therewith. In this case, while interposing alkane between the rigid printed circuit board 2 and the flexible printed circuit board 5, they are heated to a temperature not lower than the boiling point of alkane. Thus, alkane intrudes into the surface of the @PEEK 12, and as a result, a layer containing alkane dispersed therein is formed on the surface of the @PEEK 12. The dispersion layer thus formed exhibits a modulus of elasticity lower than the initial modulus of elasticity possessed by the @PEEK 12. That is, the adhesiveness of the @PEEK 12 (to the insulating substrate 10) can be improved by forming the dispersion layer on the surface of the @PEEK 12.

Then, the conductive patterns 13 are formed on the flexible printed circuit board 5 in such a manner that they correspond to the conductive patterns 11 of the rigid printed circuit board 2. Subsequently, the solder resist 16 is formed on the flexible printed circuit board 5 except the lands 13a of the conductive patterns 13 and the portions between the lands 13. The flexible printed circuit board 5 thus formed is aligned and overlapped with the rigid printed circuit board 2. At this point, the two boards 2, 5 are aligned so that a specific distance is defined between the edge of the flexible printed circuit board 5 and the solder resist 20 excluding the protruding portions thereof and so that the end portions of the protruding portions reach the connecting surfaces of the two boards 2, 5.

Next, referring to FIG. 4, the connecting portion, et which the rigid printed circuit board 2 and the flexible printed circuit board 5 are overlapped with each other, is heated while applying pressure by a thermocompression bonding tool (pressurizing member) 21. The glass transition temperature of the @PEEK 12 is in a range of 150 to 230 C, and the thermo-compression bonding tool 21 applies pressure to the connecting portion while controlling the temperature to fall in a range not lower than the fusing temperature of the solder 14 and not lower than the glass transition temperature of the @PEEK 12. For instance, the heat temperature is in a range of 240 to 400 C, and the heating and pressurization is continued for 5 to 15 seconds The thermo-compression bonding tool 21 is a pulse heating type in the present case.

The thermo-compression bonding tool 21 is explained in more detail below.

As shown in FIG. 4, the thermo-compression bonding tool 21 is formed to have an edge portion that is tapered and is placed at the front edge side of the flexible printed circuit board 5.

Therefore, whenthethermo-compressionbondingtool 21 pressurizes the flexible printed circuit board 5, large pressure is applied to portions corresponding to the middle portions of the conductive patterns 13 (lands 13a), formed on the flexible printed circuit board 5, as compared to the end portions of the conductive patterns 13.

Because of this, solder fuses and moves from a side of the middle portions of the conductive patterns 13 receiving larger pressure, toward a side of the end portions thereof receiving smaller pressure. Specifically, in the portion corresponding to the end portions of the conductive patterns 13, the pressure applied to the flexible printed circuit board 5 is small, and the edge portion of the board 5 is deformed along the shape of the thermo-compression bonding tool 21. In consequence, solder fillets14aareformedontheendportionsoftheconductivepatterns 13, thereby reducing the amount of solder extruded onto the insulating substrate 10 of the rigid printed circuit board 2.

While connecting the lands lla, 13a of the conductive patternsll, 13byfusedsolder14underheatingandpressurization by the thermo-compression bonding tool 21, the @PEEK 12 constituting the insulating substrate of the flexible printed circuit board 5 is softened and deformed to closely adhere to the connecting surface of the rigid printed circuit board 2. The thermo-compression bonding region bonded by the thermo-compression bonding tool 21 is shown in FIG. 5, where deformed @PEEK 12 adheres to the rigid printed circuit board 2 at portions exposed between the conductive patterns 11.

In this point, in the present embodiment, the protruding portions of the solder resist 20, which is disposed on the rigid printed circuit board 2, extend among the conductive patterns llwith the end portionsintervening between the adhering(bonding) surfaces of the two printed circuit boards 2, 5. Accordingly, even if the amount of the solder 14 supplied between the lands lla, 13a of the printed circuit boards 2, 5 is excessive so that the solder 14 extrudes onto the insulating substrate 10 of the rigid printed circuit board 2, the protruding portions of the solderresist20securelypreventbridgesfrombeingformedbetween neighboring lands lla.

In addition, the protruding portions of the solder resist are terminated so that the end portions thereof intervene between the adhering surfaces of the two printed circuit boards 2, 5. ] Because of this, the two printed circuit boards 2, 5 can adhere to each other with a sufficient area and a sufficient strength.

Here, the effect of the solder resist 20 with respect to the adhesive strength between the two printed circuit boards 2, is explained below. The solder resist 20 is constructed by, for instance, denatured epoxy resin used as a principal component to which filler, organic solvent, setting agent, antifoaming agent, ' etc., are added. The antifoaming agent contained in the solder I resist 20 significantlyweakens the adhesive strength with respect I to the @PEEK constituting the flexible printed circuit board 5 as a substrate material. Therefore, if the solder resist 20 having I the above-described composition intervened, along the conductive patterns 11, between the two printed circuit boards 2, 5 at the entire area of the thermo-compression bonding region, a sufficient adhesive strength could not be attainedwhen the two printed circuit boards 2, 5 were adhered to each other by softened and deformed i @PEEK.

In the present embodiment, because the solder resist 20 is formed in such a manner that only the end portions of the protruding portions thereof intervenes between the adhering I surfaces of the printed circuit boards 2, 5, the area of the solder resist 20 interposed between the two printed circuit boards 2, can be reduced as small as possible. It is most preferred that the end portions of the protruding portions of the solder resist contact the edge face of the flexible printed circuit board not to intervene between the adhering surfaces of the printed circuit boards 2, 5. Even in this case, the solder resist 20 can prevent the formation of bridges when the solder 14 extrudes onto theinsulatingsubstratelO, providedthattheendportionscontact the edge face of the flexible printed circuit board 5.

However, in the above-described structure, even slight positional slippage between the two boards 2, 5 generated in formation of the solder resist 20 or in alignment of the boards 2, 5 can produce a gap between the end portions of the protruding portions of the solder resist 20 and the edge face of the flexible printed circuit board 5. Therefore, in the present embodiment, the end portions of the solder resist 20 are disposed between the adhering surfaces of the two boards 2, 5 so that no gap is produced between the solder resist 20 and the flexible printed circuit board 5 even when a certain positional slippage arises.

Also, in the present embodiment, the protruding portions ofthesolderresist20arerespectivelydisposeddefiningspecific gaps from the conductive patternsll,on the rigid primed circuit board 2. When the solder 14 is fused to move on the conductive patterns 11, the solder 14 spreads wider than the conductive patterns 11 because pressure is applied to the flexible printed circuit board 5. In this case, if there is no gap between the solder resist 20 and the conductive patterns 11, there arises a case where solder receives resistance from the end portions of the solder resist 20 and extrudes in the vicinity of the end portions to form bridges. Therefore, a specific gap (more than 30pm) should be provided between each end portion of the solder resist20andeachconductivepatternlltopreventsuchphenomenon.

Although the preferred embodiment of the present invention is explained as above, the present invention is not limited to the above-described embodiment but may be changed in various manners.

For instance, in the above-described embodiment,the rigid printed circuit board 2 and the flexible printed circuit board are adhered to each other byntilizing the thermoplastic properly of the substrate material for the flexible printed circuit board 5, while performing the connection of the lands by the solder.

However, in the connection between the first primed circuit board and the second printed circuit board, both the printed circuit boards may be flexible printed circuit boards respectively made ofthermoplasticresin. In the case of utilizing the rigid printed circuit board, a ceramic substrate or a metal base substrate may be used as an insulating substrate thereof in place of the resin substrate.

Also, in place of @PEEK, polyether imide (PEI) or polyether ether ketone (PEEK) may be used alone as an insulating resin material for the flexible printed circuit board. Further, otherthermoplasticresinssuchaspolyethylenenaphthalate( PEN), polyethylene terephthalate ( PET), and liquid crystal polymer may tee used es an insulating materiel for the flexible printed circuit board. Otherwise, aninsulatingsubstrateoftheflexibleprinted circuit board may be composed of a polyimide substrate laminated with a layer made of a thermoplastic resin material containing at least one of PEEK, PET, PEN, and PET. The layer made of the thermoplastic resin material can be adhered to the polyimide substrateby adhesiveorthe like. Becausethepolyimide substrate has a thermal expansion coefficient of about 15 to 20ppm close to that (17 to 20ppm) of copper that is frequently used as wiring members, peeling off (separation), warping and the like of the flexible printed circuit board can be prevented.

In addition, in the above-described example, the solder 14 is supplied onto the lands lla of the conductive patterns 11 on the rigid printed circuit board 2; however, it may be supplied onto the lands 13a of the conductive patterns 13 on the flexible primed circuit boards. Otherwise,thesolder14 may tee provided on both the lands lla and 13a.

The shape of the lands lla and 13a may be either square or round, or the lands lla and 13a may have different shapes from one another. Especially from the view of preventing formation of solder bridges, each conductive pattern (land) is preferably tapered, i.e., decreased in width from the middle portion where thermo-compression bonding is performed toward the end portion thereof.

Accordingly, a sufficient distance between neighboring two conductive patterns can be secured easily at the edge portion of the thermocompression bonding region where solder bridges are liable to be formed. In this case, the shape of the solder resist disposed in the neighboring conductive patterns may be rectangular or be increased in width from the thermo-compression bonding region toward the end portions of the conductive pasterns in correspondence to the shape of the conductive patterns.

Further, in place of the solder resist 16 for the flexible primed circuit board5,acoverlayer mace of thermoplastic resin as described above may cover the conductive patterns 13. Solder resist containing denatured epoxy resin as a principal component cannot exhibit a sufficient adhesive strength with respect to epoxy resin es a substrate materialconstituting the rigid primed circuit board or solder resist having the same composition and formed on the substrata. As opposed to this, when the conductive patterns 13 on the flexible printed circuit board 5 are covered with the cover layer made of the above-described thermoplastic resin (@PEEK, PEEK, PEI, PEN, PET), the coverlayer firmly adheres to the epoxy resin constituting the rigid printed circuit board, wherebytheadhesivestrengthbetweenthetwoboardscanbeenhanced largely.

The solder resist 20 formed on the conductive patterns 11 of the rigid printed circuit board 2 may be replaced with the cover layer made of the thermoplastic resin as described above.

Inthiscase,becausetheadhesivenesswithrespecttothesubstrate material of the flexible printed circuit board 5 is enough, the length of each protruding portion of the cover layer disposed between neighboring two conductive patterns 11 may be changed appropriately in a range of a length substantially equal to that oftheconductivepatternslltoalengthwithwhichtheendportion thereof can intervene between the adhering surfaces of the two printed circuit boards 2, 5.

Meanwhile, it is not always necessary to provide the solder resist or the cover layer covering the conductive patterns 11, 13 on the respective printed circuit boards 2, 5. In this case, the solder resist or the thermoplastic resin should be disposed only between the respective neighboring conductive patterns 11 on the rigid printed circuit board 2 as solder resist preventive portions.

(Second Embodiment) A second preferred embodiment of the present invention is explained with reference to FIGS. 6A, 6B, and 7, in which the sameorsimilarpartsasthoseinthefirstembodimentaredesignated with the same reference numerals.

In the second embodiment, the conductive patterns (Cu patterns) 13 on the flexible printed circuit board 5 are covered with a protective film 116 except for the lands 13a as shown in FIG. 7. The protective film 116 is made of the same material (thermoplastic resin) as that constituting the flexible printed circuit board 5. The conductive patterns (Cu patterns) 11 on the rigid printed circuit board 2 are covered with solder resist 120 except for the lands lla.

Further, a solder plating 7 is formed on the lands 13a of the conductive patterns 13 on the flexible printed circuit board 5,andasolderleveler8isformedonthelandsllaoftheconductive patterns 11 on the rigid printed circuit board 2. The lands lla andthelandsl3aelectricallycommunicatewithoneanotherthrough the solder plating 7 and the solder leveler 8. Solder (tin-lead eutecticsolderhavingafusingtemperatureof183C)maybeformed on et least one side ofthe lands lla and the lands 13a. The other structural features aresubstantially the same as those in the first embodiment.

Next, the connecting method between the flexible printed circuit boards and the rigid printed circuit board2is explained.

First,the conductive patterns 11 are formed on the rigid printed circuit board 2 except a connecting surface front edge portion of the rigid printed circuit board 2. Accordingly, glass epoxy resin is exposed on the front edge portion of the rigid printed circuit board 2. After that, the solder resist 120 is formed on the rigid printed circuit board 2 except not only the front edge portion where the conductive patterns 11 are not formed but also the lands lla of the conductive patterns 11 and portions exposed between the lands lla, thereby covering the conductive patterns 11 with the solder resist 120. Further, the solder leveler 8 is formed on the lands lla of the conductive patterns 11.

On the other hand, the conductive patterns 13 are formed on the flexible printed circuit board 5 to correspond to the conductive patterns 11 on the rigid printed circuit board 2. The protective film 116 made of SPEED is formed on the conductive patterns 13 except the lands 13a and the portions exposed between the lands 13a. After that, the solder plating 7 is formed on the lands 13a.

A flux or a hydrocarbon compound such as an alkane is applied to at least one of the solder plating 7 and the solder leveler 8 so as to secure Nettability thereof. Especially in case of applying a hydrocarbon compound such as alkane, the hydrocarbon compound is preferably applied not only to the solder plating 7 or the solder leveler 8 but also to the entire area of the overlapping surfaces of the two boards 2, 5.

In this case, as described in the first embodiment, while interposing alkane between the rigid printed circuit board 2 and the flexible printed circuit board 5, they are heated to a temperature not lower than the boiling point of alkane. Thus, alkane intrudes into the surface of @PEEK forming the flexible printed circuit board 5, and as a result, a layer containing alkane dispersed therein is formed on the surface of the @PEEK. The dispersion layer thus formed exhibits a modulus of elasticity lower than the initial modulus of elasticity possessed by the @PEEK. That is, the adhesiveness of the @PEEK to an adhered layer (glass cloth-based epoxy resin) can be improved by forming the dispersion layer on the surface of the @PEEK.

The flexible printed circuit board 5 thus formed is aligned and overlapped with the rigid printed circuit board 2. At this point, the two boards 2, 5 are aligned so that a specific gap is defined between the edge face of the flexible printed circuit board 5 and the edge face of the solder resist 120. As a result, a part of the conductive patterns 11 on the rigid printed circuit board 2 is exposed from the gap.

Next, referring to FIGS. 6A and 6B, the connecting portion, where the rigid printed circuit board 2 and the flexible printed circuit board 5 are overlapped with each other, is heated while applying pressure by a thermo-compression bonding tool 22. The glass transition temperature of @PEEK 12 forming the flexible printed circuit board 5 and the protective film 116 is in a range of 150 to 230 C, end the thermocompressionbonding tool 22 applies pressure to the connecting portion while controlling the temperaturetofallinarangenotlowerthanthefusingtemperature of the solder7 end notlower then the grass transition temperature of @PEEK. For instance, the heat temperature is in a range of 240 to 400 C, and the heating and pressurization is continued for5to 15 sec. The thermo-compression bonding tool22 is apulse heating type in the present case.

Thethermo-compressionbondingtool22isexplainedinmore detail below.

Referring to FIG. 6A, similarly to the first embodiment, the thermocompression bonding tool 22 is formed to have an edge portion that is tapered (rounded) and is placed at the front edge side of the flexible printed circuit board 5. Therefore, when the thermo-compression bonding tool 22 pressurizes the flexible printed circuit board 5, large pressure is applied to a portion corresponding to the middle portions of the conductive patterns 13 (lands 13a), formed on the flexible printed circuit board 5, as compared to the portion corresponding to the end portions of the conductive patterns 13.

Because of this, solder fuses and moves from a side of the middle portions of the conductive patterns 13 which receives a larger pressuretowardasideoftheendPortions thereof which receives a smaller pressure. In the portion corresponding to the end portions of the conductive patterns 13, the pressure applied to the flexible printed circuit board 5 is small, and the edge portion of the board 5 is deformed along the shape of the thermo-compression bonding tool 22. In consequence, an amount of solder extruded from the conductive patterns 13 onto the insulating substrate of the rigid printed circuit board 2 can be reduced, thereby suppressing formation of solder bridges from being produced between neighboring conductive patterns.

While connecting the lands lla, 13a of the conductive patterns 11, 13 by fusing the solder 7 and the solder leveller 8 due to thermal compression by the thermo-compression bonding tool 22, SPEED 12 constituting the insulating substrate of the flexibleprintedcircuitboard5andtheprotectivefilmll6thereof issoftenedanddeformedtocloselyadheretotheconnectingsurface of the rigid printed circuit board 2.

In the present embodiment, because SPEED is used for the protective film 116, the protective film 116 firmly adheres to the front edge portion of the rigid printed circuit board2. Thus, theinsulating properly et the front edge side of the rigid primed circuit board 2 can be attained and simultaneously the adhesive strength between the two boards can tee enhanced. That is, because the protective film 116 and the portion of the flexible printed circuit board 5 that is deformed to intrude among the lands lla cancloselyadheretotherigidprintedcircuitboard2,theadhering area between the two boards is increased to increase the adhesive strength between them.

Also in the present embodiment, because the two boards 2, are aligned to define a predetermined gap between the edge face of the flexible printed circuit board 5 and the edge face of the solder resist 120, the gap can be utilized as a space (escape space) into which surplus solder escapee when the amount of solder onthelands lla,13ais excessive. This gap can prevent formation of solder bridge, in cooperation with the point that the thermo-compression bonding tool 22 applies lower pressure to the edge portion of the flexible printed circuit board 5.

In addition, whether both thelands are connected with each other through solder or not can be checked by appearance of the shape of solder fillet that is formed in this gap by thermo-compression. The reason is explained below.

In order to electrically connect the two boards 2, 5, after solder is applied to one or both of the lands lla, 13a, the lands lla, 13aaresuperposedtoeachother,andareheatedandpressurized by the thermocompression bonding tool 22 so that the solder is fused. At this point, if solder applied to one ofthelands adheres to (wets) the other one of the lands or solder on the other one of the lands, the connection through the solder is effectively achieved.

However, there is a case where solder applied to one of the lands does not adhere to (wet) the other one of the lands or solder on the other one of the lands due to an oxide film formed on the surface thereof. In this case, solder flows toward the gap as described above due to the pressurization of the thermo-compressionbondingtool22. Then, becausethesolderdoes not wet the other land or the solder on the other land, the solder keeps separating from it in the gap. Thus, whether the connection through solder can be achieved effectively or not can be checked by the shape of the solder fillet formedin the gap. Specifically, if the solder fillet formed in the gap contacts both the upper andlowerlandslla,13awithoutseparatingfromthem, itis decided that the connection is good (effective). On the other hand, if the solder fillet separates from the other land or solder on the other land, it is decided that the connection is failure (ineffective).

After that, the gap in which the solder fillet is formed is sealed with insulating resin or the like. Accordingly, an insulating property of the electric connecting portion between the two boards can be secured. When the insulating property of the electric connecting portion is not required (such as a case whereitisaccommodatedina housing haying insulating properly)' this sealing step need not be performed.

(Third Embodiment) Nexttathirdpreferredembodimentofthepresentinvention is explained with reference to FIGS. 8A and 8B, mainly in points different from the second embodiment. The same points as in the second embodiment are not reiterated.

Specifically, the third embodiment differs from the second embodimentinapointsthat,asshowninFIGS.8Aand8B,aprotective film 216 for the flexible printed circuit board 5 is made of not @PEEK but solder resist. The two boards 2, 5 are aligned so that a specific gap is provided between the edge face of the solder resist 120 and the edge face of the conductive patterns 13 and so that another specific gap is provided between the edge face of the solder resist 216 formed on the conductive patterns 13 and the edge face of the conductive patterns 11 formed on the rigid printed circuit board 2. Then, the two boards 2, 5 are connected to each other by the thermo-compression bonding tool 22.

Accordingly, two escape spaces for surplus solder are providedbythegapbetweenthesolderresist120andtheconductive patterns 13 and the gap between the solder resist 216 and the conductive patterns 11. As a result, short-circuit between neighboring wiring members caused by surplus solder can be prevented more securely.

The solder resist 216 is formed from, for instance, denatured epoxy resin as a principal component, and additives such as filter, organic solvent, setting agent, and the like.

When adhering the solder resist 216 to the rigid printed circuit board 2 made of glass epoxy resin by heating and pressurizationafterthesolderresist216isformedontheflexible printed circuit board 5, the adhesive strength is insufficient es compared tothatin the second embodiment because both materiels have thermosetting properties. However, the solder resist 216 can be used as a protective film for the conductive patterns 13 on the flexible printed circuit board 5 at least in case where excessive stress is not applied to the connecting portion between the two boards 2, 5.

In the above-described embodiments, the thermo-compression bonding tool is tapered at the edge portion thereof so that it can apply larger pressure to the portion corresponding to the middle portions of the conductive patterns; however, the thermo-compression bonding tool may have a stepped portion or a recess portion in place of the tapered portion to exhibit the above effect. In the third embodiment, two gaps are provided between the solder resist 120 and the conductive patterns 13 and between the solder resist 216 and the conductive patterns llas escape spaces for surplus solder. The two spaces for surplus solder may be provided in the second embodiment as well.

While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.

Claims

Claims 1. A method of connecting printed circuit boards, comprising:

forming a first conductive pattern including a first plurality of lands, on a first printed circuit board that is made of a thermoplastic resin; forming a second conductive pattern including a second plurality of lands, on a second printed circuit board; forming a protective film on the second conductive pattern of the second printed circuit board except on a connecting portion where the second conductive pattern is to be connected to the first conductive pattern of the first printed circuit board; applying solder to at feast one side of the first plurality of lands and the second plurality of lands; overlapping the first plurality of lands with the second plurality of lands to interpose the solder between the first plurality of lands and the second plurality of lands, and to form a gap between an edge face of the first printed circuit board and an edge face of the protective film, the gap exposing the second conductive pattern therefrom; and pressurizingandheatingtheconnectingportionofthefirst printed circuit board and the second printed circuit board at a predetermined temperature to electrically connect the first plurality of lands and the second plurality of lands through the solder, andtoadherethefirstprintedcircuitboardtoaconnecting surface of the second printed circuit board by the thermoplastic resin of the first printed circuit board that is softened and deformed at the predetermined temperature, the predetermined temperature being equal to or higher than a glass transition temperatureofthethermoplasticresinandbeingequaltoorhigher than a fusing temperature of the solder, wherein: the gap between the edge face of the first printed circuit board and the edge face of the protective film works as an escape spaceinto which surplus solder can tee extruded from the connecting portion when the connecting portion is pressurized.
2. The connecting method of claim 1,further comprising: forming another protective film made of a thermoplastic resin,to cover the first conductive pattern on the first printed circuitboard, beforethefirstprintedcircuitboardisoverlapped with the second printed circuit board, wherein: the another protective film is adhered to the connecting surface of the second printed circuit board.
3. The connecting method of any one of claims 1 and 2, further comprising sealing the gap between the edge face of the first printed circuit board and the edge face of the protective film.
4. Theconnectingmethodofanyoneofclaims 1 to 3, wherein electrical connection between the first plurality of lands and the second plurality of lands is achieved simultaneously with adhesion of the first printed circuit board to the second printed circuit board.

-

-

-

_., --
5. A method of connecting printed circuit boards, substantially as described herein and with reference to figures 6A to 7 hereof.
6. A connecting structure of printed circuit boards substantially as described herein and with reference to figures 6A to 7 hereof.