Method for producing a printed circuit board with a heat radiating structure and a printed circuit board with a heat radiating structure

A first surface of a double-sided printed circuit board has a soldering land for heat radiation, which serves as a mounting surface for an electronic part. A land for solder absorption is formed on the second surface facing the mounting surface. Viaholes are provided and open in both the soldering land for heat radiation and the land for solder absorption at the opposite ends. Molten solder flows out from the openings of the viaholes and spreads on the land for solder absorption to suppress formation of solder balls. Cream solder is applied to the outer surface of the land for solder absorption to embed the solder and to form a solder layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a heat radiating structure of a printed circuit board and a printed circuit board producing method and is particularly designed to improve the heat radiating capability of a heat generating electronic part, such as an IC chip, to which a large current is applied and to prevent an occurrence of problems at the time of producing a printed circuit board having the heat radiating capability.

2. Description of the Related Art

A printed circuit board has a mounting surface for receiving and electronic part. The mounting surface has a land for soldering the electronic part. A through hole is formed in the land and heat is radiated to the other surface through the through hole.

For example, Japanese Unexamined Patent Publication No. H09-148691 andFIGS. 8(A),8(B) and8(C) herein disclose a heat generating element1mounted on a double-sided circuit board2. More particularly, the circuit board2has opposite first and second surfaces2aand2b. A first copper foil flat pattern3is provided the first surface2aof the circuit board2and the underside of the heat generating element1is secured to the first copper foil flat pattern3using solder or paste. The first copper foil flat pattern3is connected electrically to a second copper foil flat pattern5on the second surface2bthrough viaholes4. Conductive layers4aare formed on the inner circumferential surfaces of the viaholes4by plating. Thus, heat generated by the heat generating element1is radiated from the second copper foil flat pattern5on the second surface2bvia the first copper foil flat pattern3and viaholes4is transferred efficiently from the first copper foil flat pattern3through the viaholes4and is radiated from the second copper foil flat pattern5on the second surface2b. However, Japanese Unexamined Patent Publication No. H09-148691 simply discloses that the heat generating element1is secured to the copper foil flat pattern3using paste, solder or the like, and a specific securing method using solder or paste is unclear.

Further, Japanese Unexamined Patent Publication No. 2004-127992 discloses a printed circuit board with through holes that penetrate from the top surface to the under surface. The under surface is placed on a base plate. A resin paste is printed from the topside by a squeegee so that the resin paste fills the through holes and prevents solder flowing into the through holes from forming protuberances at the underside.

The method of using a paste or solder for securing the heat generating element1and the copper foil flat pattern3to a printed circuit board having the conductor patterns on both surfaces is unclear in Japanese Unexamined Patent Publication No. H09-148691.FIGS. 9A to 9Dshow how molten solder might flow into the viaholes4while securing the copper foil flat pattern3to the heat generating element1provided with a heat sink for heat radiation and electrical connection on the underside thereof. In such a case, the molten solder might be solidified at the underside and so-called solder balls7amight project at the underside.

Specifically, cream solder7is applied to lands6that are solder-connected to lead terminals1aof a heat generating element1on one surface2aof a circuit board2and a copper foil flat pattern3for heat radiation as shown inFIG. 9A; a first reflow process is carried out by heating the circuit board2with the heat generating element1placed on the upper surface thereof as shown inFIG. 9(B); and the lead terminals1aof the heat generating element1and the underside of the heat generating element1are soldered respectively to the lands6and the copper foil flat pattern3.

The solder applied to the copper foil flat pattern3melts and can flow into the viaholes4. Thus, the solder may spill from the openings surrounded by a copper foil flat pattern5on the second surface2b, and solidifies as the solder balls7ashown inFIG. 9C.

In this state, a second reflow process is carried out after cream solder is applied to the lands on the second surface2bso that the copper foil pattern on the second surface2bis soldered to terminals and an electronic part.

At this time, a metal mask9is mounted and the cream solder is applied by a squeegee10as shown inFIG. 9(D). However, the solder balls7acause the metal mask9to become uneven. Thus, the metal mask9and squeegee10may be damaged while applying the cream solder.

The heat of the reflow process separates the cream solder into solder and flux for facilitating the soldering. The flux flows into the viaholes4together with the molten solder to adhere to the copper foil flat pattern5on the underside. The flux is adhesive, and hence the metal mask becomes difficult to remove after the cream solder is applied with the metal mask mounted on the second surface, which presents a problem of reducing operability.

The through holes may be filled beforehand to prevent the formation of the solder balls7ato close the through holes, as disclosed in Japanese Unexamined Patent Publication No. 2004-127992. However, this leads to an increased production cost because a resin applying step needs to be added.

The present invention was developed in view of the above problems, and an object thereof is to prevent improve the production process of a printed circuit board.

SUMMARY OF THE INVENTION

Accordingly, solder that flows into viaholes is prevented from being formed into solder balls. Thus, a metal mask does not become uneven and the metal mask and a squeegee used for cream application are not damaged when mounting the metal mask to apply cream solder to lands.

The invention relates to a method for producing a printed circuit board with opposite first and second surfaces and at least one heat radiating structure. The method includes preparing the printed circuit board with at least one conductor pattern and at least one land made of conductive foil formed on one or both surfaces of an insulating substrate. At least one soldering land for heat radiation is formed on a part of the first surface and defines a mounting surface for an electronic part. At least one land for solder absorption is formed on the second surface. At least one viahole has opposite ends that open in the soldering land for heat radiation and the land for solder absorption. The method then includes placing a metal mask on the first surface and applying cream solder to the lands and the soldering land portion for heat radiation. The method continues by placing the electronic part on the first surface, melting the cream solder in a first reflow process to solder-connect lead terminals of the electronic part and the respective lands, causing the molten solder leaking out towards the second surface through the viaholes to flow to the land for solder absorption and to adhere near the openings in a substantially flat manner, placing a metal mask on the second surface of the printed circuit board, applying cream solder to the land for solder absorption to cover the solder solidified near the openings of the viaholes, and melting the cream solder in a second reflow process to form a solder layer on the land for solder absorption.

According to a preferred embodiment of the invention, the method further comprises forming at least one solder resist on solder unnecessary parts of the printed circuit board to surround at least part of the soldering land for heat radiation and/or the land for solder absorption.

The first reflow process preferably comprises forming a solder layer adhering to the soldering land for heat radiation and the underside of the electronic part.

As described above, the solder melted in the first reflow process may flow into the viaholes and may leak out from the openings at the second surface. However, the land for solder absorption is provided at these openings and exposes the foil. Thus, the leaked-out solder spreads along the outer surface of the foil and deposits at the peripheries of the openings in a flat manner without forming solder balls. The absence of solder balls assures that the metal mask mounted on the second surface of the printed circuit board will not deform to become uneven. In addition, the cream solder also is applied to the land for solder absorption and the metal mask is formed with openings in a part corresponding to the land for solder absorption. Therefore the metal mask does not become uneven.

Damage of a squeegee and the metal mask can be prevented in the process of applying the cream solder to the upper surface of the metal mask by the squeegee. Further, the metal mask is not mounted on the land for solder absorption. Hence, there is no likelihood that the metal mask adheres because of flux and is made difficult to remove. In this way, problems that have occurred in the conventional production process can be solved, and production costs can be reduced by improving production efficiency.

A thickness of the cream solder applied for the first reflow process preferably is substantially equal to the thickness of the metal mask used in connection therewith.

The molten solder leaking through the viaholes and towards the second surface preferably forms solder deposited portions having a projecting height that is smaller than thickness of the metal mask used in connection with the second reflow process.

Cream solder for the second reflow process preferably is applied to the land for solder absorption up to substantially the same height as the metal mask while at least partly covering solder deposited portions near the openings of the respective viaholes.

The invention also relates to a heat radiating structure for a double-sided circuit board having conductor patterns on one or both surfaces. At least one soldering land for heat radiation is provided in an area of a second surface of the printed circuit board facing the underside of an electronic part. Viaholes penetrate the printed circuit board. Each viahole has one end that opens in the soldering land for heat radiation. At least one land for solder absorption is provided on the second surface of the printed circuit board at the viaholes. At least one solder layer is formed by reflowing cream solder applied to the soldering land for heat radiation between the underside of the electronic part and the soldering land for heat radiation. The melted solder leaks out through the viaholes, spreads on the land for solder absorption and adheres to the land for solder absorption in a substantially flat manner. The solder layer formed by reflowing the cream solder applied in a manner to cover the solder adhering in the substantially flat manner is provided on the land for solder absorption.

The heat radiating structure for the printed circuit board preferably is produced by the above-described producing method.

Specifically, it is preferable that the land for solder absorption faces the soldering land for heat radiation in the thickness direction of the printed circuit board while having substantially the same area as the soldering land for heat radiation.

Moreover, several viaholes preferably are arranged at specified intervals in forward and backward directions and/or transverse direction. The outer edge of the land for solder absorption is at a specified distance from the viaholes located at the outer ends in forward and backward direction and/or transverse direction.

The land for solder absorption has a wide area including the openings of all the viaholes and the outer edge thereof is at the specified distance from the openings of the viaholes located closest to this outer edge. Thus, molten solder and flux leaking from the openings of the viaholes is cannot adhere to the outer surface of the solder resist at the outer periphery.

Lead terminals preferably project from the outer side surface of the electronic part and are solder-connected to respective land portions provided on the one surface of the printed circuit board. The electronic part includes a heat radiating member formed of a radiation slug or heat sink on the underside thereof, and the solder layer on the soldering land for heat radiation is secured in surface contact with the heat radiating member.

The electronic part is provided with the heat radiating member on its underside, and the heat radiating member can be brought reliably substantially into surface contact with the solder layer formed on the outer surface of the soldering land for heat radiation. Therefore the heat radiating property of the electronic part can be improved.

As described above, the soldering land for heat radiation is provided at the underside of the electronic part generating heat and the land for solder absorption is provided on the other surface electrically connected to the soldering land through the viaholes. Solder in the solder layer between the underside of the electronic part and the soldering land for heat radiation is melted by heating and may leak out to the other side through the hollow parts of the viaholes. This solder spreads along the surface of the land for solder absorption and preventing the formation of protuberant solder balls. As a result, damage to the metal mask and squeegee can be prevented when the metal mask is mounted on the other surface and the cream solder is applied by the squeegee. Therefore, production efficiency can be improved.

These and other objects, features and advantages of the invention will become more apparent upon reading of the following detailed description of preferred embodiments and accompanying drawings. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A double-sided printed circuit board in accordance with the invention is identified by the numeral100inFIG. 1(A). The circuit board100has a substrate12with opposite first and second surfaces12aand12b. The substrate12is made of an insulating material, such as epoxy. First and second conductor patterns13A and13B are provided respectively on the first and second surfaces12a,12bof the insulating substrate12. The conductor patterns13A and13B are made of conductive material, preferably conductive foil material, such as copper foil, and are referred to collectively by the numeral13. An electronic part20is mounted on the first surface12a. The electronic part20includes an IC chip to which a large current can be applied.

Lead terminals20b,20cproject from the bottom left and right surfaces of a case20aof the electronic part20, and a heat sink21is attached to a surface of the electronic part20that faces towards first surface12aof the substrate12of the printed circuit board100. The lead terminals20b,20cof the electronic part20are solder-connected to lands16,17of the conductor patterns on the first surface12afor mounting the electronic part20on the first surface12a.

A soldering land22for heat radiation is defined by exposing the copper foil on the first surface12aof the substrate12of the printed circuit board100. Thus, the soldering land22for heat radiation faces the mounting side of the electronic part20where the heat sink21is provided. The soldering land22for heat radiation is surrounded by a solder resist18. Viaholes24are formed through the substrate12. Each viahole24has a first opening24aat a portion of the first surface12aof the substrate12corresponding to the soldering land22for heat radiation. Each viahole24also has a second opening24bat the second surface12aof the substrate12. The viaholes24are arranged to define a matrix array at substantially the same intervals in forward and backward directions and transverse directions. In this embodiment, as shown inFIG. 2, four viaholes24are arranged in transverse direction and three are arranged in forward and backward directions to provide a total of twelve viaholes24. However, the number of the viaholes24can be suitably selected according to the size of the electronic part20and/or the heat to be dissipated. Each viahole24has a conductive inner surface formed, for example, by plating (e.g. “Through Hole Plated” (THPlated)). The viaholes24are not used as component holes, but rather serve as interlayer connections for connecting conductive layers or patterns on the two surfaces12aand12b.

The copper foil at portions of the second surface12bof the substrate12adjacent the second openings24bof the viaholes24is exposed to define a land25for solder absorption. The land25for solder absorption has substantially the same area as the soldering land22for heat radiation and the two lands22,25are substantially registered in the thickness direction TD.

The outer periphery of the land25for solder absorption is determined by a solder resist26and is at a specified distance L from the viaholes21closest to this outer periphery. The land25for solder absorption and the soldering land22for heat radiation have substantially the same rectangular shape as the outer shape of the case20aof the electronic part20.

A solder layer27formed by reflowing cream solder is provided between the soldering land22for heat radiation and the heat sink21on the side where the electronic part20is to be mounted. The solder that becomes molten during reflowing flows as part of a first unitary solder matrix into the first openings24aof the viaholes24, along hollow parts enclosed by conductive layers28plated on the inner circumferential surfaces of the viaholes24, out through the second openings24bof the viaholes24and onto the soldering land22for heat radiation. As a result, the hollow parts of the viaholes24are filled with solder30. The solder30that flows out from the second openings24bof the viaholes24spreads along the copper foil surface of the land25for solder absorption to define substantially flat and low mountain-shaped solder depositions31. The lateral extension of each solder deposition31is at least about three times more than the height extension, more preferably at least about four times and most preferably at least about five times.

A second unitary matrix of solder32is provided on the land25for solder absorption by applying the cream solder sufficiently to cover the solder depositions31and then reflowing the cream solder.

The method for producing the double-sided printed circuit board100is illustrated inFIGS. 2 to 7. More particularly, the insulating substrate12is provided with copper foils laminated onto both opposite surfaces12a,12b. Through holes are formed at specified positions on the insulating substrate12by a drill, a laser cutting tool or the like. The conductive layers28are formed on the inner circumferential surfaces of the through holes by plating to form the viaholes24. Further, the copper foil patterns13A,13B are formed to have specified circuit configurations on both surfaces12a,12bof the circuit board preferably by etching. Subsequently, the solder resists18,26are formed on the surfaces12a,12bwhile leaving the soldering portions to attain the state shown inFIGS. 2(A) to 2(C).

As shown inFIGS. 2(A) to 2(C), the substantially rectangular soldering land22for heat radiation is formed on the first surface12aof the substrate12and substantially surrounds the first openings24aof the viaholes24to define an area that will substantially face a placing portion of the electronic part20. The soldering land portion22for heat radiation is surrounded by the solder resist18. Further, the copper foils of the lands16,17for solder connection with the lead terminals of the electronic part20are exposed at the opposite left and right sides of the soldering land22for heat radiation.

On the other hand, the land25for solder absorption is formed on the second surface12bto surround the second openings24bof the viaholes24and is surrounded by the solder resist26. The land25for solder absorption has substantially the same shape as the soldering land22for heat radiation and is registered with the soldering land22in the thickness direction TD of the insulating substrate12.

A metal mask35is mounted to expose only the soldering portion on the first surface12awhere the electronic part20is to be mounted, as shown inFIG. 3. The metal mask35has openings35ain its surface facing parts where the solder resist18is not provided, i.e. the soldering land22for heat radiation and the lands16,17.

Subsequently, cream solder36is put on a side of the outer surface of the metal mask35and is applied to the outer surface of the metal mask35by a squeegee or resilient scraping tool38as shown inFIGS. 4(A) and 4(B). The cream solder36is applied to the outer surfaces of the soldering land22for heat radiation and to the lands16,17at the openings35a. The thickness of the applied cream solder36substantially equals the thickness of the metal mask35. The metal mask35is removed after the cream solder36is applied.

The electronic part20then is placed as shown inFIG. 5so that the leading terminals20b,20ccontact with the cream solder36on the outer surfaces of the lands16,17, and the heat sink21on the underside of the electronic part20contacts the outer surface of the cream solder36on the outer surface of the soldering land22for heat radiation.

Heating then is performed at a specified temperature using heating means (not shown) to melt the cream solder36to perform a first reflow process. Thus, the lead terminals20b,20cof the electronic part20are solder-connected to the lands16,17using the solder in the cream solder36, as shown inFIG. 6.

Further, the cream solder36applied on the soldering land22for heat radiation melts to adhere the heat sink21and the copper foil of the soldering land22for heat radiation. The cream solder36solidifies upon removal from the reflow oven to become the solder layers27.

The molten cream solder36flows into hollow parts of the viaholes24exposed at the soldering land22for heat radiation and fills the hollow parts while adhering to the conductive layers28on the inner circumferential surfaces. The solidified cream solder36becomes the solders30.

The molten solder that flows into the viaholes24may also flow out from the second openings24bat the land25for solder absorption, but spreads along the copper foil because the copper foil is exposed on the surface of the land25for solder absorption. As a result, the molten solder spreads to form flat mountain-shaped projections about the openings24binstead of becoming protuberant solder balls as in the prior art.

The flat solder depositions31are formed at the second openings24bof the viaholes24at the land25for solder absorption when the molten solder is solidified and are part of a unitary matrix of solder that extends through the viaholes24. The height of the solder depositions31is smaller than thickness “t” of a metal mask40.

The metal mask40then is mounted on the second surface12band cream solder41is applied by the squeegee38, as shown inFIGS. 7(A) and 7(B).

The metal mask40has an opening40afacing the land25for solder absorption, and the height of the solder depositions31is shorter than the height of the metal mask40. Thus, the metal mask40can cover in a flat state substantially without becoming uneven due to solder balls as in the prior art. Accordingly, the cream solder41can be applied smoothly without damaging the metal mask40and squeegee38.

The cream solder41is applied to the land25for solder absorption to substantially the same height as the metal mask40while covering the solder depositions31centered on the openings24bof the respective viaholes24. Thus, even if flux F leaks to the outer surfaces of the solder depositions31, it is substantially embedded in the cream solder41and does not leak out to the outer surface of the cream solder41.

The metal mask40is removed after the application of the cream solder41. At this time, the flux F does not contact the metal mask40since the metal mask40has the opening and the solder depositions31are embedded in the cream solder41.

A second reflow process is carried out using the heating means to melt the cream solder41. As a result, the cream solder41is secured to the solder depositions31of the land25for solder absorption. The copper foil of the land25for solder absorption then solidifies to form the solder layer32.

The land25for solder absorption is connected through the viaholes24to the soldering land22for heat radiation. Thus, the molten solder that flows out to the second surface from the viaholes24can spread along the copper foil surface of the land25for solder absorption to prevent the formation of the solder balls projecting from the second surface.

Further, the land25for solder absorption is provided in a part where the molten solder flows out from the viaholes24. Thus, the metal mask40to be mounted on the solder resist26on the second surface has the opening in its part facing the land25for solder absorption and, hence, the metal mask40is not mounted on the solder depositions31formed by the flown-out and solidified solder. Thus, there is no likelihood of deforming the metal mask due to solder balls or protuberances, which have been a problem of the prior art. As a result, the damage of the metal mask40and squeegee38can be prevented when applying the cream solder41to the outer surface of the metal mask40by the squeegee38, and production efficiency can be improved.

The invention is not limited to the above embodiment. For example, even in the case where lead terminals project from the entire circumference of the case of the electronic part, a soldering land may be provided in an area surrounded by lands solder-connected to the lead terminals with the outer periphery thereof determined by a solder resist

The land for solder absorption on the second surface may be larger than the soldering land for heat radiation if possible in terms of space. In such a case, heat radiation capability can be improved further.

Furthermore, the land for solder absorption having a large area may double as a ground circuit or a power supply circuit.