METHOD OF MANUFACTURING A PRINTED CIRCUIT BOARD

Abstract: A method of manufacturing a printed circuit board including the step of selectively electroplating a high thick solder (340) onto selected conductive projections (230). The selective electrodeposition is performed by electroless applying a conductive layer (310) onto the board (100), previously provided with circuit conductors (220) and conductive projections (230), providing a high thick insulating mask (320) onto the conductive layer (310) leaving exposed the selected conductive projections (230) and electroplating the high thick solder (340). The method of manufacturing an electronic card by surface mounting of components (SMT) onto the printed circuit board (100) obtainable by the above-described method further includes the step of dispensing a flux, preferably a no clean flux dispensed by syringe, onto the board (100), avoiding the solder paste or adhesive screening.

Description: METHOD OF MANUFACTURING A PRINTED CIRCUIT BOARD The present invention relates to a method of manufacturing a printed circuit board and to a method of manufacturing an electronic card; particularly, it relates to a method of manufacturing a printed circuit board for surface mounting of components thereon. A printed circuit board (PCB) is an electronic package including a circuitized substrate for carrying one or more components, such as chips, transistors, capacitors, resistors and so on, to be connected to the circuit conductors thereof; the assembled device composed by the printed circuit board with the components connected thereto is generally called an electronic card. In the Surface Mounting Technology (SMT), the substrate surface of the PCB is provided with conductive projections, such as pads and lands (when surrounding a through-hole of the board), for connecting to electrical contacts of the components. Solder is deposited onto such conductive projections, for assembly of components onto the board, providing mechanical and electrical connection. The SMT process commonly requires the board to be coated with electroless conductive material, typically copper. A further layer of conductive material is then electroplated onto the board and subsequently etched through an etching mask. After stripping the etching mask, an insulating material is applied onto the whole board and exposed with the exception of the selected conductive projections, in order to leave the same conductive projections free after developing. The solder may then be deposited onto the conductive projections, previously finished with different methods known in the art, such aε ENTEK or HAS (Hot Air Solder Level). The ENTER method is described in "Surface investigation of copper in printed circuit board" - A.Manara, V.Sirtori - Surface and Interface analysiε - 1990 - Vol.15, Pag. 457-462, while the HASL method is described in "Soldering in electronics" - R.J. Klein Wassink - Ed. Electrochemical Publications. A critical step in the SMT processes known in the art is the solder paste screening required as a first step in the assembly process of the finished printed circuit board. This step depends on many factors, including the solder paste rheology, that is its visco-elastic properties. The solder paste rheology is itself affected by a lot of parameters, as vehicle chemical composition, powder granulometry, vehicle powder mixing process. In addition, the screening operation is itself influenced by many factors, such as the machine setup, shape of stencil window, stencil cleaning frequency, operator skill. A further emerging technology in the SMT field is the so called Flip Chip Attachment (FCA). It consists in bonding directly chips, provided with several ball contacts, onto small PCB conductive projections. This is a very complicate and difficult operation because of the very small dimensions of the conductive projections. Presently, the bonding of the chips onto the conductive projections is done with several different techniques, like the deposit of the solder by a dedicated and expensive machine that injects the melted solder on the conductive projections, or the screening of conductive adhesives or solder paste. The drawbacks of these methods are the cost of the solder injection machine and of the conductive adhesives and the high complexity and criticalness of the conductive adhesive and solder paste screening. A method of electroplating solder onto selected areas of the conductors of a printed circuit board is described in US Patent N.4,304,640. The method includes the steps of providing printed circuit conductors on the board, electroplating a conductive layer onto the whole surface, forming an insulating mask and electroplating the solder onto the selected areas of the conductors. However, the described process is effective for selectively electroplating only a thin layer of solder onto the circuit conductors; the deposited layer can be uεed exclusively to protect the electroplated conductive layer (typically copper) from oxidation. The above described process cannot be applied for particular or general soldering processes, such aε SMT, FCA, or similar. A selective electrodeposition of a high thick layer of solder onto the conductive projections of a PCB for surface mounting of components thereon is not known in the art. It should be noted that the electrodeposition of a high thick layer implies a longer and more critical process (higher adherence and sealing betv/een the insulating mask and the substrate) and the availability of a different insulating material (thicker and water soluble). The above drawbacks of the prior art are overcome by the invention aε claimed. Accordingly, the present invention provides a method of manufacturing a printed circuit board for surface mounting of components thereon, said method including the stepε of providing circuit conductors and conductive projections onto said board and selectively electroplating a high thick solder onto said conductive projections. The method according to the present invention provides for the selective electrodeposition of high thick solder only onto selected conductive projections and not onto circuit conductors; particularly, the method can be used to electroplate solder onto selected pads dedicated to the components soldering on both sideε of a double-sided printed circuit board and in the holes of the same board. The method according to the present invention replaces the successive solder paste screening performed in the SMT process with the selective electrochemical deposition (ECD) of high thickness solder. The solder layer is electroplated only on the conductive projections dedicated to the component soldering, presently covered with the solder paste. The elimination of the screening improves significantly the procesε yield and it reduces the assembly cycle time and the man power. Presently one operator must be dedicated to the solder paste or the conductive adhesive screener, whose setup (the matching of the board conductive projectionε with stencil window) is very critical because it iε required a high mechanical precision on large surfaces. Moreover the stencil cleaning, while the machine is operating, is continuously necessary and so the stencil washing at the end of the production with a lot of time losing. Preferably, said step of selectively electroplating a high thick solder includes the stepε of electroleεs applying a conductive layer onto εaid board, providing a high thick insulating mask onto said conductive layer leaving exposed said selected conductive projectionε, electroplating said high thick solder onto said selected conductive projections and stripping said insulating mask. The solder selective electroplating is obtained using the current process steps, excepting the electroless copper etching, only changing the process step sequence. No additional cost iε then involved by the process according to this embodiment of the invention. Typically, said insulating mask has a thickness comprised between 50 μm and 200 μm, with a preferred value of 75 μm. In an advantageous embodiment of the present invention, said insulating mask is a dry film. The dry film offers a higher adherence and sealing with the subεtrate, it is thicker and water soluble; in addition, the dry film deposition offers a good geometrical definition. Preferably, the above described method further includes the step of electroplating a conductive material onto εaid selected conductive projections before the step of electroplating said high thick solder. This conductive material is useful to ensure a better electrical connection between the circuits on either face of the board. In a typical process of manufacturing a PCB, said step of providing circuit conductors and conductive projections includes the steps of providing a conductive coating onto said board, providing an etching mask onto said conductive coating for masking said circuit conductors and conductive projections, etching said board and stripping said etching mask. Advantageouεly, εaid board iε a multilayer board. The use of several layers permits to reduce the additional cost involved by the process according to the present invention. The manufacturing cost of a PCB is in fact proportional to the number of layers included in the same board; on the contrary, the additional cost involved by the selective electrodeposition of the solder iε a fixed amount. Therefore, the uεe of a multilayer board reduceε the additional coεt impact with regard to the total cost of the board. In a different embodiment of the present invention, a method of manufacturing an electronic card by surface mounting of components onto a printed circuit board is provided, said method including the steps of manufacturing said board according to the above described method and then dispensing a flux onto said board. An advantage of the process according to the present embodiment of the invention with reεpect to the prior art is due to the far less critical flux dispensing operation with regard to the solder paste screening of the methods known in the art. The flux is indeed a homogeneous organic material (contrarily to the solder paste and the conductive adheεive that are a suεpenεion of a powder in an organic matrix) whoεe rheology is stable and well controlled. Different kind of fluxeε can be uεed in the above described methods, either water εoluble or no clean. Preferably, εaid flux is a no clean flux; a further advantage of thiε embodiment of the invention iε that no cleaning operationε are necessary and a εingle operator is enough to manage contemporary the flux dispensing and component placement machine. The flux can be dispensed by several methodε, εuch aε screening, spray, roll, syringe. In a particular advantageous embodiment, said flux is dispensed by a syringe; the flux dispenεing by syringe is a very simple, quick, repeatable and clean method. Moreover, the printed circuit board and the electronic card obtainable by the above described methods are disclosed. Variouε embodimentε of the invention will now be described in detail by way of examples, with reference to accompanying figures, where: Fig. 1 is a schematic representation of a PCB after drilling; Fig.2a, 2b and 2c εhow the stepε of providing circuit conductors and conductive projections onto the board; • Fig.3a, 3b, 3c, 3d and 3e depict the stepε of selectively electroplating the high thick solder onto the conductive projections; Fig.4a and 4b show the final stepε according to an embodiment of the preεent invention. With reference now to the figureε and in particular with reference to Fig.l, a schematic representation of a PCB after drilling is depicted (all previous production operations are well known in the art and are therefore omitted in the present description). The multilayer printed circuit board 100 is shown through a cross-section of a portion thereof. The method of this embodiment of the present invention will be described with reference to the production of a multilayer double sided printed circuit board having a through-connection between the two circuits on the opposite faces of the board. The use of several layers per itε to reduce the additional coεt involved by the process according to the present invention. The manufacturing cost of a PCB is in fact proportional to the number of layers included in the same board; on the contrary, the additional coεt involved by the selective electrodeposition of the solder is a fixed amount. Therefore, the use of a multilayer board reduces the additional coεt impact with regard to the total cost of the board. However, tnose skilled in the art will appreciate that the present invention may be uεed for the manufacturing of different kind of poards; for example it can be uεed with a board having a different number of layerε, including a εingle layer board and even with a εingle sided board, wherein the components are connected only onto one face thereof. The multilayer PCB 100, been formed using conventional procedureε known in the art, includeε several insulating substrates 110 carrying circuit conductors 120. An electrical conductive coating layer 130, typically copper, haε been applied onto either board surface. An aperture 140 is drilled through the board 100 in register with two or more conductors to be connected to each other. With reference now to Figures from 2a, 2b and 2c, the stepε of providing circuit conductors and conductive projections onto the board are shown. Referring in particular to Fig.2a, an etching mask 210, such aε a dry film or wet coating, iε provided onto the board 100 according to a conductor and conductive projection pattern to be formed thereon; the etching mask 210 is then exposed and developed. The coating layer 130 is subsequently etched in Fig.2b; the etching mask 210 then provideε for the formation of the desired circuit conductors 220 and conductive projections 230. With reference then to Fig.2c, the etching mask 210 iε εtripped, obtaining the external layer pattern (PCB layout) according to artwork requirementε. Referring nov to Figureε from 3a to 3e, the εteps of εelectively electroplating the high thick solder onto the conductive projections are depicted. Starting from Fig.3a, both faceε of the board 100 and the wallε of the aperture 140 (hole barrel) are rendered continuouεly conductive by electroleεs applying a conductive layer 310, typically electroleεε copper, palladium or carbon reεin. The conductive layer 310 iε applied on the entire surface to obtain cond civeness and to allow successive electroplating; it εhould be noted that, according to thiε embodiment of the present invention, the electroless conductive layer is applied after the etching of the conductive coating for providing the circuit conductors and conductive projections, on the contrary of the SMT methods known in the art. Once the conductive layer 310 has been applied, a high thickness inεulating aεk 320 is provided in Fig.3b; thiε step includeε the application, exposure and development of the insulating maεk 320. Preferably, thiε εtep is performed with a high thick water soluble dry film to cover all the surfaceε of the PCB 100, plated with the electroless conductive layer 310, excluding only the areas where it is necessary to obtain the electroplating deposition, such as conductive projections and holes. The dry film offers a higher adherence and sealing with the subεtrate; in addition, the dry film iε thicker and water soluble. The dry film deposition offers a good lateral geometrical definition, in the order of 100-200 μm, typically of 120 μm. The inεulating mask 320 iε provided with a thickness usually between 50 and 200 μm, preferably of 75 μm. This high thickness insulating mask 320 allows a high thick solder electrodeposition on the selected conductive projections, which iε necessary for asεembly of the components onto the board. In the example shown in Fig.3b, the insulating mask 320 is formed on each face leaving clear a margin surrounding each conductive projection 230, including the ones at the end of the aperture 140. Referring now to Fig.3c, the board 100 is immersed in an electroplating bath (typically a copper bath) with the continuous conductive layer 310 as a cathode. The walls of the aperture 140 and the exposed margins of the conductive layer 310 are first electroplated with a conductive material 330, in this example copper, so as to build-up a conductive surface on the hole barrel and conductive projections; this building-up is useful to ensure a better electrical connection between the circuits on either face of the board 100. Further to this, in Fig. 3d high thick solder 340 iε electroplated on top of the conductive material 330. A solder alloy is commonly used; typically, the solder alloy is a lead- tin solder alloy or a lead-free εolder alloy, εuch aε tin- bismuth. The electrodeposited solder has usually a thickness between 50 and 120 μm, typically 60 μm; this high thickness of the electrodeposited εolder allowε then the assembly of the components onto the board. The insulating mask 320 iε εtripped after the electroplating operations, as shown in Fig.3e; the board 100 is then provided with high thick solder selectively electroplated only onto selected conductive projections 230 and hollowε 140 and not onto circuit conductors 220- It should be noted that the alloy selective electroplating is obtained by the current process stepε used in a common SMT proceεs, excepting the electroleεs copper etching, only changing the process step sequence; no additional cost is then involved by the process according to this embodiment of the invention. With reference now to Fig.4a and 4b, the final steps according to an embodiment of the present invention are shown. Referring in particular to Fig. 4a, the conductive layer 310 in the areas between the conductive projections and the hollows is removed with the aid of a suitable acid; typically, this step is performed by a dedicated etching bath, used to remove only electroless copper or palladium or carbon resin. After said conductive layer haε been removed, the εolder iε reflowed or pickled in order to achieve and to maintain the solderability performance. A solder mask 410 iε then provided in Fig.4b, as in the usual PCB procesε production flow; the double-sided printed circuit board 100 is then ready for use. The printed circuit board 100 is typically used for the manufacturing of an electronic card, by assembling different components onto the board 100. In the SMT procesε, the electrical contacts of the components are connected, through tne depoεited high thick solder 340, to the conductive projections 230 provided on the substrate surface 110 of the board 100; particularly, in the FCA procesε, the chips, provided with several ball contacts, are bonded onto small PCB conductive projectionε 230. The electronic card manufacturing procesε εtartε with the step of dispensing a flux in order to activate the solder and to provide adhesivity. The board 100 can then be used for connecting the desired components onto the relative conductive projections 230. It should be noted that the process according to the present invention does not require the solder paste or adhesive screening used in common SMT processes, or the injection of the melted solder onto the conductive projections used in some FCA processes. This avoids the high cost of the solder injection machine and the high complexity of the conductive adhesive or solder paste screening step; the elimination of the screening improves significantly the process yield and it reduces the assembly cycle time and the man power. In the methods known in the art, one operator must be dedicated to the solder paste or the conductive adhesive screener, whose setup (the matching of the board conductive projections with stencil window) is very critical because it is required a high mechanical precision on large surfaces; Moreover the stencil cleaning, while the machine is operating, is continuously necessary and so the stencil washing at the end of the production with a lot of time losing. On the contrary, the flux dispensing step is a far less critical operation; the flux is indeed an homogeneous organic material (contrarily to the solder paste and the conductive adhesive that are a suεpension of a powder in an organic matrix) whose rheology is stable and well controlled. Different kind of fluxes can be used in the above described methods, either water soluble or no clean; preferably, the SMT process includes the εtep of diεpensing a no clean flux, so that no cleaning is required in the whole electronic card assembly proceεε; a single operator is then enough to manage contemporary the flux dispensing and component placement machine. The flux can be dispenεed by several methods, such as screening, spray, roll, syringe. In the flux dispensing by syringe, thiε device is moved by an automatic and programmable machine, which is able to move trough the conductive projections with high precision; the method is very simple, quick, repeatable and clean. Following the flux diεpenεing εtep, different componentε, such aε chips, transistors, capacitors, resiεtorε and so on, are placed onto the board 100, connected to the related conductive projectionε 230. The electronic card manufacturing process is then completed by reflowing the high thick solder 340 provided onto the conductive projectionε 230, in order to provide mechanical and electrical connection with the componentε placed onto the board 100.

Claims: CLAIMS 1. A method of manufacturing a printed circuit board (100) for surface mounting of components thereon, said method including the steps of: providing circuit conductors (220) and conductive projections (230) onto said board (100), selectively electroplating a high thick solder (340) onto said conductive projections (230). 2. The method according to Claim 1, v/herein said step of selectively electroplating a high thick solder (340) includes the εteps of: electroless applying a conductive layer (310) onto said board (100), providing a high thick insulating mask (320) onto said conductive layer (310) leaving exposed said selected conductive projections (230), electroplating said high thick solder (340) onto said selected conductive projections (230), stripping said insulating mask (320). 3. The method according to Claim 1 or 2, wherein εaid insulating mask (320) has a thickness comprised between 50 μm and 200 μm. 4. The method according to Claim 3, wherein εaid inεulating mask (320) has a thickness of 75 μm. 5. The method according to any Claim from I to 4, wherein said insulating mask (320) is a dry film. 6. The method according to any Claim from 1 to 5, further including the step of electroplating a conductive material (330) onto said εelected conductive projections (230) before the step of electroplating said high thick solder (340). 7. The method according to any Claim from 1 to 6, wherein εaid εtep of providing circuit conductors (220) and conductive projections (230) includes the steps of: providing a conductive coating (130) onto said board (100); providing an etching mask (210) onto said conductive coating (130) for masking said circuit conductors (220) and conductive projections (230); etching said board (100); stripping said etching mask (210). 8. The method according to any Claim form 1 to 7, wherein said board (100) is a multilayer board. 9. A method of manufacturing an electronic card by surface mounting of components onto a printed circuit board (100), said method including the steps of: manufacturing said board (100) according to any Claim from 1 to 8; dispensing a flux onto said board (100). 10. The method according to Claim 9, wherein said flux iε a no clean flux. 11. The method according to Claim 9 or 10, wherein εaid flux iε diεpensed by a syringe. 12. The printed circuit board (100) obtainable by the method according to any Claim from i to 8. 13. The electronic card obtainable by the method according to anv Claim from 9 to 11.

Inventors: MAZZONI ROBERTO, MONZANI ADELIO, SIRTORI VITTORIO, MAZZONI, ROBERTO, MONZANI, ADELIO, SIRTORI, VITTORIO
Applicants: IBM, IBM SEMEA S P A, MAZZONI ROBERTO, MONZANI ADELIO, SIRTORI VITTORIO, IBM SEMEA S.P.A., INTERNATIONAL BUSINESS MACHINES CORPORATION, MAZZONI, ROBERTO, MONZANI, ADELIO, SIRTORI, VITTORIO
Application year: 1995
Main classification: H05K3
IPC codes: H05K3