WO2005094147A1

WO2005094147A1 - A process of forming a solder mask

Info

Publication number: WO2005094147A1
Application number: PCT/NL2005/000213
Authority: WO
Inventors: Paul Herman Guillaume Binda; Tosko Alexandar Misev
Original assignee: Dsm Ip Assets B.V.
Priority date: 2004-03-25
Filing date: 2005-03-21
Publication date: 2005-10-06

Abstract

The invention relates to a process of forming a solder mask on a printed circuit board comprising the steps of (a) charging powder paint particles in the presence of carrier particles (b) feeding the charged powder paint particles with carrier particles to a transporter (c) (optionally) transferring the charged powder paint particles from the transporter onto a transfer medium and (d) applying the powder paint particles to a printed circuit board to form a layer of powder paint particles.

Description

A PROCESS OF FORMING A SOLDER MASK The present invention relates to a process of forming a solder mask on a printed circuit board (PCB). A solder mask is formed on conductor circuits of a printed circuit board for ensuring insulation between the conductor circuits, for protecting the conductor circuits from peeling or oxidation, and/or for preventing unnecessary solder from sticking on the conductor circuits at the time of soldering operation. Processes of forming a solder mask on a printed circuit board are known in the art. For example, a screen printing process is typically well-known. However, the screen printing process provides substantially low resolution. It develops defects like blur, pinholes, bleed, spread or the like, resulting in failure to deal with diminishing distances between conductor circuits or increasing densities of the printed circuit board that has taken place concomitant with the recent high density mounting of circuits. US 4,230,793 describes a process, wherein a printed circuit board is coated with a solution of a photopolymer by applying curtain coating technology. The solvent is removed, the photopolymer is subsequently cured with ultraviolet light through a mask, whereby the photopolymer allocated in the positions of the PCB that need to be soldered remains uncured, and can be developed and dissolved. The use of liquids has a number of disadvantages. For example, the thickness of the solder mask is rather thin, whereby the proper amount of electric insulation, heat resistance and resistance to organic solvents, etc., cannot be obtained. If the substrate has any through-holes, the photosensitive composition flows into these through holes and gives rise to various problems. EP 0 541 111 discloses a process of forming a solder mask by using a powder paint composition. The powder paint composition is applied to the PCB at a temperature above the softening point of the powder paint to form a coating of powder paint, the coating is irradiated with UV-light through a pattern mask, the coating is developed with a medium which dissolves the region not irradiated by the UV rays, and the remaining irradiated portion of the coating on the printed circuit board is cured at a temperature higher than the curing temperature of the powder paint composition. The use of a powder paint known so far is restricted to powder paint composition that need to be UV cured in a first step, followed by a lengthy thermal post cure step. Moreover application of the powder on the PCB is slow and turns out to be difficult in practice. Also it is impossible to apply the powder in a pattern on the printed circuit board, thereby leaving free the holes and other patterns for soldering electronic components. One object of the present invention is to provide an improved process for applying a uniform layer of a powder paint composition on a printed circuit board. Another objective of the present invention is to provide a process to develop the powder solder mask. The present invention provides a process of forming a solder mask on a printed circuit board which process comprises the following order of steps a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter and c applying the powder paint particles to a printed circuit board to form a layer of powder paint particles. In a preferred embodiment of the invention the process of forming a solder mask on a printed circuit board comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c transferring the charged powder paint particles from the transporter onto a transfer medium and d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles. According to a further preferred embodiment of the invention the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c applying the powder paint particles to a printed circuit board to form a layer of powder paint particles, d curing the layer by subjecting the layer to heat and/or ultra violet irradiation and e developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating. According to another preferred embodiment of the invention the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c transferring the charged powder paint particles from the transporter onto a transfer medium d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles, e curing the layer by subjecting the layer to heat and/or ultra violet irradiation and f developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating. According to another preferred embodiment of the invention the process of forming a solder mask on a printed circuit board comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c optionally transferring the charged powder paint particles from the transporter onto a transfer medium d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles, e sintering and/or melting the to be developed layer of powder paint particles into the required pattern by use of heat as generated with a laser f developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating by removing the non-sintered powder paint particles and g curing the patterned layer by subjecting the layer to heat and/or ultra violet irradiation. According to another preferred embodiment of the invention the curing takes place by subjecting the layer to heat. Preferably, the charging of powder paint particles in the presence of carrier particles takes place via tribo-electrical charging. The formation of a layer of powder paint particles containing a pattern can be performed in different ways. According to a preferred embodiment of the invention the process for the formation of a layer of powder paint particles containing a pattern comprising the steps of

1 applying the powder paint particles to a printed circuit board to form a substantially uniform layer of powder paint particles and 2 provide a solder mask pattern in the layer of powder paint particles by selectively removing particles from the substantially uniform layer of powder paint particles. Step 2 may be performed in different ways. For example, by approaching the layer of powder paint particles with a device having a three dimensional structure in the form of a negative of the pattern to be applied to the solder mask, whereby the structure is oppositely charged relative to the charge of the powder paint particles, and whereby the powder paint particles are selectively transferred to the device, thereby leaving behind the solder mask pattern in the layer of powder paint particles. The device may have the shape of a panel, having pins and/or protuberances directed to the surface of the PCB coated with the powder paint particles. The pins and protuberances contain an adjustable charge opposite to the charge of the powder, so that electrostatic forces will transfer powder particles from the printed circuit board to the pins and protuberances of the device, when the device is approaching the printed circuit board in a vertical manner. After selective transfer of the powder paint particles to the device, the printed circuit board contains a layer of powder paint particles that contains a pattern. The curing of the powder paint by heat and/or ultraviolet radiation will generate a printed circuit board with a solder mask. A different embodiment of the device may be a device operated in a dynamic way that is able to make different patterns. The powder coating layer may be formed into a pattern for example by moving a device containing movable pins/protuberances that have opposite polarity to the charged powder paint particles over and in the surface of the PCB whereby the movement of the device in combination with the movement of the pins/protuberances causes selective removal of powder particles from the PCB. The movement of the pins may be controlled by a computer. Different patterns can also be made by for example subjecting different charges to the pins, depending on time or position relative to the printed circuit board. Such pins may operate like a matrix printer, whereby a computer contains an electronic picture of the pattern to be applied to the printed circuit board, and controls the charge of the pins of the device for selectively removing powder paint particles from the printed circuit board. The pins can also have the form of stamps with a specific surface area to selectively remove powder paint particles from the surface areas of the PCB that should also not be coated with a solder mask. Another preferred embodiment of the invention is a process comprising the steps of a electrically charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c applying a substrate to the transfer medium, wherein the substrate contains a positive image of the pattern of the solder mask, whereby holes in the image on the substrate correspond to the holes in the pattern of the solder mask to be applied on the printed circuit board, d applying a uniform layer of powder paint particles to the substrate attached to the transfer medium, e applying the pattern of powder paint particles from the substrate to the printed circuit board and f curing the layer of powder paint particles on the printed circuit board by subjecting the layer to heat and/or ultra violet irradiation. According to a preferred embodiment after step e, another step comprising removing the residual powder paint particles in the holes of the substrate and as attached to the transfer medium, is present. This process is advantageously when a series of PCB's are coated with the aid of the same substrate. A hole may have any shape, for example a circle, a cut, or irregular area, as defined by the pattern to be applied to the solder mask. The substrate may be an aluminum sheet as used as "entry or backing material" in the mechanical drilling process to produce solder holes in a PCB. After the drilling process this sheet contains an exact image of the dimensions and of the position of the solder holes in the PCB. In addition this sheet can be further processed to match the complete image of the areas that should not be coated by punching out the corresponding areas from the aluminum sheet. The support substrate may also be made from other materials, such as for example paper, board, plastic and metal. The substrate preferably contains holes that match with the solder holes and other areas that should not be coated with solder mask. The dimensions of these holes can be set in such a way as to have a perfect registration of the solder mask around the so-called copper lands that should be free of solder mask. By applying a uniform layer of powder paint particles on the support substrate, the substrate is covered with powder paint. Transfer of powder paint particles from the substrate to the PCB, will generate a pattern, since only the powder paint particles from the surface of the substrate will be transferred to the PCB, but powder particles residing in the holes of the substrate and as deposited onto the transfer medium will not be transferred, due to the larger distance between these powder paint particles and the PCB: the electrostatic forces for transfer of these particles are too low to transfer the powder paint particles to the PCB. Powder paint particles that stay behind in the holes of the support substrate and on the transfer medium are preferably removed before applying a new layer of powder paint composition onto the support substrate. Removal can be done in different ways, for example by brushing the particles away, by changing the charge of the substrate, by sucking the particles with for example vacuum. The thickness of the substrate can vary depending on its conductivity, the charge on the powder paint particles and the applied voltage to transfer the powder paint particles to the PCB. According to a further preferred embodiment of the invention developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating takes place with use of a laser. A suitable laser to be used for ablation of the coating material is for example a CO₂ laser. According to a further preferred embodiment of the invention the PCB is pretreated. Suitable pretreatment processes include for example wetting the surface of the PCB with a polar liquid, such as for example water or alcohol and ketones with for instance a soaked cloth. The obtained increased surface conductivity will increase the homogeneity and the thickness of the applied layer of powder coatings as electrostatically applied from the magnetic brush unit. Another way of pretreatment to increase the applied homogeneity and the amount of powder paint particles as applied from the magnetic brush unit is by applying charge to the PCB with a corona discharge device. The applied charge facilitates powder application when this charge is opposite to the charge of the powder paint particles from the magnetic brush unit. A printed circuit board is a material that contains a dielectric core that has been coated or impregnated with resin. The dielectric material is usually woven glass fibers or paper. Different combinations of these two materials and the substitution of various resin systems can alter the electrical, physical, performance and cost characteristics of the material. The type of material employed for a specific purpose depends on the function of the PCB, design requirements and how it will be manufactured. FR4 is the designation given to the most widely used material for the PCB industry. It is constructed of multiple plies of epoxy resin-impregnated woven glass cloth. ln a next step of the PCB production process, the base laminate is coated with copper by either electrolytic deposition or by laminating a resin coated copper foil onto the base laminate. In a consecutive step a printed circuitry is generated on the board by the print and etch process. The present invention describes an improved process of applying a solder mask layer on the printed circuitry as a protective layer. The solder mask is applied on the entire surface of the PCB, except where soldering is expected to occur during the assembly operation. Powder paint particles are particles that have a glass transition temperature above room temperature, and that can be cured to, for example, a coating layer by ultraviolet radiation or heat. The glass transition temperature typically lies in the range between 40 °C and 140 ^CC. The powder paint particles generally include an oligomer or polymer having functional groups that can react to form a polymeric structure. The reaction may for example be a radical polymerization reaction or a cationic polymerization reaction. The functional groups may be unsaturated groups, like for example vinylether, acrylate, methacrylate, or cationically curable groups such as example epoxy and oxetane. Examples of suitable polymers include unsaturated polyesters, unsaturated polyacrylates, acrylated polyesters, bisphenol A or F type epoxy resins, brominated epoxyresins, dicyclopentadiene phenolic resin and novolac type epoxy resins, saturated carboxylated polyester resin/triglycidylisocyanurate (TGIC), saturated carboxylated polyester resin/epoxy resin, saturated carboxylated polyester resin/crosslinker containing hydroxylamide group, saturated carboxylated polyester resin aliphatic oxirane, saturated hydroxylated polyester resin/isocyanate, polyester resin/diglycidyl phtalate containing crosslinker, saturated hydroxylated polyester resin/hexamethoxy-methylmelamine (HMMM), saturated hydroxylated polyester/glycoluril(derivative), saturated hydroxylated polyester/benzoguanamine (derivative), saturated hydroxylated polyester resin/amino resin, saturated hydroxylated polyester resin/phenolic resin, epoxy resin/amino resin, epoxy resm/phenolic resin, epoxy resin/anhydride, epoxy resin (selfcrosslinking), epoxy resin/dicyanamide (derivatives) , phenolic resin (selfcrosslinking), epoxy ester resin/amino resin, amino resin/isocyanate, acrylamide resin (selfcrosslinking), acrylic resin/hydroxy-functional compound, unsaturated acrylic resin (selfcrosslinking), unsaturated acrylic resin/vinylether, unsaturated polyester resin/vinylether and saturated epoxidized acrylic resin/dodecanedicarboxylic acid. The median particle size (by volume) of the powder paint particles X_50,3 (as defined according to the description and notation at pages 12-14 of Mechanische Verfahrenstechnik by Prof. Rumpf (Carl Hansen Verlag, 1975)) can be for example below about 200 μm, and preferably, is between about 5 and about 60 μm. Characteristics of powder paint particles, the process to make the particles and the method of application are generally disclosed in Misev, Powder Coatings, Chemistry and Technology, Wiley, 1991. Powder paint compositions and processes to produce powder paint compositions are also disclosed in for example US 6342273. This reference also describes the process of charging powder paint particles and the application to substrates like metal, textile, plastic, wood, board or paper-like materials. US-A- 6342273 does not teach the coating of a PCB substrates with powder paint particles. PCB substrates have the additional difficulty that they possess both highly conductive parts and regions with non-conductivity. The process according to the present invention also uses carrier particles. Carrier particles can be either magnetic or non-magnetic. Preferably, the carrier particles are magnetic particles. Suitable magnetic carrier particles have a core of, for example, iron, steel, nickel, magnetite, γ-Fe₂O₃, or certain ferrites such as for example CuZn, NiZn, MnZn and barium ferrites. These particles can be of various shapes, for example, irregular or regular shape. Generally, these carrier particles have a median particle size between 20 and 700 μm. Preferably, the carrier particle size distribution is narrow and more preferably the ratio X₇₅,₃ :X_25|3 <2. Exemplary non-magnetic carrier particles include glass, non-magnetic metal, polymer and ceramic material. Non-magnetic and magnetic carrier particles can have similar particle size. For direct application without a transfer medium, on a metal substrate, the carrier particles should be preferably non-conductive and they should have a well-defined high resistivity of, for example, 10⁹ -10¹¹ Ohm at 10V potential and a break-through voltage above 1 ,000V (measured with a c-meter supplied by Epping GmbH). In case of use of a transfer medium the carrier particles can be conductive or non-conductive. Preferably, carrier particles having high voltage break through are used so that high electric fields can be used between transport means and substrate or transfer media to achieve a thick powder layer. A developer comprises powder paint particles and carrier particles. A development process is a way of developing and a development unit is a complete system comprising of, for example, a transporter, mixing screw(s), a supply device, blades, detectors and the like. Other examples are described in GB-A-2097701 , U.S-A-4147127 and U.S-A- 4131081. In the present invention the development process can be either one- component or two-component. According to a preferred embodiment of the invention the two-component development process, in which the carrier particles are mixed with the powder paint particles, is used. Alternatively, also all kinds of mono component development, for example, magnetic conductive, magnetic insulative or non magnetic development as disclosed on pages 203 ff of L.B. Schein (Electrography and, Development Physics, pages 32-244, Volume 14, Springer Series in Electrophysics 1988) or binary developers consisting of for example magnetic toner and carrier as described in U.S-A-. 5,516,613 can be used. Preferably, a combination of powder paint particles having a X₅₀,₃ below 80 μm and a X_95|3 below 120 μm and carrier particles having a X₅₀,3 below 180 μm and a X₉₅,₃ below 200 μm is used. In the two-component developer the amount of powder paint particles can be, for example, between about 1 and about 50 wt. % and preferably between about 5 and about 25 wt. % (relative to the amount of developer). It is an advantage of the process according to the invention that it is possible to use powder paint concentrations well in excess of 10 wt. %. Consequently, the amount of carrier particles can be between about 50 and about 99% by weight (relative to the amount of developer) and preferably is between about 75 wt. % and about 95 wt. %. The powder paint concentration can be externally or internally (in the development unit) controlled. External control can be effected by measurement of layer thickness of uncured or cured powder by, for example, optical, photo thermal or dielectrical means. Internal control can be carried out in the developer station by means of powder paint concentration control by any suitable means like inductive control (see, for example, U.S. 4,147,127 and U.S. 4,131 ,081) or volume control. In a two-component development process the powder paint particles are triboelectrically charged by intensive mixing and friction with the carrier particles. A preferred process according to the present invention thus comprises charging of the powder paint particles by intensive mixing and friction with magnetic carrier particles, transport of carrier particles and powder paint particles with the aid of a magnetic roller and subsequent application of the powder paint particles to a substrate by means of an electric field between the substrate and the magnetic roller. The coating may cover the full area of the substrate or only part of it (spot coating). In case of spot coating any image creating technology, for example as described in the above mentioned Schein, may be used to create an image on a suitable transfer medium or as first transfer medium in case of two step transfer as described before. The carrier particles may be dosed to a mixing arrangement in which one or more means of intensive mixing such as, for example, worm wheels are present along with a magnetic roller. Suitable mixing arrangements are described in, for example, "Proceeding of IS&T's Seventh International Congress on "Advances in Non Impact Printing Technologies", Vol. 1 , pages 259-265. Next, an amount of powder paint particles is fed into the mixing arrangement which is selected so as to obtain a powder paint particles concentration of, for example, about 5-15 wt. % powder paint relative to the amount of carrier particles. In this way, a developer is formed. During the intensive mixing and friction in the mixing arrangement, due to the action of the worm weels, the carrier particles and the powder paint particles become oppositely (tribo) electrically charged. A layer of electrically charged powder paint particles covers the carrier particles. The carrier particles subsequently act as carrier for the charged powder paint particles. Next, the developer is fed to the magnetic roller, in which a brush-like structure, also known as the magnetic brush, is formed. The magnetic roller transports the brush to the contact area with the substrate or transfer medium. In this way, powder paint particles, as well as carrier particles, become available at the contact area with the substrate or transfer medium. By applying a sufficiently strong electric field between the magnetic roller and the substrate or transfer medium the powder paint particles can be drawn from the brush to the substrate or transfer medium, onto which they adhere electro statically. In the case a transfer medium is used, the powder paint particles are subsequently transported to the contact area of the transfer medium and the substrate and next transferred to the substrate via one of the aforementioned transfer processes. Finally, the magnetic brush is scraped off the magnetic roller as it is returned into the mixing bin. As a result, a layer of powder paint particles forms on the substrate, which layer is substantially free from carrier particles, and can then be further processed and cured. Eventually an apparatus which can catch carrier particles can be introduced such as for example a catching equipment for residual carrier, as described in "Proceeding of IS&T Eighth International Congress on Advances in Non Impact Printing Technologies", pages 391-393. The thickness of said layer of powder paint particles can, for example, be controlled via the electric field strength between the magnetic roller and the substrate, or the transfer medium, the magnitude of the charge on the powder paint particles (e.g. by varying the concentration and the mixing time) and the roller speed. Depending on the conductivity of the substrate the electric field between the substrate and the transfer medium or means of transport can be applied with suitable processes like for instance a corona discharge or a moving or fixed counter electrode (see for instance the above mentioned Schein, pages. 36-37 and 47). US-A-4717639 is directed to the application of a resist image. A resist image is completely different from a solder mask. A solder mask is a coating that is placed onto the defined copper circuit and the insulating board onto which the copper circuit is formed whereas a resist image is a temporary coating as applied on copper cladded board material. The resist image is applied onto the copper in order to either protect the to be formed circuit from the etching chemicals in the so-called panel plating process, or to protect the whole of the copper surface, except for where the circuit should be formed in the so-called pattern plating process. In a next step of the PCB manufacturing process the solder mask is applied. US-A-4717639 does not refer to solder mask application. US-A- 4717639 refers to soldering as in the electrical deposition of tin/lead from a solution onto the whole copper circuit. Such a treatment is required to protect the copper circuit during the etching process as meant to wash away the resist image and the copper beneath the resist image. The tin/lead deposit on the residing copper circuit is then removed to fully expose the copper circuitry. This tin/lead-treatment is completely different from the soldering as applied onto the PCB after application of the solder mask. In this case soldering is the application of hot solder onto the parts of the printed circuit to which an electrical component will be attached. All the other parts of the PCB, meaning the electrical circuitry as formed by the copper and the exposed board will be protected by a so-called solder mask, before the hot soldering process takes place. Furthermore the function of the solder mask is to protect the printed circuit board during its working life against thermal, mechanical, electrical stress and chemicals, besides having a nice appearance. The invention will be elucidated with the following non-limiting examples. Examples

Example 1 A powder coating composition was prepared by premixing 300 parts by weight of a carboxyl functional polyester resin Uralac P 5981 (DSM Resins) with acid value 75 mg KOH/g, Tg 58° C (by DSC), 300 parts by weight of epoxy resin Epikote 3003 (Shell Chemie BV), 9 parts by weight of flow control agent Modaflow Powder III (Monsanto Co.) and 4.5 parts by weight of benzoin as a degassing agent (GCA Chemical) in a Henschel batch mixer and next melt kneaded in a Buss-Ko-Kneader PLK 46 at 115 ° C. The cooled extrudate was milled first in a hammer mill to a particle size below 3 mm and then fed into a Retsch ZM100 mill after which the ground powder was sieved over a 70μm sieve, obtaining powder with a medium particle size of 45 μm and d75.3/d25.3 ratio of 3.3.

Example 2 Preparation of a Carrier 992 parts by weight Cu-Zn-ferrite powder, having a median particle size of 81 μm and a ratio X₇₅,₃/X₂₅,₃ of 1 ,4 (both measured with the laser granulometer Cilas HR 850), were dry coated with 8 parts by weight polyvinylidenedifluoride (Kynar 301 F™) by mixing both materials in a Lδdige mixer and coating the polymer on the surface of the ferrite in a rotary kiln at 200°C under Nitrogen to obtain a carrier with a median size of 80 μm, a ratio X₇₅,₃/X₂₅,₃ of 1.4, a resistance of 1.1*10¹⁰ Ohm at a potential of 10V and a break-through voltage above 1 ,000V (both measured in a c-meter of Epping GmbH).

Example 3

Preparation of a developer A 500 gram mixture or developer of 20% by weight of the powder composition made according to Example 1 and 80% by weight of the carrier according to Example 2 were brought into a magnetic brush unit available as the LD-tester (developer life time tester) from Epping GmbH, mounted at a distance of 3 mm of a rotating rubber coated metal drum. The rubber coating on the drum is available from Ruma, Hoogeveen, NL product code: 607-50. The rubber is coated with an anti-stick coating: product code: Rumacoat 101F). The developer was mixed in the LD-tester for 1 minute at a rotating speed setting of 90, which is equivalent to 150 RMP or a surface speed of the drum of 57 m/min. The charge-to-mass ratio of the powder coating was determined to be +13 μC/g with a device available as the Epping Q/m-meter. (Epping Q/m meter experimental details: 60μm sieve material was used, with a blow-off pressure of 5 bar for 90' seconds) .

Example 4 Applying powder homogeneously onto a PCB substrate The developer as prepared in Example 3 was used to fully coat a 1mm thick FR-4 based PCB substrate. This PCB was attached to the rubber coated metal drum.

The PCB was wiped with a cloth, soaked in water to get rid of any residual charges, due to handling of the PCB, and to increase its surface conductivity. The rotation speed of the drum (i.e. coating speed) was set at 3 m/minute; the speed of the magnetic brush was set at 57 m/minute in the same direction as the drum. The magnetic brush unit was set at a positive potential of 2.5 kV, whilst the rubber coated metal drum was grounded, resulting in a development potential of +2.5 kV. After passing the magnetic brush unit one time the PCB was coated on both the copper and plastic parts with a layer of powder paint particles.

Example 5

Developing the solder mask coating on the PCB substrate The layer of powder paint particles as applied in Example 4 was selectively sintered/melted onto the PCB substrate with use of a CO₂-laser, that was programmed to develop the specific pattern of the solder mask, as the heating source. In a next step the powder paint particles that were not irradiated by the laser and therefore not sintered onto the PCB substrate, where mechanically removed, whereby the solder mask was developed. In a subsequent step, the developed powder coating solder mask was cured by heat in a convection oven at 160°C for 12 minutes. The obtained solder mask was on average 45 μm thick and had a good resistance against a 50/50 Pb/Sn molten solder of 260°C. Example 6

Developing the solder mask coating on the PCB substrate by laser ablation The developer as prepared in Example 3 was used to fully coat a 1 mm thick FR-4 based PCB substrate. The PCB was wiped with a cloth, soaked in water to get rid of any residual charges, due to handling of the PCB, and to increase its surface conductivity. The rotation speed of the transfer drum (i.e. coating speed) was set at 3 m/minute; the speed of the magnetic brush was set at 57 m/minute in the same direction as the drum. The magnetic brush unit was set at a positive potential of +2500 V, whilst the rubber coated metal drum was set at a potential of +1500 V. The to be coated PCB substrate was grounded. The rubber coated metal drum was coated with powder paint particles, under the voltage drop between the magnetic brush and rubber drum. The powder paint particles then jumped onto the PCB substrate under the subsequent voltage drop between the rubber drum and the grounded PCB substrate when the latter was brought into contact and passed over the rubber drum. Consequently, a homogeneous powder layer was applied onto the PCB substrate, whilst the solder holes were substantially free from powder particles. This layer of powder paint particles was cured by heat in a convection oven at 160°C for 12 minutes. In a subsequent step the solder mask pattern was developed by laser ablation with a CO₂ laser that was programmed in such a way that the solder holes and solder pads were freed of coating material. The obtained solder mask was on average 30 μm thick and had a good resistance against a 50/50 Pb/Sn molten solder of 260°C.

Example 7

Preparation of a support substrate A flexible, approximately 0.25 mm thick aluminium sheet, as used as entry material in the hole-drilling process of the PCB, was obtained together with a matching 1 mm thick FR-4 PCB as obtained after the drilling process. Besides the printed circuit, this PCB contained solder holes, only. The solder holes had a diameter of 1 mm. After printing and etching, round copper connections with a diameter of 1.5 mm were generated around the holes. In order to obtain a perfect registration of the powder coating solder mask around the copper connections, the holes in the aluminium entry sheet were extended to 1.6 mm. A representative part of the aluminium sheet was cut into a sheet of 10 cm in width and 20 cm in length.

Example 8

Coating on a support substrate A 500 gram mixture or developer of 20% by weight of the powder composition made according to Example 1 and 80% by weight of the carrier according to Example 2 were brought into a magnetic brush unit available as the LD-tester (developer life time tester) from Epping GmbH, mounted at a distance of 3 mm of a rotating rubber coated metal drum. The rubber coating on the drum is available from Ruma, Hoogeveen, NL product code: 607-50. The rubber is coated with an anti-stick coating: product code: Rumacoat 101 F). The developer was mixed in the LD-tester for 1 minute at a rotating speed setting of 90, which is equivalent to 150 RMP or a surface speed of the drum of 57 m/min. The charge-to-mass ratio of the powder coating was determined to be +13 μC/g with a device available as the Epping Q/m-meter. (Epping Q/m meter experimental details: 60μm sieve material was used, with a blow-off pressure of 5 bar for 90' seconds) The support substrate as prepared in Example 7 was mounted on the rotating rubber drum. The rotation speed of the drum (i.e. coating speed) was set at 7 m/minute; the speed of the magnetic brush was set at 57 m/minute in the same direction as the drum. The doctor blade of the magnetic brush was adjusted to a distance of 2.0 mm to the magnetic roller.The magnetic brush unit was set at a positive potential of 2.0 kV, whilst the rubber coated metal drum was grounded, resulting in a development potential of +2.0 kV. After passing the magnetic brush unit, the support substrate was coated fully on its continuous surface, whilst some powder was deposited in the holes and onto the surface of the rubber coated metal drum.

Example 9 Transferring the powder coating on the support substrate to the PCB. The support substrate was left on the rubber coated metal drum of the LD-tester. The rubber coated drum, and consequently the support substrate, was set at +2.5kV. The corresponding part of the PCB was mounted around an earthed steel drum and surface-pretreated with a negatively charging corona discharge from a point-electrode set at 50 kV, for 5 seconds, at a distance of 20 cm from the PCB, and rotated over its corresponding image on the support substrate. Under the influence of the generated electrostatic field strength the powder was developed from the support substrate onto the PCB, thereby leaving the copper connects and the solder holes completely free of powder, as the powder as residing in the holes and to the rubber coated drum experienced a too weak electrical field strength, due to their increased distance compared to the surface of the support substrate, to electrostatically develop onto the PCB. The applied powder coating layer was cured in a convection oven at 160°C for 12 minutes. The resulting solder mask was perfectly registered around the copper connects surrounding the solder holes. The solder mask had an average layer thickness of 45 μm. The coated PCB was dipped for 10 seconds into a 50/50 Pb/Sn molten solder of 260°C. The produced solder mask did not show any blisters, spots of delaminating and peeling from the PCB surface.

Example 10 Transferring the powder coating on the support substrate to the PCB. The support substrate as coated in Example 7 was taken of the rubber coated metal drum of the LD-tester and placed on a sheet of insulating material. Subsequently, the support substrate was electrically attached to a voltage generator and set at +2.5kV. The, earthed, corresponding part of the PCB was pretreated with a negatively charging corona discharge from a point-electrode set at 50 kV, for 5 seconds, at a distance of 20 cm from the PCB. The PCB was exactly positioned above the support substrate and brought into contact with charged powder particles on the support substrate. Under the influence of the generated electrostatic field strength the powder was developed from the support substrate onto the PCB, thereby leaving the copper connects and the solder holes completely free of powder. The powder as residing on the edges of the support substrate experienced a weak electrical field strength, due to their increased distance compared to the surface of the support substrate, to electrostatically develop onto the PCB. The applied powder paint layer was cured in a convection oven at 160°C for 12 minutes. The resulting solder mask was perfectly registered around the copper connects surrounding the solder holes. The solder mask had an average layer thickness of 45 μm. The coated PCB was dipped for 10 seconds into a 50/50 Pb/Sn molten solder of 260°C. The produced solder mask did not show any blisters, spots of delaminating and peeling from the PCB surface. Example 11

Producing the solder mask pattern on the fully coated powder coated PCB substrate. A metal wire with a diameter of 1.7 mm was attached to a negative voltage-generating device. The voltage on the wire was set at 1.5 kV and position exactly above a solder hole and corresponding copper land of the PCB as described in Example

4. Due to the strong negative voltage on the (blunt) tip of the wire, the positively charged powder coating particles attached to the PCB jumped to and sticked onto the wire.

Thereby leaving the solder hole and corresponding copper land complete free of powder coating particles. This step was repeated to free a number of solder holes and copper lands of powder coating particles. In this way the pattern of the solder mask was obtained.

The applied powder coating layer was cured in a convection oven at 160°C for 12 minutes.

The resulting solder mask was perfectly registered around the copper connects surrounding the solder holes. The solder mask had an average layer thickness of 45 μm.

The coated PCB was dipped for 10 seconds into a 50/50 Pb/Sn molten solder of 260°C. The produced solder mask did not show any blisters, spots of delaminating and peeling from the PCB surface.

Claims

1. A process of forming a solder mask on a printed circuit board comprising the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter and c applying the powder paint particles to a printed circuit board to form a layer of powder paint particles.

2. A process according to Claim 1 characterized in that the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c transferring the charged powder paint particles from the transporter onto a transfer medium and d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles. 3. A process according to any one of Claims 1-2 characterized in that the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c applying the powder paint particles to a printed circuit board to form a layer of powder paint particles d curing the layer by subjecting the layer to heat and/or ultra violet irradiation and e developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating.

4. A process according to any one of Claims 1-3 characterized in that the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c transferring the charged powder paint particles from the transporter onto a transfer medium d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles e curing the layer by subjecting the layer to heat and/or ultra violet irradiation and f developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating. A process according to any one of Claims 1-4 characterized in that the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c optionally transferring the charged powder paint particles from the transporter onto a transfer medium d applying the powder paint particles to a printed circuit board to form a layer of powder paint particles e sintering and/or melting the to be developed layer of powder paint particles into the required pattern by use of heat as generated with a laser f developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating by removing the non- sintered powder paint particles and g curing the patterned layer by subjecting the layer to heat and/or ultra violet irradiation. 6. A process according to any one of Claims 1-5 characterized in that the process comprises the steps of a charging powder paint particles in the presence of carrier particles b feeding the charged powder paint particles with carrier particles to a transporter c applying a substrate to the transfer medium, wherein the substrate contains a positive image of the pattern of the solder mask, whereby holes in the image on the substrate correspond to the holes in the pattern of the solder mask to be applied on the printed circuit board, d applying a uniform layer of powder paint particles to the substrate attached to the transfer medium, e applying the pattern of powder paint particles from the substrate to the printed circuit board and f curing the layer of powder paint particles on the printed circuit board by subjecting the layer to heat and/or ultra violet irradiation.

7. A process according to any one Claims 1-6 characterized in that the process of applying the powder paint particles to a printed circuit board to form a layer of powder paint particles additionally comprises the step of providing a solder mask pattern in the layer of powder paint particles by selectively removing particles from the substantially uniform layer of powder paint particles.

8. A process according to any one Claims 1-7 characterized in that charging powder paint particles in the presence of carrier particles takes place via tribo-electrical charging.

9. A process according to any one Claims 1-8 characterized in that the printed circuit board is pretreated with a polar liquid or a corona discharge device.

10. A process according to any one Claims 3-9 characterized in that developing the layer into a pattern comprising areas containing powder coating and spots that do not contain powder coating takes place with use of a laser.

11. A printed circuit board containing a solder mask as obtained by a process according to any one of Claims 1-10.