Method of manufacturing a multi-layer printed circuit board

A method of manufacturing a multi-layer printed circuit board by bonding together a plurality of circuit board layers, each of which includes a substrate and a conductive circuit pattern on at least one surface of the substrate, includes the steps of: coating the surface of the substrate with a continuous layer of conductive material, masking the layer with a resist, etching away a part of the conductive material so as to obtain the circuit pattern with conductive parts separated by gaps, and filling the gaps with an electrically insulating adhesive material to a level that is at least equal to the thickness of the layer of conductive material. The resist is left on the conductive parts and the adhesive material is selected to be chemically compatible with the resist.

METHOD OF MANUFACTURING A MULTI-LAYER PRINTED CIRCUIT BOARD

The invention relates to a method of manufacturing a multi-layer printed circuit board by bonding together a plurality of circuit board layers each of which includes a substrate and a conductive circuit pattern on at least one surface of the substrate, the method comprising the steps of:coating the surface of the substrate with a continuous layer of conductive material,masking the layer with a resist,etching away a part of the conductive material so as to obtain the circuit pattern with conductive parts separated by gaps andfilling the gaps with an electrically insulating adhesive material to a level that is at least equal to the thickness of the layer of conductive material.

A manufacturing method of this type has been described in US 2005/121225 A1.

WO 2013103298 A1 describes a manufacturing method wherein ink jet printing technologies are used for masking the conductive layer. After etching, the resist is stripped off, and several layers each of which consists of a substrate and at least one circuit pattern are laminated one upon the other, with films of fibre-reinforced epoxy resin intervening between the adjacent layers. These films serve as bonding layers, so that the multiple layers can be bonded together by applying heat and pressure.

U.S. Pat. No. 7,713,862 B2 and U.S. Pat. No. 7,285,305 B2 disclose manufacturing methods, in which ink jet printing technology is also used for directly printing the conductive circuit patterns, and the multi-layer board is built up step by step by successively forming insulating layers at circuit patterns one upon the other.

It is an object of the invention to provide a method of the first-mentioned type which permits to produce multi-layer printed circuit boards with improved quality and yield.

According to the invention, in order to achieve this object, the resist is left on the conductive parts and the adhesive material is selected to be chemically compatible with the resist.

This method has the advantage that the conductive material of the circuit pattern and the adhesive material in the gaps, together, can form a flat surface, which permits to bond the various layers together more reliably and without warping. In particular, each layer will have a constant thickness, regardless of local variations in the distribution of conductive parts and gap. For example, when, in a plurality of layers, each layer has densely packed conductive tracks with only narrow gaps therebetween on one side and only very few conductive tracks with large gaps therebetween on the other side, each layer will nevertheless have a uniform thickness on its entire area, so that no warps will be caused by thickness variations that accumulate over the thickness of the stack. As another advantage, it is not necessary to strip off the resist after the etching step.

Another advantage is that, as the adhesive material is electrically insulating, it will reliably insulate adjacent conductive tracks from one another, even when the gap between the tracks is very narrow and even when pressure is applied in the bonding step and the conductive material of the tracks is squeezed and forced to spread in width direction, with the result that the gaps between the tracks become even narrower.

More specific optional features of the invention are indicated in the dependent claims.

The adhesive material filling the gaps may form part of the surface of the circuit board layer and may directly constitute the bonding layer that is used for bonding the various circuit board layers together. As an alternative, an additional bonding layer may be provided to cover the conductive tracks and also the adhesive material in the gaps. In this case, the adhesive material in the gap will bond to the bonding layer on the one side and to the substrate on the other side, so that the various circuit board layers can be held together with high bonding strength.

The resist may be covered with a thin layer of the adhesive material so as to forma continuous bonding layer. As an alternative, the resist may have adhesive properties itself, so that the gaps may be filled with the insulating adhesive material to the level of the top surface of the resist, and the resist and the insulating adhesive material, together, will form the bonding layer. In a specific embodiment, the material forming the resist may be identical with the material filling the gaps.

In another embodiment, the resist and the material filling the gaps may constitute adhesive components that undergo a chemical reaction with one another in the bonding process. Similarly, when the substrates of the circuit board layers carry conductive patterns on both sides, the material forming the bonding layer between the two circuit board layers may be constituted by a two-component system, with one component forming the surface of one of the circuit board layers and the other component forming the surface of the other circuit board layer. In the bonding process, the two components will react with one another to form the bond.

Similarly as the resist, the material filling the gaps may be applied onto the substrate by means of a printing technique such as ink jet printing or electrostatic printing.

The invention also discloses a multi-layer printed circuit board that is obtained by the process described above.

FIG. 1illustrates an example of a multi-layer printed circuit board having three circuit board layers10with bonding layers12intervening therebetween. Each circuit board layer10comprises a substrate14that may be formed for example by a glass fibre-reinforced epoxy resin and carries an electrically conductive circuit pattern16on both its main surfaces, Each circuit pattern16comprises a number of conductive parts18, e.g. in the form of tracks, that are separated by smaller or wider gaps20.

The gaps20are filled with an electrically insulating adhesive material that, in this example, is identical with the material of the bonding layers12. In the bonded state shown inFIG. 1, the material filing the gaps of the conductive patterns16of two adjacent layers10is merged with the bonding layer12that is arranged between these two circuit board layers10, so that the material filling the gaps and the material of the bonding layer12form a one-piece body22that bonds the two layers together and electrically insulates their circuit patterns from one another.

FIGS. 2 to 6illustrate a sequence of steps for preparing one of the circuit board layers10shown inFIG. 1.

As is shown inFIG. 2, the plate-like substrate14is coated with a continuous layer of electrically conductive material16′ on both sides. Then, as is shown inFIG. 3, the conductive material16′ is subjected to a physical and/or chemical treatment such as pre-etching, for obtaining a surface24that assures good adhesion of a resist26that is applied in a subsequent step, as shown inFIG. 4.

The resist26is applied in a pattern that corresponds to the desired circuit pattern16shown inFIG. 1and masks the conductive material16′ in those areas where the conductive parts18(FIG. 1) shall be formed. A printing technique such as electrostatic printing or ink jet printing may be used for applying the resist26, as is known in the art. Then, the parts of the conductive material16′ that are not covered by the resist26are etched away. The result is shown inFIG. 5. What is left of the conductive material16′ forms the conductive parts18, each of which is still covered with the resist26. Thus, the gaps20are formed between the conductive tracks.

Then, as is shown inFIG. 6, the circuit board layer10is completed by filling the gaps20with an electrically insulating adhesive material28which, in the present example, is identical with the material of the resist26. In this example, the gaps20are filled up to a level that corresponds to the combined thicknesses of the conductive parts18(former thickness of the layer of conductive material16′) and the resist26, and the material filling the gaps20merges with the resist to form a continuous flat surface that extends over the entire area of the layer10.

Finally, a plurality of such layers10, generally with different circuit patterns16, are stacked one upon another and bonded by applying heat and/or pressure, so that the layers of adhesive material28of adjacent circuit board layers10are merged with one another and are cured to form the one-piece bodies22shown inFIG. 1.

The adhesive material28may be applied in the form of droplets by means of ink jet printing. In order to obtain the required layer thickness in the regions of the gaps20, the drop size of the ink jet printer may be adjusted appropriately, or the adhesive material28may be applied in two or more layers in subsequent printing steps.

In a modified embodiment, the adhesive material28may be applied by means of another printing technique such as electrostatic printing. In this case, the adhesive material28will originally be a solid state, in the form of fine particles comparable to toner particles for electrostatic printing. The particle size may be adjusted to obtain the appropriate layer thickness. A fusing step in which the particles are fused to form a continuous solid layer may in this case be combined with the bonding step in which heat and pressure are applied, anyway.

The examples that have been described above may be modified in various ways.

For example, in the step shown inFIG. 6, the adhesive material28may be applied not only in the gaps20but also in the form of a thin layer covering the resist26that was already present on the conductive parts18.

The adhesive material28may also be different from the material forming the resist26, provided that these materials are chemically compatible with one another in the sense that they can be merged to form a one-piece body by fusing, sintering or chemical reaction.

In another embodiment, starting from the condition shown inFIG. 5, the resist26may be removed before the adhesive material28is applied. Then, the adhesive material28may cover the conductive parts18in a thin layer so as to obtain a continuous bonding layer. I this case, of course, the adhesive material28needs not to be compatible with the resist26.

As another alternative, the gaps20may be filled with the adhesive material28only to the level that corresponds to the thickness of the conductive parts18, so that these conductive parts and the adhesive material28, together, form a smooth surface. Then, an additional bonding layer in the form of a thin film may be laminated onto the smooth surface before the circuit board layers10are bonded together.

If the layers10are formed with a conductive pattern16only on one side, e.g. only on the respective top side, an additional bonding layer may also be dispensed with. Then, the bonding layer would be formed only by the adhesive material28in the gaps20and would be interrupted by the conductive parts13.

Another example of a method according to the invention will now be described in conjunction withFIGS. 7 to 10. The first steps of this method correspond to the steps that have been described above in conjunction withFIGS. 2 to 5.

FIG. 7illustrates a state where the resist26has been stripped off, leaving only the substrate14with the conductive patterns16formed thereon.

In this example, it shall be assumed that the conductive patterns16of two adjacent circuit board layers10are to be contacted with one another at specified positions. For this reason, as is shown inFIG. 8, electrically conductive bumps30are printed onto the top surface of some of the conductive parts18, e.g. by means of an ink jet printer.

As an alternative, the bumps30may be formed by cover the part of the conductive layer where the interconnection should be made by printing a resist on it, then etching away the top part of the uncovered conductive tracks, leaving a minimum needed thickness of material for the conductive parts18. In order to prevent under-etching and undesired change of width of the conductive parts18, the gaps20between the tracks should be filled, prior to etching process, with resist to the minimum height that should remain. Next the resist is removed and all gaps are filled with adhesive material until the top surfaces of the bumps are flush with the top surface of the adhesive material.

In a subsequent step, illustrated inFIG. 9, the gaps between the conductive parts18are filled with a first adhesive material28a, and the conductive parts13are covered by the same adhesive material28aup to a level that corresponds to the combined thickness of the conductive parts13and the bumps30, so that the top surfaces of the bumps are flush with the top surface of the adhesive material28a. In this way, first layer10aof the multi-layer printed circuit board is completed.

According to another method for forming the bumps30and achieving the condition shown inFIG. 9, the complete surface of the printed circuit board is covered with an adhesive layer, but holes are left in the adhesive layer in the places where the bumps30shall be formed. The next step is to fill the holes with solder and/or grow conductive material via electrochemical processes until the holes are filled to the rim with conductive material.

FIG. 9further shows another layer10bthat is to be bonded together with the layer10a. The layer10bis prepared in the same way as the layer10awith the only difference that a different adhesive material28bis used for filling the gaps20and covering the conductive parts18. The adhesive materials28aand28bare selected to form reaction partners of a two-component adhesive. Thus, when the layers10aund10bare bonded together, as has been shown inFIG. 10, a bonding layer32is formed at the boundary between the two layers10aand10bdue to chemical reaction of the materials28aand28bwith one another. Further, due to the heat and pressure applied in the bonding step, the bumps30are fused with the conductive parts18on which they have been formed and with the bumps and conductive parts of the adjacent layer, so that a through-contact34is formed for electrically connecting the circuit patterns of the two layers.

Of course, a multi-layer printed circuit board with more than two layers can be formed by stacking alternating layers10aand10band bonding them all together, with formation of bonding layers32at the boundary of each pair of adjacent circuit board layers. The bumps30that are exposed in the top surface and the bottom surface of the multi-layer board may be used for contacting the circuit patterns. Of course, if desired, the insulating adhesive materials on the top surface of the topmost substrate14and the bottom surface of the lowermost substrate14may be omitted or may be used just for filling the gaps20between the conductive parts, so that, for example, electronic components may be surface-mounted on these circuit patterns.

The resist26and the adhesive materials28,28a,28bmay be selected from a large variety of suitable materials. Some general features of preferred materials are described below.

The resist26that is used for masking the conductive material16′ should have a good adhesion to the surface of the conductive material16′ (e.g. copper) or to the treated surface24. It is therefore preferred that this resist comprises one or more adhesive substances (including reactive adhesive substances) such as acrylics (including light-curable acrylics) epoxy resins, urethane acrylics, cyano-acrylates or a combination of these components. The material may be selected to have a good instantaneous (intrinsic) adhesion, or adhesion may be induced by heat and/or light treatment or by a chemical reaction of one or more components in the resist or by a reaction of resist ingredients with components on the (treated) copper surface. Typically, the adhesive components should comprise more than 50% (by volume or by weight) of the total material of the resist.

The adhesive materials28,28aand28bthat are used for filling the gaps between the conductive parts should have a good adhesion to the surface of the substrate14and also to the resist26, if the adhesive material is to cover the resist, Therefore, this material should also contain one or more adhesive substances (including reactive substances), such as acrylics (including light-curable acrylics), epoxy resins, urethane acrylics, cyano-acrylates or a combination of these components. As with the resist26, these adhesive components may have a good intrinsic adhesion, or adhesion may be induced or improved by a heat and/or light treatment or by a chemical reaction of one or more components in the material or by a reaction of ingredients with components on the surface of the substrate14and/or components of the resist26. Again, the content of adhesive components in the material28,28aor28bshould be more than 50% (by weight or by volume) of the total material.

Depending upon the method with which the resist26and the adhesive material28,28a,28bare applied, it may be useful to add additional ingredients to the material. For example, when ink jet technology is used for applying the material, suitable additives would be: a solvent or a diluent (reactive or not) for adjusting the viscosity of the liquid base material as required for ink jet printing; stabilizers and/or inhibitors for preventing premature reaction of the components of the material; flow control and/or gelling agents ensuring a desired flow of the material on the surface onto which they are printed, for regulating coagulation, spread and feathering of the droplets; surfactants for ensuring proper droplet formation and minimizing the creation of satellite droplets; and dyes or pigments for giving the layer a specific colour.

In case of electrostatic printing, suitable additives would be: resins (reactive or not) for making the material solid under normal printing conditions, so as to form toner particles of the required size and suitable for development on a charge image; charge control agents or surface coating agents for regulating the electrical charge of the toner particles so that the toner image can be developed in accordance with the requirements for electrophotographic printing; conductive components or surface coatings for regulating the electrical charge of the toner particles (of course, charge control agents and conductive components should be added only in an amount small enough to assure that the resulting layer will be electrically insulating as a whole); magnetic components; dyes or pigments.