A METHOD FOR DRYSENSITIZATION OF AN INSULATING SURFACE AND A POWDER FOR USE WITH THE METHOD

Abstract: In order to sensitize the surface of an insulating substrate with a view to subsequent electroless metallization, powder particles are transferred to the surface by means of a xerographic process. The particles carry the sensitizing compound, at least on their surface. After fixing of the transferred powder, conventional electroless metallization may take place. The xerographic transfer enables selective plating, since any master that might be copied by the xerographic process may also be transferred to the surface of the substrate. The powder used with the process may be manufactured by acting on an existing plastic powder by a sensitizing metal compound in an organic solution having at least a superficial influence on the individual powder particles.

Description: Powder for Use with the Method. Method for dry sensitization of an insulating surface and powder to be used with the method. The invention relates to a method for dry, selective sensitization of the surface of an insulating substrate for electroless deposition of a metal, including transfer of a metal compound to the surface. Within electronics, it has for a long time been desirable to be able to transfer conducting materials in a prescribed pattern (selectively) with good adherence to the surface of a insulating substrate. In par¬ ticular because the prescribed pattern mentioned might have details that do not touch other parts on the pattern, electroless deposition has been used. For this can be used for example masking of those parts of the surface which should not be metallized or/and sensitization of the parts which should be mettalized. It is a long know method to use aqueous sensitization solutions and lately (e.g., in Danish Patent 132 801) pure organic solutions are described. In the above patent, an ex¬ tremely lucid exposition of the function of these solutions, together with advantages and disadvantages of aqueous and organic solvents, has been made. The method, where aqueous solvents are used, is in brief the fol¬ lowing: +2 (1) Immersing in a Sn solution (sesitizer), rinsing. (2) Immersing in an activator solution (e.g., a Pd compound), rinsing. (3) Immersing in a metallizing bath. The method, where an organic solvent is used, is based upon direct transfer of, e.g., Pd ions to the surface of the substrate by evapo¬ rating the organic solvent, which might also have etched the surface of the substrate. OMPI ■ty IPO . These methods have the disadvantage that the precision of the metal lization is determined by the method- used for transfer of the sensiti¬ zed pattern. As an example can be mentioned serigraphy, where the reso¬ lution will be no better than the mesh gauge of the silk screen. Also confluency and diffluency before drying may occur. Another problem in connection with the above methods is that the adherence in practice does not appear to be very ggd - a problem which is well know from non- selective metallizing. The solutions of this problem are, for example, described in SE Patent 350 774 (rubbing-in together with slight grin¬ ding), GB Patent 1324653 (ion bombardment), DE AS 1571 802 (deposi¬ tion of a binding layer prior to atomization of metal particles). Transfer of the pattern may also be carried out by a selective de- sensitization by ultraviolet light, as described in the Proceedings of the conference Physics in Industry 1976: "New Photoimaging Processes for Industrial Use - a Link between Basic and Appleid Research", pp. 101-105. Still another methods for transfer of a conducting pattern to the surface of a substrate consists in electrostatic precipitation of a masking layer, which is a positive picture of the desired conductive pattern on an existing layer of Cu on the insulating substrate. Accor¬ ding to, US Patents 2 966 429 and 2910 351, the layer is produced by confluency during the melting of an electrostatically transferred thin layer of a dielectric powder. After removal by etching of the non-mas¬ ked Cu layer, the conducting pattern will .be left. In the latter of the above patents, reference is made to a non-published US patent applica¬ tion 472622, which alledgedly describes a photo-electrostatic process involving the surface of the substrate to be covered by a photo-conduc- ting, insulating layer (probably as konow from the ZnO process also knoe as "Electrofax"), The same patent describes that a dielectric pow¬ der in an ensuing partial process for electroplating with silver may be mixed with an Ag compound which, after melting-in with the dielectric powder, is reduced to metallic Ag in a bath. In this way, the conduc¬ ting pattern obtained in the first partial process is covered by Ag particles, which may then be electroplated with other materials. It should be borne in mind that this partial process implies the presence of a conducting layer - in the description Cu - under the transferred and by reduction created Ag particles. The procedure described in the latter two US patents is timeconsu- ming, but provides good results when coarse details such as printed circuits with satisfactory adherence are produced. However, both pro¬ cedures are using stationary "pattern electrodes" for the transfer of the pattern, which makes it difficult to alter the conductive pattern. Actually, the transfer only consists in selectively to protect parts of a laminated layer of Cu against being etched awayj hence the good ad¬ herence may be ascribed to the lamination process. The purpose of the invention is to avoid the disadvantages mentio¬ ned above and to obtain a transfer of a pattern to a surface of an in- silating substrate, which is more frexible (i.e., easy to alter) and which provides a better adherence and greater possibilities of obtai¬ ning finer details than do the hitherto known procedures. This is obtained with the procedure of the invention, which is spe¬ cial in that the transfer is made by the use of a xerographic process for the transfer of a dielectric powder containing a metal compound, after which the transferred powder is exposed to a treatment which will secure the adherence to the surface. In claim No 2 is stated a powder to be used in the procedure accor¬ ding to the invention. This powder consists of a thermoplastic plastic +2 material with an Sn compound precipitated on the surface of each pow¬ der particle. In claim No 3 is stated a powder to be used in the procedure accor¬ ding to the invention. This powder consists of a thermoplastic plastic +2 material with an Sn compound distributed in each powder particle. In claim No 4 is stated a powder to be used in the procedure accor¬ ding to the invention. This powder has the property that, when in a molten state, it wets the surface to be sensitized. In claim No 5 is stated that the treatment after the transfer of the powder according to the invention consists in treating the sub¬ strate and powder with an organic solvent to obtain adherence. In claim No 6 is stated a procedure according to the invention which will establish adherence between powder and substrate surface by melting of the sunstrate surface. In claim No 7 is stated a powder to be used in the procedure accor¬ ding to the invention. This powder contains an infrared-absorbing ma¬ terial. In claim No 8 is stated a powder to be used in the procedure accor¬ ding to the invention, the essential component of this powder being a material with large loss-angle at microwave frequencies. The invention will be further described in the following with spe¬ cification of subprocesses and examples of proparations which have ap¬ peared to be particularly advantageous in connection with the reduc¬ tion to practice of the invention. What makes possible to utilize a procedure as specified in the pre- O- sent invention, and which has not been known hitherto, is that it still may be advantageous to use a three-step process as that described in the introduction, but that it is possible to avoid the wet process at the transfer of the Sn compound, the sensitizing part of the process, and the subsequent selective desensitizing. The adherence between the Sn compound and the surface of the substrate in the wet process is not overwhelmingly good, but quite surprisingly it has appeared that transfer performed according to the invention gives an adherence, which is extremely good, i.e., that the Pd and Cu atoms transferred in the following subprocesses have a better fixation to the substrate than it has been possible to obtain so far. The xerographic process has appeared to be well-suited for transfer of the sensitizing Sn compound as it provides both a suitable cover¬ age of the insulating substrate and a suitable resolution, which are normally conflicting claims. Furthermore, many of the plastic qualities used for toning powders in xerographic copying machines are well suited as regards adhesion to a large number of different substrate materials. As the example, reference is made to those mentioned in US Patent 2966 429; furthermore, it will be a simple matter for the man skilled in -the art to check the usability of commercially available plastic ma¬ terials. Good results are obtained with such different materials as po- lyacrylates, polyamides, polyvinylclorides, and phenol resin. As usab¬ le substrate materials may be mentioned polyesters, polyachrylates, po- lyvinylchlorides, polycarbonates, i.e., a great deal of the materials used in practical electronics. The fixation to the surface of the insulating substrate can partly be obtained by methods, which by and large are well known, by powders not containing sensitizing metal compounds, typically by "burning down". ^UR£ ^ _O H_ Sfa. WΪPCT ' ' ' This means that each particle of the powder is heated so that when melt¬ ing, it spreads on the surface to a degree which depends on the wetting (the surface tension). According to the invention, the same demands are not made to a pore-free confluencing that are mede in the procedures that are based on using the layer obtained as a mask prior to etching. This is probably connected with the sensitizing mechanism which, rather than demanding an even surface as a basis for the subsequent electroless metal deposition, uses the metal-compound particles deposited on the surface as a seed. This means that the demands on the adherence are greater than the demands on the spreading. With a view to obtaining a local heating of the transferred partic¬ les, materials have been added to them which absorb the energy radiation, the wavelength of which depends ,q>n the application and combination of substrate/powder. In cases where the powder material is not thermoplas¬ tic, it will be expedient to let each powder particle heat the surface of the substrate in this area of contact, and a suitable choice of soft¬ ening temperature for the substrate will make possible a melting-down in this case. This method is just a special case of the method that con¬ sists in heating all of the surface to the melting point. In circumstances where heating is not desirable, fixation can be made by letting the powder on the substrate pass an atmosphere with a suit¬ able vapour pressure of an organic solvent which, in the well-known way, makes the substrate and particles stick to one another, and subse¬ quently evaporate the solvent. There are several ways of producing the prepared powders used accor¬ ding to the invention. The methods used so far at the development of the method have all had the purpose of securing that each powder particle, at least on its surface, has had a reasonably even distribution of Sn OMPI WIP compounds. This can be obtained in two principally different ways: - either directly by impregnating the surface of each particle, or by producing a solid mass aiming at an even distribution of the sensitizing compound in the plastic material which is subsequently pulverized in the usual way to obtain a suitable size of the particles. This size is.de¬ termined by the construction of the xerographic transfer apparatus and is a well-known technique within the subject in question. Example 1 Production of a sensitizing powder, aiming at an even distribution of tin compounds in the material: Add to 10 g of a plastic material 0.05 - 0.5 g of SnCl 2 , 2H-0. This mixture can be wet- or dry-ground in an aqueous or organic liquid. Then the liquid may be removed by a conventional drying technique, e.g. evap¬ oration, filtration, or spray-drying. Impregnation of the individual plastic particles with a tin compound may, according to the invention, use as its starting point already ex¬ isting powders, but may also be done at the production of the plastic powder. A more appropriate way of obtaining the desired distribution of tin in the individual plastic particle is to do it during the production of the powder. This can, for example, be achieved by atomizer-drying (spray-drying) the plastic-containing solution, which also contains a tin compound. As certain Sn compounds, e.g. SnCl ? , 2H_0 are soluble in organic solvents, it is possible to influence the powder individually by a sol¬ vent, which makes the particles quell and hence absorb the tin compounds which are dissolved in the organic solvent. Evaporation of the solvent OMPI . /,. IPO , * creates the desired distribution of SnCl ? , 2H„0. Example 2 A plastic powder, which quells when an organic solvent is used, is sus¬ pended in such a solvent, to which is added a saturated solution of SnCl„, 21- O in methanol without precipitation of tin compounds. After a certain time of reaction, the solvent can be removed by e.g. filtering or spray-drying of the suspension, which now contains the prepared pow¬ der. The quantity of tin to be added depends very much on the combina¬ tion of plastic material and solvent and their relative volumes and also on the method of drying the powder. In so far as the plastic material can be completely dissolved in an organic solvent, a tin compound, also in an organic solvent, may be added. Then the prepared powder can be obtained by a conventional process of drying, e.g. by spray-drying of the plastic-containing solution. Another method, which has been successfully used, is a ' chemical pre¬ cipitation of an Sn compound on the powder particles during contiuous stirring. Example 3 Precipitation of a tin compound on each plastic particle. 10 g plastic powder is suspended in 200 ml methanol. While stirring, a saturated solution of SnCl„, 2H„0 in methanol is added, the quantity de¬ pending on the size of the powder particles. The weight of the SnCl„, 2I-L0 may be e.g. 1 - 10?ό of that of the plastic powder. While stirring, an alcalic agens agent is added, causing the suspended plastic powder to act as a scavenger for the precipitation of hydroxy-tin-chlorides. The powder prepared in this way can then be dried, and if necessary ground WIPO and sieved. Through the development of the method according to the invention, sensitization of an insulating surface has been obtained with a view to a subsequent electroless metallization, performed by known methods, without demanding preceding special masters or masks, as any pattern which can be copied by a xerographic process, can be transferred to the desired substrate surface. The method does not put heavy demands on res- sources nor is it detrimental to environment, as an etching-off of the superfluous metal layer is avoided. Furthermore, the metal layer trans¬ ferred by electroless metallization has a much better adherence to the surface of the substrate (e.g. as measured by the Scotch-Tape method) than layers of the same thickness, which have been transferred by con¬ ventional electroless deposition or by evaporation or vpd- ethods (vpd: vapour-phase deposition). In addition, the present method makes much less demands on- the cleaning of the surface of the substrate prior to metallization than do the methods mentioned above.

Claims: P A T E N T C L A I M S 1. Method for a dry, selective sensitization of the surface of an in¬ sulating substrate with a view to electroless deposition of a metal, characterized by the transfer taking place by the use of a xerographic process for the transfer of a dielectric powder containing a metal com¬ pound, after which the transferred powder is exposed to a treatment that will secure adherence to the surface of the substrate. 2. A powder to be used with the method according to claim 1, its main constituent being a plastic material, characterized by the metal compound being an Sn compound precipitated on the surface of each powder particle, the plastic material being thermoplastic. 3. A powder to be used with the method according to claim 1, its main constituent being a plastic material, characterized by the metal com¬ pound being an Sn compound, which is distributed in each powder par¬ ticle, the plastic material being thermoplastic. 4. A powder according to claims 2 or 3, characterized by a wetting of the surface to be sensitized by the molten plastic material. 5. A method according to claim 1, characterized by an adhesion between the dielectric powder and the surface of the substrate being established by the influence of an organic solvent. 6. A method according to claim 1, characterized by an establishing of the adherence between the surface of the substrate and the powder par¬ ticles, at least by local melting. 7. A powder for the use with the method according to claims 1 or 6, characterized by the powder particles containing an infrared-absorbing material. 8. A powder for the use with the method according to claims 1 or 6, characterized by its main constituent being a material with a large loss angle at microwave frequencies.

Inventors: SORENSEN GUNNAR, SVENDSEN LEO GULVAD, SORENSEN G, SVENDSEN L
Applicants: NESELCO AS, SORENSEN G, SVENDSEN L
Application year: 1980
Main classification: H05K3
IPC codes: C23C18, G03G9, H05K3