FR2477829A1 - Hybrid microwave circuit mfr. using serigraphic layers - uses laser to machine notches from layers of conducting ink obtained by serigraphic process - Google Patents

Abstract

The circuit has a conductive band (1) applied across the surface of a substrate (3) by serigraphic printing using a conductive ink. The applied band is notched to provide a capacitive effect, with the notches (2) formed by laser machining. Pref. the conductive band (1) is applied to an Al substrate (3) by an atomic printing machine before firing at a temp. of about 850 deg.C. for 30 minutes. The machining of the notches (2) is effected by an automatic laser appts. using a YAG laser with a mean power of between 1 and 3 watts and a frequency of 1 to 5 Kg giving a spot with about 50 microns. The machining process can be also used to separate an applied conductive band into halves, or for selective material removal from an applied band for impedance adjustment. The mfg. process can be fully automated.

Description

REALIZATION OF A MICROWAVE CIRCUIT IN PRINTED LAYERS.

The invention relates to a process for producing small spaces, in layers obtained by screen printing an ink on a substrate, then baking. The invention relates more particularly to the production of microwave circuits, for example filters of the micro-ribbon type, provided with small notches, so as to have a capacitive effect.

 The invention relates to electronics, more specifically to hybrid microcircuitry.

When making microcircuits, it is frequently necessary to make small spaces, either to separate two conductive strips or to make notches of a predetermined shape, so as to obtain, for example, microwave filters. By small dimensions is meant here dimensions less than or equal to 100 microns.

 According to the prior art, the patterns were produced by vacuum evaporation and it was easy to obtain, from a given mask, thin layers of desired geometric shape. In addition, photogravure methods made it possible to subsequently obtain the formation of small spaces.

But these methods of vacuum deposition and photoengraving are now proving to be too expensive; compared to screen printing.

 The screen printing process, well known to those skilled in the art, makes it possible to obtain, by passing through a relatively fine mesh screen, an ink sufficiently viscous not to spread too much, the reproduction of the defined pattern. on the screen. Baking is then carried out, in an atmosphere which may be, for example, air or nitrogen, so as to eliminate the organic vehicle from the ink, and to obtain a solid layer, of a thickness substantially equal to 10 microns. .

 This screen printing process has recently been applied to the production of microcircuits, since it is of immediate industrial application and less expensive than the previous processes; however, the implementation of this new process does not make it possible to obtain free spaces (not covered with ink) of small size, that is to say whose dimensions are less than a hundred microns, because the dimensions of the meshes of the screen printing screen are approximately of this order of magnitude. The article published in EMI October 1, 1975, under the title "Screens: essential tools for screen printing thick layers" by F. FRANCONVILLE describes the general properties of screen printing screens and cites in particular that a good rule is to use a mesh opening 2.5 to 5 times larger than the average particle size. The particles being typically in the form of spherical grains of the order of ten microns, it can easily be deduced that the opening is about fifty microns. It is also indicated that the screen printing screens marketed generally have an opening density of 325 mesh, a unit which defines the number of openings per linear inch (2.54 cm), that is to say about a calculated opening of 50 microns.

 The invention aims to overcome the above drawbacks, by proposing a method of producing small spaces in screen-printed layers, which is simple and economical.

This process is remarkable in that it consists in removing the material from the layer, in determined locations, by means of a laser beam.

 This method finds a particularly advantageous application in the production of microwave circuits, such as filters in the form of micro-ribbons provided with small notches.

 The description which follows, with reference to the appended drawings, will allow a better understanding of how the invention is implemented.

 FIG. 1 represents a microwave frequency filter, in the form of a microstrip, provided with so-called spur notches.

 Figure 2 shows an interdigitated transducer.

 Figures 3 and 4 show a coupler.

 According to the prior art, the creation of small spaces is obtained at the time of the deposition of thin layers, by evaporation under vacuum, through an adequate mask, or subsequently to the deposition, by conventional photogravure techniques.

 However, such methods prove to be too expensive for mass market products, as electronic components are today.

 The realization of hybrid microcircuits, by processes as conventional and inexpensive as screen printing has made it possible to develop a whole range of components (conductors, resistances, capacities ...), but the dimensions of the mesh of the screens prevent the production of conductive strips. close together, or the formation of notches in order to obtain high definition microwave circuits.

Microwave filters are known from the prior art and the article published in Microwaves will be cited as an example,
Optics and Acoustics November 1977, Vol.l, nO 6, under the title "Design of microstrip spur-line band stop filters" by RN Bates which describes a particular type of filters provided with notches known as spurs, and which one shown in Figure 1.

 According to this figure, a conductive strip 1 is provided with notches 2, called spurs, since they consist of a first straight slot perpendicular to the edge of the strip, and a second straight slot parallel to the edge of the strip, in the middle of it.

The assembly is produced on a substrate 3, for example in alumina.

The dimensions of the slots may be different.

The process for producing such a structure, in accordance with the present invention, consists in producing the conductive strip 1, by screen printing of a conductive ink, for example a conductive ink with copper, described in patent number 2,305,478 deposited on March 25, 1975, in the name of the Applicant, and cooking under nitrogen, for 30 minutes at a temperature in the region of 8500C.

Then, by means of a focused laser beam, of average power 1 to 5 W, an adjustment frequency 1 to 5 kHz, and a cutting speed of 2 mm.s 1, for a diameter of the spot close to 50 microns, the removal of the material at the determined locations.

For larger widths, several successive partially overlapping passages can be used.

The operations can be fully automated, whether by screen printing, from automatic machines marketed for example by the firm DE HAART, cooking under nitrogen, or volatilization of material by laser beam.

A YAG laser allows in particular the production of these notches, the wavelength (1.06 / μm) of the radiation of this type of laser making it easy to focus> automatic machine tools provided with such a laser and a table already sold.

Such a process can be used for the manufacture of microwave filters but also for the formation of any small space. Thus, it is possible to separate two conductive strips after deposition, and even to rectify strips so as to adjust their impedances to a precise value.

 The list of applications is not limited to the examples cited, and those skilled in the art will not do inventive work in the production of a variant which is not essentially different. I1 also emerges from the field of the invention, the production of antennas as described in European patent application 5.642, filed on November 28, 1979, where the formation of notches can be carried out according to this process; similarly, the production of a structure as shown in FIG. 2, and which represents an interdigital transducer can be carried out according to this method.

It should also be added that the existing automations allow extremely varied notch geometries such as those represented in FIG. 3 (broken line) or in FIG. 4 (curve), and which represent a coupler.

Claims (3)

CLAIMS: 1. A method of producing small spaces, in layers obtained by screen printing and then baking, of ink on a substrate, characterized in that the material of the layer is removed in determined locations, by means of 'a laser beam.  2. Application of the method according to claim 12 to the production of microwave circuits, in the form of micro-ribbons provided with small notches, so as to have a capacitive effect, characterized in that said micro- ribbon by serigraphy of a conductive ink on a generally alumina substrate, through a serigraphy screen, and then the ink is baked so as to obtain a solid layer, then the material is removed the layer, in determined locations, by means of a laser beam. 3. Microwave filters, of the micro-ribbon type provided with small notches, obtained by implementing the method according to claim 1 or 2.