SURFACE MOUNTING PROCESS OF COMPONENTS FOR OSCILLATORS ON PRINTED CIRCUITS USING CONDUCTIVE EPOXY ADHESIVE

Abstract: The following description relates to a surface mounting process for components of microwave frequency generation circuits on printed circuits for which a conductive epoxy adhesive is used as a welding element. The adhesive (R1, R2, R3, R4) is applied on the printed circuits (C1) by screen printing process. Moreover an insulating epoxy adhesive (P) is used to glue several components (T1, T2) on the printed circuit board in order to increase the adhesion.

Description: SURFACE MOUNTING PROCESS OF COMPONENTS FOR OSCILLATORS ON PRINTED CIRCUITS USING CONDUCTIVE EPOXY ADHESIVE Field of the Invention The present invention relates to the sector of circuits with surface mounting for high frequency applications, and more precisely to a surface mounting process of components for high frequency generation circuits on printed circuits, using a conductive epoxy adhesive. In the digital radio systems the high frequency generation circuits such as local oscillators carry out one of the most important and critical tasks to obtain the required system characteristics. In order to get an appropπate project of frequency generation circuits at high frequency it is necessary to take into account different constraints as for example phase noise, long term frequency stability versus temperature, short term frequency instability always from the point of view to reduce the realization costs. About the short term frequency instabilities these may occur in phase jumps or oscillation frequency of short duration casually generated due to the mechanical or thermic stresses either by the layer on which the layout of the oscillator is performed or depending of used components or of the alloys by which the components are soldered to the circuit, or of the general mechanic structures housing the circuit itself. These transients of short duration and rare occurrence over long periods of time and varying environmental conditions, generally caused by rapid variations of environmental temperature can be properly modeled as Frequency Steps (FS) in the internal frequency generation circuits. Normally the occurring frequency jumps are in the relative order of 1-2 ppm, i.e. 1 kHz for each GHz of oscillation frequency value. These frequency steps become phase error transients internal to the carrier recovery loop utilized for coherent demodulation. They might cause an unwanted rotation of the I/O demodulation axis, leading to an "eye closing" conditions and hence to errors bursts or even to synchronization losses. The number of this errors is proportional to the level/order of modulation, due to the higher number and therefore to the relative proximity of the various points of the constellation of the modulation system. At the actual state of the art regarding this sector there are no known examples for a solution of the technical problem of the frequency jumps caused by short term instability. For the solution of said technical problem the discovery by the inventors has been determining that the frequency jumps are mainly due to: • imperfect mechanical contacts within the oscillator resonator or in the tuning mechanism; • solder joints, especially with reference to the normal and widespread use of tin alloys for soldeπng; • unstable RF load impedance (frequency pulling); • unstable reference crystal oscillator monitoπng load impedance; • unstable power supply voltage (frequency pushing), • electromagnetic interference; • mechanical shocks and vibrations Summary of the Invention According to the present invention the above-descπbed technical problem has been solved by a surface mounting process for the components of the frequency generation circuits on pπnted circuits using a conductive epoxy adhesive instead of tin alloys, according to claim 1 and the appended claims. The surface mounting process as the object of the present invention presents the main advantage to be of simple application, to use mateπal to be easily found in commerce, such as multilayer pπnted circuits, even for highly professional applications, obtaining a substantial control of realization costs. Brief descπotton of the Drawings The invention, together with further objects and advantages thereof, may be understood with reference to the following descπption taken in conjunction with the accompanying drawings, and the several figures of which like referenced numerals identify like elements, and in which: the figures 1 - 4 show the different phases of the process being the object of the invention. The process according to the invention is based on the following steps realized one after the other A) Realization of frequency generation circuits and in particular of high frequency oscillators on traditional substrate, that is generally multilayer pπnted circuits with dimensions and routes of the track equivalent to the alumina substrate For instance fig 1 shows a cross section of a pπnted circuit CS with 4 layers realized in the known way, by S1 , S2, S3 are indicated three layers of isolating mateπal put between the metal layers C1 , C2, C3, C4 setting up the conductive tracks. The upper track C1 is evidenced as section of a series of circuit pads realized in the known way. The printed circuit is for instance realized on circuit board multiplexed FR4, plated with Ni - Au. B) Screen printing process operation using electrical and thermal conductive epoxy adhesive on printed circuit. In the here described example the adhesive is one component, silver filled. Referring to fig. 2 the screen printing process is realized with a mask being the negative of the printed circuit itself, for example using a stainless steel stencil. On said mask M are realized the holes or apertures B1 , B2, B3, B4 placed in correspondence of the metallic circuit pads C1 of the printed circuit where the components have to be soldered. Moreover the holes or openings are of a smaller dimension than that of the circuit pads so that a crown of about 100 μm will remain covered by the mask. This is to guarantee that after the deposition the conductive adhesive will not go over the circuit pads due to the pressure carried out for the positioning of the components. The stencil M is placed in contact with the printed circuit. The screen printing process is realized by putting the adhesive on the stencil, for example with a steel squeegee, single or double element (typical thickness 100 μm), in double wet pass. Proceeding in this way the adhesive penetrates in the holes of mask. At the end the stainless steel stencil will be taken away with the result of the deposition of the adhesive in the wanted points on the circuit pads corresponding to the holes of the mask shown in fig. 3 with the areas R1 , R2, R3, R4. C) - Positioning of the glue points or dots on the printed circuit in correspondence of the body of some components as the integrated circuits in surface mounting, as for example evidenced in fig. 3 by the dot P in order to increase the adhesion of said components which otherwise could be critical as a consequence to mechanical solicitations the printed circuits are subject to. The adhesive consists for example of isolating epoxy adhesive. The positioning of the isolating epoxy adhesive is carried out by a machine of the pick-and-place type. D) Surface mounting of all components on the circuit board by a machine of the pick-and-place type. Fig. 4 shows the example of surface mounting of the components T1 e T2 on the conductive adhesive of the circuit pads R1 , R2, R3, R4 respectively. T2 is moreover placed on the isolating adhesive dot P. E) Curing of the adhesives. This is carried out using an air or nitrogen forced oven. A curing cycle is implemented which foresees an initial transient starting from environmental temperature and amviπg at about 150 β C within 5 minutes and a stay at that temperature for about 5 minutes at 150°C. At the end of the reticulation cycle the welding of all components on the pπnted circuit is realized. F) The final scoπng of the pπnted circuit by the circuit board. While a particular emoodiment of the present invention has been shown and descπbed, it should be understood that the present invention is not limited thereto since other embodiments may be made by those skilled in the art without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims. So is it for example possible to use other types of conductive epoxy adhesives such as for example bi-component ones. Other types of substrate may be used, as far as they are suitable for the use at high frequency. The screen pπnting process may be realized by a mesch screen with appropnate thickness emulsion.

Claims: CLAIMS 1. Surface mounting process for components adapted to implement a frequency generation circuit at high frequency on printed supports, characterized in that said components are mounted on printed circuits by the use of conductive epoxy adhesive as welding element. 2. Surface mounting process according to claim 1 , characterized in that said conductive epoxy adhesive is applied on the pπnted circuits by a screen printing process. 3. Surface mounting process according to claim 2, characterized in that said screen printing process includes the following steps: application on said printed circuit of a mask ( ) provided with holes or apertures (B1, B2, B3, B4) placed in coσespondence of the metallic circuit pads (C1) of the pπnted circuit where said components must be soldered; - laying of the adhesive on the mask (M) so as to make it penetrate in said holes; taking away of the mask with the result of the deposition of the adhesive on the circuit pads (R1 , R2, R3, R4); surface mounting of the components on the pπnted circuit; - cuπng the adhesive realizing said welding of the components. 4. Surface mounting process according to claim 3, characteπzed in that moreover an isolating epoxy adhesive is used to glue on the pπnted circuit at least some of said components, in order to increase the adhesion, putting it on said pπnted circuit before said surface mounting of the components on the pπnted circuit. 5. Surface mounting process according to claim 3, characterized in that said Mask (M) is a stainless steel stencil. 6. Surface mounting process according to claim 3, characterized in that said mask (M) is a mesch screen with appropriate thickness emulsion. 7. Surface mounting process according to claim 3, characterized in that said holes or apertures (B1, B2, B3, B4) placed in correspondence of the metallic circuit pads contacts (C1) of the printed circuit, have a smaller dimension as that of the circuit pads, so that a crown remains, covered by the mask, of about 100 μm on the edges of the circuit pads. 8. Surface mounting process according to claim 3, characterized in that said laying of the adhesive on the mask (M) is carried out with a steel squeegee, single or double element. 9. Surface mounting process according to claim 3 or 4, characterized in that said curing of the adhesive is carried out using an air or nitrogen forced oven implementing a curing cycle providing for an initial transient starting from environmental temperature and arriving at about 150°C within 5 minutes, and a stay at this temperature for about 5 minutes. 10. Surface mounting process according to whatsoever previous claim, characterized in that said conductive epoxy adhesive is silver filled one component.

Inventors: RICCI SERGIO, TROMBETTA ANTONELLA, RICCI, SERGIO, TROMBETTA, ANTONELLA
Applicants: ITALTEL SPA, RICCI SERGIO, TROMBETTA ANTONELLA, ITALTEL S.P.A., RICCI, SERGIO, TROMBETTA, ANTONELLA
Application year: 1997
Main classification: H05K3
IPC codes: H05K1, H05K3