GB2251677A

GB2251677A - Drying thick-layer printed circuit boards

Info

Publication number: GB2251677A
Application number: GB9127253A
Authority: GB
Inventors: Der Lippe Norbert Von; Rudolf Graener; Peter Sommer
Original assignee: Rheinmetall GmbH
Current assignee: Rheinmetall Industrie AG
Priority date: 1991-01-09
Filing date: 1991-12-23
Publication date: 1992-07-15
Also published as: FR2672182A1; DE4100392A1; JPH05152723A; GB9127253D0

Abstract

Thick-layer pastes containing solvents are dried more satisfactorily and before being burnt onto steel hybrid printed circuit boards by the use of a high-frequency alternating electric field which induces eddy-current effects in the board to produce heating so that undesirable cavitation and fissuring accompanying evaporation of the solvent are avoided. <IMAGE>

Description

2231077 1 TITLE Method for Drying Thick-lgyer Pastes This invention

relates to a method for drying thicklayer pastes containing solvents used on steel hybrid printed circuit boards for electrical and electronic circuits.

Systems are known wherein thick-layer pastes are applied to steel hybrid printed circuit boards for the production of electronic circuits using surface mounted devices (M). This is usually done by imprinting thicklayer pastes containing solvents onto the printed circuit boards, for example by a screen printing process. Before the stoving of these thicklayer pastes in an oven of a customary type they are usually dried in advance in a normal drying oven in order to evaporate the solvents.

Hitherto the desired heating of the printed circuit board provided with the thick-layer paste was carried out using infra-red or similar heat radiators directed onto the printed side. This heating process which was effected from above suffered from the drawback that an excessively rapid drying of the pastes on the surface occurred, forming a kind of skin which prevented solvent and gas bubbles from emerging from within the 1 2 paste. This has the disadvantage of causing either the formation of cavities in the paste or conglomerations of small gas bubbles to form larger ones, which on emerging through the already dried surface of the thick-layer paste lead to undesirable fissures and craters.

One of the objects of this invention is to provide a simple means of drying a thick-layer paste, by the evaporation of the solvents contained therein prior to the actual stoving process, without producing cavities or fissures in the paste.

According to this invention there is provided a method for the drying of thick layer pastes containing solvents on a steel hybrid printed circuit board for electronic circuits or components wherein a printed circuit board after application of a thick-layer paste is subjected, prior to a stoving process, to a highfrequency alternating electric field.

The particular ad,antage of the method according to this invention is that for the purpose of drying applied thick-layer pastes the steel hybrid printed circuit boards are subject to a high frequency (HF) alternating field. This HF alternating field induces in the steel hybrid printed circuit boards a voltage producing a short circuit current inside the metal plate as a result of which the latter heats up and the solvents in the paste

1 evaporate evenly without the formation of pores or cracking.

This method according to this invention has proved particularly advantageous using frequencies of about 10 KHz and also provides a simple means of obtaining any desired degree of heating in the steel hybrid printed circuit board up to a temperature of about 1500C by varying the amplitude of the HF alternating field. it has been found in practice that this temperature is a particularly suitable one for the pre-drying operation.

To enable this invention to be more clearly understood reference is now made to examples of the method which are hereinafter described with reference to the accompanying drawings, wherein- Figure 1 shows an example of an arrangement for application of the method of this invention, and Figure 2 shows 6 further arrangement for the application of the method of this invention.

Referring to the drawings, Figure 1 shows a steel hybrid printed circuit board 10 to which a coating 12 of a thick-layer paste containing a solvent has already been applied. Before the stoving of the coating 12 the board 10 is introduced, in accordance with this invention, into i 4 - a high-frequency alternating field of which the lines of magnetic force are referenced 16. As shown in Figure 1, this high-frequency alternating field can be generated, for example, by applying an alternating voltage U to a coil 14.

The lines of magnetic force 16 of the high-frequency alternating field permeate the steel hybrid printed circuit board causing eddy currents to form in the said board 10 which heat the latter more or less from within to a desired temperature.

Figure 2 illustrates a further arrangement for the application of the method according to this invention for drying the coating 12 on a steel hybrid printed circuit board 10. In this case the coil 14 is wound on a Ccone 18 and the printed circuit board 10 is introduced into the gap of the cone 18 between an upper pole piece 20 formed and a lower pole piece 22. After the application of a high-frequency alternating voltage U to the coil 14 a high-frequency alternating field is formed between the pole pieces 20,22, this field being indicated by the lines of magnetic force 16 and resulting in the formation of the aforementioned eddy currents in the steel hybrid printed circuit board 10 and thus heating this latter.

The arrangements shown in Figures 1 and 2 for the i - application of the method of this invention are illustrated by way of an example. Without limiting the scope of the principle of the invention, still further examples are conceivable, provided only that the coated steel hybrid printed circuit board 10 is subject to a high-frequency alternating field for the purpose of drying the thick-layer pastes. It would thus be possible to provide coils 14 (similar to that shown in Figure 1) above or to the side of the steel hybrid printed circuit board 10.

A simple means of varying the amplitude of the alternating field acting on the printed circuit board 10 is to vary the amplitude of the alternating voltage U applied to the coil 14 so that the desired degree of heating for the board 120 can be adjusted or selected with ease.

Due to the simplicity and efficiency the method of this invention is capable of integration into an automated process for the steel hybrid printed circuit boards described and can, where necessary, be simply performed in an input area of a stoving oven of the customary type.

6 1

Claims

Method for the drying of thick layer pastes containing solvents on a steel hybrid printed circuit board for electronic circuits or components wherein a printed circuit board after application of a thick-layer paste is subjected, prior to a stoving process, to a high-frequency alternating electric field.
2. Method in accordance with Claim 1, wherein frequency of the alternating electric field is about 10 KHz.
3. Method in accordance with Claim 1 or 2, wherein the amplitude of the alternating electric field is adjustable.
4. Method in accordance with Claim 3, wherein the amplitude of the electric field is adjusted to produce a temperature of substantially 1500C in the printed circuit board.
5. Method for the drying of thick layer pastes on printed circuit boards substantially as described herein and exemplified with reference to the drawings.

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6. Printed circuit boards when processed by a method in accordance with any preceding claim.