WO2002098193A1

WO2002098193A1 - A method for applying thick copper on substrates

Info

Publication number: WO2002098193A1
Application number: PCT/SE2002/001014
Authority: WO
Inventors: Lars Drugge; Per Ferm
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2001-05-28
Filing date: 2002-05-27
Publication date: 2002-12-05
Also published as: SE0101868D0; TW593778B; SE0101868L

Abstract

A method of providing thick copper conductors on a ceramic substrate is achieved by the steps of applying a seed layer in the form of a conductor pattern (2) on the ceramic substrate (1), applying an autocatalytic copper layer (3) on the conductor pattern (2) and on the substrate (1) outside the conductor pattern (2), applying a plating resist (4 to the areas that are not to be provided with thick copper conductors (5). Subsequently, copper is applied to the areas not protected by the plating resist (4), thus providing the thick copper conductors (5). The plating resist (4) is removed and finally the remaining autocatalytic copper (3) outside the conductor pattern (2) is removed. By the method is obtained a ceramic substrate (1) comprising a conductor pattern (2) on which is provided an autocatalytic copper layer (3) on which is provided thick copper conductors (5).

Description

A METHOD FOR APPLYING THICK COPPER ON SUBSTRATES

TECHNICAL FIELD

The invention relates to a method of providing thick copper conductors for high current applications on ceramic substrates.

BACKGROUND OF THE INVENTION

In order to be able to produce competitive DC/DC modules and/or other efficiency demanding and space-saving components for e.g. telecommunications, it is crucial to find techniques for the manufacture of densely packed circuit boards with low resistance conductors. This is usually achieved by means of so-called thick film technology on ceramic substrates. In order to further save space and reduce costs it is often desirable and necessary to incorporate printed components in a circuit pattern. One requirement for this is that the material in the printed components is compatible with the manufacturing process of the circuit.

Known techniques for the production of the above mentioned types of circuits have included either conductors with high current capacity without printed components, or conductors with low to medium current capacity in combination with printed resistors. The most common methods for manufacture of copper conductors with high current capacity, i.e. thick copper films on ceramic substrates have been by means of:

• Direct Bonded Copper (DBC) • Copper Clad (includes electroplating with subsequent firing)

• High Temperature thick film copper

• Plated Copper on Thick Films (PCTF)

Most of the above mentioned techniques (except PCTF) include a heat treatment at high temperature in order for the copper to adhere to the ceramic substrate, which hitherto has made it difficult or impossible to use a combination of thick copper conductors and printed resistors and crossovers.

Of the above mentioned processes PCTF is of special interest, especially the variant that is described in U.S. Patent No. 5,298,687. The process according to this patent does not include any heat treatment of the ceramic substrate but does require an additional step to apply short-circuiting conductors, so called links, which can be difficult to remove. This short-circuiting of the circuit is necessary in order to facilitate plating of the ceramic substrate.

The short-circuiting links demand space, which in turn results in a reduction of the packing density on the ceramic substrate, i.e. less circuits per substrate. One additional problem is that after the links are removed silver contacts are exposed and the silver might, when exposed to humidity and voltage, start to migrate and short-circuit the circuits on the ceramic substrate. Therefore it demands an extra step of plating with nickel and/or gold in which the exposed silver contacts are covered.

SUMMARY OF THE INVENTION The object of the invention is to eliminate the above mentioned problems and bring about a method for production of dense circuit boards for high current applications, that allow for a combination of thick copper conductors together with printed resistors and/or other printed components.

Among other things, the invention involves printing a circuit pattern by means of a seed layer of silver or the like on the ceramic substrate, on which a layer of autocatalytic copper has good adhesion, instead of firing the final copper pattern to achieve good adhesion to a ceramic substrate, as in the Copper Clad-technology. This also makes it possible to include printed components on the substrate without the risk for damage during the otherwise used firing. Instead of printing temporary short-circuiting silver links, as in the mentioned PCTF-technology, the circuit pattern can be short-circuited by covering the entire surface of the substrate with a thin layer of autocatalytic copper.

The areas on the ceramic substrate not to be further plated with copper are subsequently covered with a so-called plating resist. A subsequent electroplating builds up the copper thickness on the remaining areas.

One of the advantages of the invention is that it is space-saving since there is no need for temporary short-circuiting connections or a so-called bus bar on the substrate for connection of the circuit prior to the final copper plating.

Another advantage is that, since no consideration has to be made for current accessing the link system, the invention allows for a larger number of circuits per ceramic substrate, thus enabling the manufacture of denser circuit boards.

One further advantage is that risk for short-circuiting by unwanted copper treeing is eliminated since no plating resist edge is connecting two conductors.

Finally, since the conductance of the autocatalytic copper is at least 10 times better than the link system of e.g. PCTF a more even copper plating is achieved.

With the invention the conductors achieve a similar conductor quality as the conductors in the copper clad technology, still keeping the possibility to include standard thick film resistors and crossovers.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described more in detail below with reference to the appended drawing on which Figs. 1-6 illustrate successive steps of the method according to the invention. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Figs. 1 - 6 schematically illustrate those process steps, according to the invention, that are included, upon manufacture of thick conductors on a ceramic substrate.

In a first step of the method according to the invention, illustrated in Fig. 1, a so- called seed layer of silver in the form of a conductor pattern 2 is printed on a ceramic substrate 1. This is performed in order to provide a layer that has good adhesion both to the substrate 1 and to copper, and to enable the actual plating.

The process is not limited to just include silver as a seed layer but can include any other material that has good adhesion to the ceramic substrate 1, can act as terminal for printed components and at the same time can bond copper to its surface.

After the conductor pattern 2 is applied to the ceramic substrate 1, according to the invention, the substrate 1 is lowered into a chemical bath for a predetermined time in order to apply an approximately 1 μm thick layer of autocatalytic copper 3 on the substrate 1 and the conductor pattern 2 as illustrated in Fig. 2.

It is also possible to utilize other methods for applying the autocatalytic copper layer 3.

The autocatalytic copper layer 3 is applied in order to facilitate short-circuiting of the conductor pattern 2 and thereby prepare the ceramic substrate 1 for a further build-up of the thickness of the copper conductors by subsequent electrolytic plating or metallization. It is understood that the autocatalytic copper layer 3 is in contact with the conductor pattern 2.

In the next process step, illustrated in Fig. 3, a so-called plating resist 4 is applied on the autocatalytic copper layer 3 on the areas of the ceramic substrate 1 outside the conductor pattern 2. This plating resist consists of a standard polymer material well known in the plating industry. The plating resist 4 is applied in order to protect and cover those areas that are not intended to be plated in a later process step i.e. areas outside the conductor pattern 2.

Subsequently, as illustrated in Fig. 4, copper is applied on the autocatalytic copper layer 3 on the conductor pattern 2 that in the previous step has not been covered by plating resist 4. Thus, copper conductors 5 are obtained. The thickness of the copper conductors 5 is increased to a desired value. The application of copper for the copper conductors 5 can be achieved by means of electroplating or metallization. It is understood that other application methods can be utilized.

The desired value of the thickness of the copper conductors 5 is usually in the interval from 70 μm to 200μm.

The plating resist 4 is then removed by means of a chemical process, as illustrated by Fig. 5. After that, copper remains only as the thick copper conductors 5 and the short-circuiting autocatalytic copper layer 3 on the substrate 1.

The chemical process can include a lowering of the ceramic substrate into one or more subsequent chemical baths that dissolve the plating resist.

Finally, as illustrated in Fig. 6, the autocatalytic copper layer 3 that is outside the conductor pattern 2 is removed. This can be achieved by etching the entire surface of the substrate.

By the above described process steps a ceramic substrate 1 with thick copper conductors 5 is obtained, comprising a seed layer in the form of a conductor pattern 2 under the copper conductors 5 and a layer of autocatalytic copper 3 between the conductor pattern 2 and the thick copper conductors 5. The ceramic substrate 1 can also include various printed components (not shown) e.g. resistors that are connected to the conductor pattern 2 in Fig.l. When and if such components are included on the substrate 1, the plating resist 4 is applied to these as well.

The invention is not limited to the above described and in the drawings shown embodiment, but can be modified within the limits of the appended claims.

Claims

1. A method of providing thick copper conductors for high current applications on a ceramic substrate (1), comprising applying a seed layer in the form of a conductor pattern (2) on the ceramic substrate (1), characterized by the steps of:

-applying a layer of autocatalytic copper (3) covering the conductor pattern (2) and the rest of the substrate (1),

-applying a plating resist (4) on the autocatalytic copper layer (3) outside the conductor pattern (2), -applying thick copper conductors (5) on the autocatalytic copper layer (3) on the conductor pattern (2),

-removing the plating resist (4) from the autocatalytic copper layer (3) outside the conductor pattern (2), and

-removing the autocatalytic copper layer (3) from the substrate (1) outside the conductor pattern (2) .

2. The method according to claim 1, characterized by the application of the copper conductors (5) being achieved by electroplating or metallization.

3. The method according to claim 1, characterized by removing the autocatalytic copper layer (3) by etching.

4. The method according to claim 1, characterized by applying silver as material for the conductor pattern (2).

5. A ceramic substrate (1) with thick copper conductors (5) for high current applications, comprising a seed layer in the form of a conductor pattern (2) under the thick copper conductors (5), characterized by an autocatalytic copper layer (3) between the conductor pattern (2) and the thick copper conductors (5).

6. The ceramic substrate (1) according to claim 5, characterized by the conductor pattern (2) consisting substantially of silver.

7. The ceramic substrate (1) according to claim 5, characterized by the autocatalytic copper layer (3) having a thickness of approximately 1 μm.

8. The ceramic substrate according to claim 5, characterized by the thickness of the thick copper conductors (5) being in the interval 70μm to 200μm.