T 0607/90 06-03-1991

1. The appeal is admissible.

2. Inventive step

2.1. From document D1 a semiconductor device is known, which has all the features defined by the pre-characterising parts of Claims 1 of the main and first and secondary auxiliary requests.

The subject-matter of Claims 1 of all said three requests differs from the device of document D1 in the identically worded characterising part, i.e. in that:

(a) "said conductive block (19) has a concave portion housing resilient means (23) for exerting pressure on either said insulating plate (17) or said main electrode plate means (16a, 16b);" and in that b) "said resilient means is a coned disc spring for exerting pressure over a ring-shaped area."

2.2. The remaining documents on file do not come closer to the device claimed in Claim 1. In particular neither document D2 nor any other document cited in the European Search Report disclose a semiconductor device with a ring-shaped control electrode placed in between a first and a second main electrode or with a coned disc spring.

2.3. Starting - in correspondence with the description of the present application - from document D1, the objective problem underlying the present invention is to avoid irregularities in the size of the control electrode plate and its insulating plate as the result of tolerances in the manufacturing process, i.e. to avoid misalignments which may either cause damage to the insulating plate or an insufficient contact pressure between control electrode and semiconductor body; see also the description, page 4, lines 1 to 17 and page 5, lines 16 to 19.

The above deficiencies will be found automatically in practice by a skilled mechanical engineer who is regarded by the Board to be the competent expert for the manufacturing process of the device components outside the semiconductor wafer. Thus, no contribution to inventive step is to be found in the recognition of this problem.

2.4. The above-mentioned problem is solved according to the identical characterising parts of Claims 1 of all three requests by housing a coned disc spring (feature (b) in para. 2.1) in the conductive block for exerting pressure on the insulating plate or main electrode plate means (feature (a) in paragraph 2.1).

2.5. It is known from document D3, in particular page 190, paragraph 2, to balance manufacturing tolerances by a coned disc spring, i.e. by feature (b). The teaching of document D3 is kept quite general and not specified to one particular use. In the Board's view, a solution of a general mechanical problem in a non-specific field must be considered to form part of the general technical knowledge of a mechanical engineer; see also Decision T 195/84, OJ EPO 1986, 121, in particular point 8.4. Hence, no inventive step can be seen in the fact that the same skilled person - a mechanical engineer - who discovers the technical problem, applies his general expert knowledge in order to solve it.

Even if - in view of the Appellant's argument in paragraph VII(c) - a semiconductor specialist became aware of the above problem of manufacturing tolerances, he would nevertheless not be the specialist whose knowledge and ability have to be taken into account in the assessment of whether the solution involves an inventive step. If the problem prompts the person skilled in the art to seek its solution in another technical field, the specialist in that field is the person qualified to solve the problem; see also Decision T 32/81, OJ EPO 1982, 225. Hence, again, the question whether the provision of a coned disc spring implies an inventive step would have to be decided on the knowledge and abilities of the mechanical engineer.

In the Board's view, a mechanical engineer is conscious of the fact that a coned disc spring exerts a uniform pressure along its annular contact area on the basis of its axial symmetry and will automatically apply it where such uniform pressure has to be realised. The recognition of the necessity for and the advantages of a pressure uniformity for a control contact is held to fall within the general routine capacities of the skilled person. Hence, though there may be a host of other solutions, nothing inventive can be seen in selecting among known technical means a particular one which appears to be the most appropriate for a given purpose on the basis of its well known properties. Thus, nothing inventive can be seen in feature (b) of the characterising part.

2.6. Once having in an obvious way decided to balance unwanted manufacturing tolerances by a coned disc spring, a skilled person will consult the prior art about possibilities for housing spring means in the field of semiconductor devices and take into account document D2, from which feature (a) of the characterising part is known, i.e. a conductive block which has a concave portion housing resilient means for exerting pressure on an insulating plate; see document D2; figure 2. The conversion of the known central circular concave portion into an annular concave portion for housing the selected coned disc spring as well as the realisation of its alternative site "for exerting pressure on said main electrode means" represent, in the Board's view, constructional routine measures of no inventive merit, so that the skilled person would arrive at the subject-matter of Claims 1 of all three requests without exercising an inventive step.

2.7. The Appellant has produced no evidence that a prejudice against the use of coned disc springs - or generally of resilient means - in high current semiconductor devices existed in the art and was generally valid, i.e. that it was a widespread opinion which was generally shared by all experts in the particular field; see also the Decision T 19/81, OJ EPO 1982, 51. Moreover, in the Board's view, the skilled person can be regarded as able to dimension the conductive block and eventually provide additional cooling means in such a way, that also for high current applications the working temperature within the conductive block stays low and allows the elastic properties of a spring to be maintained. Contrary to the Appellant's view, the lack of a description of this application in a prior art document does not prove the existence of a prejudice, cf. paragraphs VII(a) and (b) above.

2.8. For the above reasons, the Board regards it as obvious to make use of a coned disc spring known from document D3 for balancing manufacturing tolerances and to house such a spring in a concave portion of a conductive block in analogy to the teaching of document D2.

3. Therefore, Claims 1 of the main and first and second auxiliary requests are considered to lack an inventive step and not to be allowable with regard to Articles 52(1) and 56 EPC. Claims 2 to 14 of these requests fall because of their dependence on the respective Claim 1.