T 0488/99 (Bump heights/FUJITSU) 03-09-2002

1. The appeal meets the requirements of Articles 106 to 108 and Rule 64 EPC and is therefore admissible.

2. Amendments

Claim 1 according to the main request corresponds to claim 1 as filed. Claim 1 according to the auxiliary request contains the features of claims 1 and 4 as filed together the feature on page 4, lines 16 to 18 of the application as filed (step of removing oxide). Thus, the requirements of Article 123(2) EPC are met.

3. Inventive step - Main Request

The only issue in the present appeal is that of inventive step.

3.1. Document D4 was considered the closest prior art in the decision under appeal. It discloses a method of assembling two components where solder bumps on one component are brought in contact with corresponding solder bumps on the other component. In order to accurately control the distance between the two components to be assembled, a number of stepped electrodes 11 - 14, 11' - 14' with known heights are formed at the periphery of one of the two components. When the components are brought to a distance equal to the height of one of the stepped electrodes 11 - 14, 11' - 14', this electrode contacts the other component and a current can be detected through the electrode using an ammeter M, M' (cf. page 5, last paragraph to page 6, second paragraph). Thus, as the two components are brought closer together, successively shorter electrodes will establish contact with the other component.

In the assembling method of document D4, the heights of all the bumps are measured. Based on the measured solder bump heights, one of the stepped electrodes 11. - 14, 11' - 14' is chosen which has a height corresponding to a distance between the two components at which all the solder bumps are in contact with their respective counterparts (cf. translation, page 6, third paragraph). The two components are then pressed together until the chosen electrode is in contact with the counter metal pad 15, 15' and conducts current. The distance between the assembled components is at most equal to the height of the chosen electrode and is thus equal to or smaller than the size of the smallest pair of corresponding bumps.

3.2. In this connection the appellant argued firstly that in the method of document D4, the distance between the two components is not determined using the results of the measuring of the heights of the bumps, since the heights of the stepped electrodes are fixed before the heights of the bumps are known, and therefore, the distance between the components cannot be chosen freely (cf. item VII(a) above). The expression in claim 1, "the distance L1 being determined using the results of the measuring and being equal to or less than the smallest measured bump height (H)...", means that the distance L1 is based directly on the measurement of the bump height so that an arbitrary value of the distance L1 as defined in the claim, as against the preselected values of the heights of the stepped electrodes, can be chosen.

3.2.1. It is established practice in the EPO to give the words in a claim the meaning and scope which they normally have in the relevant art, unless the description gives the words a special meaning by explicit definition or otherwise (cf. e.g. Guidelines C-III, 4.2 and T 0415/93, reasons 4). Furthermore, in order to meet the requirements of Article 84 EPC, the claims should be clear in themselves without having to resort to the description for an interpretation (cf. T 1129/97, OJ EPO 2001, 273, reasons 2.1.3). An interpretation of the claims using the description and drawings, as provided for in Article 69(1) EPC, concerns the determination of the extent of protection whenever this is at issue and not the definition of the matter to be protected by a claim.

3.2.2. In the present case, the application in suit neither contains any special definition of the term "determine" nor any other disclosure which would justify a narrow interpretation. Furthermore, the term "determined" was interchanged by the appellant in the appeal procedure with "selected", which suggests that a narrow interpretation of "determined" was not intended.

3.2.3. In the method of document D4, the distance between the two components is selected by selecting one of the stepped electrodes. Although the method of document D4 only allows the distance between the components to be chosen from a limited number of predetermined values, it does not change the fact that the results of the measurements of the bumps in the method of document D4 are used as input for determining which of the stepped electrodes has to be selected, which in turn determines the distance L1 between the components. Thus, the distance L1 in document D4 is determined using the results of measurement of the bump heights as in the method of claim 1.

3.3. The appellant further argued that the method according to document D4, in contrast to the claimed method, is restricted to components having pair of bumps which are larger than the smallest of the stepped electrodes. If one pair of bumps were smaller than the smallest electrode, it would be impossible to select a distance smaller than the smallest pair of bumps, and the components to be connected would have to be discarded (cf. item VII(b) above).

3.3.1. The Board, however, notes that the method according to claim 1 does not exclude a lower limit of acceptable bump sizes. Therefore, this argument cannot be accepted for this reason alone.

Furthermore, contrary to the contentions of the appellant, the Board is satisfied that the method according to document D4 will work properly for most units, and therefore in most cases accomplishes the task of connecting the two components. According to document D4, the size of the largest of the stepped electrodes should be set larger than pairs of bumps, and the difference in height between the largest and the smallest of the stepped electrodes should be set so "that it exceeds the variation of the heights of plural bumps" (cf. translation, page 5, second paragraph, last sentence). Thus, it is ensured that for most units, there will be at least one of the stepped electrodes which has a height smaller than that of the smallest pair of bumps.

3.4. Document D5 discloses a method of measuring the height of a bump electrode using light incident at an oblique angle. The height is obtained from measuring the length of the shadow of the bump (cf. abstract). The method is described as being suitable for automation. Document D5, however, does not disclose that these measurements are used for bonding components together.

3.5. As apparent from the above discussion, document D4 is undisputably the closest prior art.

The method of claim 1 according to the main request differs from that of document D4 in that (i) the distance between the two components is set to be equal of smaller than the smallest measured bump height, whereas in document D4, due to the "double bump" configuration, the distance between the two components becomes less or equal to the smallest sum of the heights of the respective top and bottom bumps 3, 6.

3.6. The above difference is a direct consequence of the fact that both components 1, 2 in the unit of document D4 have bumps. Therefore, the technical problem relates to modifying the process of document D4 for a unit having solder bumps on only one of the two components to be connected.

3.7. At the priority date of the application in suit, it was commonly known in the art to employ flip-chip bonding techniques having solder bumps on only one of the two components to be bonded (cf. for example document D2). In the light of the simplified structure of such arrangement compared to the "double bump" configuration shown in document D4, it appears to be obvious to consider a "single bump" alternative whenever appropriate.

A skilled person faced with the task of applying the teaching of document D4 to a structure where only one of the components has solder bumps, would in accordance with the teaching of document D4 chose the distance between the components to be such that all the bumps are in contact with the two components. In other words, the distance is chosen to be equal to or less than the smallest measured bump height.

3.8. The appellant's argument that the skilled person would be unable to modify the teaching of document D4 to a "single-bump" configuration cannot be followed by the Board (cf. item VII(c) above), since, as mentioned under item 3.3.1 above, the teaching of document D4 regarding the heights of the stepped electrodes is sufficiently detailed so the skilled person is able to modify it for the case when only a single bump is used for each contact. The skilled person would therefore as a matter of routine understand that the highest of the stepped electrodes should be higher than the highest bump, and that the difference between the highest and the lowest of the stepped electrodes is greater than the variation in heights of a single bump.

Therefore, in the Board's judgement, the subject matter of claim 1 according to the main request does not involve an inventive step within the meaning of Article 56 EPC.

4. Inventive step - Auxiliary Request

4.1. The method of claim 1 according to the auxiliary request specifies in addition to that of the main request that (ii) only one of the two components has bumps; (iii) oxide film is removed from the bumps; (iv) the distance L1 between the components is the result of applying a predetermined pressure; and (v) the distance L1 is determined for each unit fabricated.

Feature (v) is known from document D4. Thus, the method of claim 1 according to the auxiliary request differs from that of document D4 in the above features (i) to (iv) mentioned above (cf. also item 3.4 above for feature (i)).

4.2. As discussed in the application in suit (cf. application as published, column 1, line 55 to column 2, line 18), it is necessary that oxide on the solder bumps has to be removed either by etching or using flux in order to ensure proper bonding. For a process where no flux is used at the bonding operation, it is known in the art that the oxide film on the bumps must be removed prior to the bonding operation.

Feature (iv) (applying a predetermined pressure) has the advantage that after finding out the correspondence between the applied pressure and the resulting distance between the components, it is possible to carry out the assembly without having to resort to a complicated structure of stepped electrodes.

In view of the differences with respect to the method of document D4, the technical problems relate addressed by the claimed method relate to (a) providing a method of assembling a unit having solder bumps on only one of the two components, while (b) ensuring that the solder bonds are reliable without using flux at the bonding operation, and (c) simplifying the process so that the provision of stepped electrodes or the like on the components is not required.

4.3. Document D4 discloses that pressure has to be applied to the two components when the bumps are brought in contact with each other (cf. translation, page 6, second paragraph). It is however not suggested in document D4 that a predetermined value of the applied pressure selected in order to obtain the required distance between the components for contacting all the bumps. On the contrary, document D4 consistently discloses that it is necessary to provide a rather elaborate construction of stepped electrodes on one of the two components for monitoring the actual distance between the two components as the assembly operation is performed. The Board therefore agrees with the submissions of the appellant that the skilled person would not find any hint in document D4 to control the distance between the components as a function of applied pressure (cf. item VII(d) above).

Therefore, in the Board's judgement, the subject matter of claim 1 according to the auxiliary request involves an inventive step within the meaning of Article 56 EPC.