" SINTERING PRESS FOR SINTERING ELECTRONIC COMPONENTS ON A

SUBSTRATE "

[0001] The present invention concerns a sintering press for sintering electronic components on a substrate.

[0002] As is well known, in some electronics applications, integrated electronic components, e.g. diodes, IGBTs, thermistors, MOSFETs, are fixed to a substrate by the interposition of a sintering paste. In order for each component to be sintered correctly, it must be pressed onto the substrate while it is at a sintering temperature, for example greater than 200°C.

[0003] A sintering press usually comprises a base that forms a pressing plane on which one or more substrates are positioned. The press is equipped with a pressing unit provided, for each substrate, with one or more presser members controlled for example by a hydraulic circuit to exert a predetermined pressure on the electronic components to be sintered.

[0004] In some embodiments of the press, the base is further equipped with one or more load cells suitable to detect the sum of the forces exerted by the presser members on the electronic components for each substrate in order to monitor the correct operation of the press. The load cells are electronic components that have to operate at temperatures much lower than the sintering temperature .

[0005] One of the problems that afflict sintering presses of the type described above is therefore keeping the load cells at a much lower working temperature than the sintering temperature, even though the load cells are operatively connected to the pressing plane, as they must detect the correct application of the force exerted by the pressing members on the substrates.

[0006] The object of the present invention is to propose a press capable of solving such a problem.

[0007] Said object is achieved with a press according to claim 1. The dependent claims describe preferred embodiments of the invention.

[0008] The features and advantages of the sintering press according to the invention will become evident from the description hereinafter of its preferred embodiments, provided by way of indicative and non-limiting examples, with reference to the accompanying figures, wherein:

[0009] - Figure 1 is an axial section of the press according to the invention; and

[0010] - Figure 2 is a perspective view of a reaction element .

[0011] In said drawings, a sintering press according to the invention has been indicated collectively at 1. [0012] The press is suitable for sintering electronic components 10 on a substrate 12.

[0013] In one embodiment, the press 1 is designed to perform the simultaneous sintering of electronic components on a plurality of substrates 12.

[0014] The substrates 12 hold the electronic components 10 to be sintered (e.g. IGBTs, diodes, thermistors, MOSFETs) placed on a layer of sintering paste. The components 10 must be processed with a predefined surface pressure, for example 30 MPa, at a predefined temperature, for example 260°C, for 180 to 300 seconds.

[0015] The electronic components 10 must be pressed with a force directly proportional to their projection surface, taking into account that the components are of thicknesses varied by families.

[0016] The sintering press 1 comprises a framework 8, for example that extends vertically along a press axis X, which extends vertically and supports a pressing unit 14 at the top and a reaction base 60 at the bottom that supports at least one substrate 12, preferably a plurality of substrates 12.

[0017] One or both of the pressing unit 14 and the base 60 is movable with respect to the other along a press axis X to bring the electronic components to be sintered 10 substantially in contact with the pressing unit 14 to then carry out the pressing.

[0018] In one embodiment, the pressing unit 14 comprises, for each substrate 12, one or more presser members suitable to apply the necessary sintering pressure on the electronic components 10.

[0019] In one embodiment, the pressing unit 14 comprises a multi-rod cylinder 20 equipped with parallel and independent presser rods 28. Each presser rod 28 is coaxial and barycentric to a respective electronic component 10 to be sintered and has a thrust section proportional to the force to be exerted on the respective electronic component 10, the area of each electronic component to be sintered being known. The term

"barycentric" means that each presser rod 28 has a rod axis that coincides with the barycenter of the respective electronic component 10.

[0020] In one embodiment, the presser rods 28 are driven by a pressurized control fluid. For example, the presser rods 28 communicate with a compression chamber 30 wherein the control fluid is introduced and wherein a suitable control element is housed to transfer the pressure exerted by the control fluid to the presser rods. For example, this control element is in the form of a membrane 32. When the compression chamber 30 is pressurized to the sintering pressure, the membrane 32 deforms by pressing against the rear ends 28' of the presser rods 28 for a transfer of the sintering pressure to each presser rod 28.

[0021] Naturally, other presser rod actuation systems may also be used.

[0022] In accordance with one aspect of the invention, the press comprises at least one reaction element 40, that extends along an element axis parallel to the press axis X between a first element end 40' and a second element end 40". The first element end 40' forms a support plane for a respective substrate 12.

[0023] A load cell 50 is operatively connected to the second element end 40". The load cell 50 is suitable to detect, by means of the reaction element 40, the force exerted by one or more presser rods 28 of the pressing unit 14 on the electronic components 10 to be sintered located on the substrate 12.

[0024] One should note that the load cell 50 may be used, according to the specific requirements, simply to detect that a pressure has been applied to the respective substrate, thus in ON/OFF operation mode, or to detect the value of the pressure applied, for example by a feedback pressure control.

[0025] In one embodiment, the load cell 50 is housed in a cell holder plate 52 operatively connected to a cooling circuit 54.

[0026] Each reaction element 40 is supported slidably in an element plate 70. The term "supported slidably" does not mean that the element must necessarily slide in the element plate 70, but that this element is inserted in a guided way in a respective guide seat made in the element plate without being bound thereto. In effect, as will be described below, the element plate 70 must ensure heat transmission therefrom to the reaction element 40, keeping the latter in the correct position parallel to the press axis X, but at the same time without affecting the force detected by the respective load cell 50.

[0027] One should note that, in a preferred embodiment, the second end 40" of the reaction element is always in contact with the load cell 50 whereby, during the pressing step, the reaction element 40 undergoes a substantially null or negligible axial displacement. In this case, the reaction element 40 provides a real contrast to the force exerted by the presser member, which is then completely absorbed by the electronic components to be sintered.

[0028] In one embodiment, the element plate 70 is heated by a heating circuit 72 suitable to bring the element plate 70 and, by conduction, the reaction elements 40 to the required sintering temperature. For example, the heating circuit 72 is embedded in a heater body 74 placed around the element plate 70.

[0029] Each reaction element 40 has a heating portion 40a that passes through the element plate 70 and that is suitable to transmit by conduction the heat of the element plate 70 to the respective substrate 12.

[0030] The reaction element 40 has moreover a cooling portion 40b ending with the second end 40" and shaped in such a way as to dissipate the heat transmitted from the element plate 70 to the heating portion 40a.

[0031] For example, the heating portion 40a and the cooling portion 40b are arranged consecutively.

[0032] In one embodiment, the heating portion 40a has an axial extension substantially equal to or slightly greater than the thickness of the element plate 70. For example, the heating portion 40a ends with the first end 40' of the reaction element 40, which protrudes axially from the element plate 70.

[0033] In one embodiment, the element plate 70 and the cell holder plate 52 are separated axially from each other by a separation fluid suitable to dissipate the heat of the reaction element, for example air.

[0034] For example, the cooling portion 40b has an extension substantially equal to the distance between the element plate 70 and the cell holder plate 52. [0035] In one embodiment, the heating portion 40a is prism shaped. For example, the heating portion 40a has an axial extension greater than the diameter of the support plane of the substrate.

[0036] In one embodiment, the cooling portion 40b comprises an axial succession of dissipating disks 44 that extend coaxially to the element axis.

[0037] In one embodiment, the second end 40" of the reaction element 40 is equipped with an infrared screen 46 facing the load cell 50.

[0038] In one embodiment, the heating circuit 72 is suitable to heat the element plate to a working temperature between 240°C and 290°C.

[0039] For example, the element plate is made of a metallic material with high thermal conductivity.

[0040] For example, the heating circuit 72 comprises electrical resistors controlled by resistance thermometers .

[0041] In one embodiment, the cooling circuit 54 is suitable to keep the cell holder plate 52 at a temperature of about 25°C.

[0042] For example, the cooling system is based on the circulation of a coolant conditioned by a chiller.

[0043] The reaction element, combined with the heatable element plate and the coolable cell holder plate, therefore allows to:

[0044] - provide for the heating of the sintered substrate through the transmission of heat by conduction from the element plate to the heating portion of the reaction element;

[0045] - counteract the sintering pressure applied by the upper presser members;

[0046] - transmit the counteracting force to the load cell ;

[0047] - reduce the heat transmission to the load cell .

[0048] The cooling circuit may therefore keep the load cell at an acceptable working temperature, for example 60°C without excessive energy expenditure.

[0049] To the embodiments of the sintering press according to the invention, a person skilled in the art, to satisfy contingent needs, may make modifications, adaptations and replacements of elements with others that are functionally equivalent, without departing from the scope of the following claims. Each of the features described as belonging to a possible embodiment may be implemented independently from the other described embodiments.