DK178412B1

DK178412B1 - Temperature Sensor as Flip Chip on a Printed Circuit Board

Info

Publication number: DK178412B1
Application number: DK201270292A
Authority: DK
Inventors: Karlheinz Wienand; Gernot Hacker; Stefan Dietmann
Original assignee: Heraeus Sensor Technology Gmbh
Priority date: 2011-06-01
Filing date: 2012-05-31
Publication date: 2016-02-08
Also published as: AT511506A2; DE102011103827B4; DK201270292A; CN102809441B; CN102809441A; AT511506A3; AT511506B1; DE102011103827A1; AT511506B9

Abstract

To produce a temperature sensor, in which a conductor track (42), which has at least 100 Ohm and is made of platinum, is connected via contact fleids (41) to two conductor tracks (12, 16) made of copper, and in which the conductor track (42) made of platinum, which is implemented ifl thin-film technology as a platinum track that is at least 10 mm long, 3 to 50 pm wide and 0.1 to 5 pm thick on a rectangular surface area measuring 1 to 10 mm of a 0.1 to 1 mm thick ceramic plate (40), transitions at its two ends into contact fleids (41) that are 20 to 500 times wider, according to the invention these two widened fields (41) ifl the interior of these fleids (41) are structured, a metal paste is applied to the two internally structured flelds, and the metal paste is burned in, whereby the thick film (3) / pad, which is generated from the paste, contains an oxidic component and is fixed to the oxidic surface of the ceramic plate (40) which is freely accessible due to the internal structure of the contact fields (41).

Description

The present invention relates to temperature sensors comprising a printed circuit board for fixation between two leads of a cable and to the production thereof, in which a conductor track made of platinum is connected via two contact fields to two conductor tracks of the printed circuit board, wherein the platinum track, which is implemented in thin-film technology as a platinum track that is 0.1 to 1 m long, 10 to 100 pm wide and 1 to 5 pm thick on a rectangular surface area measuring 1 to 10 mm2 of a ceramic plate 0.1 to 1 mm thick, transitions at its two ends into contact fields that 2 to 5 times wider.

DE 39 39 165 C1 shows a temperature sensor in which a plastic film is connected at the front and back to a connecting cable, and a component is arranged on one side of the film and is connected to the connecting cable via a front and a back conductor track. However, the cable contacting impedes automatic manufacturing. While such flexible films are described in other documents as flexible printed circuit boards in the sense of items that are flexible, in contrast to a plate, the adjective thus describes the differences as compared to the plate, and therefore neither a plate nor a printed circuit board.

DE 87 16 103 U1 describes the double-sided contacting of a shunt resistor via conductor tracks of a circuit board, wherein contact is made via a through-connecting to the conductor track on the back of the circuit board. With respect to lastingly reliable and precise temperature measurement, as well as reproducibility and a robust design, this assembly offers room for improvement.

DE 295 04 105 U1 shows a terminal board having a meandershaped current path between a connecting cable and a functional component. No rear contacting is provided.

DE 31 27 727 relates to a device for measuring temperature, in which resistors are arranged on the front and the back of a substrate board and are electrically contacted in each case on the corresponding sides. No additional printed circuit board is provided.

EP 0 809 094 discloses a method for producing a sensor assembly for temperature measurement, comprising a temperature-sensitive shunt re- sistor which comprises a thin metal film as the resistance layer and contact surfaces on a ceramic substrate, wherein the resistance layer is covered by an electrically insulating protective layer, and the contact surfaces are connected so as to conduct electricity and are directly connected in a mechanically secure manner to conductor tracks, which are electrically insulated from each other, on a high-temperature-resistant circuit board. Contact is made with the shunt resistor at one end of the circuit board. At the end of the circuit board facing away from the shunt resistor, contact surfaces are arranged for connecting a carrier or cable. A still moist thick film conductive paste is applied to the circuit board on the contact surfaces used for establishing contact and attaching the shunt resistor, directly prior to applying the shunt resistor to the high-temperature-resistant board, with the free contact surfaces of the shunt resistor being placed on the circuit board and burned into the board at temperatures up to 1000°C, so that it is contacted and attached. In one embodiment, a plug contact surface is disposed in each case on the front and back of the substrate board. However, the method is complex.

DE 197 42 236 discloses a temperature sensor comprising an elongated printed circuit board, which has at least one conductor track on a substrate made of temperature-resistant materials having an electrically insulating surface, wherein two terminal contact fields connected to the conductor track are arranged on the surface for the purpose of electrical connection by means of a melting process with ends of terminal leads of a connecting cable. A first terminal contact field is placed on the front and a second terminal contact field is placed on the back of the circuit board. The printed circuit board is made of epoxy, triazines, polyimides or polytetrafluoroethylene. As seen from above, the conductor track has a meander-shaped design and is configured as a plane in the region of the terminal contact fields. This created a sensor for the permanent reliable precise measurement of temperatures, which has a simple, robust design and offers a high quality.

AT 502636 relates to the production of a temperature sensor, wherein a connecting cable is connected to each end of a front and back current path on a plastic strip. The chips are attached to each other in an appeal ing manner, and very economically, via the contact surfaces on the conductor tracks. The flip chip is a bridge attached to contact fields between the conductor tracks on the plastic strip. The electrical contacts therefore must be able to bear mechanical load. For this reason, a complex metallization is carried out on and around the narrow sides of the chips. Such treatments of chips following the separation thereof are complex and may impair the quality.

The object of the present invention is to increase quality and simplify mass production.

So as to achieve the object, the two widened fields of the thin-film platinum structure have an inner structure, and more particularly a lattice structure, by means of which a composite material of a metal paste, which is applied as a thick film and which contains an oxidic component, is fixed to the freely accessible oxidic surface of the substrate.

According to the invention, flip chips comprising contact fields that are not designed over the entire surface area, but are internally structured, and more particularly have a frame or a lattice structure, are attached in a mechanically stable manner without metallization of the end faces. According to the invention, the chip is finished on a panel until the separation thereof.

The object is achieved by the characteristics of the independent claims. Preferred embodiments are described in the dependent claims.

In mass production, the two widened fields of the thin platinum film are internally structured, in particular in a lattice shape, then a metal paste, and more particularly a silver or platinum paste, is applied to the two internally structured fields, optionally beyond the outer edges of the fields, the metal paste is then burned in, whereby the thick film, which is generated from the paste and contains an oxidic component, is fixed to the oxidic surface of the substrate which is freely accessible due to the inner structure of the contact fields.

The inner structure is preferably a platinum lattice that is structured using thin-film technology. A simple frame made of Pt already increases the strength of the contacts over conventional contact fields, especially when paste is printed beyond the edge of the contact field for this purpose. Comb or meander structures are likewise suitable for contact fields according to the invention. The platinum structure generated from the thin film is continuous or present as one piece.

Ag, Pt, Pd, Ni, Au and Ir are suitable metal components of the metal paste. Platinum pastes, and more particularly platinum pastes containing silver or palladium, or silver pastes containing Pt and optionally Pd, have been used successfully. Other metals either worsen the measurement accuracy or, for example Al, Cu, Fe, worsen the mechanical stability. Glass or glass ceramic materials are suitable binding agents for the paste.

The noble metal paste of the contact fields (pads) allows mechanically stable soldering of the flip chip, using soft solder, to contact fields on a copper circuit board. Attachment of the flip chips by means of the contact fields thereof saves complex solder connections via the end faces, which are customary for SMD components.

Tried and tested soft solder contains tin as the primary component, and silver, copper or lead as the secondary component or secondary components. The soft solder, or the bond thereof to the burned-in platinum paste, then forms the weak spot in terms of mechanical stability. It must therefore be applied as thin as possible, yet thick enough for it to withstand the various expansions that occur due to differing thermal expansions of the different materials of the bonds.

The bridge chip of the temperature sensor comprising a conductor track made of platinum is attached via contact fields to two conductor tracks made of copper, wherein this attachment forms the electrical connection of the platinum conductor track and the copper conductor track.

The substrate of the flip chip is a ceramic plate which is 0.1 to 1 mm, and more particularly 0.3 to 0.5 mm, thick and has a rectangular surface area measuring 1 to 10 mm2, and more particularly 2 to 5 mm2, with a preferred side ratio of 1.2 to 2.5, and more particularly 1.3 to 2.0. Substrates/ceramic plates that are too thin or too long are more difficult to process and become mechanically unusable if they may generate short-circuits due to a lack of rigidity. Thicker substrates worsen the measurement accuracy, as do square ones. While plastic substrates are less complex to produce, they are not suitable for platinum conductor tracks. Conductor tracks made of platinum simplify temperature measurement as compared to other conductor tracks.

On this substrate of the flip chip, the conductor track made of a thin platinum film, which is implemented as a curvy platinum track which has at least 100 Ohm, and more particularly 500 to 10000 Ohm, and is at least 10 mm, and preferably 20 to 500 mm, and still more preferably 40 to 200 mm long, and 3 to 100 pm, and more particularly 20 to 30 pm wide, and 1 to 5 pm, and more particularly 1 to 3 pm thick, transitions at the two ends into contact fields that are 20 to 500 times wider, and more particularly 50 to 200 times wider. Especially thinner, but also shorter, narrower and wider platinum tracks worsen the measurement accuracy. Longer platinum tracks require larger substrates. Thicker platinum tracks require more thin-film coatings. Thick-film platinum tracks worsen the measurement accuracy. If the thin-film contact fields are wider, less space remains for the platinum track. Narrower thin-film contact fields worsen the measurement accuracy.

A respective Ag/Pt, Pt/Ag or Ag-Pt-Pd thick film is attached in particular to the two fields at the end of the platinum track. These improve the mechanical stability. The material selection is very crucial for this purpose. A transition is created from the platinum lattice and the substrate surface, and the transition can be soldered more easily and with more stability using soft solder.

The thick films are attached to each contact field of a circuit board and thus also hold the ceramic substrate on the circuit board.

The circuit board is a glass fiber-reinforced plastic strip, in particular made of epoxy, triazine, polyimide or fluoropolymer. Bismaleimide-Triazine epoxy resin (BT epoxy) is epoxy, triazine and polyimide. Many other plastic materials cannot withstand the thermal requirements.

A conductor track made of copper runs both on the front and back on the plastic strip. The conductor tracks preferably have a curvy structure, for example in the form of meanders, in particular between the contact fields 11 and 13. The measurement accuracy is the best when using pure copper, and not perhaps when using silver. The dependence of the measuring sensitivity on additives in the copper is low in relation to the dependence thereof on the dimensions of the conductor track, printed circuit boards or substrates. Guiding one of the two conductor tracks on the front of the printed circuit board and the other conductor track on the back thereof assures permanently good electrical insulation from each other and simplifies the connection of the leads of a cable by pushing the substrate between the leads, or pushing a lead in each case on one side of the printed circuit board. This self-centering assures not only simple attachment of the leads that is protected from short circuits, but also stabilizes the circuit board held between the leads, thus precisely fixing the board and allowing it to be inserted more easily in a protective conduit. The self-centering in the protective conduit causes secure spacing of the conductor tracks from the protective conduit. This saves spacers or protective layers and is permanently secure.

In this respect, a self-centering use of the circuit board for fixation between two leads of a connecting cable is made possible.

The rear conductor track is connected to a contact field on the front of the circuit board by means of through-connecting, and the three other ends of the two conductor tracks are structured as wide contact fields which are coated with tin solder, wherein for attachment of the ceramic plate the two contact fields are coated with a tin solder, which is used to solder on the thick platinum film. For stability reasons, the attachment on the contact field and the current feed-through are spaced from each other. The contact fields of this plastic circuit board are covering all over and allow simple, efficient contacting and sufficient stability of the contacts. Larger contact fields or wider conductor tracks, worsen the measurement accuracy, as do narrower conductor tracks.

The conductor track on the back between the through-connecting and the contact field is arranged in a rectilinear manner in the vicinity of the center of the plastic circuit board only if the resistance could no longer be neglected as compared to the shunt resistor due to the narrow track width. The conductor tracks preferably do not run directly on top of each other, but paral- lei offset from each other next to the center. This again increases the measurement accuracy.

The substrate is attached in the region of the contact fields parallel via the printed circuit board. In the regions of the curves of the conductor track, the plastic circuit board is free of conductor tracks and electrically insulating.

The electrical resistance of the Pt thin film structure is several times greater than the resistance of all other conductors and contacts combined.

The invention will be described hereafter based on examples with references to the figures.

FIG. 1 shows an exploded view of the layers of a sensor; FIG. 2 shows a shunt resistor of the sensor; FIG. 3 shows the bridge construction of the shunt resistor on the circuit board; and FIG. 4 shows the side of a circuit board for attaching a shunt resistor.

Example 1 A glass fiber-reinforced BT-epoxy film 10, which is coated on both sides with 50 pm copper, is structured to form recurring strip-shaped units 1. Each unit 1 is provided with a contact feed-through 15 at one end and with an elongated, large contact field 11, 17 in each case on the front and back at the other end, the contact field taking up almost the entire width of the strip. The large contact field 11 is connected via the conductor track 12 to the small contact field 13, which extends transversely over almost the entire width of the strip. At the back, the contact feed-through 15 is connected to the large contact field 17 via the conductor track 16. Two smaller contact fields 13, 14, which extend transversely to the longitudinal direction of the circuit board 1, are created on the front and are bridged with a flip chip 4. The contact feedthrough 15 is provided at one of the small contact fields 14. A meandershaped 1 mm wide conductor track 12 is structured between the other small contact field 13 and the large contact field 11.

The contact fields are coated with soft solder 2. The soft solder 2, no- tably of the small contact fields 13, 14, contains tin, silver and copper.

The flip chip 4 is only attached to contact fields 41. As compared to conventional SMD components, the flip chip 4 does not contain any solder contacts at the end faces. In mass production, the flip chips 4 are attached to the panel.

So as to produce the flip chips, a 2 pm platinum thin film is lithographically structured on a 0.5 mm thick ceramic plate in respective 2 x 1.5 mm units to form curvy platinum tracks, each 50 mm long and 20 pm wide and having a resistance of approximately 1000 ohm, as meanders 42 in each case between two contact fields 41. The contact fields 41 are structured at each of the two longitudinal ends of each rectangular unit in lattice form. An Ag-Pt paste is applied to and burned in this lattice. As a result, the thick silver-platinum film designed as a pad 3 adheres particularly securely to the ceramic substrate 40 in the lattice gaps. The units are then separated to form chips 4. The separated chips 4 are attached with the burned-in Ag-Pt paste/pad 3 on the soft solder 2 to the panel via the small contact fields 13, 14 made of copper. For this purpose, the separated chips are not first metallized on and around the narrow sides thereof, which is customary for corresponding SMD components.

The panel is separated. The large terminal contact fields 11, 17 are fixed in pairs between two-core cables 5 between the lead pairs 51. The equiped printed circuit board 1 thus automatically becomes centered between the leads of a cable. Self-centered insertion in a metallic conduit follows on the cable. The self-centering and the flip chip attachment prevent short circuits in the metal conduit. The appeal of this technique is the simplicity thereof as a basis for high safety, because no components are required to space conductor tracks from the conduit.

Flexible printed circuit board would be unusable because they are not plates and therefore lack the rigidity necessary for mass production.

A glass passivation 61 or glass ceramic 61 for protecting the shunt resistor 42 protects the shunt resistor from chemical attacks. Mechanically, the shunt resistor is protected by the metal-free region between the small contact fields 13, 14 on the film 10. An epoxy resin serves as a covering varnish 62, 63 and protects the circuit board from chemical attack. It is protected mechanically due to the self-centering feature.

Example 2 A glass fiber-reinforced BT-epoxy film, which is coated on both sides with 50 pm copper, is structured to form small units 1. Each unit 1 is provided with a contact feed-through 15 at one end and with a large contact field 11,17 in each case on the front and back at the other end. At the back, the contact feed-through 15 is connected to the contact field 17 via a meander-shaped conductor track 16. Two smaller contact fields 13, 14, which extend 1 mm in the longitudinal direction of the circuit board 1, are created on the front and are bridged with a flip chip 4. The inner one of the small contact fields 13 is connected via a meander-shaped conductor track 12 to the large contact field 11 during the structuring of the copper coating. The current feed-through 15 is provided at the outer of the small contact fields 14. A narrow conductor track 16 is structured between the contact feed-through 15 and the large contact field 17.

The contact fields 11, 13, 14 and 17 are coated with soft solder. The soft solder 2 of the small contact fields is a tin alloy containing silver or copper.

The flip chip 4 is only attached via the pads 3 fixed to the substrate at the contact fields 41. As compared to conventional SMD components, the flip chip 4 does not contain any solder contacts at the end faces. In mass production, the flip chips 4 are attached on the panel.

So as to produce particularly small flip chips, a 1 pm platinum thin film is lithographically structured on a 0.3 mm thick ceramic plate into 1.5 x 1 mm units to form curvy platinum tracks, each 30 mm long and 20 pm wide and having a resistance of approximately 1000 ohm, as meanders in each case between two contact fields. The contact fields are structured at each of the two longitudinal ends of each rectangular unit as combs that are open toward the outside. An Ag-Pt-Pd paste is applied to these combs up to the edges of the end faces and burned in. The burned-in paste/pad 3 adheres in the structured gaps of the platinum contact fields 41 directly and particularly securely to the ceramic substrate 10 of the flip chip 4. At the end, the units are separated into chips, without being metallized thereafter at the narrow sides. The separated chips 4 are attached only with the burned-in Ag-Pt-Pd paste/pad 3 on the soft solder 2 to the panel via the small contact fields 13, 14.

The panel is divided into rows of two, in which the large terminal contact fields 10, 17 are directed outward. These terminal contact fields 11, 17 are fixed in pairs between two-core cables between the lead pairs and are then separated. The equiped circuit board is thus automatically centered between the leads of a cable. Self-centered insertion in a metallic conduit follows on the cable. The self-centering and the flip chip attachment prevent short circuits in the metal conduit. The appeal of this technique is the simplicity thereof as a basis for high safety, because no components are required to space conductor tracks from the conduit, and a covering varnish 62, 63 as protection from chemical effects has no mechanical requirements because of the self-centering feature.

Reference numerals: 1 Circuit board 10 Film 11 Large contact field 12 Conductor track 13,14 Small contact fields 15 Contact feed-through 16 Conductor track 17 Large contact field 2 SAC soft solder 3 Silver-platinum thick film pad 4 Flip chip 40 Substrate/ceramic plate 41 Structured Pt contact field 42 Pt meander

Claims

A method of manufacturing a temperature sensor wherein a conductor path (42) having at least 100 Ohms and is made of platinum is connected via contact fields (41) to two conductor paths (12, 16) made of copper, and the conductor path (42) made of platinum implemented in thin film technology as a platinum web of at least 10 mm in length, 3 to 50 µm in width and 0.1 to 5 µm in thickness on a rectangular surface area measuring 1 to 10 mm 1 mm thick ceramic plate (40), which at its two ends passes into contact fields (41) which are 20 to 500 times wider, characterized by structuring these two extended fields (41) within these fields (41), applying a metal paste to the two internally structured fields and burning the metal paste, whereby the thick film (3) generated from the paste contains a component comprising oxide and attached to the surface comprising oxide of the ceramic plate (40) which is freely available on because of the internal structure of contact field the (41).

Method according to claim 1, characterized in that the metal paste is applied beyond the outer edge of the contact field.

Method according to claim 1 or 2, characterized in that the metal paste is a silver paste containing platinum and optionally palladium.

A method according to any one of claims 1 to 3, characterized in that the metal paste (3) of the contact fields is soldered to the contact fields (13, 14) with a soft solder (2) of a copper coating on a band-shaped paste plate (10).

A method according to any one of claims 1 to 4, characterized in that the brazing solder (2) comprises tin as the primary component and at least one metal from the group consisting of silver, copper, lead as a secondary component.

A temperature sensor, wherein a platinum conductor (42) is connected via contact fields (41) to two copper conductors (12, 16), a 0.1 to 5 micron thin film made of platinum structured on a rectangular surface area which measures 1 to 10 mm 2 of a 0.1 to 1 mm thick ceramic plate (40), as a curved platinum web (42) at least 10 mm long and 3 to 50 µm wide, including 0.1 to 2 mm wide contact fields (41) at the two ends of the platinum web (42), characterized in that these two platinum fields (42) have an internal structure by means of which a composite material of a metal paste is burnt as a thick film (3) and contains a component comprising oxide, is attached to the freely accessible surface comprising oxide of the substrate (40).

Temperature sensor according to claim 6, characterized in that the conductor path (42) made of Pt is arranged on a ceramic plate (10) and the length-to-width ratio is 1.2 to 3.0, wherein a respective Ag-Pt or The Ag-Pt-Pd thick film (3) is attached to the two fields (42) at the ends of the platinum web (41), the thick film being in turn attached to a respective contact field (13, 14) of a circuit board (1) and thus also holding the ceramic substrate (40) on the circuit board (1), and the circuit board (1) is a glass fiber reinforced plastic band (10), on which a conductor path (12, 16) made of copper runs both on the front and back, and the conductor path (16 ) at the back is connected by a through connection (15) to a contact field (14) at the front of the circuit board (1), and the three other ends of the two conductor paths (12, 16) are made wider to form contact fields ( 11, 13, 17), wherein the two contact fields (13, 14) for fixing the ceramic plate (40) are coated with a solder (2), by means of which a silver-platinum thick film (3) is soldered on.

Temperature sensor according to claim 6 or 7, characterized in that at least the short conductor path (12) between the contact field (13) and the contact field (11) is arranged in a curved manner on the plastic printing plate (1).

A temperature sensor according to any one of claims 6 to 8, characterized in that the contact fields (11, 13, 14, 17) cover more than 2/3 of the board width and the conductor paths (12, 16) cover less than 1/3.

Use of a temperature sensor according to any one of claims 6 to 9, characterized in that the circuit board (1) is fixed between two wires of a connecting cable.