Electrical solder connection, sensor with a solder connection and method of manufacture

Abstract

A solder connection that connects a contact pad on a first surface to a strip-shaped conductor. The strip-shaped conductor has a widening at one end and a through opening therein. A solder ball is located in the opening and connects the end of the conductor to the contact pad.

Claims

1. A solder connection for a flux-free process, the solder connection being formed between a first surface on which a contact pad is arranged and a strip-shaped conductor, wherein the strip-shaped conductor has at one end a widening and a through-opening, wherein a solder ball is arranged in the through-opening and connects the end of the strip-shaped conductor to the contact pad, wherein the strip-shaped conductor is planar, wherein a side wall of the through-opening is vertical, and wherein the strip-shaped conductor is electrically insulatingly covered except at the vertical side wall of the through-opening, by a substrate of the strip-shape conductor, a protective layer, or both.

2. The solder connection according to claim 1, wherein the strip-shaped conductor is formed as a conductor path on the substrate that is likewise strip-shaped, and wherein the through opening extends through the widening and the strip-shaped substrate.

3. The solder connection according to claim 2, wherein the strip-shaped substrate comprises a flexible material which is a plastic film.

4. The solder connection according to claim 1, wherein the strip-shaped conductor comprises a conductor path comprising copper (Cu), which is provided with a solderable coating in the region of the through-opening.

5. The solder connection according to claim 4, wherein the solderable coating comprises gold (Au) in the region of the through-opening.

6. The solder connection according to claim 1, wherein the strip-shaped conductor is formed as a conductor path on a likewise strip-shaped carrier of polyimide.

7. The solder connection according to claim 1, wherein the diameter of the through-opening is between 50 m and 1 mm.

8. The solder connection according to claim 1, wherein the strip-shaped conductor is formed as a conductive path that runs parallel to the first surface or is bent upwards from the first surface.

9. The solder connection according to claim 1, wherein the contact pad has a thickness of 50 nm to 500 nm.

10. A sensor having the solder connection according to claim 1, the sensor comprising a sensor housing in which are arranged a printed circuit board with a microprocessor, a substrate with a functional layer having a sensor function, a contact pad connected to the functional layer or a conductive structure on or in the functional layer, and a strip-shaped conductor which connects the contact pad to the circuit board, wherein the conductor is connected to the contact pad by means of the solder connection.

11. A method of manufacturing a solder connection, providing a substrate having a contact pad on a surface, placing a strip-shaped conductor having a widening at one end with a through-opening therein on the surface with the contact pad in such a way that the through-opening is arranged above the contact pad, wherein the strip-shaped conductor is planar, wherein a side wall of the through-opening is vertical, and wherein the strip-shaped conductor is electrically insulatingly covered except at the vertical side wall of the through-opening by a substrate of the strip-shaped conductor, a protective layer, or both, printing one or more liquid solder balls into the through-opening from above by laser solder jet bonding, and allowing the solder balls to cool, whereby an electrically conductive.

Description

(1) The figures are schematic and not to scale, so that neither absolute nor relative size indications can be taken from them. Identical or similarly acting parts are marked with the same reference sign in different figures.

(2) FIG. 1 shows a surface with a contact pad

(3) FIG. 2 shows a strip-shaped conductor in plan view

(4) FIG. 3 shows a strip-shaped conductor according to FIG. 2 after placement on the surface according to FIG. 1

(5) FIG. 4 shows from FIG. 3 after inserting a solder ball into the opening

(6) FIG. 5 shows the temperature/time behavior at a solder connection when using different soldering techniques

(7) FIG. 6A shows a top view of a conductor path

(8) FIG. 6B shows the conductor path of FIG. 6A in a cross-sectional view

(9) FIG. 7 shows a sensor in whose sensor housing a flexible strip-shaped conductor is electrically connected to a contact via a solder connection.

(10) FIG. 1 shows a surface OF with a contact pad KP applied to it. The surface is, for example, the surface of a functional layer of a sensor element and may comprise, for example, a thermistor material such as an NTC or a PTC material. The contact pad is made of metal such as Al or Ni and has a solderable coating. Alternatively, the contact pad may be made entirely of solderable metal. For a sensor application, when the surface OF is a functional sensor layer, a thickness of the contact pad of 50 nm to 500 nm is sufficient.

(11) FIG. 2 shows a flat strip-shaped conductor SL in plan view. One end of the strip-shaped conductor has a widening VB in which a through-opening OE is arranged. At least the inner edges of the opening are provided with a solderable coating LB. The solderable coating can overlap the conductor at the edge of the opening. In the widening VB, the conductor SL may have a round outline as shown when viewed from above. Accordingly, the opening may also have a round opening cross-section. The side walls of the opening may be vertical.

(12) However, the outline of the widening may also have a different and, for example, rectangular shape.

(13) The conductor may comprise Cu. The solderable coating may comprise Au. The conductor may be made entirely of metal. However, it may also be formed as a conductor path on a substrate STS, which is also strip-shaped. Preferably, the conductor SL, conductor path LB and substrate STS are flexible.

(14) FIG. 3 shows a strip-shaped conductor SL according to FIG. 2 from above, which has been placed on the surface OF according to FIG. 1 in such a way that the contact pad KP is arranged below the opening OE. Preferably, the opening is centered on the contact pad so that the contact pad completely fills the cross-sectional area of the opening as seen from above.

(15) FIG. 4 shows the arrangement of FIG. 3 after a solder ball SB has been placed in the opening OE and soldered the conductor to the contact pad via the solderable coating LB.

(16) The solder ball largely fills the opening but may still protrude beyond its upper edge.

(17) Depending on the size of the cross-sectional area of the opening OE or depending on the diameter of a round opening, the breakaway force of the solder connection can be adjusted. Corresponding solder balls can then have diameters from 50 m to 1 mm. With, for example, solder balls with a diameter of 350 m strong solder connections can be obtained which resist shear forces of up to approx. 300 cN per solder ball. The pull-off forces that can be achieved are correspondingly high, also at approx. 300 cN per solder ball.

(18) Depending on the solder alloy selected, the melting point of the solder and thus the temperature resistance of the solder connection can be adjusted. With the solder alloy Au80Sn20, the solder connection can be set stable up to 280 C. Other alloys allow low melting points down to a minimum of 100 C.

(19) A preferred process for making the solder connection is laser solder jet bonding. In this process, only a short and selective thermal load is applied to the surface, rather than a complete load on the component/surface over a longer period.

(20) FIG. 5 shows a comparison of the temperature/time behavior of a solder connection when using different soldering techniques. The highest temperature load is experienced by the surface and thus the entire component during reflow soldering in a reflow oven. Curve 3 illustrates the temperature behavior over time, e.g. 60 seconds, which is required for reliable soldering.

(21) Curve 2 represents a solder iron soldering process in which the solder is melted with a heating head via pressure contact. Here, the thermal stress is shorter compared to reflow soldering, but it cannot be controlled as well.

(22) Curve 1 shows only a single narrow temperature peak, as is usually measured in laser solder jet bonding. This shows that the thermal stress occurs only selectively and very briefly (<1 s), and that the amount of heat introduced corresponds only to that of the imprinted liquid solder ball, which is small and dissipates very quickly by means of dissipation.

(23) Laser solder jet bonding has the further advantage that it can be carried out without contact. Thus, there is no mechanical stress on the surface or even on the solder connection during the soldering process.

(24) FIG. 6A shows a top view of a strip-shaped conductor as already shown in FIG. 2.

(25) FIG. 6B shows the strip-shaped conductor SL of FIG. 6A in a cross-section along the line AA shown in FIG. 6A. The cross-section shows a substrate STS, also strip-shaped, on which a thin conductor LB is deposited. The substrate is, for example, a polyimide film which can also withstand mechanical stress due to bending or vibration without damage. The opening OE in the conductor path LB also passes through the STS with the same cross-section. It can be seen here that the solderable coating BL completely covers the side walls of the conductor path in the opening.

(26) FIG. 7 shows a sensor in whose sensor housing a flexible strip-shaped conductor SL electrically connects a contact pad on a functional layer FS of a sensor element to a contact on a PCB via a solder connection. The functional layer is the functional core of the sensor element and is located within a sensor housing GH. In spatial proximity to the sensor element, the PCB is arranged in the housing. It can be arranged arbitrarily with respect to the surface of the functional layer, for example as shown or vertically with respect thereto. The flexible strip-shaped conductor SL also allows bent or angled conductor runs without damage.

(27) The functional layer is arranged at one end of the housing, at the so-called sensor head SK. The sensor head can be designed for a pressure contact, which can, for example, determine the temperature of a surface by pressing on the sensor head.

(28) However, the sensor head can also be immersed in a medium or determine the temperature of the atmosphere for measurement.

(29) Although only described in more detail for a temperature sensor, the solder connection presented is also suitable for any other sensors. It is characterized by small space requirements and simple manufacturing. The manufacturing process can be carried out without mechanical and thermal stress on the solder connection and is therefore particularly suitable for mechanically and thermally sensitive components or sensors.

LIST OF REFERENCE SIGNS

(30) 1, 2, 3 temperature/time measurement curves AA sectional plane BL solderable coating FS functional layer GH sensor housing KP contact pad LB conductor path OE through opening OF first surface PCB printed circuit board SB solder ball SK sensor head SL strip-shaped conductor STS strip-shaped substrate/carrier VB widening