Lead for a thermopile

Abstract

There is provided a lead for a thermopile, said lead extending through a sheet-like substrate, where the lead comprises a plurality of separate metal nanowires of which at least one nanowire is able to conduct electricity through the substrate, characterized in that the number of metal nanowires in the lead is from 150 to 300.

Description

[0001] Lead for a thermopile

[0002] FIELD OF THE INVENITON

[0003] This invention relates to an electric lead (conductor), a thermocouple and a thermopile.

[0004] BACKGROROUND

[0005] Thermopiles are very useful as temperature- and IR sensors, for example in gas sensors, or other types of sensor applications. Current thermopiles are rather sensitive to damage as the leads are exposed. Often these thermopiles need to be enclosed in a housing in order to be protected, which leads to a bulky sensor product. There is therefore need for small and robust thermopile that can be used without a housing. There is also needed a thermopile with short response time, that can be manufactured in a cost-effective manner, and that can be shaped in different forms. There is also a need for a thermopile that can withstand repeated cycles of soldering during incorporation of the thermopile in components.

[0006] A more compact thermopile would be desirable. For example, a compact thermopile is more easy to integrate into portable devices (for example mobile phones) where the size is of great importance.

[0007] The current invention solves this and other problems.

[0008] SUMMARY OF THE INVENTION

[0009] In a first aspect of the invention there is provided a lead for a thermopile, said lead extending through a sheet-like substrate, where the lead comprises a plurality of separate metal nanowires of which at least one nanowire is able to conduct electricity through the substrate, characterized in that the number of metal nanowires in the lead is from 150 to In various embodiments, the number of nanowires is from 150 to 280. In various embodiments, the number of nanowires is from 160 to 260. In various embodiments the number of nanowires is from 160 to 250, or from 160 to 240.

[0010] In various embodiments, the number of nanowires per area in a plane that is perpendicular to the lead is from 4000000 / cm2to 6 000000 / cm2.

[0011] In various embodiments, the area of a cross section of the lead, in a plane that is perpendicular to the lead, is from 3 000 pm2to 8 000 pm2

[0012] In various embodiments, the thickness of the nanowires is from 500 to 1500 nanometres. In various embodiments, the thickness of the nanowires is from 800 to 1200 nanometres. This provides a thermopile with a short response time.

[0013] In various embodiments, the length of the lead is from 50 to 500 micrometres.

[0014] In a second aspect of the invention there is provided a thermocouple for a thermopile comprising a first lead and second lead according to the first aspect of the invention, where the first and second leads are made of metals with different Seebeck coefficients, and where the first and second leads are electrically connected at the first side or the second side.

[0015] In a third aspect of the invention there is provided a thermopile comprising a plurality of serially connected thermocouples according to the second aspect of the invention. In various embodiments, the thermopile has a response time of at most 800 ms.

[0016] In a fourth aspect of the invention there is provided an IR sensor comprising a thermopile according to the third aspect of the invention.

[0017] In a fifth aspect there is provided a method for sensing IR radiation comprising allowing IR radiation to impinge on an IR sensor according to the fourth aspect of the invention and detecting the voltage created in the thermopile. In a seventh aspect of the invention there is provided a soldered electronic component comprising a thermopile according to the third aspect of the invention. The soldered electronic component may be an IR sensor.

[0018] In an eight aspect of the invention there is provided a method for producing a soldered electronic component comprising the steps a. providing a thermopile according to claim the third aspect of the invention, b. providing a substrate, c. soldering the thermopile to the substrate.

[0019] In various embodiments, the soldering is carried out by reflow soldering. In various embodiments, the reflow soldering subjects the thermopile to a temperature of from 230 C° to 260 C°. In various embodiments, reflow soldering takes place at least twice.

[0020] The lead, and a thermocouple and a thermopile that incorporates such a lead, has several advantages. A thermopile using the inventive leads can be made small and flat. A protective housing is not needed. The thermopile is thin and can be manufactured in many different shapes. The thermopile can be manufactured in a cost-effective manner.

[0021] The inventors have found, trough experiments, that nanowires may be fragile but that the disclosed lead, thermocouple and thermopile can withstand repeated soldering.

[0022] Brief iption of

[0023] The accompanying drawings form a part of the specification and schematically illustrate preferred embodiments of the invention and serve to illustrate the principles of the invention.

[0024] Figs, la and lb are schematic drawings of a cross section of lead, which is not to scale.

[0025] Fig. 2 is a schematic cross section a thermocouple, which is not to scale. Fig. 3 is a schematic cross section along the line A-A in in Fig. 2, showing a part of a substrate.

[0026] Fig. 4 is a schematic cross section of a detail of a thermopile, which is not drawn to scale. Figs. 5 is a schematic side view of a thermopile.

[0027] Fig. 6 is a schematic top view of the thermopile of Fig. 5.

[0028] Fig. 7 is a schematic drawing showing how the area of a cross section of a lead may be determined.

[0029] Fig. 8 is a SEM image of a plurality of exposed leads, showing the top parts of the leads and a plurality of nanowires.

[0030] Fig. 9 is a graph showing a soldering profile. The nominal profile temperature (solid black line) as a function of time is shown together with measured temperature (dotted line) during one soldering cycle.

[0031] Fig. 10a and 10b are graphs showing the mean resistance (10a) and mean IR responsivity (10b) of the different datasets. Error bars for each dataset show a 95% confidence interval of the means.

[0032] Fig 11 is a graph showing the response time of a thermopile.

[0033] DETAILED DESCRIPTION

[0034] With reference to Figs 1-3, the lead (or conductor) 1 comprises or consists of a plurality of metal nanowires 2 in a substrate 3. The substrate 3 may be a film, a sheet or a board, where a film or a sheet is preferred. The substrate 3 may be film-like or sheet-like. The substrate 3 may have a first side 4 and second side 5 which are substantially parallel. The substrate 3 may be considered to have an even thickness. The nanowires 2 are approximately perpendicular to the first side 4 and the second side 5 of the substrate 3. The lead 1 is approximately perpendicular to the first side 4 and the second side 5 of the substrate 3. Base layer 10 is a metal layer from which the nanowires 2 may be grown, as described below. Each of first side 4 and second side 5 has a surface, where the surfaces of first and second side 4,5 are parallel or essentially parallel. The nanowires 2 may extend from the first side 4 to the second side 5. The nanowires 2 are connected at the first side 4 and the second side 5 of the substrate 3 such that the nanowires 2 are electrically connected such that a current can flow from the first side 4 to the second side 5 or vice versa trough the lead 1. Hence the plurality of nanowires 2 essentially works as one single lead 1. Nanowires 2 can for example be connected on the surface of the substrate 3, or just below the surface of the substrate 3. The connection makes the plurality of nanowires 2 work as one single lead 1 for electricity. The connections on the first side 4 or the second side 5 of the substrate 3 can for example be made by connector 8 (see below). Base layer 10 may serve as connection between nanowires 2 forming one lead 1.

[0035] Any suitable thickness of the substrate 3 may be used. The thickness of substrate 3 is preferably from 50 pm to 500 pm, where 75 pm to 200 pm is preferred, and where from 100 pm to 150 pm is even more preferred. The length of the nanowires 2 matches the thickness of the substrate 3, and is thus preferably from 50 pm to 500 pm, where 75 pm to 200 pm is preferred, and where from 100 pm to 150 pm is even more preferred, and where fromllO pm to 140 pm may be even more preferred.

[0036] The material of the substrate 3 should be essentially electrically non-conducting. The material should also preferably have low thermal conductivity. Suitable materials of the substrate 3 include various polymer materials, in particular poorly conducting or non-condu- cive polymer material such as, for example, polyimide, polycarbonate or epoxy, where polyimide is preferred. Aromatic polymers may be used. The material should be compatible with various techniques used in manufacturing of integrated circuits such as electrodeposition (including electroplating), sputtering, etching etc. Etching of the substrate 3 may for example be carried out using alkaline and / or oxidizing wet environments.

[0037] In some embodiments the substrate 3 may be flexible. This may be advantageous in some applications and / or during production. Also, flexibility results in a more robust thermopile 9. Examples of flexible materials for substrate 3 includes polyimide and polycarbonate, for example a film of one of these substances. The thickness of each of the nanowires 2 is preferably from 500 nanometres to 1500 nanometres, more preferred from 800 nanometres to 1200 nanometres, and even more preferably from 900 nm to 1100 nm. The thickness refers to the maximum thickness of one of the nanowires 2 in a lead 1. This results in a lead 1 with good thermoelectric properties where the thermopile 9 has high sensitivity. This results in a thermopile 9 with short response time. For example, the thermopile 9 can provide a sufficient signal to detect presence at room temperature if a hand is swept in front of a sensor that incorporates the thermopile 9. Moreover, the nanowires with this thickness are also durable and robust and are able to withstand reflow soldering.

[0038] The response time of the thermopile is preferably at most 1000 ms, more preferably at most 800 ms, even more preferably at most 700 ms and most preferably at most 650 ms. The response time may be measured as the time to reach 95% of final voltage when the field of view of the thermopile is a black body with a temperature that is between 20°C and 50°C higher than the temperature of the thermopile, where the black body is not in contact with the thermopile and the field of view of the thermopile is completely occupied by the direct radiation from the black body.

[0039] The cross section of the nanowire 2 may be more or less circular. This can be obtained by radiation and etching as described below.

[0040] The metal nanowires 2 of one lead 1 may be parallel or almost parallel, but that is not an absolute requirement. Thus, the nanowires may be at slight angles from each other as seen in Fig. 8.

[0041] Returning to Figs 1-3, the plurality of nanowires 2 are preferably separated from each other to a great extent in the substrate 3. Preferably nanowires 2 are separated by the material of the substrate 3, such that each nanowire 2 has its own tunnel through the substrate 3. Hence no nanowires 2 are in contact with each other at least in one cross section of the lead 1 (Figs 3 and 7). The positions of the nanowires 2 within lead 1 may be randomly distributed (Poisson distribution). The density of nanowires 2 determined as number of nanowires per area in a plane that is perpendicular to the lead (Fig 3), may be from 3 000 000 / cm2to 10 000000 / cm2, more preferably from 5 000 000 / cm2to 8 000000 / cm2, even more preferably from 4 000 000 / cm2to 6 000 000 / cm2, and most preferably from 4 000000 / cm2to 5 500 000 / cm2.

[0042] The perpendicular cross section 6 of the lead lmay have any suitable shape. The cross section 6 may be for example approximately circular, oval or square, where circular is preferred. The cross section 6 can be circular as shown in Figs 3, 6 and 7-8. A suitable shape may be obtained by using an appropriate mask when etching the substrate 3 as described below.

[0043] The cross section 6 is preferably such that the plurality of nanowires 2 are clustered. Fig 3 shows two examples of such clusters.

[0044] The area of the cross section 6 of the lead may preferably be from 1000 pm2to 50000 pm2, more preferably from 2 000 pm2to 30000 pm2, and even more preferably from 3 000 pm2to 8 000 pm2and most preferably from 3 500 pm2to 5 000 pm2.

[0045] The diameter of the cross section 6 - when it is a circle - can be from for example from 30 to 150 pm, where from 40 to 100 pm is preferred and where from 60 to 80 pm is even more preferred and from 70 to 80 pm is even more preferred.

[0046] A suitable number of nanowires 2 in the lead 1 may be from 150 to 300, more preferably from 150 to 280, even more preferably from 160 to 260 and most preferably from 160 to 250 or from 150 to 240, or from 160 to 240 or from 169 to 240 or from 180 to 220. This ensures that the final lead 1 has enough nanowires 2 that extends from the first side 4 to the second side 5 after downstream processing (such as soldering) and for the lifetime of the thermopile 9. Again, it is to be noted that Figs. 1-4 are schematic and only shows a few nanowires 2. Fig la and lb, for example, shows three nanowires 2.

[0047] The inventors have realized, through experiments, that during manufacturing some nanowires 2 may not be complete such that they do not reach from a second side 5 to the first side 4 of the substrate 3 due to imperfection in the manufacturing process (nanowire 2a in Fig lb). Moreover, some nanowires 2 may degrade during downstream processing, such as soldering. Furthermore, the inventors have realized that nanowires 2 may break during its lifetime (nanowire 2b in Fig lb). The indicated preferred numbers of nanowires 2 in each lead 1 refer to the total number of nanowires 2 in the lead 1 and also includes faulty and non-conducting nanowires 2, for example nanowires that are broken (nanowire 2b) or that have not completely grown from the first side 4 to the second side 5 (or vice versa) (nanowire 2a). The lead 1 must have at least one complete nanowire 2 that goes all the way from the first side 4 to the second side 5 in order to conduct electricity.

[0048] The number and thickness of nanowires 2 in a lead 1 and the thickness of the lead may be determined, for example, by etching away the upper part of the substrate 3 to expose parts of the nanowires 2 and capturing scanning electron microscopy (SEM) images of the upper surface of the lead 1. This is described below with reference to Example 2. Alternatively, a cut across the lead 1 can be made along a plane that is perpendicular to the lead 1 before capturing SEM images. The thickness of nanowires 2 is preferably measured approximately in the middle between the first side 4 and the second side 5.

[0049] With reference to Fig. 7, the area of a cross section of a lead 1 may be determined in an image of a cross section of a lead 1 by drawing lines 12 between the outer nanowires 2 that make up the lead 1. The area of the shape may then be determined using image analysis, for example area analysis tools available in image analysis software packages. Because of the separation of leads 1 in a typical thermopile it is straightforward to determine which nanowires 2 participate in the lead 1.

[0050] With reference to Figs. 2 and 3, a thermocouple 7 has two leads la lb, where the first lead la comprises or consists of nanowires 2 of a first metal with a first Seebeck coefficient and the second lead lb comprises or consists of nanowires 2 with a second Seebeck coefficient, where the first and second Seebeck coefficients are different. The skilled person can select suitable pairs of metals. The metal is preferably chosen so that it can be used in an electrodeposition method for growing the nanowires 2. Preferably the first metal is chosen from the group consisting of copper, antimony, cromel (NiCr), SbzTes, and the second metal is chosen from the croup consisting of nickel, constantan (CuNi), PbTe, CoSbs and BizTes. The two leads la lb are for example connected with connector 8, which may be in a shape sometimes referred to as "dogbone". Suitable materials for connector 8 is for example copper or gold. Connector 8 may for example be a part of base layer 10 or a layer added later by for example sputtering or other methods.

[0051] When there is a temperature difference between the first side 4 and the second side 5 a voltage potential will be generated over the ends of leads la, lb as arranged in Fig. 2.

[0052] With reference to Figs. 4-6, a thermopile 9 may comprise a plurality of serially connected thermocouples 7 for example as schematically shown in Fig. 4. Leads la and la' are made of a first metal whereas leads lb and lb' are made of a second metal, as discussed above. The connections are made such that the current alternatingly flows through connectors 8 on the first side 4 and the second side 5 of the substrate 3 as shown in Figs 4 and 5. A suitable number of thermocouples 7 may be connected to form the thermopile 9. The individual thermocouples pairs 7 of the thermopile 9 are coupled to provide a voltage potential that is sufficient to be detected. The current may be detected. The number of leads 1 in the thermopile 9 may be for example at least 50, more preferably at least 100, even more preferably at least 200, and most preferably at least 250. The thermopile 9 may have for example from 50 to 500 leads, more preferably from 200 to 350 leads and most preferably from 250 to 300 leads. The number of thermocouples 7 is half the number of leads 1.

[0053] The sensitivity of the thermopile may be at least 3 pV / K more preferably at least 8 pV / K, more preferably at least 10 pV / K.

[0054] The substrate 3 is film-like or a sheet-like. It may be cut into any suitable shape. Fig. 6 shows a quadratic thermopile 9 with a quadratic detection surface 11 but the substrate 3 can be shaped in any suitable shape. The detection surface 11 of the thermopile 9 (as formed by the surface of the first side 4 or the second side 5) can have any suitable shape such as circular, rectangular, oval etc. Flexibility of the substrate 3 furthermore provides the option to bend the substrate 3 to obtain a thermopile 9 with a variety of shapes. An additional layer may be added to the detection surface 11 of the thermopile 9, such as a heat sink layer. The detection surface 11 of the thermophile may have any suitable size. The inventors have found that a useful thermopile 9 can be made as small as having a detection surface of from 10 to 20 mm2, more preferably from 15 to 18 mm2.

[0055] The thermopile 9 may be a part of an IR or heat sensor that comprises various other downstream components connected to the thermopile 9 such as an amplifier, an A / D converter, a processor, etc. for processing the signal from the thermopile 9.

[0056] The leads 1 of the thermopile 9 are preferably approximately perpendicular to the surface of the substrate 3. Therefore, when the thermopile 9 is used in a sensor, the thermopile 9 will typically be mounted such that the leads 1 of the thermopile 9 will be essentially parallel to the direction of sensing, for example the direction 30 of the incoming IR. One of the first side 4 and the second side 5 will be the hot side, and the other side will be the cold side. Thus, the thermopile 9 will be an "axial" thermopile and not a "lateral" thermopile.

[0057] The lead 1, thermocouple 7 and the thermopile 9 can be produced as described in W02004098256 and corresponding patents (for example granted patent US7176578), in particular Figs 1, 8 and 9 and corresponding text. For example, a film, for example a film of a polymer substrate, such as polyimide film, is exposed to radiation, for example ion radiation, for example from 200-7000 MeV per ion, for example 1000 MeV129Xe27+to create latent nanotracks in the film. The nanotracks are then subjected to etching to create nanosized through-holes in the film. Sodium hypochlorite or potassium oxide may be used for etching at this step. Longer etching times leads to increased diameter of the nanowires 2.

[0058] Nanowires 2 may be formed by electrodeposition of metal in the nanotracks. The metal may be deposited in separate sets of nanotracks 1 by using masks / photoresists or other photolithographic technologies. Nanowires 2 may be grown from a base layer 10. Base layer 10 may for example be obtained by sputtering or electrodeposing a suitable metal, for example copper, onto the surface of substrate 3. The thermopile is highly durable and resistant to degradation due to heating, for example during reflow soldering in an oven.

[0059] It is also provided a soldered electronic component comprising a thermopile comprising nanowires 2. The soldered electronic component may for example be a heat sensor or an IR sensor. The electronic component may comprise a matrix such as a printed circuit board to which the thermopile 9 and other electronic components are attached with the use of soldering. Hence, the electronic component may comprise solidified solder.

[0060] There is also provided a method for producing a soldered electronic component comprising the steps a) providing a thermopile 9, b) providing a substrate or matrix (such as a printed circuit board (PCB), or a blank for a PCB) and c) soldering the thermopile to the substrate or matrix.

[0061] Soldering preferably takes place using reflow soldering as is known in the art, preferably reflow soldering. Reflow soldering involves applying solder to a matrix, such as a circuit board, typically using a stencil, then placing components for the electronic component, such as a thermopile 9, so that at least some part (connecting parts) of the component attach to the solder; and then heating the component and the matrix in an oven. Typically, a predetermined heat profile is used. Suitable maximum temperatures are known in the art and depend on the solder used and may be up from 230 C° to 260 C° or from 230 C° to 240 C°. The reflow soldering may be lead-free soldering.

[0062] In some embodiments, reflow soldering takes place at last two times. Repeated reflow soldering may be used to first attach components to a first side of a PCB and then to a second side of the PCB. Sometimes repeated reflow soldering is used to add or replace components, which may make it necessary to subject some components to high temperatures more than two times. A plurality of leads was created in a 125 pm polyimide film substrate as follows. The entire film was irradiated with 1000 MeV129Xe27+. A mask with a plurality of circular apertures with a diameter of 75 pm was placed on the film to define a plurality of circular apertures. The film was then subjected to etching using sodium hypochlorite with an addition of boric acid to lower the pH. A copper base layer was added onto one side of the film. Nanowires of a first metal were grown from the base layer in the set of wells formed by the apertures, to form a first set of leads. The process was repeated to form nanowires of a second metal in a second set of defined surfaces aperture, to form a second set of leads. The first and second set of leads were connected by addition of copper connectors to form a thermopile. The process is described in W02004098256 (see above). A total of 280 leads (140 thermocouples) were serially connected to form the thermopile.

[0063] A film from example 1 were subjected to oxygen etching to remove parts of the upper layer of the film to expose the upper parts of the leads and the nanowires. SEM images where captured. Fig. 8 shows an example of a plurality of leads, where each lead comprises a plurality of nanowires. The number of nanowires was counted, and the thickness of the nanowires was measured. The number of nanowires in each lead was from 169 to 240. The thickness of the nanowires was approximately 960 nm.

[0064] A number (sample set, n = 19) of thermopiles from Example 1 were surface mounted by reflow soldering to printed circuit boards (PCB) using the solder heating profile described in figure 9. Lead free SAC-305 solder paste was applied using a 0.12 mm thick stainless stencil. The PCBs were then mounted into a measurement setup where measurements of IR responsivity and resistance could be done.

[0065] The measurement setup comprised a calibrated multimeter capable of measuring the voltage output, U, and resistance, R, of the sensor. Infrared radiation was emitted from a black body source at 120 °C, having an emissivity of 0.95, onto the sensor. The sensor and the PCB was mounted on a temperature regulated block, having a temperature of 32 °C. For this configuration, a radiant power, P, could be expressed, and the IR responsivity, was then defined as:

[0066] S = U / P

[0067] To study the effects on resistance and IR responsivity when the sensor was resoldered the trial consisted of an initial characterization of S and R by means of the measurement setup, as stated above, on the sensor sample set after the initial soldering. The sensor sample set was then resoldered and subsequently measured, and this was repeated 5 times, producing two different datasets, table 1, of R and S, each having 19 values of the 6 different conditions.

[0068] Table 1.

[0069] Example 4

[0070] The response time for contact less detection was determined for a thermopile according to Example 1. Briefly, a Fluke 4180 calibrator was placed in the field of view of the thermopile approximately 3 cm from the surface of the thermopile. The thermopile was kept at 26°C and the temperature of the black body of the calibrator was 50°C. The ambient temperature was 25°C. The shutter of the calibrator was opened, and the response time as an increase in voltage in the thermopile was detected. The time for reaching 95% of the final voltage was recorded. A representative response is shown in Fig 11. The response time was determined to be 647 ms.

[0071] 5 Results and conclusions

[0072] The result of the trial is presented in figure 10a and 10b and shows the statistics of the datasets. Tolerance limits (dashed lines) for both resistance (0.4 to 5 kohm) and IR responsivity (0.109 to 0.120 V / W) are also shown. Notably, for both resistance and IR responsivity, w the means and confidence limits fall within the limits of the tolerances. It was found that even after 5 resoldering cycles, resistance and IR responsivity fall within the requirements. It can therefore be concluded that nanowire sensors with 169 to 240 nanowires per aperture surprisingly give robust and durable performance, even after multiple resoldering cycles, at temperatures typical for lead-free soldering.

[0073] 15

[0074] While the invention has been described with reference to specific exemplary embodiments, the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention. The invention is generally defined by the claims.

Claims

CLAIMS1. A lead for a thermopile, said lead extending through a sheet-like substrate, where the lead comprises a plurality of separate metal nanowires of which at least one nanowire is able to conduct electricity through the substrate, characterized in that the number of metal nanowires in the lead is from 150 to 300.

2. The lead according to claim 1 where the number of nanowires is from 150 to 280.

3. The lead according to claim 1 where the number of nanowires is from 160 to 260.

4. The lead according to claim 1 where the number of nanowires is from 160 to 250.

5. The lead according to any one of claims 1 to 4 where the thickness of the nanowires is from 500 to 1500 nanometres.

6. The lead according to claim 5 where the thickness of the nanowires is from 800 to 1200 nanometres.

7. The lead according to any one of claims 1 to 6 where the number of nanowires per area unit in a plane that is perpendicular to the lead is from 4000000 / cm2to6 000000 / cm2.

8. The lead according to any one of claims 1 to 7 where the area of a cross section of the lead, in a plane that is perpendicular to the lead, is from 3 000 pm2to 8 000 pm29. The lead according to any one of claims 1 to 8 where the length of the lead is from 50 to 500 micrometres.

10. A thermocouple for a thermopile comprising a first lead and second lead according to any one of claims 1 to 9, where the first and second leads are made of metals with different Seebeck coefficients, and where the first and second leads are electrically connected at the first side or the second side.

11. A thermopile comprising a plurality of serially connected thermocouples according to claim 10.

12. The thermopile according to claim 11 which has a response time of at most 800 ms.

13. A soldered electronic component comprising a thermopile according to claim 11 or 12.

14. The soldered electronic component according to claim 13 which is an IR sensor.

15. A method for producing a soldered electronic component comprising the steps of a) providing a thermopile according to claim 11 or 12, b) providing a substrate, c) soldering the thermopile to the substrate.

16. The method of claim 15 where the soldering is carried out by reflow soldering which subjects the thermopile to a temperature of from 230 C° to 260 C°.

17. The method of claim 16 where reflow soldering takes place at least twice.

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