DE102020105180A1 - Method for producing a solder connection, method for separating at least one component from a contact surface, printed circuit board and field device - Google Patents

Abstract

The invention relates to a method for producing a solder connection (1) comprising the steps: - providing solder (5) on a contact surface (2), which solder contains bismuth (Bi) with a proportion of at least 50 percent by weight; - arranging a component (4) on the contact surface (2); - Carrying out a soldering process, comprising the steps: - Melting a solder (5), during which melting the solder (5) is brought to a maximum temperature (Tmax), which is the maximum temperature ( Tmax) is above the liquidus temperature (LQ) of the solder (5), - Subsequent cooling of the solder (5), during which cooling the solder (5) solidifies so that the solder connection (6) between the contact surface (2) and the Component (4) is formed, the cooling taking place at a cooling rate (dT / dt), which cooling rate (dT / dt) is greater than 6 ° C / s. The invention also relates to a method for separating a component (4) from a contact surface (2). Furthermore, the invention relates to a conductor plate (3) and a field device (11) of automation technology with a printed circuit board (3).

Description

The invention relates to a method for producing a solder connection between a contact surface of a circuit board and a component arranged on the contact surface. The invention also relates to a method for separating at least one component soldered onto a respective contact surface of a printed circuit board from the respective contact surface. The invention also relates to a printed circuit board and a field device in automation technology with a printed circuit board.

Soldering is a thermal process with which a bonded joint is obtained.

Printed circuit boards with components soldered on them are used in a wide variety of configurations in the automation technology field devices manufactured and sold by the Endress + Hauser Group. Field devices are used to determine and / or monitor process variables. In principle, all devices that are used close to the process and deliver or process process-relevant information are referred to as field devices. These are, for example, level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc., which record the corresponding process variables level, flow, pressure, temperature, pH value and conductivity. Field devices often have a sensor unit, in particular at least temporarily and / or at least in sections, which is in contact with a process medium and which is used to generate a signal that is dependent on the process variable. Furthermore, these often have an electronic unit arranged in a housing, the electronic unit serving to process and / or forward signals generated by the sensor unit, in particular electrical and / or electronic signals.

Electronic units often include printed circuit boards with components soldered onto them in one or more soldering processes. Different soldering methods are known from the prior art, the selection of which is determined by the application, in particular the components to be soldered and / or the solder.

A soldering process that is widespread in industrial production lines for the production of printed circuit boards is reflow soldering or reflow soldering, in which surface-mountable components, so-called 'surface mounted devices' (for short: SMD components), are placed directly on the intended Contact surfaces are soldered. For this purpose, the SMD components are automatically placed on the contact surface provided with solder paste on the circuit board with automatic placement machines and soldered together with a reflow soldering process in a reflow soldering oven. This creates a large number of solder connections at the same time.

In addition to the SMD components, there are special components that have larger dimensions due to their function. These components are preferably designed as Through Hole Technology components - THT components for short. THT components are typically soldered using wave soldering. The circuit board is moved over a so-called solder wave, which is generated by pumping liquid solder through a narrow gap.

In order to incorporate THT components in a manufacturing process dominated by a large number of SMD components, what has become known as backside reflow soldering and, for example, in the patent DE 102 11 647 B4 method described is used. In backside reflow soldering, THT components arranged on the first surface are soldered overhead from the second surface of the circuit board together with SMD components arranged on the second surface in a common reflow process. This enables the cost-effective production of printed circuit boards with mixed components on both sides.

In the event that not all components can be integrated into a common reflow soldering process, selective soldering processes are used, in which a soldering point can be individually processed in a targeted manner. In the DE 10 2006 035 528 A1 For example, the selective wave soldering of temperature-sensitive THT components is disclosed. Selective wave soldering is a special case of the wave soldering mentioned above.

Both reflow soldering and wave soldering have in common that the solder (or the item to be soldered) is exposed to a so-called temperature profile during a soldering process. This describes a temperature-time curve during soldering. The temperature profile typically includes preheating of the solder and / or the item to be soldered, the achievement of a maximum temperature which is above the liquidus temperature of the solder, and subsequent cooling. In the prior art, efforts are made to achieve low cooling rates in order to produce solder connections of sufficiently high quality, i.e. for example high-strength solder connections with sufficiently high mechanical resistance.

Such solder connections, however, have the disadvantage that, in order to remove the component from the contact surface, its solder connection (s) must be melted again. On the one hand, this is energy-intensive and, on the other hand, it can produce toxic fumes. To protect the environment and to improve the possibility of recycling printed circuit boards, it is desirable to provide a simpler way of removing the components. This applies in particular to printed circuit boards which are used in devices with a comparatively short lifespan, such as between 2 and 10 years.

The invention is based on the object of specifying a method for producing a solder connection which is easy to separate or of specifying a simple method for separating a component from its respective contact surface.

The object is achieved by a method for producing a solder connection between a contact surface of a circuit board and a component arranged on the contact surface, a method for separating at least one component from a contact surface, a circuit board and a field device of automation technology with a circuit board.

• - Bereitstellen von Lot auf der Kontaktfläche, welches Lot Bismut (Bi) mit einem Anteil von zumindest 50 Gewichtsprozent aufweist;
• - Anordnen eines Bauelements auf der Kontaktfläche;
• - Durchführen eines Lötprozesses, umfassend die Schritte:
  • -- Aufschmelzen des Lots, bei welchem Aufschmelzen das Lot auf eine maximale Temperatur gebracht wird, welche maximale Temperatur oberhalb der Liquidustemperatur des Lots liegt,
  • -- Anschließendes Abkühlen des Lots, bei welchem Abkühlen das Lot erstarrt, so dass die Lotverbindung zwischen der Kontaktfläche und dem Bauelement gebildet wird, wobei das Abkühlen mit einer Abkühlrate erfolgt, welche Abkühlrate größer als 6° C/s (d.h. Grad Celsius oder Kelvin pro Sekunde) ist.

With regard to the method for producing a solder connection between a contact surface of a circuit board and a component arranged on the contact surface, comprising the steps:

• - Provision of solder on the contact surface, which solder has bismuth (Bi) in a proportion of at least 50 percent by weight;
• - Arranging a component on the contact surface;
• - Carrying out a soldering process, comprising the steps:
  • - melting of the solder, during which melting the solder is brought to a maximum temperature, which maximum temperature is above the liquidus temperature of the solder,
  • - Subsequent cooling of the solder, during which the solder solidifies so that the solder connection is formed between the contact surface and the component, the cooling taking place at a cooling rate which is greater than 6 ° C / s (ie degrees Celsius or Kelvin per second).

Investigations by the applicant have shown that the combination of the high bismuth-containing solder (ie at least 50 percent by weight) and the high cooling rate greater than 6 ° C / s (ie degrees Celsius or Kelvin per second) result in a brittle solder joint will. Surprisingly, the solder with a high bismuth content, provided it is cooled sufficiently quickly, leads to the brittle solder joint.

In fracture mechanics, a sudden material failure is referred to as brittle fracture. In the stress-strain diagram, brittle materials are characterized by a steep rise in Hook's straight line, at the end of which the break occurs without plastic deformation.

The great advantage of a brittle solder connection is that the component can be neatly separated from the contact surface under a defined force without the need to remelt the solder connection.

The brittle solder connection is nonetheless stable for the intended use time of the printed circuit board under the intended use conditions.

"Intended conditions of use of the circuit board" means, for example, that in the event that the circuit board is used in an electronics unit of a field device in automation technology, the circuit board is exposed to the conditions specified for the field device, such as mechanical vibrations. These are not sufficient to achieve the force required for the brittle fracture.

The method according to the invention is suitable both for the aforementioned reflow soldering and for wave soldering or other types of selective soldering. With regard to the method according to the invention, these differ essentially in whether the solder is provided first and then the component on the contact surface (such as, for example, with reflow soldering), or vice versa, the component is provided first and then the solder (as with wave soldering ).

In one embodiment of the method, the solder provided also contains tin (Sn) with a proportion of at least 40 percent by weight.

In one embodiment of the method, the solder provided also has silver (Ag), with a maximum proportion of 2 percent by weight.

An example is a solder alloy with percentages by weight of Bi: 58 and Sn: 42 (bismuth-tin eutectic) or a bismuth-tin-silver solder with percentages by weight of Bi: 57, Sn: 42 and Ag: 1. These solders have a liquidus temperature of 139 ° C. The silver content is used to reduce the brittleness and should therefore amount to a maximum of 2% in one embodiment.

In one embodiment of the method, the solder provided essentially only has bismuth and tin. This has the advantage of high availability and that it is significantly cheaper than solders that also contain silver.

In one embodiment of the method, the cooling rate is greater than 10 ° C./s.

In one embodiment of the method, the cooling rate is set by supplying liquid nitrogen to the solder connection.

Gaseous nitrogen can be used (for example as a cover medium) in the wave or reflow soldering systems mentioned above. Nitrogen is supplied in a liquid state from a liquid tank. The liquid nitrogen is now preferably also used to cool down soldered items that have already been soldered. The nitrogen is heated to its gaseous state and can be used as a gaseous covering medium for a subsequent soldering process.

In one embodiment of the method, the cooling rate is less than 50 ° C./s (degrees Celsius per second).

• - Tennen des zumindest einen Bauelements von der Kontaktfläche, bei welchem Trennen unter einer Einwirkung einer Kraft auf die Lotverbindung ein Sprödbruch der Lotverbindung erzeugt wird.

With regard to the method for separating at least one component from a contact surface, the object is achieved by a method for separating at least one component soldered onto a respective contact surface of a printed circuit board from the respective contact surface, a solder connection between the component and the contact surface being established by means of a method according to the invention , comprising the step:

• Separation of the at least one component from the contact surface, in which separation under the action of a force on the solder connection, a brittle fracture of the solder connection is produced.

In one embodiment of the method for separating, in the event that several components are separated from their respective contact surfaces, a large part of the components is separated essentially simultaneously from the respective contact surface by means of brittle fracture, with an essentially simultaneous force acting on the respective solder connection.

In one embodiment of the method for separating, the method is carried out at temperatures which are at least 50 ° C., in particular at least 100 ° C. below the liquidus temperature of the solder.

As mentioned above, the liquidus temperature of the high-bismuth-containing solder is approx. 140 ° C. The method can preferably be carried out, for example, at room temperatures of approx. 20 ° C, i.e. a clean, environmentally friendly separation of the component from the circuit board is made possible.

In one embodiment of the method for separating, the force is applied to the solder connection (s) by means of an impact of the printed circuit board in a free fall of the printed circuit board onto an impact surface.

In particular, the force is applied by means of a free fall in a falling device, in which falling device the impact surface is arranged essentially perpendicular to the plane of the printed circuit board. The impact surface is designed, for example, as a surface of a block at the foot of the impact device.

In one embodiment of the method for separating, the force is applied to the solder connection (s) by means of an impact of the printed circuit board accelerated with a centrifugal force on an impact surface.

In one embodiment of the method for separating, the force is applied to the solder connection (s) by means of a bend in the printed circuit board.

In one embodiment of the method for separating, the force is applied to the solder connection (s) by means of sonication with ultrasound or infrasound.

• - eine Kontaktfläche;
• - ein Bauelement, das mittels einer Lotverbindung mit der Kontaktfläche stoffschlüssig verbunden ist,

wobei die Lotverbindung mittels eines erfindungsgemäßen Verfahrens hergestellt ist.With regard to the circuit board, the object is achieved by a circuit board having:

• - a contact surface;
• - a component that is materially connected to the contact surface by means of a solder connection,

wherein the solder connection is produced by means of a method according to the invention.

With regard to the field device of automation technology, the object is achieved by a field device of automation technology, with an electronics unit, the electronics unit having the circuit board according to the invention.

The invention and further advantageous refinements are explained in more detail below on the basis of exemplary embodiments. The same parts are provided with the same reference symbols in all figures; If it is necessary for clarity or if it appears to be useful in some other way, the reference symbols already mentioned are dispensed with in the following figures.

• 1 a, b, c: Eine Schnittansicht einer erfindungsgemäßen Lotverbindung.
• 2: Ein Temperatur-Profil des erfindungsgemäßen Verfahrens zur Herstellung der Lotverbindung;
• 3: Ein Flussdiagram des erfindungsgemäßen Verfahrens zur Herstellung der Lotverbindung und des erfindungsgemäßen Verfahrens zum Trennen zumindest eines auf einer jeweiligen Kontaktfläche einer Leiterplatte aufgelöteten Bauelements von der Kontaktfläche;
• 4 a, b, c: Verschiedene Ausgestaltungen des erfindungsgemäßen Verfahrens zum Trennen zumindest eines auf einer jeweiligen Kontaktfläche einer Leiterplatte aufgelöteten Bauelements; und
• 5: Ein Feldgerät der Automatisierungstechnik mit einer erfindungsgemäßen Elektronikeinheit.

It shows:

• 1 a, b , c : A sectional view of a solder connection according to the invention.
• 2 : A temperature profile of the method according to the invention for producing the solder connection;
• 3 : A flow diagram of the method according to the invention for producing the solder connection and the method according to the invention for separating at least one component soldered onto a respective contact surface of a circuit board from the contact surface;
• 4 a, b , c : Various configurations of the method according to the invention for separating at least one component soldered onto a respective contact surface of a circuit board; and
• 5 : A field device of automation technology with an electronic unit according to the invention.

In 1a , 1b are steps in the manufacturing process for making a solder joint 1 between a contact surface 2 a circuit board 3 and one on the contact surface 2 arranged component 4th shown, with the circuit board 3 is shown in a sectional view. A component 4th will be on the contact surface 2 equipped. Then (or, depending on the soldering process, before assembling the component 4th ) a plumb bob 5 provided. According to the invention, it is a solder with a high bismuth content 5 ie with a proportion of at least 50 percent by weight. The component 4th is then soldered in a conventional bulk or selective soldering process to create a solder connection 6th to produce, again referring to the wave or reflow soldering mentioned at the beginning.

All common soldering processes have in common that an in 2 The temperature profile shown in more detail is controlled. This is a temperature-time function that describes at what point in time during soldering the item to be soldered (e.g. the component 4th ) and / or the plumb bob 5 which temperature is experienced, for example, in a first section of the temperature profile, preheating occurs, followed by heating to a maximum temperature Tmax . This is typically above the liquidus temperature LQ of the lot 5 . This is followed by controlled cooling, ie with a defined, set cooling rate dT / dt . As a cooling rate dT / dt In a soldering process, the maximum change in temperature over time is referred to as the cooling process, immediately afterwards at the point in time at which the liquidus temperature is reached LQ is present.

According to the invention, the high bismuth solder is now 5 with a cooling rate dT / dt combined that is greater than 6 ° C / s. This is significantly greater than the cooling rates commonly used in the prior art.

Surprisingly, investigations by the applicant have shown that this results in a brittle solder connection 6th is produced.

In one embodiment, the cooling rate is increased with the supply of liquid nitrogen 7th set. Of course, the invention is not limited to this embodiment.

All solder connections are preferred 6th of the component 4th Solder connections produced with the method according to the invention 6th . The component 4th can be very easily achieved by means of a brittle fracture in the solder joint 6th clean from the circuit board under the action of a controlled force 3 be separated again. In the case of brittle fracture, the solder joint becomes brittle 6th separated into two or more parts. This is in 1c shown.

The steps of the process for producing the brittle solder joint 6th are also once again as a flow diagram in 3 shown. They include: A: Providing the high bismuth solder 5 , B: Providing the component 4th , C: melting and D: cooling at the cooling rate according to the invention greater than 6 ° C./s. Depending on the soldering process, step A takes place before B or vice versa.

After the end of the intended service life of an electronic device in which the circuit board 3 is used (typically 2 to 10 years), the component can 4th very easy from the circuit board 3 due to brittle fracture of the solder joint 6th removed.

This takes place in step E: separating the at least component 4th from the contact surface 2 at which separation under the action of a force F. on the solder connection 6th a brittle fracture of the solder connection 6th is produced.

The power F. can be applied to the solder connection (s) 1 using different methods, which are shown in 4a-c is shown in more detail.

4a shows a drop device 8th into which the circuit board 3 is used and then under the action of the acceleration due to gravity g over a predefined distance of the fall device 8th is dropped until the circuit board 3 on an impact surface AF (e.g. an end face of a block of wood). The impact on the impact surface AF leads to a shear force acting on the solder joint 6th acting force F. , which causes the brittle fracture in the Solder connection 6th caused. This allows the component 4th very easily from the circuit board at room temperature without exposure to additional heat 3 be separated.

The circuit board 3 can also be done using a centrifuge prior to impact with an impact surface AF be accelerated. By means of the centrifuge, accelerations greater than the acceleration due to gravity g can be achieved.

Most of the components are preferred 4th , 4a , 4b , ... by means of such brittle solder connections 6th , 6a , 6b , ... on their respective contact surfaces 2 , 2a , 2, b, ... (not shown) soldered on. This allows the majority of the components 4th , 4a , 4b , ... (for example all components 4th , 4a , 4b , ...) under the action of the force F. essentially simultaneously from the circuit board 3 removed.

So it becomes a much simplified process for dismantling many or all of the components 4th , 4a , 4b , ... from a circuit board 3 allows: In the event that the circuit board 3 a variety of components 4th , 4a , 4b , ... (not shown), they can be removed from the circuit board essentially at the same time 3 removed.

In 4b becomes the circuit board 3 in a holding device 21 clamped. The power F. on the solder connection 6th is made by means of a bend, e.g. a single-point or multi-point bend, of the printed circuit board 3 , applied, which leads to the brittle fracture, whereby the components 4th , 4a , 4b , 4c by brittle fracture of their solder connections 6th , 6a , 6b , 6c essentially simultaneously from the circuit board 3 be separated.

Another way to apply the force F. is in 4c shown. Using a loudspeaker 22nd becomes sound S. , for example. Ultrasound or infrasound, onto which in a holding device 21 clamped circuit board 3 upset. This causes the force leading to brittle fracture F. , making the components 4th , 4a , 4b , 4c by brittle fracture of their solder connections 6th , 6a , 6b , 6c essentially simultaneously from the circuit board 3 be separated.

The electronics unit 10 comprising the arrangement of the component 4th and circuit board 3 is preferred in a field device 11 used in automation technology. Such a field device 11 automation technology is in 5 shown in more detail. The field device 11 has a sensor unit that is in contact, in particular, at least temporarily and / or at least in sections with a process medium 17th which is used to generate a measurement signal that represents the process variable, for example electrical and / or electronic.

The one in a transmitter housing 9 of the field device 11 arranged electronics unit 10 is used for processing and / or forwarding the from the sensor unit 17th generated measurement signals. The electronics unit 10 includes the circuit board 3 with the component arranged on it 4th .

In the in 5 The embodiment shown has the field device 11 another, as a display / input unit 20th designed electronics unit 10 on, with a (touch) display mounted on it. The electronic unit according to the invention 10 comprising the component 1 and the circuit board 2 can of course also be used as a display / input unit 20th be designed.

List of reference symbols

1

Solder connections

2,2a, 2b

Contact surfaces

3

Circuit board

4,4a, 4b, ...

Components

5

Lot

6,6a, 6b, ..

Solder connection

7th

nitrogen

8th

Drop device

9

Transmitter unit

10

Electronics unit

11

Field device

17th

Sensor unit

20th

Display / input unit

21

Holding device

22nd

speaker

Tmax

maximum temperature

LQ

Liquidus temperature

dT / dt

Cooling rate

AF

Impact area

F.

force

S.

sound

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 10211647 B4 [0007]
• DE 102006035528 A1 [0008]

Claims (15)

A method for producing a solder connection (1) between a contact surface (2) of a circuit board (3) and a component (4) arranged on the contact surface (2), comprising the steps of: - Provision of solder (5) on the contact surface (2), which solder has bismuth (Bi) in a proportion of at least 50 percent by weight; - Arranging the component (4) on the contact surface (2); - Carrying out a soldering process, comprising the steps: - melting of the solder (5), during which melting the solder (5) is brought to a maximum temperature (Tmax), which maximum temperature (Tmax) is above the liquidus temperature (LQ) of the solder (5), - Subsequent cooling of the solder (5), during which cooling the solder (5) solidifies, so that the solder connection (6) is formed between the contact surface (2) and the component (4), the cooling at a cooling rate (dT / dt), which cooling rate (dT / dt) is greater than 6 ° C / s. Procedure according to Claim 1 , wherein the provided solder (5) additionally comprises tin (Sn) with a proportion of at least 40 percent by weight. Procedure according to Claim 1 or 2 , the provided solder (5) additionally comprising silver (Ag), with a proportion of a maximum of 2 percent by weight. Method according to at least one of the preceding claims, wherein the cooling rate (dT / dt) is greater than 10 ° C / s. Method according to at least one of the preceding claims, wherein the cooling rate (dT / dt) is set by supplying liquid nitrogen (7) to the solder connection (6). Method according to at least one of the preceding claims, wherein the cooling rate (dT / dt) is less than 50 ° C / s. Method for separating at least one component (4) soldered onto a respective contact surface (2) of a printed circuit board (3) from the respective contact surface (2), wherein a solder connection (1) between the component (4) and the contact surface (2) is made by means of a Method according to at least one of the Claims 1 until 6th is made, comprising the step: - Separating the at least one component (4) from the contact surface (2), in which Tennen under the action of a force (F) on the solder connection (1), a brittle fracture of the solder connection (1) is generated. Procedure according to Claim 7 In the event that several components (4,4a, 4b, ...) are separated from their respective contact surfaces (2,2a, 2b, ...), a large part of the components (4,4a, 4b, ...) .) is separated essentially simultaneously from the respective contact surface (2,2a, 2b, ...) by means of brittle fracture, with an essentially simultaneous action of a force (F) on the respective solder connection (6,6a, 6b, ...) ). Method according to at least one of the Claims 7 until 8th , the method being carried out at temperatures which are at least 50 ° C., in particular at least 100 ° C. below the liquidus temperature (LQ) of the solder (5). Method according to at least one of the Claims 7 until 9 , wherein the force (F) is applied to the solder connection (s) (6,6a, 6b, ..) by means of an impact of the circuit board (3) in a free fall of the circuit board (3) on an impact surface (AF), in particular by means of a free fall in a falling device (8), in which falling device (8) the impact surface (AF) is arranged essentially perpendicular to the plane of the printed circuit board (3). Method according to at least one of the Claims 7 until 10 , wherein the force (F) on the solder connection (s) (6, 6a, 6b, ..) is applied by means of an impact of the printed circuit board (3) accelerated with a centrifugal force on an impact surface (AF). Method according to at least one of the Claims 7 until 11 , the force (F) being applied to the solder connection / s (6,6a, 6b, ..) by means of a bend in the printed circuit board (3). Method according to at least one of the Claims 7 until 12th , the force (F) being applied to the solder connection (s) (6,6a, 6b, ..) by means of ultrasound or infrasound (S). Printed circuit board (3), comprising: - a contact surface (2); - A component (4) which is materially connected to the contact surface (2) by means of a solder connection (6), the solder connection (6) by means of a method according to at least one of the Claims 1 until 6th is made. Field device (11) of automation technology with an electronic unit (10), the electronic unit (10) following the printed circuit board (3) Claim 14 having.

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