DE102020129831A1 - Process for soldering a component onto a surface of a first printed circuit board - Google Patents

Abstract

The invention relates to a method for soldering a component (B) onto a surface of a first printed circuit board (LP1), the component (B) having a multiplicity of solder depot-free connections (A) on an end face of the component (B), the Surface of the first printed circuit board (LP1) has a plurality of contact surfaces (K) which are provided for soldering on the solder depot-free connections (A). The method has at least the following steps: applying a first solder (L1) to the terminals (A) of the component (B) (1); remelting the first solder (L1) on the terminals (A) at a remelting temperature which exceeds the liquidus temperature of the first solder (L1), the remelted first solder (L1) forming a solder depot (D) (2); and applying a second solder (L2) to the contact surfaces (K) of the first printed circuit board (LP2), the second solder (L2) having a liquidus temperature which is at least 50°C higher than the liquidus temperature of the first solder (L1) (3 ).

Description

The invention relates to a method for soldering a component onto a surface of a first printed circuit board, the component having a multiplicity of solder deposit-free connections on an end face of the component, the surface of the first printed circuit board having a multiplicity of contact surfaces which are provided for soldering the solder deposit-free connections are.

In process and automation technology, field devices are often used to determine and/or monitor process variables. In principle, all devices that are used close to the process and supply or process process-relevant information are referred to as field devices. These are, for example, level meters, flow meters, pressure and temperature meters, pH and redox potential meters, conductivity meters, etc., which record the corresponding process variables level, flow, pressure, temperature, pH value, redox potential or conductivity. Field devices often have a sensor unit that is in contact with a process medium at least temporarily and/or at least in sections, which is used to generate a signal that is dependent on the process variable. Furthermore, field devices often have an electronics unit arranged in a housing, the electronics unit being used to process and/or forward signals generated by the sensor unit, in particular electrical and/or electronic signals. For this purpose, the electronics unit often has a printed circuit board with components arranged on it.

A high quality of the solder joints between the connections of the component and the contact areas of the printed circuit board is of great importance for the reliability of the electronic unit or the field device. The quality of the solder joints depends, among other things, on whether the distance between the connection and the contact surface (the so-called solder gap height) is large enough. If the solder gap height is too small, stress-induced cracks can occur in the solder joint due to temperature fluctuations over time. In particular, if the component is heavy and the resulting low solder gap height of the soldering point, there is a risk that undesirable air inclusions, so-called voids, will form within the soldering points. This is particularly true when the component leads are located beneath the component. Investigations by the applicant have shown that in the case of such heavy components, even with vacuum soldering methods, voids within the soldering point cannot be completely avoided.

The object of the present invention is therefore to specify a method which enables a stable and low-void solder joint between a component and a printed circuit board.

• - Aufbringen eines ersten Lots auf die Anschlüsse des Bauelements,
• - Umschmelzen des ersten Lots auf den Anschlüssen bei einer Umschmelztemperatur, welche die Liquidustemperatur des ersten Lots überschreitet, wobei das umgeschmolzene erste Lot ein Lotdepot formt,
• - Aufbringen eines zweiten Lots auf die Kontaktflächen der ersten Leiterplatte, wobei das zweite Lot eine Liquidustemperatur aufweist, die mindestens 50°C höher ist als die Liquidustemperatur des ersten Lots,
• - Positionieren des Bauelements relativ zur ersten Leiterplatte derart, dass die Oberfläche der ersten Leiterplatte der Stirnfläche des Bauelements zugewandt ist, und
• - Verlöten der Anschlüsse mit den Kontaktflächen in einem Lötofen unter Vakuum bei einer Löttemperatur, welche oberhalb der Liquidustemperaturen des ersten Lots und des zweiten Lots liegt.

The object is achieved according to the invention by a method for soldering a component onto a surface of a first printed circuit board, the component having a multiplicity of solder depot-free connections on an end face of the component, the surface of the first printed circuit board having a multiplicity of contact surfaces which are used for soldering on the solder depot-free connections are provided, the method comprising the following steps:

• - Applying a first solder to the connections of the component,
• - remelting the first solder on the terminals at a remelting temperature which exceeds the liquidus temperature of the first solder, the remelted first solder forming a solder depot,
• - applying a second solder to the contact surfaces of the first circuit board, the second solder having a liquidus temperature which is at least 50°C higher than the liquidus temperature of the first solder,
• - positioning the component relative to the first printed circuit board in such a way that the surface of the first printed circuit board faces the end face of the component, and
• - Soldering the connections to the contact surfaces in a soldering oven under vacuum at a soldering temperature which is above the liquidus temperatures of the first solder and the second solder.

The great advantage of the method according to the invention is that the liquidus temperature of the first solder is at least 50° C. below the liquidus temperature of the second solder. Typical soldering temperatures in soldering ovens, for example reflow soldering ovens, are around 245-260°C. A solder with a liquidus temperature that is below typical soldering temperatures, for example far below 245°, can thus be selected as the first solder. Due to the low liquidus temperature of the first solder, it is possible to remelt the first solder on the connections without the need for high soldering temperatures, which represent a high thermal load for the component. Pre-soldering of the connections is generally not possible, particularly for components that experience a maximum of one reflow process, ie that may only be exposed to the typical temperatures of a reflow process once. In the method according to the invention, however, the thermal load when pre-soldering the connections of the Component significantly reduced by the low liquidus temperature of the first solder. In particular, the liquidus temperature of the first solder is below 180°C.

The remelting of the first solder reduces the amount of flux present in the first solder, since the flux at least partially evaporates during the remelting due to the increase in temperature to at least the liquidus temperature of the first solder. The solder depots obtained by remelting the first solder have a highly solderable surface. This highly solderable surface of the solder deposits makes subsequent soldering with the second solder, which is provided on the contact areas of the first circuit board, easier. The vacuum applied in the soldering oven under which the terminals are soldered to the contact pads facilitates the exit of the flux from the first solder and from the second solder during soldering. In this way, soldering points between the connections and the contact surfaces are obtained by the method according to the invention, which are characterized by an almost complete absence of air inclusions. The soldering points therefore consist almost completely and continuously of the mixture of the first solder and the second solder and are accordingly more stable than soldering points which are traversed by air inclusions.

In one embodiment of the method according to the invention, a first stencil is used to apply the first solder to the connections. The first solder can thus be applied to the connections as solder paste using the first stencil.

In a further embodiment of the method according to the invention, a second stencil is used to apply the second solder to the contact surfaces. The second solder can thus be applied to the contact surfaces as solder paste by means of the second stencil.

An alloy of Bi, Sn and Ag is advantageously used as the first solder.

An alloy of Bi, Sn and Ag in the ratio Bi:Sn:Ag=57.6:42:0.4 at% is preferably used as the first solder. Such an alloy has a liquidus temperature of about 142°C. The first solder on the terminals is therefore remelted at a relatively low temperature compared to conventional remelting processes. As a result, the thermal load on the component is lower than usual.

In one possible embodiment of the method according to the invention, the liquidus temperature of the first solder is around 140°C. The liquidus temperature of the first solder is consequently significantly lower than typical soldering temperatures. Due to the low liquidus temperature of the first solder, the component experiences a thermal load that is far below the thermal load in typical soldering processes.

An alloy of Sn, Cu and Ag is advantageously used as the second solder.

An alloy of Sn, Cu and Ag in the ratio Sn:Cu:Ag=96.5:3:0.5 at% is preferably used as the second solder. Such an alloy has a liquidus temperature of about 217°C.

In one possible embodiment of the method according to the invention, the liquidus temperature of the second solder is approximately 217°C.

In a further embodiment of the method according to the invention, the second solder has a liquidus temperature which is at least 70° C. higher than the liquidus temperature of the first solder.

The remelting of the first solder is preferably carried out in a reflow soldering oven or a curing oven. Due to the rather low liquidus temperature of the first solder, the first solder does not necessarily have to be remelted in a reflow soldering oven, but can also be done in a curing oven that can heat to the required liquidus temperature.

In one possible embodiment of the method according to the invention, the connections are soldered to the contact surfaces in a soldering oven under vacuum in a reflow oven.

When soldering the connections to the contact surfaces, a vacuum of 5 to 10 mbar is advantageously built up, which is maintained for 5 to 15 s.

In a further embodiment of the method according to the invention, a second printed circuit board with an integrated circuit arranged on the second printed circuit board is used as the component, the end face of the component being formed by the surface of the second printed circuit board, and the plurality of connections of the component forming a Land Grid Array (LGA) arranged on the front face.

• 1: ein Ausführungsbeispiel des erfindungsgemäßen Verfahrens.
• 2: eine schematische Darstellung des Bauelements vor dem Aufbringen des ersten Lots.
• 3: eine schematische Darstellung des Bauelements nach dem Aufbringen des ersten Lots.
• 4: eine schematische Darstellung des Bauelements nach dem Umschmelzen des ersten Lots.
• 5: eine schematische Darstellung der ersten Leiterplatte vor dem Aufbringen des zweiten Lots.
• 6: eine schematische Darstellung des Bauelements und der ersten Leiterplatte vor dem Verlöten der Anschlüsse und der Kontaktflächen.

Furthermore, the present invention is to be based on the following figures 1-6 be explained in more detail. Show it:

• 1 : an exemplary embodiment of the method according to the invention.
• 2 : a schematic representation of the component before the application of the first solder.
• 3 : a schematic representation of the component after the application of the first solder.
• 4 : a schematic representation of the component after the remelting of the first solder.
• 5 : a schematic representation of the first circuit board before the application of the second solder.
• 6 : a schematic representation of the component and the first circuit board before the soldering of the connections and the contact surfaces.

The method according to the invention can be used for all types of first printed circuit board LP1 and components B, in particular for those first printed circuit boards LP1 and components B that are used in field devices. The method according to the invention is used for soldering a component B onto a surface of a first circuit board LP1, the component B having a multiplicity of solder depot-free connections A on an end face of the component B, the surface of the first circuit board LP1 having a multiplicity of contact areas K which are provided for soldering the solder depot-free connections A. A second printed circuit board LP2, for example, can be used as the component B, on which an integrated circuit is arranged and whose end face is formed by the surface of the second printed circuit board LP2. The multiplicity of terminals A of such a component B is designed as a land grid array (LGA) arranged on the end face.

In 1 an exemplary embodiment of the method according to the invention is shown. In a first step 1 of the method according to the invention, a first solder L1 is initially applied to the multiplicity of connections A of the component B, which can be done, for example, by means of a first template S1. The applied first solder 1 is then remelted in a second step 2, so that a solder depot D is obtained on each connection A. For this purpose, a remelting temperature is selected which is higher than the liquidus temperature of the first solder L1. The first solder L1 can be remelted in a reflow soldering oven or a curing oven, for example. In a further third step 3, a second solder L2 is applied to the contact surfaces K of the first printed circuit board LP2, with a second template S2 optionally being able to be used for applying the second solder L2. The second solder L2 is characterized in that it has a liquidus temperature that is at least 50° C., in particular at least 70° C., higher than the liquidus temperature of the first solder L1. In the fourth step 4 of the method according to the invention, the component B and the first printed circuit board LP1 are aligned with one another in such a way that the surface of the first printed circuit board LP1 faces the end face of the component B. Then, in a fifth step 5, the connections A can be soldered to the contact surfaces K. The soldering is performed under vacuum and at a soldering temperature that is above the liquidus temperatures of the first solder L1 and the second solder L2. For example, a vacuum of 5 to 10 mbar is selected for soldering, which is maintained for 5 to 15 s.

In 2 shows a schematic representation of the component B before the application of the first solder L1. In this case, a first template S1 is arranged in the area of the connections A, so that the first solder L1 can be applied to the connections, for example by means of a squeegee. In the schematic representation in 3 the first solder L1 has been applied and the first stencil S1 has been removed. After the first solder L1 has been remelted, solder depots D are obtained on the terminals A of the component B, as in FIG 4 shown schematically. The application of the second solder L2 to the contact areas K of the first circuit board LP1 is shown schematically in FIG 5 represented by a second template 2. After the application of the second solder L2 and the removal of the second template S2, the component B and the first printed circuit board LP1 are aligned with one another, so that the solder depots D and the second solder L2 each face one another, as in FIG 6 shown schematically. The connections A can then be soldered to the contact surfaces K.

For example, an alloy of Bi, Sn and Ag can be used as the first solder L1, in particular in a ratio of Bi:Sn:Ag=57.6:42:0.4 at%. An alloy of Sn, Cu and Ag, for example, can be used as the second solder L2, in particular in a ratio of Sn:Cu:Ag=96.5:3:0.5 at%.

Optionally, the liquidus temperature of the first solder L1 can be around 140° C., so that no high thermal load occurs on the component B when the first solder L1 is remelted. The liquidus temperature of the second solder L2 can be around 217° C., for example.

Reference List

B

component

A

connections

P

solder stop

S1

first template

S2

second template

LP1

first circuit board

LP2

second circuit board

K

contact surface

D

solder depot

L1

first lot

L2

second lot

Claims (14)

Method for soldering a component (B) onto a surface of a first printed circuit board (LP1), the component (B) having a multiplicity of solder depot-free connections (A) on an end face of the component (B), the surface of the first printed circuit board (LP1 ) has a multiplicity of contact surfaces (K), which are provided for soldering on the solder depot-free connections (A), the method comprising the following steps: - Applying a first solder (L1) to the terminals (A) of the component (B) (1), - remelting the first solder (L1) on the terminals (A) at a remelting temperature which exceeds the liquidus temperature of the first solder (L1), the remelted first solder (L1) forming a solder depot (D) (2), - Applying a second solder (L2) to the contact surfaces (K) of the first printed circuit board (LP2), the second solder (L2) having a liquidus temperature which is at least 50°C higher than the liquidus temperature of the first solder (L1) (3 ), - Positioning the component (B) relative to the first printed circuit board (LP1) in such a way that the surface of the first printed circuit board (LP1) faces the end face of the component (B) (4), and - Soldering the connections (A) to the contact surfaces (K) in a soldering furnace under vacuum at a soldering temperature which is above the liquidus temperatures of the first solder (L1) and the second solder (L2) (5). procedure after claim 1 , A first template (S1) for applying the first solder (L1) to the terminals (A) is used. Method according to at least one of the preceding claims, in which a second template (S2) is used to apply the second solder (L2) to the contact surfaces (K). Method according to at least one of the preceding claims, in which an alloy of Bi, Sn and Ag is used as the first solder (L1). procedure after claim 4 , with an alloy of Bi, Sn and Ag in the ratio Bi:Sn:Ag=57.6:42:0.4 at% being used as the first solder (L1). procedure after claim 4 , the liquidus temperature of the first solder (L1) being around 140°C. Method according to at least one of the preceding claims, in which an alloy of Sn, Cu and Ag is used as the second solder (L2). procedure after claim 7 , whereby an alloy of Sn, Cu and Ag in the ratio Sn:Cu:Ag = 96.5: 3: 0.5 at% is used as the second solder (L2). procedure after claim 7 , where the liquidus temperature of the second solder (L2) is about 217°C. Method according to at least one of the preceding claims, wherein the second solder (L2) has a liquidus temperature which is at least 70°C higher than the liquidus temperature of the first solder (L1). Method according to at least one of the preceding claims, wherein the remelting of the first solder (L1) is carried out in a reflow soldering oven or a curing oven. Method according to at least one of the preceding claims, in which the connections (A) are soldered to the contact surfaces (K) in a soldering oven under vacuum in a reflow oven. Method according to at least one of the preceding claims, wherein soldering the connections (A) to the contact surfaces (K) builds up a vacuum of 5 to 10 mbar, which is maintained for 5 to 15 s. Method according to at least one of the preceding claims, wherein a second printed circuit board (LP2) with an integrated circuit arranged on the second printed circuit board (LP2) is used as the component (B), the end face of the component (B) passing through the surface of the second printed circuit board (LP2) is formed, and wherein the plurality of terminals (A) of the component (B) is designed as a land grid array (LGA) arranged on the end face.

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