WO2020012226A1

WO2020012226A1 - Additivated solder paste and process for applying a reactive additive element for selective control of soldering temperature on the reflow soldering method

Info

Publication number: WO2020012226A1
Application number: PCT/IB2018/055184
Authority: WO
Inventors: Delfim FERNANDES SOARES; José Carlos FERNANDES TEIXEIRA; Isabel Alexandra CARVALHO CEREJEIRA FONTES DELGADO; José Ricardo BARROS ALVES; Hugo Sérgio PITÃES FIGUEIREDO; Daniel ARAÚJO BARROS; Pedro Emanuel ABREU RIBEIRO; Paulina ARAÚJO CAPELA
Original assignee: Bosch Car Multimedia Portugal, S.A.; Universidade Do Minho
Priority date: 2018-07-10
Filing date: 2018-07-13
Publication date: 2020-01-16

Abstract

The present application concerns an additivated solder paste and a process to apply a reactive additive element for the control of the soldering temperature of electronic components on the reflow soldering method by the use of metallic particles deposited at the interface between the Printed Circuit Board pad, and the solder paste. The aim is to promote a transient melting reaction between the pad, the reactive additive element and the solder paste. Then the preliminary transient liquid phase reacts with the solder paste particles, and behaves as a catalyser for the solder alloy melting. The selective additive element addition, at the interface, allows a concentrated effect resulting in an enhanced degree of the initial melting temperature decrease at that zone. The technique is able to be applied on selected zones of a PCB. This way, during Surface Mount Devices reflow soldering, the PCB thermal gradients effects are minimized.

Description

DESCRIPTION

"ADDITIVATED SOLDER PASTE AND PROCESS FOR APPLYING A REACTIVE ADDITIVE ELEMENT FOR SELECTIVE CONTROL OF SOLDERING TEMPERATURE ON THE REFLOW SOLDERING METHOD"

Technical field

The present application relates to an additivated solder paste comprising a reactive additive element, as well as a process for applying the reactive additive element for a selective control of the soldering temperature of electronic component on the reflow soldering method.

Background art

On the reflow soldering method thermal heterogeneity usually occurs at Printed Circuit Boards (PCB) zones with different components density, mass and volume, leading to different heat demands along the board. The constant progress in the reduction of component sizes increases the susceptibility to thermal gradients, and their damage. When regions with higher heat demand occur, higher maximum temperature and longer times are necessary on the process. Satisfying the soldering conditions of those regions, without overheating the opposite ones, is usually a compromise relationship that many times cannot be achieved without recurring to other techniques, like the rearrangement of the component layout on the PCB or manually thermal shielding the less heat demand regions from the convection heat currents inside the reflow oven. Whatever technique is applied it usually passes through a costly iterative testing procedure and rises the production costs .

Document US 8834747 B2 describes the use of tin nanoparticles, and other electrically conductive particles, e.g. copper, with size below 25 nm to reduce the melting temperature of bulk tin (232°C) for values as low as 200°C with 10 nm particles. Compositions can contain whisker suppressant to eliminate or reduce the formation of tin whiskers after tin nanoparticles fusion. This document also comprises methods for use of this technique. The present application differs from this technology since it describes an additivated solder paste comprising metallic particles and does not comprise a surfactant or a way to control the alloy start melting temperature in a selective way over the PCB .

Document US 8840700 B2 describes metallic particle transient liquid phase sintering compositions containing blended formulations of metal and metal alloy components that form interconnected conductive metallurgical networks with increased stability, resistance to thermal stress and ability to mitigate CTE mismatch between materials. The present application differs from the technology disclosed since it relates to an additivated paste comprising a reactive additive element ranging from 0.3 to 10.0% in wt%.

Document US20090107584 (Al) discloses a solder composition constituted by a mixture of a flux and different metallic particles, with diameters less than 100 nm. The flux is chosen to have at least one reactive group, which reacts with at least one metal. Metal particles comprise an alloy selected from tin/silver/copper alloys, bismuth/indium/tin alloys, bismuth/indium/antimony alloys and combinations thereof. The melting temperature is under 190°C. The present application differs from the technology disclosed since it relates to an additivated paste comprising a reactive additive element ranging from 0.3 to 10.0% in wt% and particles ranging from 0.025 to 45 pm in diameter. Summary

The present application relates to an additivated solder paste for selective control of soldering temperature comprising a solder paste, and a reactive additive element in an amount ranging from 0.3 to 10.0% in wt% in relation to the metal content of the solder paste.

In one embodiment the solder paste comprises the alloy SnAgCu .

In another embodiment the alloy SnAgCu composition is: Sn from 94.00 to 97.50 %, Ag from 2.00 to 4.50 % and Cu from 0.25 to 1,00 %.

In yet another embodiment the reactive additive element is selected from a list of Au, Bi, Ga, In, Ni, Sb, Zn or combinations thereof.

In one embodiment the reactive additive element is in nano- or microparticle form.

In one embodiment the particles size ranges from 0.025 to 45 pm in diameter.

The present application relates to an additivated solder paste to be used in the reflow soldering method for Surface Mount Devices.

The present application also relates to a process for applying a reactive additive element of an additivated solder paste for selective control of soldering temperature on the reflow method, comprising the following steps: the reactive additive element particles are mechanically mixed in the solder paste producing the additivated solder paste;

application of the additivated solder paste on the selected soldering surfaces by stencil printing;

second stencil printing stage with standard solder paste that covers all soldering surfaces.

In one embodiment the amount of additivated solder paste is defined by the stencil geometry and thickness.

In another embodiment the amount of applied additivated solder paste is defined according to the melting temperature decrease desired effect.

Additionally, the present application also relates to a process for applying a reactive additive element for selective control of soldering temperature on the reflow method, comprising the following steps:

the reactive additive element particles are mixed in a colloidal flux suspension producing a colloidal flux suspension, which is a doped flux;

application of the doped flux on the selected soldering surfaces;

standard solder paste is applied on all soldering surfaces by stencil printing.

In one embodiment the doped flux is applied with a dispensing system.

In another embodiment the dispensing system is a syringe, jet machine or similar technology. In yet another embodiment the amount of reactive additive element is controlled by the volume of applied doped flux.

In one embodiment the amount of applied doped flux is controlled according to the melting temperature decrease desired effect.

General description

The present application relates to an additivated solder paste comprising a reactive additive element and processes to apply said reactive additive element for the reflow soldering method. The technology disclosed herein is particularly directed to the reflow soldering method and Surface Mount Devices (SMD) in the production of electronic PCB assemblies. Due to the drawbacks in thermal heterogeneity in reflow soldering technology, the objective of the present additivated solder paste is to promote a transient liquid phase (TLP) effect, which decreases the maximum temperature during the reflow soldering technique. This technical solution is achieved by adding a reactive additive element, selected from a list of metallic elements, to a solder paste by at least two different processes .

The inventive aspects of the present technology are the ability to target only critical components along with the possibility to control the degree of melting temperature decrease :

1 - Selective application of the reactive additive element only in the components where it is necessary to reduce the melting temperature;

2 - It is possible to adjust the quantity of additive element applied on each pad, in relation to the temperature variation necessary in each component: i) By adjusting the quantity added in the first stage - in an application by stencil printing;

ii) By the number of droplets applied in each pad, in an application by dispensation.

Brief description of drawings

The present technology is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings .

Figure 1 illustrates the thermal behaviour as obtained by Differential Scanning Calorimetry (DSC) , more specifically the solder alloy fusion temperature range, of three different solder pastes: alloy 1- standard process SAC type solder paste, alloy 2 and 3 - SAC type solder paste additivated with up to 2 wt% of In plus Bi particles added respectively into the mixture and to the pad/solder interface .

Figure 2 represents an additive element application process by means of two stage stencil printing: A, B) on the first printing stage, a doped solder paste is applied; C, D) a second printing stage is carried out over the first one, with a bigger volume of standard process solder paste. The references are as follows: 1 - First Stencil, 2 - Paste with particles, 3 - Copper pad, 4 - PCB, 5 - Standard solder paste, 6 - Second Stencil.

Figure 3 depicts an additive element application process by means of a targeted deposition. A, B) Using a syringe, jet machine or similar technology, a doped solder flux is applied on the solder pad. C, D) After that, a bigger volume of standard solder paste is applied over the doped flux, by means of a traditional stencil printing method. The references are as follows: 7 - Syringe/jet machine, 8 - Flux with particles, doped flux, 9 - Copper pad, 10 - PCB, 11 - Stencil, 12 - Standard solder paste.

Figure 4 depicts the thermal behaviour as obtained by Differential Scanning Calorimetry (DSC) , of the first and second heating cycles of SAC type additivated solder paste (alloy 2), showing that TLP effect is only relevant on the first heating cycle.

Figure 5 shows the evolution of the soldering process in laboratorial conditions of a component being soldered with SAC type solder paste (A) and SAC type solder paste with up to 2.0 wt% Bi plus In as additive elements applied by the two stage printing method (B) . An arrow indicates the melting start temperature on both soldering processes.

Description of embodiments

Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

The present application discloses an additivated solder paste comprising a reactive additive element and processes to apply said reactive additive element on the reflow soldering method to selectively control, more specifically decrease, the maximum temperature used on the thermal cycle. The present technology provides transient liquid phase (TLP) effect to produce a control of the initial melting temperature during the reflow soldering method.

This effect is obtained by the use of a reactive additive element selected from a list of Au, Bi, Ga, In, Ni, Sb, Zn or combinations thereof.

The present technology allows the selection of the component pads to be additivated and the degree of the reflow temperature change, by the control of the reactive additive element type and quantity on each soldering pad. The technology allows a precise control of the soldering temperature of the components of SMD, avoiding local circuit boards overheating and to obtain the desired ranges of temperatures in the various phases of reflow soldering.

In one embodiment, the present application concerns the development of a new concept to selectively decrease and control the melting temperature of a solder paste containing the alloy SnAgCu (SAC) , in any different variation of the proportion of each of the three elements: Sn, Ag and Cu, on the reflow soldering method in the production of electronic PCB assemblies. In one embodiment the SAC alloy composition is: Sn from 94.00 to 97.50%, Ag from 2.00 to 4.50%, and Cu from 0.25 to 1.00%.

Therefore, to decrease the melting temperature of the solder paste, metallic nano or microparticles of the additive element are applied on the selected soldering surfaces. In one embodiment the particles size ranges from 0.025 pm to 45 pm in diameter.

The amount of reactive additive element is adjusted according to the temperature reduction that is needed, for a specific component on the PCB. A relationship can be established for each type of additive for given processing parameters, such as soldering heating rate, type of component as in size, shape, volume and thermal mass, and the component position on the PCB.

In one embodiment the amount of reactive additive element ranges from 0.3 to 10.0% in wt% in relation to the metal content of the solder paste. In this application, standard solder paste is to be understood as solder paste without the reactive additive element .

In a typical reflow soldering method two joints should be considered: the PCB pad substrate/solder and the solder/component surface. The additive effect provided with this technology firstly affects the PCB substrate/solder joint decreasing the initial melting temperature.

In the case of SAC solder paste, the additive element reacts with tin, from the solder alloy, and produces a transient liquid phase (TLP) at a lower temperature than the standard solder paste alloy melts. That liquid acts as a melting reaction catalyst for the standard solder paste alloy. This transition can be observed as an endothermic event in differential scanning calorimetry (DSC) as presented in Figure 1. The transient liquid phase only exists for a short period of time as a result of the reaction with tin and copper, from solder paste alloy and substrate, respectively.

The amount of reactive additive element is selected in order to produce a continuous liquid formation: initially from the TLP effect, followed by the standard solder paste alloy melting, such as alloys 2 and 3 on Figure 1. The reactive additive element does not require having a melting point lower than the solder alloy. That effect allows the decrease of the alloy melting temperature at the solder- substrate interface, in a controlled way. The degree of temperature reduction can be adjusted to the real component thermal cycle on the reflow soldering method, by controlling the fraction of additive. The amount of the reactive additive element addition is limited to a defined range in order to produce no major deviations from the standard solder paste chemical composition.

The reactive additive element for selective control of soldering temperature on the reflow soldering method is applied in at least two processes:

In one process, metallic nano- or microparticles of the reactive additive element are mechanically mixed in the solder paste. The additivated solder paste is then applied on the selected soldering surfaces by means of stencil printing. The position and amount of additivated solder paste, on selected pad substrates, is defined by the stencil geometry and thickness. The amount of applied additivated solder paste is also defined according to the melting temperature decrease desired effect. After that, another stencil printing stage (second stencil printing stage) is carried out over the first one with standard solder paste, that covers all soldering surfaces. The scheme of this process is presented in Figure 2. The geometry and dimensions of the first printing stage define the final concentration of additive at the interface and on the final solder. Based on this process, a concentrated effect on melting temperature is obtained at the solder/substrate interface, due to the local initial higher additive amount. The geometry and dimensions parameters are adjusted according to the desired temperature melting effect, which is related to the amount of additive particles on the soldering surface.

In a second process, metallic nano- or microparticles of the reactive additivated element are mixed with flux producing a colloidal flux suspension, which is a doped flux, that is applied in selected soldering surfaces. In one embodiment the doped flux is applied with a dispensing system. In one embodiment the dispensing system is a syringe or jet machine, as in Figure 3. This machine pours droplets of doped flux on the soldering surfaces. Standard solder paste is then printed by traditional stencil printing techniques over the doped flux. This step covers all soldering surfaces.

The amount of applied doped flux is controlled according to the melting temperature decrease desired effect. During the first heating on the reflow cycle the additive element and solder paste melt. By diffusional homogenization the final alloy chemical composition is produced, solder paste + additive element, at the solder joint. After cooling, subsequent temperature thermal cycles, even beyond the original standard alloy melting start temperature, do not reproduce the original initial melt temperature. This means that, as expected, the TLP effect does not occur on a second heating cycle, as can be seen on Figure 4.

Both processes described above are especially suitable to control the solder melting temperature on selected components of a PCBA, whose thermal behaviour is critical, allowing to solder larger sized components, that otherwise could not be soldered. This way all the PCB components might be simultaneously soldered, maintaining the reflow thermal cycle, namely the peak temperature, within processing limits that are tolerable for the components and PCB, and/or exempt other, more difficult and expensive, production techniques to protect smaller components from overheating .

The first process, the double stencil solder paste printing, is also suitable to be used on all the soldering surfaces of a PCB assembly in order to reduce standard thermal cycle temperature and, thus, associated costs, without changing the standard solder paste chemical composition of the process and without a big impact on chemical bulk composition of the final soldered joint.

Example 1

This example relates a soldering processes done in controlled laboratory conditions. SAC solder paste highly doped with particles was printed on a PCB using a stencil tool. Over that preliminary printing stage, using a bigger gap stencil tool, another printing process was performed with standard solder paste. After that, a ceramic component was placed over the printed board. The assembly was then heated to a maximum temperature of 260°C on a controlled temperature and atmosphere oven. A digital contrast camera captured and recorded the process. The control sample was made by repeating the process without the addition of the particles, thus, exempting the first printing stage. The result is shown on Figure 5, where the melting process of both samples are compared. It is clearly seen that the particle addition at the interface allows a decrease on the initial melting temperature of 5°C.

Example 2 A second example can be considered as the application of the disclosed technology in a standard industrial facility. A standard PCB assembly production process by reflow technology was carried out with changes on reflow thermal cycle (decrease of the maximum temperature) in order to intentionally cause soldering problems on bigger components. The PCB assemblies produced this way constituted the control samples. Another series of the same PCB, components and production parameters was produced, but using the two stage printing process, where on the first stage the particles were applied, in the form of a highly doped solder paste, and on the second stage the usual process with standard solder paste.

This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.

Claims

1. Additivated solder paste for selective control of soldering temperature comprising a solder paste, and a reactive additive element in an amount ranging from 0.3 to 10.0% in wt% in relation to the metal content of the solder paste.

2. Additivated solder paste according to claim 1, wherein the solder paste comprises the alloy SnAgCu.

3. Additivated solder paste according to the previous claim, wherein the alloy SnAgCu composition is: Sn from 94.00 to 97.50 %, Ag from 2.00 to 4.50 % and Cu from 0.25 to 1,00 %.

4. Additivated solder paste according to any of the previous claim, wherein the reactive additive element is selected from a list of Au, Bi, Ga, In, Ni, Sb, Zn or combinations thereof.

5. Additivated solder paste according to any of the previous claims, wherein the reactive additive element is in nano- or microparticle form.

6. Additivated solder paste according to any of the previous claims, wherein the particles size of the reactive additive element ranges from 0.025 to 45 pm in diameter .

7. Use of the additivated solder paste described in claim 1, in the reflow soldering method for Surface Mount Devices .

8. Process for applying a reactive additive element of an additivated solder paste for selective control of soldering temperature on the reflow method described in claims 1 to 7, comprising the following steps:

the reactive additive element particles are mechanically mixed in the solder paste producing the additivated solder paste;

9. Process according to the previous claim, wherein the amount of additivated solder paste is defined by the stencil geometry and thickness.

10. Process according to any of the claims 8 to 9, wherein the amount of applied additivated solder paste is defined according to the melting temperature decrease desired effect.

11. Process for applying a reactive additive element for selective control of soldering temperature on the reflow method described in claims 1 to 7, comprising the following steps:

application of the doped flux on the selected soldering surfaces;

standard solder paste is applied on all soldering surfaces by stencil printing.

12. Process according to the previous claim, wherein the doped flux is applied with a dispensing system.

13. Process according to the previous claim, wherein the dispensing system is a syringe, jet machine or similar technology .

14. Process according to the any of the claims 11 to 13, wherein the amount of reactive additive element is controlled by the volume of applied doped flux.

15. Process according to claim 11, wherein the amount of applied doped flux is controlled according to the melting temperature decrease desired effect.