JP2012047680A - Method for evaluating solder flux, and solder flux - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for simply evaluating changes with age such as increase in viscosity during storage and handling of a solder paste, and wettability or the like during reflow, and also to provide a solder flux for the solder paste which is superior in coating properties and wettability.SOLUTION: The solder flux is heated, and the active degree of the flux is evaluated by measuring the ion conductivity in two different temperature ranges, a room temperature range and a high temperature range, thereby selecting the solder flux in response to use of the solder paste.

Description

  The present invention relates to a solder flux evaluation method and solder flux. More specifically, a method for evaluating a change over time such as an increase in viscosity of the solder paste during storage of the solder paste and handling in a printing press, a method for evaluating wettability during reflow, and a solder paste excellent in wettability. It relates to solder flux.

Solder paste in which solder alloy powder and flux are kneaded is applied or printed on a printed circuit board and mounted on the printed circuit board by mounting or mounting the electronic component on the printed circuit board. .
In such solder paste used for electronic component joining, there is no change in the viscosity of the solder paste during handling using a printing machine, etc., maintaining good coatability for a long time, and meltability during reflow It is required that (wetability) does not decrease.

  In order to solve these problems, for example, Patent Document 1 discloses that the dissociation constant is 2.5 or less as a thickening inhibitor and is compatible with the resin component constituting the flux, and is uniform in the flux. A solder paste containing a soluble carboxylic acid or derivative thereof has been proposed. Patent Document 2 proposes a solder paste containing an aliphatic tricarboxylic acid as an activator.

  In recent years, the use of Pb has been regulated from the viewpoint of environmental protection, and the use of lead-free solder alloy powder containing no Pb has been promoted, and the solder alloy powder mainly composed of Sn has been adopted. Yes. Since the solder alloy powder containing Sn as a main component has lower wettability and higher melting point (eg, 217 ° C.) than the conventional Sn—Pb solder alloy powder, it is used for the conventional solder alloy powder containing Pb. However, it is difficult to cope with lead-free solder alloy powder.

JP-A-5-318176 JP 2006-110580 A

In the solder pastes of Patent Document 1 and Patent Document 2, the flux for solder paste may react with the solder alloy powder and thicken during storage or handling using a printing machine. In addition, since the active component is consumed by this, there is a problem that, when the solder paste is reflowed, there is not enough active component to remove the oxide film on the surface of the solder powder, and the solder paste does not melt.
And when evaluating the flux used for such a solder alloy powder, the solder alloy powder and the flux are actually mixed to produce a solder paste, stored at room temperature, whether the viscosity changes, or melted at a high temperature It was not efficient because there was no way other than trying to see if it could be done.

  The present invention has been made in view of the above-described problems, and provides a method for easily evaluating changes over time such as viscosity increase during storage and handling of solder paste, wettability during reflow, and the like. An object of the present invention is to provide a solder flux for a solder paste having excellent properties and wettability.

The solder flux evaluation method of the present invention is characterized by measuring the relationship between the temperature of the solder flux and the ionic conductivity.
Thus, the activity of the flux at a specific temperature can be obtained by measuring the ionic conductivity of the solder flux. Thereby, before producing solder paste, the flux according to the use of solder paste can be selected.

Further, in the solder flux evaluation method of the present invention, the measurement of the ionic conductivity is performed in two different temperature regions, a normal temperature region where a solder paste produced using the solder flux is stored or handled and a high temperature region where the solder paste is reflowed. It is good to implement so that it may contain.
Measure the ionic conductivity of the flux in the temperature range of the solder paste to be produced, including the temperature during handling or storage in a printing machine (room temperature) and the temperature (high temperature) when mounting electronic components (reflow). As a result, the activity of the flux in these temperature ranges can be obtained. It is possible to select a solder flux that can remove the oxide film and have good wettability.

In addition, the solder flux of the present invention has an ionic conductivity of 0.05 mS / m or less in a normal temperature range in which the solder paste to be produced is stored or handled, and an ionic conductivity in a high temperature range to be reflowed is 1.0 mS / m. It is the above.
Such solder flux has a reduced ion dissociation of the active component in the solder flux when the solder paste is handled using a printing machine (normal temperature range). The reaction can be prevented and an increase in the viscosity of the solder paste can be prevented. At the same time, at the time of reflowing the solder paste (high temperature range), the flux has sufficient activity to remove the oxide film on the surface of the solder alloy powder, so that it can have good wettability.

  Moreover, the solder flux of this invention is a solder flux for lead-free solder pastes, Comprising: It is good in the said normal temperature range being 25-40 degreeC and the said high temperature range being 180-230 degreeC.

The solder paste of the present invention is a lead-free solder paste, and is preferably formed into a paste by mixing the solder flux and solder alloy powder containing 95 mass% or more of Sn and not containing Pb.
For lead-free solder alloy powders containing 95% by mass or more of Sn and not containing Pb, a highly active solder flux is required, and during reflow (high temperature range) while suppressing an increase in viscosity during storage (normal temperature range) It was difficult to obtain good wettability. On the other hand, the solder paste using the above-mentioned solder flux contains lead-free solder alloy powder, but prevents changes over time such as viscosity increase, and stable applicability when used in a printing machine. And good wettability can be obtained during reflow.
The lead-free solder alloy powder containing no Pb indicates that the Pb content in the solder alloy powder is less than 1000 ppm.

  Moreover, the manufacturing method of the mounting board | substrate of this invention mounts an electronic component using the said solder paste, It is characterized by the above-mentioned.

  According to the present invention, the flux activity in a specific temperature region can be obtained by measuring the ionic conductivity of the solder flux. Therefore, before producing the solder paste, the flux corresponding to the application of the solder paste Can be selected. Furthermore, for the solder paste to be produced, the ionic conductivity of the flux in the temperature range including the temperature during handling or storage in a printing machine (room temperature) and the temperature during reflow when mounting electronic components (high temperature) is measured. As a result, flux activity in these temperature ranges can be obtained, so that stable changes in coating properties can be obtained by preventing changes over time such as viscosity increase at room temperature, and the oxide film on the surface of the solder alloy powder is removed at high temperatures. Thus, it is possible to determine a solder flux that can have good wettability.

It is a figure explaining the measuring method of the ionic conductivity of the solder flux of one Embodiment of this invention.

Hereinafter, a solder flux evaluation method according to an embodiment of the present invention will be described with reference to the drawings.
The solder flux evaluation method of the present embodiment is to heat the flux and handle or store the produced solder paste using a printing machine or the like (normal temperature range) and when mounting electronic components (reflow) In this method, the ion conductivity is measured in a temperature range including when the solder paste is melted (high temperature range), and the flux is selected according to the application of the solder paste from the relationship between the temperature of the flux and the ion conductivity.

  An activator that works to remove oxide films such as organic acids and halogen compounds contained in the flux is decomposed into ions when it reaches the temperature range including when the solder paste melts. The solderability is improved by attacking and removing the oxide film. However, the flux decomposed into ions during the handling of the solder paste in a printing machine or the like reacts with a metal such as Sn in the solder paste to generate a compound, or reacts with oxygen in the air. In addition, the solder paste viscosity is increased, the applicability is deteriorated, and the activator in the flux is consumed. Cannot melt.

  Therefore, in this evaluation method, when solder paste is handled or stored in a printing machine or the like (normal temperature range: 25 to 40 ° C.) and when the electronic component is mounted (reflow), the solder paste is melted (high temperature range). : Flux activity is measured by measuring the ionic conductivity of the flux at a temperature of 180 to 230 ° C., and a flux suitable for the solder paste is selected.

The ion conductivity of the flux can be measured by, for example, the ion conductivity measuring device 100 shown in FIG. In this measuring apparatus 100, the ion conductivity is continuously measured while heating the flux F to be evaluated to 25 ° C. to 230 ° C. using the stirrer 1 and the mantle heater 2.
An oil bath 3 surrounded by a mantle heater 2 is placed on the stirrer 1. Silicon oil 4 is stored in the oil bath 3 and a stirrer 5 a is immersed therein. It is in a heated state.
The flux F is accommodated in the test tube 6 and heated with the thermocouple 7 and the electric conductivity meter 8 inserted into the flux F in the test tube 6. At this time, the temperature of the flux F can be continuously measured by the thermocouple 7 and the ionic conductivity of the flux F can be continuously measured by the electric conductivity meter 8.
The flux F is stirred by the stirring bar 5b in the test tube 6 and heated uniformly.

Next, a method for evaluating the flux suitable for the solder paste from the ionic conductivity of the flux will be described.
The state where the ion conductivity of the flux is low is a state where the activity of the flux in which ion dissociation of the activator in the flux is suppressed is low, and the reaction with other components is suppressed. On the other hand, when the ionic conductivity is high, the active agent in the flux is actively ion-dissociated and the activity of the flux is high, and the reaction with other components is promoted.
Concerning the flux used for solder paste, when the solder paste is handled and stored using a printing machine, etc., it suppresses ion dissociation of the active agent in the flux and suppresses increase in the viscosity of the solder paste. (Viscosity stability) is required to maintain good coatability over a long period of time. Further, during reflow of the solder paste (high temperature range), it is required to promote ion dissociation of the activator in the flux and to remove the oxide film on the surface of the solder alloy powder so as to have good wettability.
Therefore, this flux evaluation method evaluates both the viscosity stability in the normal temperature range and the wettability in the high temperature range by examining the activity of the flux in the normal temperature range and the high temperature range, and provides a flux suitable for the solder paste. Select.

Next, the solder paste flux will be described.
As described above, the flux suitable for the solder paste is preferably one having a low ionic conductivity in the normal temperature range without the active agent in the flux being decomposed into ions. On the other hand, in a high temperature range, ions are separated and high ion conductivity is preferable. In particular, a flux suitable for a lead-free solder paste has an ion conductivity of a flux of 25 to 40 ° C. (normal temperature range) of 0.05 mS / m or less and an ion conductivity of a flux of 180 to 230 ° C. (high temperature range). The rate is preferably 1.0 mS / m or more.
Such a flux suppresses the ionic dissociation of the activator in the flux when the solder paste is handled using a printing machine (normal temperature range). Thus, the increase in the viscosity of the solder paste can be prevented, and good coating properties can be maintained for a long time. Further, at the time of reflowing the solder paste (high temperature region), the flux has sufficient activity to remove the oxide film on the surface of the solder alloy powder, so that good wettability can be provided.

  As described above, according to the solder flux evaluation method of the present invention, the flux activity in a specific temperature region can be obtained by measuring the ionic conductivity of the solder flux in advance. Flux can be selected in accordance with the application of the solder paste before manufacturing. And in the solder paste using such a flux, it is possible to prevent a change with time such as an increase in viscosity in a normal temperature range, to obtain a stable coating property, and to obtain a good wettability in a high temperature range. .

Next, Examples 1 to 3 and Comparative Examples 1 to 5 according to the solder flux evaluation method of the present invention will be described.
(Flux production)
The solder fluxes of Examples 1 to 3 and Comparative Examples 1 to 5 were prepared according to the ratio (mass%) in the formulation table shown in Table 1.

(Measurement of ionic conductivity)
The ion conductivity of each flux was measured using the measuring apparatus 100 shown in FIG. 30 ml of each produced flux is taken out and put into a test tube 6, and a thermocouple 7 and an electric conductivity meter 8 are inserted into the flux in the test tube 6. Next, this test tube 6 was put in an oil bath 3 heated to 300 ° C., heated to 25 ° C. to 230 ° C., and the ionic conductivity at that time was continuously measured. Table 2 shows the results at 25 to 40 ° C and the results at 180 to 230 ° C.

  As shown in Table 2, the fluxes of Examples 1 to 3 have a flux of 25 to 40 ° C. (normal temperature range) having an ionic conductivity of 0.05 mS / m or less and 180 to 230 ° C. (high temperature range). This flux is suitable for a lead-free solder paste having an ionic conductivity of 1.0 mS / m or more. On the other hand, the fluxes of Comparative Examples 1, 2, and 4 have an ionic conductivity exceeding 0.05 mS / m at room temperature, and the fluxes of Comparative Examples 1, 3, and 5 have an ionic conductivity of 1 at high temperatures. Less than 0.0 mS / m.

(Preparation of solder paste)
Next, a solder alloy powder (melting point: 219 ° C.) having an Sn-3.0 mass% Ag-0.5 mass% Cu composition and an average particle diameter of 8.0 μm is mixed with these fluxes to produce a paste. did. The mixing ratio is 88% by mass of solder powder and 12% by mass of flux.

(Viscosity stability evaluation during continuous printing of solder paste)
The viscosity of the solder paste immediately after the production was measured using PCU-205 manufactured by Malcolm. Next, using a printing machine NP-MB04 manufactured by Hitachi Plant Technology Co., Ltd., the solder paste was printed on a substrate one by one per minute, and this was repeated for 24 hours to perform continuous printing for 24 hours. Then, after 24 hours of continuous printing, the solder paste was collected and the viscosity of the solder paste was measured again. These results are shown in “Viscosity stability during continuous printing” in Table 3.
“Viscosity stability during continuous printing” shows the results of evaluating the viscosity stability of the solder paste by comparing the viscosity of the initial solder paste with the solder paste after 24 hours of continuous printing and using the increased value. It was. “◯” indicates that the viscosity increase was 0 or more and less than 30 Pa · s, and good viscosity stability was obtained. In addition, “Δ” indicates a case where it is 30 or more and less than 60 Pa · s, and “X” indicates a case where it is 60 Pa · s or more.

(Evaluation of wettability during continuous printing of solder paste)
The solder paste immediately after fabrication is printed on a Cu plate using a metal mask having a thickness of 0.2 mm and an opening diameter (hole diameter) of 6.5 mm, and reflowed by heating in nitrogen at a maximum temperature of 240 ° C. The diameter of wet spread was measured.
Next, 24-hour continuous printing of the solder paste was performed using a printing machine NP-MB04 manufactured by Hitachi Plant Technology. After 24 hours of continuous printing, the paste was collected, and the solder paste was printed again on a Cu plate using a metal mask, reflowed, and the wet spread diameter was measured. These results are shown in “Wettability during continuous printing” in Table 3.
In “Wettability during continuous printing”, the diameters after printing and after reflowing were compared, and the result of evaluating the wettability of the solder paste by the increased value was shown. “◯” indicates that when the spread amount exceeds 0 and less than 10%, good wettability was obtained. Further, “Δ” is 0% and “x” is less than 0%, and the smaller the value, the worse the wettability. Note that less than 0% indicates that the diameter after reflow is smaller than that after printing due to surface tension.

  As shown in Table 3, it can be seen that the solder pastes using the fluxes of Examples 1 to 3 have no change in viscosity, have good viscosity stability, and have good wettability. On the other hand, the viscosity stability decreased for the solder pastes of Comparative Examples 1, 2, and 4, and the wettability decreased for Comparative Examples 1, 3, and 5. In the solder pastes of Comparative Examples 1, 2, and 4 in which good results could not be obtained, a flux having an ionic conductivity exceeding 0.05 mS / m at room temperature was used. No. 5 was a flux having an ionic conductivity in the high temperature range of less than 1.0 mS / m.

  As described above, according to the solder flux evaluation method of the present invention, by measuring the ionic conductivity of the flux in advance in the normal temperature range and the high temperature range of the solder paste to be manufactured, Flux can be selected.

  In addition, this invention is not limited to the thing of the structure of the said embodiment, In a detailed structure, it is possible to add a various change in the range which does not deviate from the meaning of this invention.

DESCRIPTION OF SYMBOLS 1 Stirrer 2 Mantle heater 3 Oil bath 4 Silicone oil 5a, 5b Stirrer 6 Test tube 7 Thermocouple 8 Electrical conductivity meter

Claims (6)

  A method for evaluating solder flux, comprising measuring a relationship between a temperature of solder flux and ionic conductivity.   The measurement of the ionic conductivity is performed so as to include two different temperature regions, a normal temperature region where a solder paste prepared using the solder flux is stored or handled and a high temperature region where the solder paste is reflowed. Item 2. The solder flux evaluation method according to Item 1.   Solder characterized in that the ionic conductivity in the normal temperature range where the solder paste to be produced is stored or handled is 0.05 mS / m or less and the ionic conductivity in the high temperature range to be reflowed is 1.0 mS / m or more. flux.   The solder flux for a lead-free solder paste, wherein the normal temperature range is 25 to 40 ° C, and the high temperature range is 180 to 230 ° C.   A lead-free solder paste, wherein the solder flux according to claim 3 or 4 and a solder alloy powder containing 95 mass% or more of Sn and not containing Pb are mixed and pasted.   An electronic component is mounted using the solder paste according to claim 5.

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