JP2012192434A - Flux, solder paste, and method for manufacturing mounting substrate - Google Patents

Abstract

The present invention provides a flux for a solder paste that is excellent in applicability and wettability with little change over time such as increase in viscosity during storage and handling of the solder paste.
A flux that is mixed with a lead-free solder alloy powder containing Au and Sn and not containing Pb to form a solder paste, and has an ionic conductivity of 0.001 mS in a room temperature range where the solder paste is stored or handled. The ion conductivity in the high temperature region to be reflowed is 0.01 mS / m or more.
[Selection] Figure 1

Description

  The present invention relates to a flux evaluation method, a flux, a solder paste using the flux, and a method for manufacturing a mounting board. 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 is about 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.

JP-A-5-318176

In the solder paste of Patent Document 1, there is little risk of the solder paste flux reacting with the solder alloy powder and thickening during storage or handling using a printing machine, but the activator in the flux is not present during reflow. It is difficult to sufficiently remove the oxide film on the surface of the solder alloy powder, which may cause deterioration of the bonding reliability due to poor wetting.
The evaluation of the flux used for such a solder alloy powder is made by actually mixing the solder alloy powder and the flux to produce a solder paste and storing it at room temperature to determine whether the viscosity changes, or by heating to a high temperature. It was not efficient because there was no method other than trying to melt it.

  The present invention has been made in view of the above-mentioned problems, has little change over time such as increase in viscosity at the time of storage of solder paste and handling such as coating and printing, etc., and excellent wettability during reflow. An object is to provide a flux for solder paste.

The characteristics of the flux can be evaluated by measuring the relationship between the temperature of the flux and the ionic conductivity.
By measuring the ionic conductivity of the flux, the activity of the flux at a specific temperature can be obtained. Thereby, before producing solder paste, the flux according to the use of solder paste can be selected.

Moreover, the measurement of the ionic conductivity is good to implement so that two different temperature ranges, the normal temperature range in which the solder paste produced using a flux is handled, and the high temperature range in which reflow is handled may be included.
For the solder paste to be produced, the ion conductivity at each temperature of the flux in the temperature range including the temperature at the time of handling in a printing machine (room temperature) and the temperature at the time of mounting electronic parts (during reflow) (high temperature) By measuring, the activity of the flux in these temperature ranges can be obtained, so that the activity is low at room temperature and the change in time such as viscosity increase is prevented to obtain a stable coating property. A flux capable of removing the oxide film on the surface of the alloy powder and having good wettability can be selected.
Based on such knowledge, the following means of flux was used as a solution.

The flux of the present invention is a flux that is mixed with a lead-free solder alloy powder containing Au and Sn and not containing Pb to form a solder paste, and has an ionic conductivity in a room temperature range where the solder paste is stored or handled. It is 0.001 mS / m or less, and the ion conductivity of the reflowed high temperature region is 0.01 mS / m or more.
Such a flux suppresses the ionic dissociation of the active component in the flux when the solder paste is handled using a printing machine or the like (normal temperature range). This can prevent an increase in the viscosity of the solder paste. 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.

  Here, the said normal temperature range is 25-40 degreeC, and it is good in the said high temperature range being 260-340 degreeC.

The solder paste of the present invention is preferably formed into a paste by mixing the flux, Sn: 18 to 25% by mass, the balance being Au and lead-free solder alloy powder containing inevitable impurities and not containing Pb.
Sn: 18-25% by mass, the remainder consisting of Au and unavoidable impurities and containing no Pb, while suppressing the increase in viscosity during storage (normal temperature range), while reflowing (high temperature range) It was difficult to obtain good wettability. On the other hand, the solder paste using the flux described above contains lead-free solder alloy powder, but prevents changes over time such as an increase in viscosity, and has a stable application property when used in a printing machine or the like. It can be obtained 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, a solder paste prepared by mixing with a lead-free solder alloy powder containing Au and Sn and not containing Pb is mounted with a temperature at the time of handling in a printing machine (room temperature) and an electronic component. By measuring the ionic conductivity of the flux in the temperature region including the temperature during reflow (high temperature), the activity of the flux in these temperature regions can be obtained. It is possible to obtain a flux capable of preventing and obtaining a stable coating property and removing the oxide film on the surface of the solder alloy powder at a high temperature and having good wettability.

It is a figure explaining the measuring method of the ionic conductivity of the flux of one embodiment of the present invention.

Hereinafter, a flux and an evaluation method thereof according to an embodiment of the present invention will be described with reference to the drawings.
The flux evaluation method of the present embodiment is a method of heating the flux and of solder paste when handling the solder paste to be produced using a printing machine or the like (room temperature range) and mounting an electronic component or the like (during reflow). In this method, ion conductivity is measured in a temperature region including at the time of melting (high temperature region), and a flux corresponding to the application of the solder paste is selected 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 increases, the applicability deteriorates, and the activator in the flux is consumed, so that the solder paste cannot be removed on the surface of the solder alloy powder during reflow. Cannot melt.

  Therefore, in this evaluation method, the solder paste is melted (high temperature region: 260 to 260 ° C.) at the time of handling in a printing machine or the like of solder paste (normal temperature region: 25 to 40 ° C.) and when the electronic component is mounted (during reflow). The flux activity is measured by measuring the ionic conductivity of the flux at a temperature of 340 ° 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 340 ° 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.
Regarding the flux used in the solder paste, when handling the solder paste using a printing machine or the like (room temperature range), the ion dissociation of the activator in the flux is suppressed, and the increase in the viscosity of the solder paste is suppressed ( Viscosity stability) and maintaining good coating properties over a long period of time are required. 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, the flux suitable for the Au—Sn-based lead-free solder paste has an ionic conductivity of 25 to 40 ° C. (normal temperature range) and a flux ion of 0.001 mS / m or less, and 260 to 340 ° C. (high temperature range). A flux having an ionic conductivity of 0.01 mS / m or more is preferable.
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 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 flux in advance, so that a solder paste is produced. Before doing so, it is possible to select a flux according to the application of the solder paste. 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 flux evaluation method of the present invention will be described.
(Flux production)
The fluxes of Examples 1 to 3 and Comparative Examples 1 to 5 were produced according to the ratio (mass%) in the recipe 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 into an oil bath 3 heated to 300 ° C., heated to 25 ° C. to 340 ° C., and the ionic conductivity at that time was continuously measured. Table 2 shows the results at 25-40 ° C and the results at 260-340 ° C.

  As shown in Table 2, the flux of Examples 1 to 3 has a flux of 25 to 40 ° C. (normal temperature range) with an ionic conductivity of 0.001 mS / m or less and a flux of 260 to 340 ° C. (high temperature range). This flux is suitable for an Au—Sn lead-free solder paste having an ionic conductivity of 0.01 mS / m or more. On the other hand, the fluxes of Comparative Examples 1, 2, and 4 have an ionic conductivity in the normal temperature range exceeding 0.001 mS / m, and the fluxes of Comparative Examples 1, 3, and 5 have an ionic conductivity of 0 at a high temperature. Less than 0.01 mS / m.

(Preparation of solder paste)
Next, a solder alloy powder (melting point: 278 ° C.) having an average particle diameter of 8.0 μm having a composition of 80 mass% Au-20 mass% Sn was mixed with these fluxes to prepare a paste. The mixing ratio is 90% by mass of solder alloy powder and 10% 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 340 ° 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 0.01 mS / m at room temperature was used. In No. 5, a flux having an ionic conductivity in the high temperature range of less than 0.001 mS / m was used.

  As described above, according to the flux evaluation method of the present invention, the solder paste is stored and printed 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 produced. It is possible to select a flux for a solder paste that is excellent in change over time such as an increase in viscosity of the solder paste during handling in a machine and excellent wettability during reflow.

  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 (4)

A flux that is mixed with a lead-free solder alloy powder containing Au and Sn and not containing Pb to form a solder paste,
A flux characterized by having an ionic conductivity in a normal temperature range in which the solder paste is stored or handled is 0.001 mS / m or less and an ionic conductivity in a high temperature range to be reflowed is 0.01 mS / m or more.   The flux according to claim 1, wherein the normal temperature region is 25 to 40 ° C, and the high temperature region is 240 to 360 ° C.   The flux according to claim 1 or 2 is mixed with Sn: 18 to 25% by mass, and the remainder is made of Au and lead-free solder alloy powder containing inevitable impurities and containing no Pb. Solder paste.   An electronic component is mounted using the solder paste according to claim 3.

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