JP5783273B2 - Solder flux - Google Patents

Description

  The present invention relates to a solder flux. More specifically, the present invention relates to a solder flux for solder paste that can evaluate the occurrence of voids during reflow of the solder paste and has excellent bonding strength.

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 pastes used for joining electronic components, if voids occur after reflow (after melting) of the solder paste, the bonding strength between the solder and the electrodes of the electronic components is reduced, affecting long-term reliability. Therefore, the reduction of voids has been demanded, and the solder flux has been improved.

  For example, in Patent Document 1, an attempt is made to reduce voids by using a solder flux containing a solvent whose temperature at which the weight loss rate is 15% by mass in the measurement by the TG method is 5 ° C. or more higher than the melting peak temperature of the solder. ing.

  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.

No. 2004-108345

The weight loss rate of the solvent by the TG method of Patent Document 1 is effective when evaluating the generation of voids due to thermal decomposition of the solvent itself. However, no consideration is given to the generation of voids due to the reaction gas of the solder alloy powder and the activator.
Also, when evaluating the flux used for solder alloy powder, the solder paste is actually mixed with the flux to produce a solder paste, and whether it can be melted during reflow, or the bonding strength decreases due to the generation of voids. It was not efficient because there was no way other than trying to see if it was possible.

  The present invention has been made in view of the above-mentioned problems, and can easily evaluate the wettability and void generation during reflow of the solder paste, and is excellent in wettability and bonding strength. The purpose is to provide flux.

  When evaluating the solder flux of the present invention, the ionic conductivity in the temperature region including the melting point is measured immediately before the melting point of the solder paste produced using the solder flux.

  The activator in the solder flux is ionically dissociated at a specific temperature and reacts with oxides on the solder surface and the electrode surface to generate a reactive gas. The active power of the activator in the solder flux can be clarified by evaluating the ion dissociation state of the activator in the solder flux, that is, the ionic conductivity.

When a solder flux having a weak activity of the activator in the solder flux is used, the oxide film on the solder surface and the electrode surface cannot be sufficiently removed, resulting in insufficient wetting and increased voids. In addition, even in solder fluxes where the activator's activity continues continuously up to the melting point of the solder, the activator itself continues to decompose even when the solder's melting point is reached, so as to continue to generate reaction gas. , Voids increase.
Therefore, by measuring the ionic conductivity of the flux in a specific temperature range near the melting point determined by the metal component of the solder powder alloy to be blended in advance for the solder paste to be prepared, the activity of the flux in these temperature ranges is determined. Since it can be obtained, it is possible to specify the wettability during reflow and the occurrence of voids from the ionic conductivity, and it is possible to select a flux according to the application of the solder paste.

Further, in this solder flux evaluation method, in addition to the temperature range including the melting point, the ion conductivity in a temperature region lower than the melting point may be measured.
In this way, by measuring the ionic conductivity in two different temperature regions, the oxide film on the solder surface can be removed to have good wettability, and the generation of voids can be suppressed to prevent a decrease in bonding strength. Can be selected.

The solder flux of the invention is a solder flux for lead-free solder paste, the maximum value of the ion conductivity at 170 to 200 ° C. is a temperature region lower than the melting point be 2.0 mS / m or more And the ion conductivity in 217-250 degreeC which is a temperature range containing the said melting | fusing point is 1.0 mS / m or less, It is characterized by the above-mentioned.
Such solder flux has a sufficient activity to remove the oxide film on the solder surface in a temperature range (170 to 200 ° C.) lower than the melting point of the solder. Can have. Also, until the melting point of the solder, the active state of the flux continues continuously and the oxide film is removed and decomposition gas continues to be generated, but the temperature range (217 to 250 ° C.) including the melting point after reaching the melting point immediately before the melting point. Since the activity of the flux can be suppressed, the generation of reaction gas can be suppressed, voids can be reduced, and the solder joint strength can be prevented from being lowered.
In the solder flux of the present invention, the ionic conductivity is measured by an ionic conductivity measuring device, and the ionic conductivity measuring device can continuously measure the temperature and ionic conductivity of the heated flux. A thermocouple and electrical conductivity meter may be provided.

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 powder containing 95% by mass or more of Sn and not containing Pb, a highly active solder flux is required, and good wettability during reflow is obtained while suppressing an increase in viscosity during storage. It was difficult. On the other hand, this solder paste using the solder flux contains lead-free solder alloy powder, but in the temperature range lower than the melting point of the solder, the oxide film on the solder surface is removed and good wetting is achieved. In the temperature region including the melting point of the solder and the melting point after reaching the melting point, the generation of the reaction gas from the flux can be suppressed to reduce the voids and prevent the bonding strength from being lowered.
The lead-free solder alloy powder containing no Pb indicates that the Pb content in the solder alloy powder is less than 1000 ppm.

  The mounting board manufacturing method of the present invention is characterized in that an electronic component is mounted using the solder paste.

  According to the present invention, by measuring the ionic conductivity of the solder flux in the temperature range near the melting point of the solder paste, the activity of the solder flux in these temperature ranges can be obtained. In addition, the wettability during reflow and the occurrence of voids can be evaluated from the ionic conductivity. As a result, in the temperature range lower than the melting point of the solder, the oxide film on the solder surface is removed to have good wettability at the time of melting the solder, and in the temperature range including the melting point after reaching the melting point immediately before the melting point, the flux Therefore, it is possible to select a solder flux that can suppress generation of decomposition gas from the metal to reduce voids and prevent a decrease in solder joint strength.

It is a figure explaining the measuring method of ion conductivity.

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 this embodiment is to heat the flux and measure the ionic conductivity for two different temperature regions near the melting point of the solder paste to be produced. From the relationship between the flux temperature and the ionic conductivity, This is a method of selecting a flux according to the application of the solder paste.

The active agent that works to remove oxide films such as organic acids and halogen compounds contained in the flux is decomposed into ions, which react with the oxide films on the solder surface and the electrode surface of the electronic component. The solderability is improved by removing.
The active power of the active agent in the flux can be clarified by evaluating the state of ion dissociation of the active agent, that is, the ionic conductivity.
When using a solder flux with a weak activator in the solder flux during reflow, the oxide film on the solder surface and electrode surface cannot be removed sufficiently, resulting in insufficient wetting and an increase in voids due to the reaction gas. To do. In addition, when a flux is used in which decomposition of the activator itself continues even when the melting point of the solder is reached, the reaction gas is continuously generated, resulting in an increase in voids.

  Therefore, in this evaluation method, a temperature region including two temperature regions different from a temperature region (170 to 200 ° C.) lower than the melting point of the solder and a temperature region (217 to 250 ° C.) including the melting point immediately after reaching the melting point immediately before the melting point. The flux activity is measured by measuring the flux ionic conductivity, 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 170 ° C. to 250 ° 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, it is required to promote the ion dissociation of the activator in the flux during reflow of the solder paste and to remove the oxide film on the surface of the solder powder so as to have good wettability. Therefore, it is required to suppress generation of reaction gas to reduce voids and prevent a decrease in solder joint strength.
Therefore, in this flux evaluation method, by examining the activity of the flux in the vicinity of the melting point of the solder, both the wettability and the bondability of the solder are evaluated, and the flux suitable for the solder paste is determined.

Next, the solder paste flux will be described.
Flux suitable for solder paste is in a state where ions are separated in a temperature region lower than the melting point of the solder during reflow, and has a high ionic conductivity. In a temperature region including the melting point immediately after reaching the melting point of the solder, Those having low conductivity are preferred. In particular, the flux suitable for the lead-free solder paste has an ion conductivity of 2.0 mS / m or more in the temperature region lower than the melting point (170 to 200 ° C.) and includes the melting point immediately after reaching the melting point immediately before the melting point. The thing whose ion conductivity of the flux of a temperature range (217-250 degreeC) is 1.0 mS / m or less is preferable.
Such a flux has sufficient activity to allow the flux to remove the oxide film on the surface of the solder powder at a temperature lower than the melting point when the solder paste is reflowed. Can be given. Also, in the temperature range including the melting point just before the melting point and after the melting point is reached, the activity of the flux is suppressed and the generation of reaction gas is suppressed, so that voids can be reduced, thereby reducing the solder joint strength. Can be prevented.

  As described above, according to the solder flux evaluation method of the present invention, the flux activity in the temperature region near the melting point of the solder can be obtained by measuring the ionic conductivity of the solder flux in advance. In addition, the oxide film on the solder surface is removed in a temperature range lower than the melting point of the solder to have good wettability, and in the temperature range including the melting point after reaching the melting point immediately before the melting point, generation of decomposition gas from the flux is suppressed and voids are generated. Can be selected, and a solder flux that can prevent a decrease in the bonding strength of the solder can be selected.

Next, Examples 1 to 3 and Comparative Examples 1 to 3 according to the solder flux of the present invention will be described.
(Production of solder flux)
The solder fluxes of Examples 1 to 3 and Comparative Examples 1 to 3 were prepared according to the blending ratio (mass%) in the blending 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 into an oil bath 3 heated to 300 ° C., heated to 170 to 250 ° C., and ionic conductivity at that time was continuously measured. Table 2 shows the results at 170 to 200 ° C and the results at 217 to 250 ° C.

  As shown in Table 2, in the fluxes of Examples 1 to 3, the maximum value of the ionic conductivity of the flux at 170 to 200 ° C. is 2.0 mS / m or more, and the ionic conductivity of the flux at 217 to 240 ° C. It is a flux suitable for a lead-free solder paste of 1.0 mS / m or less. On the other hand, the flux of Comparative Examples 2 and 3 has a maximum ion conductivity of 170 to 200 ° C. less than 2.0 mS / m, and the flux of Comparative Example 1 has an ion conductivity of 217 to 240 ° C. It exceeds 1.0 mS / m.

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

(Void measurement)
The produced solder paste was printed on a bump forming substrate (mask thickness 30 μm, mask opening diameter 75 μm), and reflow was performed in nitrogen at a maximum temperature of 240 ° C. Solder bumps formed after reflow were observed with a transmission X-ray microscope, and voids present in the solder bumps were measured. And the diameter of the solder bump and the diameter of the void which exists in the solder bump were compared, and it evaluated according to the ratio of the diameter of the void with respect to the diameter of this solder bump. At this time, when a plurality of voids exist in the solder bump, the ratio between the sum of the void diameters and the solder bump diameter was determined and evaluated. These results are shown in Table 2.
“◯” indicates that when the void diameter is less than 20% of the solder bump diameter, a good solder joint with few voids was obtained. Further, “Δ” is 20 to 30%, “x” is 30% or more, and the larger the value occupied by the void, the worse the solderability of the solder.

  According to the present invention, the maximum value of the ionic conductivity of the flux at 170 to 200 ° C. is 2.0 mS / m or more, and the ionic conductivity of the flux at 217 to 250 ° C. is 1.0 mS / m or less. By using the flux, it is possible to obtain a good solder joint in which voids are suppressed.

  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 soldering flux for lead-free solder paste, the maximum value of the ion conductivity of the solder flux at 170 to 200 ° C. is a temperature region lower than the melting point is at 2.0 mS / m or more and the melting point The solder flux characterized by having an ion conductivity of 1.0 mS / m or less at a temperature range of 217 to 250 ° C.   The ion conductivity is measured by an ion conductivity measuring device, and the ion conductivity measuring device is a thermocouple and an electric conductivity meter capable of continuously measuring the temperature of the heated flux and the ion conductivity. The solder flux according to claim 1, wherein the solder flux is provided.   3. A lead-free solder paste, wherein the solder flux according to claim 1 or 2 is mixed with a solder alloy powder containing 95% by mass or more of Sn and not containing Pb, to form a paste.   An electronic component is mounted using the solder paste according to claim 3.

Images