Lead-free solder paste

Abstract

As electronic equipment has become smaller in size, printed circuit boards which cannot be subjected to cleaning have been developed, and a no-clean lead-free solder paste is becoming necessary. In order for a solder paste not to require cleaning, it is necessary that the color of the residue be transparent and that the residue be non-tacky. A maleated rosin, which is a rosin suited for no-clean paste, has a high acid value so it is not suitable for a flux for lead-free solder. As a means of suppressing a reaction between a flux containing a maleated rosin and a Sn—Ag—Cu based solder alloy powder, a Sn—Ag—Cu—Sb based solder alloy powder is used which adds 1-8 mass % of Sb to a Sn—Ag—Cu based solder alloy. As a result, it is possible to provide a solder paste which has the excellent effect that the solder paste does not easily undergo changes over time and has a long pot life.

Claims

1. A lead-free solder paste comprising: a solder alloy powder of a solder alloy consisting of 1.0-4.0 mass % of Ag, 0.4-1 mass % of Cu, 1-8 mass % of Sb, and a balance of Sn; and a flux which contains one or more rosins which collectively are the largest component of the flux in mass %, the flux containing 5-45 mass % of a maleated rosin.

2. A lead-free solder paste as claimed in claim 1 wherein the solder alloy powder contains 2-5 mass % of Sb.

3. A lead-free solder paste as claimed in claim 1 which has an increase in viscosity at 60 days at 25° C. after preparation of the solder paste with respect to its viscosity one day after preparation of at most 25%.

4. A lead-free solder paste as claimed in claim 1 wherein the flux consists of one or more rosins and at least one material selected from an activator, a thixotropic agent, and a solvent.

5. A lead-free solder paste as claimed in claim 4 wherein the flux contains a total of at least 50 mass % of the one or more rosins.

6. A lead-free solder paste as claimed in claim 1 wherein the flux contains at least one rosin selected from a polymerized rosin, a hydrogenated rosin, and a phenol modified rosin.

7. A lead-free solder paste as claimed in claim 1 wherein the flux contains a hydrogenated rosin.

8. A lead-free solder paste as claimed in claim 1 which consists of the solder alloy powder and the flux.

9. A lead-free solder paste comprising: a solder alloy powder of a solder alloy consisting of 1.0-4.0 mass % of Ag, 0.4-1.0 mass % of Cu, 1-8 mass % of Sb, at least one of Ni, Co, and Fe in a total amount of greater than 0 to at most 0.5 mass %, and a balance of Sn; and a flux which contains one or more rosins which collectively are the largest component of the flux in mass %, the flux containing 5-45 mass % of a maleated rosin.

Description

BEST MODE FOR CARRYING OUT THE INVENTION

(1) If the Sb content of a Sn—Ag—Sb alloy powder or a Sn—Ag—Cu—Sb alloy powder according to the present invention is less than 1 mass %, there is no effect of suppressing a reaction with a maleated rosin, while if the Sb content is greater than 8 mass %, solder wettability which is an advantage of a Sn—Ag solder or a Sn—Ag—Cu solder is impaired. Accordingly, the amount of Sb added to a Sn—Ag solder powder or a Sn—Ag—Cu solder powder according to the present invention is preferably 1-8 mass % and more preferably 2-5 mass %.

(2) According to the present invention, there is no direct relationship between the contents of Ag and Cu in a Sn—Ag—Sb alloy powder or a Sn—Ag—Cu—Sb alloy powder and the reactivity with a maleated rosin. Accordingly, from a technical standpoint, there is no particular need to limit the contents of Ag and Cu in a Sn—Ag—Sb alloy powder or a Sn—Ag—Cu—Sb alloy powder according to the present invention, and in a Sn—Ag—Cu—Sb alloy, selecting Ag to be 1.0-4.0 mass % and Cu to be 0.4-1.0 mass % provides a solder with excellent tensile strength and creep properties. More preferably, Ag is 3.0-4.0 mass % and Cu is 0.5-0.7 mass %. Furthermore, if at least one transition metal from the iron group such as Ni, Co, and Fe is added in a total amount of at most 0.5 mass %, an improvement in the strength of the solder alloy composition is observed.

(3) The content of a maleated rosin in a flux for a solder paste according to the present invention is at least 5 mass % and at most 45 mass % of the flux. If the content of a maleated rosin contained in a flux for a solder paste is less than 5 mass %, there is little change over time in a solder paste even with a conventional Sn—Ag alloy powder or a Sn—Ag—Cu alloy powder, and there is no particular need to add Sb. If a maleated rosin is added in excess of 45 mass % of a flux for a solder paste, even if a Sn—Ag—Sb alloy powder or a Sn—Ag—Cu—Sb alloy powder is used, it is not possible to suppress changes with the passage of time. Accordingly, the content of a maleated rosin contained in a preferred flux for a solder paste according to the present invention is at least 5 mass % and at most 45 mass %. More preferably, it is at least 5 mass % and at most 20 mass %.

(4) The present invention realizes a solder paste having little change over time and a long pot life even when a flux for a solder paste contains a maleated rosin by adding Sb, which has low reactivity with a maleated rosin, to a conventional Sn—Ag alloy powder or a Sn—Ag—Cu alloy powder.

(5) Accordingly, it is preferable to limit the content of components having high reactivity with solder powder other than a maleated rosin. Such components in flux having high reactivity with solder powder are hydrogen halide salts such as diphenylguanidine HBr. A hydrogen halide salt contained in a flux for a solder paste according to the present invention preferably has a content of less than 2 mass % of the flux. If the content of the hydrogen halide salt contained in a flux is at least 2 mass %, a reaction between the alloy powder and the flux progresses even when Sb is added, and the effects of the present invention are not realized. In the present invention, is it possible to add at most 3 mass % of an organic halogen compound such as hexabromocyclododecane, trans-2,3-dibromo-2-butene-1,4-diol, 2,3-dibromo-1,4-butanediol, 2,3-dibromo-1-propanol, or 1,3-dibromo-2-propanol as an auxiliary activator.

Example 1

(6) Examples of a flux according to the present invention and comparative examples were prepared using a maleated rosin, a polymerized rosin, a hydrogenated rosin, and a phenol modified rosin. The composition of the flux is shown in Table 1.

(7) The method of manufacturing flux was to add each rosin to a solvent, perform heating to melt the rosin, then stop heating and cool to around 100° C. When the liquid temperature reached around 100° C., a thixotropic agent was added and melted, and then an activator was added and was dissolved while stirring. After all of the materials had dissolved, the flux was cooled to solidify it by water cooling or the like.

(8) TABLE-US-00001 TABLE 1 Flux in examples Flux in comparative (mass %) examples (mass %) Function Details 1 2 3 4 1 2 3 4 Rosin Maleated rosin 20 45 15 5 3 — — 50 Polymerized rosin 30 — 20 30 30 30 30 — Hydrogenated 5 5 20 10 10 20 — rosin Phenol modified — — — 12 15 30 10 — rosin Activator Diphenyl- 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 guanidine HBr 2,3-dibromo-1,4- 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 butanediol Adipic acid 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Thixotropic Hydrogenated 4 4 4 4 4 4 4 4 agent castor oil Ethylene- 1 1 1 1 1 1 1 1 bisstearamide Solvent Diethylene glycol 38.5 43.5 38.5 36.5 35.5 33.5 33.5 43.5 monohexyl ether

Example 2

(9) Sn—Ag based solder powders and the fluxes prepared in Example 1 were mixed in the proportions shown in Table 2 to prepare a solder paste. The flux content of each solder paste was a reference value of 10.5 mass %, and the flux content was adjusted until the viscosity was around 200 Pa-sec.

(10) Each solder paste was left at room temperature for one day, and then its viscosity was measured. In addition, each solder paste was stored for 60 days in a constant temperature storage chamber set to 25° C., and then its viscosity was measured. Viscosity was measured based on the method set forth in JIS Z 3284 Appendix 6 using a PCU-205 manufactured by Malcom Co., Ltd. The results are shown in Table 2.

(11) The change over time in each solder paste was compared and the pot life was determined.

(12) TABLE-US-00002 TABLE 2 Solder powder composition Viscosity Wettability Color Flux (mass %) (mass %) (Pa-S) test of Type Content Sn Ag Cu Sb Other Day 1 Day 60 (category) residue Examples 1 Ex. 1 10.5 rem. 3.5 — 2 — 198 220 2 Transparent 2 Ex. 1 10.5 rem. 1 0.7 1 — 198 223 1 Transparent 3 Ex. 1 10.5 rem. 3.9 0.6 3 — 198 225 1 Transparent 4 Ex. 2 10.5 rem. 3.9 0.6 3 — 200 250 1 Transparent 5 Ex. 4 10.5 rem. 3.9 0.6 3 0.04 Ni 195 218 1 Transparent 6 Ex. 3 10.3 rem. 4 1 8 — 200 240 2 Transparent 7 Ex. 4 10.5 rem. 3 0.5 2 0.04 Ni 198 225 1 Transparent 8 Ex. 3 10.3 rem. 3.5 0.7 1 0.02 Co 195 240 1 Transparent Comparative 1 Comp. 1 10.5 rem. 3.9 0.6 3 — 198 220 2 Dark 2 Comp. 2 10.5 rem. 3.9 0.6 3 — 200 NM* 2 Dark 3 Comp. 3 10.5 rem. 3.9 0.6 3 — 200 NM* 2 Transparent 4 Comp. 4 10.3 rem. 3.9 0.6 3 — 200 400 2 Transparent 5 Ex. 1 10.5 rem. 3.9 0.6 0.5 — 198 350 1 Transparent 6 Ex. 1 10.5 rem. 3.9 0.6 10 — 198 220 3 Transparent *NM: Not measurable

Example 3

(13) The wettability and the color of the residue of the solder pastes manufactured in Example 2 were compared.

(14) Wettability of the solder paste was measured by the method set forth in JIS Z 3284, Appendix 10.

(15) The temperature of the solder bath was measured as 250±3° C.

(16) In the same manner as in JIS Z 3284, Appendix 10, the color of the residue of the solder paste was determined by printing solder paste using a metal mask with a thickness of 0.2 mm and holes with a diameter of 6.5 mm on a tough pitch copper plate measuring 5.0 mm×5.0 mm×0.5 mm and then performing reflow using a solder bath at a temperature of 250±3° C. The color of the flux residue after cooling was compared. The results are shown in Table 2.

(17) Summarizing the above, solder powder of an alloy having at least 1 mass % of Sb added to a Sn—Ag alloy or a Sn—Ag—Cu alloy had the effect of suppressing changes in viscosity when using a maleated rosin. However, when the Sb content is less than 1 mass %, it is not possible to suppress a change in viscosity, and when Sb is added in excess of 8 mass %, wettability of solder paste is inhibited.

(18) In addition, if the content of a maleated rosin in a solder paste according to the present invention exceeds 45 mass %, even if Sb is added, a change in the viscosity of the solder paste cannot be suppressed. A suitable amount of a maleated rosin in a solder paste according to the present invention is at least 5 mass % and at most 45 mass %.

INDUSTRIAL APPLICABILITY

(19) A solder paste according to the present invention is not limited to Sn—Ag alloys or Sn—Ag—Cu alloys and it can be applied not only to lead-free solder pastes of Sn—Ag alloys or Sn—Ag—Cu alloys but also to alloys in which Bi or In is added to a Sn—Ag alloy or a Sn—Ag—Cu alloy.