Lead-Free Solder Ball

Abstract

A lead-free solder ball is provided which suppresses interfacial peeling in a bonding interface of a solder ball, fusion defects which develop between the solder ball and solder paste, and which can be used both with Ni electrodes plated with Au or the like and Cu electrodes having a water-soluble preflux applied atop Cu. The lead-free solder ball for electrodes of BGAs or CSPs consists of 1.6-2.9 mass % of Ag, 0.7-0.8 mass % of Cu, 0.05-0.08 mass % of Ni, and a remainder of Sn. It has excellent resistance to thermal fatigue and to drop impacts regardless of the type of electrodes of a printed circuit board to which it is bonded, which are Cu electrodes or Ni electrodes having Au plating or Au/Pd plating as surface treatment.

Claims

1. A method of forming a solder joint, comprising: placing a solder ball having an alloy composition on an electrode provided on a module substrate for a BGA or a CSP; heating the solder ball to form a solder bump on the electrode; preparing a printed circuit board by applying a solder paste onto an electrode provided on the printed circuit board; mounting the module substrate onto the printed circuit board by contacting the solder bump with the solder paste, wherein the solder bump is facing downwards with respect to the printed circuit board during the mounting step; and heating the solder bump and the solder paste to solder the solder bump to the electrode of the printed circuit board, wherein the alloy composition of the solder ball consists of: 1.6-2.9 mass % of Ag; 0.7-0.8 mass % of Cu; 0.05-0.08 mass % of Ni; optionally at least one element selected from Fe, Co, and Pt in a total amount of 0.003-0.1 mass % or optionally at least one element selected from Bi, In, Sb, P, and Ge in a total amount of 0.003-0.1 mass %; and a remainder of Sn.

2. The method of forming a solder joint as set forth in claim 1, wherein the electrode of the module substrate is selected from the group consisting of: an electrolytic Ni/Au electrode, an electroless Ni/Pd/Au electrode, and a Cu—OSP electrode.

3. The method of forming a solder joint as set forth in claim 1, wherein the electrode of the module substrate is an electrolytic Ni/Au electrode.

4. The method of forming a solder joint as set forth in claim 1, wherein the electrode of the module substrate is an electroless Ni/Pd/Au electrode.

5. The method of forming a solder joint as set forth in claim 1, wherein the electrode of the module substrate is a Cu—OSP electrode.

6. The method of forming a solder joint as set forth in claim 1, wherein the solder ball has a diameter of at least 0.1 mm.

7. The method of forming a solder joint as set forth in claim 1, wherein the solder ball has a diameter of at least 0.3 mm.

8. The method of forming a solder joint as set forth in claim 1, wherein the solder ball has a diameter of at least 0.5 mm.

9. The method of forming a solder joint as set forth in claim 1, further comprising applying a flux to the electrode of the BGA or CSP prior to placing the solder ball on the electrode.

10. The method of forming a solder joint as set forth in claim 1, wherein a reflow furnace is used to solder the solder bump to the electrode of the printed circuit board.

11. The method of forming a solder joint as set forth in claim 1, wherein the alloy composition of the solder ball consists of: 1.9-2.3 mass % of Ag; 0.7-0.8 mass % of Cu; 0.05-0.08 mass % of Ni; and a remainder of Sn.

12. The method of forming a solder joint as set forth in claim 1, wherein the alloy composition of the solder ball consists of: 1.6-2.9 mass % of Ag; 0.7-0.8 mass % of Cu; 0.05-0.08 mass % of Ni; at least one element selected from Fe, Co, and Pt in a total amount of 0.003-0.1 mass %; and a remainder of Sn.

13. The method of forming a solder joint as set forth in claim 1, wherein the alloy composition of the solder ball consists of: 1.6-2.9 mass % of Ag; 0.7-0.8 mass % of Cu; 0.05-0.08 mass % of Ni; at least one element selected from Bi, In, Sb, P, and Ge in a total amount of 0.003-0.1 mass %; and a remainder of Sn.

14. A method of mounting a module substrate for a BGA or a CSP to a printed circuit board, comprising: placing a solder ball having an alloy composition on each of a plurality of electrodes provided on the module substrate for the BGA or CSP; heating the solder balls to form a solder bump on each of the electrodes; preparing the printed circuit board by applying a solder paste onto each of a plurality of electrodes provided on the printed circuit board; mounting the module substrate onto the printed circuit board by contacting the solder bumps with the solder paste, wherein the solder bumps are facing downwards with respect to the printed circuit board during the mounting step; and heating the solder bumps and the solder paste to solder the solder bumps to the electrodes of the printed circuit board, wherein the alloy composition of the solder ball consists of: 1.6-2.9 mass % of Ag; 0.7-0.8 mass % of Cu; 0.05-0.08 mass % of Ni; optionally at least one element selected from Fe, Co, and Pt in a total amount of 0.003-0.1 mass % or optionally at least one element selected from Bi, In, Sb, P, and Ge in a total amount of 0.003-0.1 mass %; and a remainder of Sn.

15. An electronic part comprising a module substrate for a BGA or a CSP mounted on a printed circuit board, wherein the module substrate is mounted on the printed circuit board through one or more solder joints formed according to the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is an example of connection between a BGA and a printed circuit board.

[0033] FIG. 2 is an enlarged view of 4 in FIG. 1.

[0034] FIG. 3 is a figure showing the occurrence of fusion defects in the bonding interface between the solder ball of FIG. 2 and solder paste.

[0035] FIG. 4 is a figure showing an electrode of a BGA of Example 2 in Table 1.

[0036] FIG. 5 is a figure showing an electrode of a BGA of Comparative Example 6 of Table 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] A solder ball according to the present invention which does not develop fusion defects and which has both excellent resistance to thermal fatigue and excellent resistance to drop impacts is preferably used for bump formation on a package part such as a BGA having electrodes on its lower surface.

[0038] If the Ag content of a Sn—Ag—Cu—Ni based solder alloy of a solder ball according to the present invention is less than 1.6 mass %, the wettability of the solder ball decreases. As a result, wettability to solder paste decreases and fusibility worsens, and fusion defects easily occur. In addition, if the Ag content is less than 1.6 mass %, the strength of solder decreases and resistance to thermal fatigue worsens. If the Ag content exceeds 2.9 mass %, the solder ball become hard and resistance to drop impacts worsens. Accordingly, an alloy for a solder ball according to the present invention preferably has an Ag content of 1.6-2.9 mass %. More preferably it is 1.9-2.3 mass %.

[0039] If the Cu content of a Sn—Ag—Cu—Ni based solder alloy of a solder ball according to the present invention is less than 0.7 mass %, the composition moves away from the eutectic point of Sn—Ag—Cu, so when the solder ball is used with a Cu electrode, Cu from the Cu electrode diffuses into the solder. As a result, a Cu6Sn5 intermetallic compound layer becomes thick at the interface with the Cu electrode, and resistance to drop impacts worsens.

[0040] If the Cu content of a Sn—Ag—Cu—Ni based solder alloy exceeds 0.8 mass %, the amount of intermetallic compounds of Cu and Ni in the solder ball increases so that intermetallic compounds of Cu and Ni precipitate on the surface of the solder balls, thereby increasing the occurrence of fusion defects. In addition, if the Cu content exceeds 0.8 mass %, the composition moves away from the eutectic point of Sn—Ag—Cu, so the Cu6Sn5 intermetallic compound easily forms in a reaction layer between the solder alloy and a Cu electrode. As a result, the Cu6Sn5 intermetallic compound which is formed in the solder bonding interface with the Cu electrode becomes thick.

[0041] Accordingly, the Cu content of a Sn—Ag—Cu—Ni based solder alloy of a solder ball according to the present invention which does not develop fusion defects and which has excellent resistance to drop impacts must be 0.7-0.8 mass %.

[0042] If the Ni content of a Sn—Ag—Cu—Ni based solder alloy of a solder ball according to the present invention is less than 0.05 mass %, resistance to drop impacts worsens. In addition, if the Ni content is less than 0.05 mass %, the effect of adding Ni is not obtained. Thus, Ni easily diffuses from a Ni electrode, and it becomes easy for intermetallic compounds to form in the interface. Therefore, the content of Ni in a Sn—Ag—Cu—Ni based solder alloy must be at least 0.05 mass %.

[0043] Similarly, if the Ni content exceeds 0.08 mass %, the amount of intermetallic compounds of Sn, Cu, and Ni in the solder ball increase so that intermetallic compounds of Sn, Cu, and Ni precipitate on the solder ball surface, and the occurrence of fusion defects increases. Furthermore, if the Ni content exceeds 0.08 mass %, the Ni concentration in intermetallic compounds formed in the bonding interface increases leading to a decrease in the bonding strength, and the hardness of solder increases. As a result, peeling at the interface more easily occurs when an impact is applied.

[0044] Therefore, the Ni content in a Sn—Ag—Cu—Ni based solder alloy for a solder ball according to the present invention must be 0.05-0.08 mass %.

[0045] At least one element selected from Fe, Co, and Pt in a total amount of 0.003-0.1 mass % may be further added to a Sn—Ag—Cu—Ni based solder alloy for a solder ball according to the present invention. Addition of the elements Fe, Co, or Pt to an alloy for a solder ball refines an intermetallic compound layer which is formed at the bonding interface and suppresses its thickness, so it has the effect of improving dropping resistance. If the content of elements selected from Fe, Co, and Pt is less than 0.03 mass %, the above-described effect is extremely difficult to obtain, while if they are added in excess of 0.1 mass %, the hardness of solder bumps increases and there is the harmful effect that interfacial peeling due to impacts develops.

[0046] At least one element selected from Bi, In, Sb, P, and Ge in a total amount of 0.003-0.1 mass % may be further added to a Sn—Ag—Cu—Ni based solder alloy for a solder ball according to the present invention. After a solder ball is mounted on a module substrate, it is determined by image recognition whether soldering has taken place. If discoloration of a solder ball such as yellowing occurs, image recognition may determine that a defect is present. Therefore, it is preferable that a solder ball not undergo discoloration during reflow. The addition of Bi, In, Sb, P, or Ge has the effect of preventing discoloration due to heat or the like, whereby errors in quality inspection of bumps can be avoided. If the content of elements selected from Bi, In, Sb, P, and Ge is less than 0.003 mass %, it is extremely difficult to obtain the above-described effect, while if they are added in excess of 0.1 mass %, the hardness of solder bumps increases, and there is a possibility of a decrease in the effect of improving dropping resistance.

[0047] A solder ball according to the present invention is used for electrodes. The diameter of the solder ball is at least 0.1 mm, preferably at least 0.3 mm, and more preferably at least 0.5 mm. In recent years, electronic equipment is becoming increasingly miniaturized, and solder balls mounted on electronic parts continue to become smaller. Solder balls for bonding of flip chips are typically 0.1 mm or smaller, while solder balls like the solder ball of the present invention which are used for electrodes of BGAs and CSPs which have a flip chip housed therein are primarily at least 0.1 mm in size.

Examples

[0048] Solder alloys having the compositions shown in Table 1 were prepared, and they were formed into solder balls having a diameter of 0.3 mm by the gas atomization method. The resulting solder balls were used to evaluate with respect to fusion defects and by a thermal fatigue test and a drop impact test.

TABLE-US-00001 TABLE 1 Number of Number Number cycles in of drops Solder composition (mass %) of fusion thermal in drop Sn Ag Cu Ni Fe Co Pt Bi In Sb P Ge defects fatigue test impact test Remarks Examples 1 Rem 1.6 0.75 0.07 0 1623 141 2 Rem 2.0 0.75 0.07 0 1900 118 3 Rem 2.5 0.75 0.07 0 1971 91 4 Rem 2.9 0.75 0.07 0 2373 66 5 Rem 1.8 0.70 0.05 0 1696 133 6 Rem 2.9 0.80 0.08 0 2296 57 7 Rem 2.5 0.70 0.08 0 2008 88 8 Rem 2.5 0.80 0.05 0 2181 82 9 Rem 2.0 0.75 0.07 0.01 0 1915 135 10 Rem 2.0 0.75 0.07 0.008 0 1935 128 11 Rem 2.0 0.75 0.07 0.05 0 1903 133 12 Rem 2.0 0.75 0.07 0.1 0 1942 115 13 Rem 2.0 0.75 0.07 0.1 0 1928 112 14 Rem 2.0 0.75 0.07 0.07 0 1930 120 15 Rem 2.0 0.75 0.07 0.003 0 1895 117 16 Rem 2.0 0.75 0.07 0.008 0 1890 116 Comparative 1 Rem 3.0 0.50 0 2464 1 examples 2 Rem 1.0 0.50 10 898 1 3 Rem 1.0 0.75 0.07 15 977 156 4 Rem 3.5 0.75 0.07 0 2489 18 5 Rem 2.5 0.60 0.07 0 1965 35 6 Rem 2.5 0.90 0.07 11 1991 29 7 Rem 2.5 0.75 0.03 0 1913 37 8 Rem 2.5 0.75 1.00 15 2156 22 9 Rem 1.2 0.50 0.05 13 1037 1 10 Rem 2.5 0.50 0.50 17 1972 1 11 Rem 3.0 1.00 0.50 20 2399 1 12 Rem 1.5 0.50 0.50 18 1244 1 Pat. Doc. 3 13 Rem 2.0 1.00 0.01 14 1890 1 14 Rem 1.5 0.65 0.07 10 1102 39 15 Rem 3.0 0.50 0.05 0 2406 1 16 Rem 2.0 0.70 0.15 16 1876 1 17 Rem 1.8 1.50 0.15 28 1650 1

[0049] 1. The number of occurrences of fusion defects was evaluated by the following procedure. Solder balls which were made using each composition were subjected to aging treatment at a temperature of 110° C. and a relative humidity of 85% for 24 hours. A solder paste was printed on a glass epoxy substrate (FR-4) measuring 36×50 mm and having a thickness of 1.2 mm with the electrode pattern of the substrate, and the solder balls which had undergone the aging treatment were mounted on the electrodes and subjected to reflow at a temperature of at least 220° C. for 40 seconds with a peak temperature of 245° C. The number of fusion defects occurring between the solder balls and the solder paste was counted using a stereomicroscope.

[0050] 2. Next, a thermal fatigue test and a drop impact test were carried out in the following manner. The solder balls of each composition which were made were used to carry out reflow soldering to a module substrate for a CSP measuring 12×12 mm using a WF-6400 flux manufactured by Senju Metal Industry Co., Ltd., thereby fabricating a CSP using each solder composition for electrodes.

[0051] 3. A solder paste was printed on a glass epoxy substrate (FR-4) measuring 30×120 mm and having a thickness of 0.8 mm with the electrode pattern of the substrate, and the CSP fabricated in Step 2 was mounted on the substrate and subjected to reflow at a temperature of 220° C. or above for 40 seconds with a peak temperature of 245° C. to fabricate a substrate for evaluation.

[0052] 4. The thermal fatigue test was carried out under the following conditions. Using the substrate for evaluation fabricated in Step 3, the resistance was continuously measured in a series circuit while a thermal load consisting of −40° C. for 10 minutes and +125° C. for 10 minutes was repeatedly applied using a Model TSA-101LA thermal shock chamber manufactured by ESPEC Corporation. It was determined that failure had taken place when the resistance exceeded 15 ohms, and the number of thermal fatigue cycles before the failure was recorded.

[0053] 5. A drop impact test was carried out under the following conditions using a substrate for evaluation like one used for the thermal fatigue test. The test method comprised securing both ends of the substrate for evaluation at a position 10 mm above a base using a special jig. In accordance with JEDEC specifications, an impact with an acceleration of 1500 G was repeatedly applied. It was determined that failure occurred when the resistance increased to 1.5 times the initial resistance, and the number of drops before the failure was recorded.

[0054] In Example 2 in Table 1, the contents of Ag, Cu, and Ni were all in suitable ranges, so the results with respect to fusion defects, thermal fatigue resistance, and drop impact resistance were all excellent. FIG. 4 shows the layer of a compound in the bonding interface in Example 2. It can be seen that a thin Cu6Sn5 intermetallic compound layer was formed in the bonding portion between a BGA electrode 9 and a solder ball 5.

[0055] In the case of the solder ball alloy compositions of Comparative Examples 1, 4, and 11 having an Ag content exceeding 2.9 mass %, although effects of improving resistance to thermal fatigue and fusion defects were exhibited, the Ag content was not optimal in order to obtain drop impact resistance. Thus, the number of drops was less than 20, and a sufficient improvement was not obtained.

[0056] For Comparative Examples 2, 3, 9, and 10, the Ag content was less than 1.6 mass %, so they had poor resistance to thermal fatigue and the number of cycles did not reach 1500. Due to a decrease in wettability caused by insufficient Ag content, there were more than 10 fusion defects, and there was no effect of suppressing fusion defects.

[0057] In Comparative Examples 5, 6, 7, and 8, although a suitable Ag content was selected, the contents of Cu and Ni were not optimized, so the effects of improving both fusion defects and resistance to drop impacts were not obtained. FIG. 5 shows a layer of compounds in the bonding interface for Comparative Example 6. It can be seen that a thick Cu6Sn5 intermetallic compound layer was formed.

[0058] It can be concluded that a solder composition consisting essentially of 1.6-2.9 mass % of Ag, 0.7-0.8 mass % of Cu, 0.05-0.08 mass % of Ni, and remainder of Sn provides a solder alloy which suppresses the occurrence of fusion defects and which has both resistance to thermal fatigue and resistance to drop impacts.

INDUSTRIAL APPLICABILITY

[0059] According to the present invention, a solder ball for electrodes is provided which has the effect of suppressing the occurrence of fusion defects and which has excellent resistance to thermal fatigue as well as excellent resistance to drop impacts when used either with Cu electrodes (Cu—OSP electrodes, i.e., ones which are coated with a water-soluble flux atop Cu) and with Ni electrodes (electrolytic Ni/Au electrodes or electroless Ni/Pd/Au electrodes). Suppressing fusion defects is associated with a decrease in the occurrence of initial failures in a manufacturing process. Until now, it was necessary to select a composition in accordance with the properties demanded of products. However, because a solder ball according to the present invention has resistance to both drop impacts and thermal fatigue, it is possible to apply the solder ball to a wide range of fields from portable devices to personal computers and vehicle mounted equipment and to the new field of mobile personal computers which is rapidly developing.

EXPLANATION OF SYMBOLS

[0060] 1 BGA part [0061] 2 mounting substrate [0062] 3 fusion of solder bump [0063] 4 fusion defect of solder bump [0064] 5 solder ball after heating for mounting [0065] 6 solder paste after heating for mounting [0066] 7 portion of a fusion defect [0067] 8 fusion-impeding compound [0068] 9 BGA electrode [0069] 10 layer of Cu6Sn5 intermetallic compound as surface treatment.