KR100366131B1 - Lead-free solder with lower melting point and lower dross - Google Patents

Abstract

The present invention relates to a lead-free solder used for soldering electronic components and the like.

The lead-free solder according to the present invention is composed of% by weight, Cu: 0.1-2.0%, In: 0.01-2.0%, P: 0.01-1.5%, Bi: 0.1-6.5%, and the balance Sn.

The Sn-Cu-Bi-In-P-based lead-free solder has a relatively low melting temperature compared to conventional Sn-Ag-based or Sn-Cu-Ag-based lead-free solders, and has less dross during soldering. When soldering electronics, it is possible to reduce the thermal damage to electronic components and to facilitate the soldering operation.

Description

Lead-free solder with lower melting point and lower dross

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to lead-free solders used for soldering electronic components, and more particularly, to lead-free solders having a low melting point and less dross during soldering.

Sn-Pb based solder has been used as the most effective bonding material for electronic devices for a long time. However, recently, when disposing of electronic devices using solder, the lead component contained in the solder is eluted by acid rain, which contaminates the groundwater, and when absorbed by the human body, it is an environmental pollutant that harms the human body such as decreased intelligence and decreased reproductive function. It is pointed out. Due to these problems, research on lead-free solders has been conducted worldwide in 1990 in the United States with the examination of lead regulation in electric and electronic solders.

Typical lead-free solders include Sn-8.8Zn (melting point 199 ° C), Sn-3.5Ag (melting point 221 ° C), Sn-0.75Cu (melting point 227 ° C), and the like. Of these alloys, Sn-8.8Zn-based solders are very poor in wettability and are not practical. On the other hand, Sn-3.5Ag and Sn-0.7Cu alloys have excellent wettability and reliability among lead-free solders, but their biggest disadvantage is that the melting point is 30-40 ° C higher than the typical Sn-37Pb solder (melting point 183 ° C). Generally, the soldering temperature for electronic components is about 50 ° C above the melting point of the solder. In addition, since electronic components have a heat resistance limit, when solders having a high melting point are used, the soldering temperature is high, which causes thermal damage to the electronic components, making it difficult to use them.

Recently, in order to lower the melting point of lead-free solder even a little, lead-free solder mainly on Sn-4.7Ag-1.7Cu ternary process was developed at Iowa University (US 5,527,628) and the like. However, this lead-free solder also has a melting point around 217 ° C, which is still about 30 ° C higher than the melting point of 183 ° C of Sn-37Pb. In addition, Japan's Senju / Matsushida holds a patent for Sn-Ag-Cu-Sb-based lead-free solders (Japanese Patent Laid-Open No. 5-50286). He holds a patent (No. 168,964). However, the melting point of these lead-free solders is still about 20-30 ° C. or more higher than that of Sn-37Pb.

Since the conventional lead-free solder has a high melting point and low wettability, current soldering production equipment causes many problems when soldering and assembling electronic products. This can be expensive because the existing soldering equipment must be replaced with lead-free soldering equipment. In particular, conventional lead-free solders generate a large amount of dross (a main component, SnO), a solder oxide known as a molten solder residue when soldering an electronic component, oxidizing the solder and incorporating it into a soldering portion, causing solder failure.

The present invention actively suppresses the occurrence of dross in the process of soldering compared to the conventional Sn-Ag-based or Sn-Cu-Ag-based lead-free solders in order to facilitate thermal damage and soldering of electronic components when soldering semiconductors or electronic products. The purpose is to provide a Sn-Cu-Bi-In-P-based lead-free solder.

In addition, the present invention provides a new Sn-Cu-Bi-In-P lead-free solder having a lower melting point than many existing lead-free solders such as Sn-Ag, Sn-Ag-Cu-Sb, Sn-Ag-Cu, etc. The purpose is to provide.

1 is a graph of the wettability result for the lead-free solder of the present invention.

2 is a graph showing the relationship between the heating temperature and the heat amount for the lead-free solder of the present invention.

Figure 3 is a photograph showing the bridge test results for the present invention and conventional lead-free solder.

4 is a surface texture photograph of a soldered joint using lead-free solder of the present invention.

Lead-free solder of the present invention for achieving the above object, by weight%, Cu: 0.1 ~ 2.0%, In: 0.01 ~ 2.0%, P: 0.01 ~ 1.5%, Bi: 0.1 ~ 6.5%, the composition containing the remaining Sn do.

Hereinafter, the lead-free solder according to the present invention will be described in detail.

First, Cu contained in the lead-free solder according to the present invention serves to lower the melting point of the solder and to improve thermal fatigue characteristics. In order to achieve this effect, Cu should be added at least about 0.1%, but if it is too large, the melting point of the solder is increased, which is undesirable, so the upper limit thereof is limited to 2.0%. More preferably, the Cu content is in the range of about 0.1 to 0.75%.

In is effective in reducing the melting point of the solder and balancing the elongation and strength of the Bi-containing solder. In order to achieve this effect, the In content should be added at least 0.01%. However, if the content of In is too high, it becomes a factor of the price increase and the competitiveness is low and the amount of dross generated is not preferable, so it is preferable to limit it to about 2.0% or less.

P acts to suppress the formation of dross (the main component is SnO), which is a solder oxide known as the molten solder residue during soldering. Dross consumes the solder by oxidizing the solder when soldering the electronic component to the substrate with the molten solder, and when soldered into the soldering portion of the electronic component may cause solder failure. Therefore, dross is an impurity that must always be removed from the solder bath when soldering. P contained in the lead-free solder of the present invention is first oxidized to P ₂ O ₅ , which is a stable oxide because of less oxide generation free energy (ΔG ^o ) than Sn, Cu, and In, thereby inhibiting oxidation of other elements. As a result, P suppresses the generation of Sn oxide, which constitutes most of the solder, thereby minimizing dross generation. In addition, when P suppresses the production of Sn oxide, the flowability of the molten solder is increased, thereby improving the wettability or spreadability of the solder and improving the bridge performance. As a result, P can suppress the reaction layer growth of the soldered part and can prevent the fall of the strength of a junction part. For this effect, P is preferably added at least about 0.01% or more. However, if the content of P exceeds 1.5%, the soldering portion is cured and brittleness is generated, which is not preferable.

If Bi is added in addition to P, the melting point of the solder can be lowered, and in particular, the wettability of the solder can be improved. For this effect, the Bi content is preferably added at least 0.1%. However, although the eutectic composition of Sn and Bi has been put into practical use as Sn-58% Bi, the addition of Bi lowers the elongation of the solder and lowers the aging resistance, so in the present invention, the Bi content is preferably limited to 6.5% or less. Do.

Hereinafter, the lead-free solder of the present invention will be described in detail by way of examples.

EXAMPLE

Lead-free solders having various compositions were prepared as shown in Table 1 below, and each of the lead-free solders was subjected to wettability, melting point, shear strength, dross generation amount, bridge test, and solder test.

division Chemical composition (% by weight) Remarks Bi Cu In P Ag Sn Invention 1 6 0.8 1.0 0.2 - Balance Invention 2 1.7 0.8 0.15 0.11 - Balance Conventional Materials 1 - - - - 3.5 Balance Conventional material 2 - 0.7 - - 3.5 Balance

Wetting test

Wetting tests were performed by applying two fluxes of VOC free and SSF to the lead-free solder of the inventive material (1) and Table 2, respectively. At this time, the temperature of the solder molten metal was 235-255 degreeC. The results are shown in FIG.

As shown in FIG. 1, the lead-free solder of the present invention exhibited good wettability at about 240 ° C., especially when SSF flux was applied than when VOC free flux was applied.

Melting point

The lead-free solder according to the invention material (1) (2) of Table 1 was subjected to differential scanning calorimetry (DSC) analysis to confirm each melting point. 2 shows the result.

As can be seen from FIG. 2, the lead-free solder of the present invention has a melting point of about 4 ° C. lower than that of a conventional representative lead-free solder (traditional material 1). In addition, the melting point is comparable to or slightly lower than that of the conventional Sn-Ag-Cu series, which is the most representative lead-free solder, having the lowest melting point. However, since Ag is not included, the price is low. Moreover, the lead-free solder of the present invention has the advantage that the melting point is lower than about 10 ℃ compared to Sn-0.2Ag-2Cu-0.85Sb (melting range 226 ~ 228 ℃) of the U.S. Kester, which dramatically reduced the Ag content. Accordingly, when using the lead-free solder of the present invention, the soldering temperature may be lowered to about 240 ° C., so that soldering may be performed at a temperature about 10 ° C. lower than that of the conventional lead-free solder.

Shear strength

A multilayer ceramic capacitor (MLCC) having a size of 3.1 × 1.5 × 0.7 mm is mounted on a copper substrate, and the board mounted with the lead-free solder of the inventive material (1) (2) is rippled at about 240 ° C. After reflow soldering, the shear strength values were examined for each case. At this time, the moving speed of the shear tip was set to 0.2 mm / min.

As a result, when the lead-free solder of Invention Material (1) and (2) was used, the shear strength values were about 1.5 kgf and 2.3 kgf, respectively.

Dross generation

Inventive material (2) and conventional material (2) of Table 1 were melted at about 250 ° C. forcibly stirred, and the amount of dross generated in each solder melt for 1 hour was investigated.

In the case of the conventional material (2), the amount of dross was generated about 0.209g per 1g of lead-free solder, whereas in the case of the invention material (2), only about 0.1246g was generated, which was reduced to about half.

As described above, the dross consumes the solder and should be periodically removed from the solder bath during the soldering operation since the dross degrades the performance of the solder when incorporated into the nip solder. Therefore, manpower and time will be required for this.

Bridge test

The bridge test was done about the lead-free solder of the invention material 2 of Table 1, and the prior art material 2 most conventionally recognized. The bridge test is a test to investigate the degree of short circuit when soldering between terminal leads of minute electronic components. It is one of the important failure tests because electronic products cannot be used when a short circuit occurs. Solder with a low bridge generation rate is a good material because there is less short circuit between terminals.

3A and 3B show the lead-free solder of the invention material 2 having a pitch spacing of 0.6 mm and 1.3 mm, respectively, and FIGS. 3C and 3D show the conventional material 2 having a pitch spacing of 0.6 mm and 1.3 mm, respectively. In the case of lead-free solder.

As a result of this bridge test, as shown in Fig. 3, both the lead-free solder of the invention material 2 and the prior art material 2 showed excellent performance without the occurrence of bridge at the pitch between 0.6 mm and 1.3 mm.

Soldering Experiment

The lead of MLCC was reflow soldered at about 240 degreeC on Cu board | substrate using the lead-free solder of invention material (1) (2). At this time, SSF was used as the flux. The surface structure of the solder joint thus soldered was observed, and the results are shown in FIG.

As shown in FIG. 4, as a result of soldering using the lead-free solder of the present invention, it was confirmed that soldering was performed very well even at about 230 to 250 ° C., which is almost the same solder temperature as the conventional Sn-Pb solder.

As described above, the Sn-Cu-Bi-In-P-based lead-free solder of the present invention has less dross in the soldering process than the conventional Sn-Ag-based or Sn-Cu-Ag-based lead-free solders, so that the soldering operation is smoothly performed. can do. In addition, the lead-free solder of the present invention has a relatively low melting temperature, so that the thermal damage of electronic components is small when soldering a semiconductor or an electronic product.

Claims (1)

Low melting point lead-free solder composed of Cu: 0.1% to 2.0%, In: 0.01% to 2.0%, P: 0.01% to 1.5%, Bi: 0.1% to 6.5%, and residual amount of Sn.