HK1053278B - A lead-free solder, method for preparing the same and method of soldering by using the same. - Google Patents

Description

Lead-free solder, method for producing the same, and soldering method using the same

Technical Field

The present invention relates to solders, and more particularly to substantially lead-free solders.

Background

Many conventional solders contain lead as its major component. Such solders often have desirable physical properties and the use of lead-containing solders is widely used in several industries, including those related to the production of printed circuit boards. For example, a solder containing 63% tin and 37% lead is typically used in wave soldering production.

However, the demand for lead-free solders is increasing due to, for example, environmental considerations, and it appears that in the next few years, the use of solders containing little or no lead in the production of many articles will be regulated by law in some countries.

Some previous attempts to develop lead-free solders have not been entirely successful. Conventional lead-free solders typically have undesirable physical properties including poor wetting, low flow, poor compatibility with existing component coatings, and excessive amounts of dross. One particular problem that has been recognized in lead-free solder applications is the occurrence of fillet mounding (biting), which in a printed circuit board tends to separate from the underlying material (e.g., nickel/gold coating) at the edges of through-hole vias through the board. Another problem is that lead-free solders tend to have a high dissolution rate for copper, so that copper dissolves into the lead-free solder from components and circuit boards in contact with the solder.

Thus, some manufacturers have come to appreciate that existing brazing processes that have been practiced effectively for many years now must be well suited for use with lead-free solders. Furthermore, existing materials used in printed circuit board production may have to be replaced so as not to conflict with the use of lead-free solder. The use of such methods and materials is widely recognized as a poor use of resources, particularly since, as noted above, the standards for products made with known lead-free solders are generally much lower than those achievable with products made with conventional lead-containing solders.

Disclosure of Invention

It is an object of the present invention to provide a lead-free solder which can be used to varying degrees as a direct replacement for conventional lead-containing solders.

Thus according to one aspect of the present invention there is provided a substantially lead-free solder comprising: 88.5% to 93.2% tin; 3.5% to 4.5% silver; 2.0% to 6.0% indium; and 0.3% to 1.0% copper.

The solder of the present invention may also include an antioxidant or anti-skinning additive, such as phosphorus or other non-metallic compounds or elements, to 0.5%.

In a preferred embodiment, the solder comprises 91.3% tin, 4.2% silver, 4.0% indium and 0.5% copper.

In another preferred embodiment, the solder comprises 91.39% tin, 4.1% silver, 4.0% indium and 0.5% copper and 0.01% phosphorus.

According to another aspect of the present invention there is provided a method of making a substantially lead-free solder, the method comprising the steps of mixing tin, silver, indium and copper: the proportion of tin in the total solder is from 88.5% to 93.2%; the proportion of silver in the whole solder is from 3.5% to 4.5%; the proportion of indium in the whole solder is from 2.0% to 6.0%; and the proportion of copper in the entire solder is from 0.3% to 1.0%.

A method of making a solder according to the present invention may include mixing less than 0.5% of an antioxidant or anti-skinning additive into the solder mixture.

A preferred method of making the solder of the present invention comprises the steps of mixing tin, silver, indium and copper: the proportion of tin in the whole solder is 91.3%; the proportion of silver in the whole solder is 4.2%; the proportion of indium in the whole solder is 4.0 percent; and the proportion of copper in the entire solder is 0.5%.

Another preferred method of making the solder of the present invention comprises the steps of mixing tin, silver, indium, copper and phosphorus: the proportion of tin in the total solder was 91.39%; the proportion of silver in the whole solder is 4.1%; the proportion of indium in the whole solder is 4.0 percent; the proportion of copper in the whole solder is 0.5%; and the proportion of phosphorus in the entire solder is 0.01%.

According to a further aspect of the invention there is provided a method of soldering comprising the step of employing a substantially lead-free solder comprising: from 88.5% to 93.5% tin; from 3.5% to 4.5% silver; from 2.0% to 6.0% indium; and from 0.3% to 1.0% copper.

Preferably, the brazing method comprises the step of using a solder comprising 91.3% tin, 4.2% silver, 4.0% indium and 0.5% copper.

Advantageously, the brazing method comprises the step of using a solder comprising 91.39% tin, 4.1% silver, 4.0% indium and 0.5% copper and 0.01% phosphorus.

Advantageously, the brazing method comprises a wave brazing step using a substantially lead-free solder.

Drawings

In order that the invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a table of wetting times (in seconds) at various temperatures for various selected solders, including the solder of the present invention;

FIG. 2 is a graph showing the data in the table of FIG. 1;

FIG. 3 is a table of maximum wetting forces at various temperatures for various different solders selected, including the solder of the present invention;

FIG. 4 is a graph showing data represented in the table of FIG. 3;

FIG. 5 is a table showing physical properties including the thermal expansion coefficients of selected various solders (including the solder of the present invention);

FIG. 6 is a graph illustrating thermal expansion data represented in the table of FIG. 5;

FIG. 7 is a table of mechanical properties including tensile strength and yield strength for various selected solders, including the solder of the present invention;

FIG. 8 is a graph showing tensile strength and yield strength data represented in the table of FIG. 7;

FIG. 9 is a table of results obtained in fillet bump testing of various selected lead-free solders, including the solder of the present invention;

FIGS. 10A and 10B are two pairs of photomicrographs at two different scales showing the fillet weld of the solder of the present invention adhered to the nickel/gold and OSP coatings (polymer coatings on copper substrates), respectively;

FIG. 11 is a chart showing the dissolution rate of copper into various different types of solders (including the solder of the present invention);

FIG. 12 is a graph showing data represented in the table of FIG. 11; and

fig. 13 is a table showing the dross levels exhibited by various solders, including the solder of the present invention.

Detailed Description

As described above, conventional lead-free solders have various disadvantages when compared to conventional lead-containing solders, including poor wettability, low fluidity, poor compatibility with existing component coatings, fillet mounding, high copper dissolution rate, and excessive dross.

However, it has been found that solders of the present invention that are composed of lead-free alloys (comprising 88.5% to 93.2% tin; 3.5% to 4.5% silver; 2.0% to 6.0% indium; 0.3% to 1.0% copper and less than 0.5% of anti-oxidants or anti-skinning additives such as phosphorus or other non-metallic compounds or elements) have greatly improved performance over existing lead-free solders. Of course, the properties of the solder of the present invention comparable to conventional lead-containing solders are wettability, flowability, compatibility with existing component coatings, fillet mounding, copper dissolution rate, and dross.

In order to demonstrate the advantageous properties of the solder of the present invention, five experiments were performed, which will be described below. These tests were conducted with the preferred embodiment of the solder of the present invention, referred to herein as ALLOY349 (ALLOY 349) which includes 91.39% tin, 4.2% silver, 4.0% indium, 0.5% copper and 0.01% phosphorus.

Test 1: wettability

The first test was a test relating to the wettability of the solder samples of the present invention compared with the wettability of some known solder samples, namely 8 existing lead-free solders and a conventional lead-containing solder.

Nine known solders are as follows:

1. lead-containing solder with the components of 63% of Sn and 37% of Pb.

2. Lead-free solder with the components of 99.3% of Sn and 0.7% of Cu.

3. A second lead-free solder with the composition of 96.5% Sn, 3.5% Ag.

4. A third lead-free solder (referred to herein as VIROMET 217) having a composition of 88.3% Sn, 3.2% Ag, 4.5% Bi, and 4.0% In.

5. A fourth lead-free solder (referred to herein as VIROMET 411) having a composition of 92% Sn, 2% Cu, 3% Ag, and 3% Bi.

6. A fifth lead-free solder (referred to herein as VIROMET 513) with a composition of 92.8% Sn, 0.7% Cu, 0.5% Ga and 6% In.

7. A sixth lead-free solder having a composition of 93.5% Sn, 3.5% Ag, and 3.0% Bi.

8. A seventh lead-free solder having a composition of 95.5% Sn, 4.0% Ag and 0.5% Cu.

9. An eighth lead-free solder having a composition of 96.0% Sn, 2.5% Ag, 1.0% Bi and 0.5% Cu.

The first aspect of the first test involves measuring solder wetting time under a variety of different temperature conditions ranging from 235 ℃ to 265 ℃ according to the ANSI/J Std-003 standard. In this test, a copper sample is immersed in a quantity of each molten solder. A sensitive force measuring device is attached to the copper sample and is arranged such that the perpendicular force on the sample can be measured and recorded.

The change in vertical force on the copper sample during immersion of the copper sample into molten solder is due to two major factors. The first factor is buoyancy, i.e. the buoyancy resulting from the upward force on the sample due to the expulsion of solder, which is equal to the weight of the solder expelled by the sample. Since the volume of the sample portion immersed in the solder and the density of the solder are known, the upward buoyancy can be calculated and taken into account.

The second factor is the force acting on the sample due to the change in the contact angle between the solder surface and the sample surface. In each particular case, the wetting time is defined as the time during which the wetting force acting on the sample equals zero.

The results of the first aspect of the first test are shown in figure 1. In short, the solder of the present invention has a wetting time at each temperature that is comparable to that of conventional leaded solders. In addition, the wetting time of the solder of the present invention is generally lower than the wetting time of any other lead-free solder. The wetting time is a measure of the speed at which solder adheres to a substance and it is clear that a short wetting time is a desirable characteristic of solder. It can thus be seen that the solder of the present invention has better overall results in the first aspect of the first test than other existing lead-free solders.

The results of the first aspect of the first test are shown in graphical form in figure 2. From this curve, it can be seen that the results showing the properties of the conventional lead-containing solder and those showing the properties of the solder of the present invention are very close to each other, as compared with those showing the properties of other lead-free solders.

A second aspect of the first test includes the measurement of the maximum wetting force 2 seconds after immersion of the sample in the respective solder. As mentioned above, the wetting force is the adhesion between the solder and the sample. It is clear that wetting forces provide a useful strength guide for solder adhesion to a substrate, and that high wetting forces are desirable properties for solder.

The results of the second aspect of the first experiment are shown in figure 3. Summarizing these results, it can be seen that the solder of the present invention exhibits the greatest wetting force at the 2 seconds after immersion of the sample, although somewhat lower, than that exhibited by conventional lead-containing solders at each temperature tested. While some existing lead-free solders show wetting forces close to those of lead solder at some temperatures, only VIROMET 217 has somewhat better overall results, and the solder of the present invention shows wetting forces close to those of conventional lead-containing solders at all tested temperatures. The properties of the solder of the present invention allow it to be used in a similar manner to conventional lead-containing solders under a variety of temperature conditions, or to be soldered under a variety of temperature conditions.

The results of the second aspect of the first test of the present invention are shown in the form of a curve in fig. 4, which clearly shows that the test results of the solder of the present invention follow the test results of conventional lead-containing solders at least more closely than the test results of other lead-free solders.

As can be seen from the results of the first test, the solder of the present invention exhibited characteristics very similar to those of the conventional lead-containing solder with respect to wettability. It is apparent that similar physical properties make the solder of the present invention suitable as a replacement for conventional leaded solders.

Test 2: mechanical Properties

The second test compares the mechanical properties of the solder of the present invention with those of conventional lead-containing solders. In this second test, various mechanical property tests were conducted in accordance with ASTM standards to compare the performance of the solder ALLOY349 of the present invention (ALLOY 349) with conventional lead-containing solders having a composition of 63% Sn/37% Pb and seven other existing lead-free solders having the following compositions:

1. first lead-free solder: 99.3% Sn; 0.7% Cu.

2. Second lead-free solder: 96.5% Sn; 3.5% Ag.

3. Third lead-free solder (referred to herein as VIROMET 217): 88.3% Sn; 3.2% Ag; 4.5% Bi and 4.0% In.

4. Fourth lead-free solder (referred to herein as VIROMET HF): 92.8% Sn; 0.7% Cu; 0.5% Ga and 6% In.

5. Fifth lead-free solder: 93.5% Sn; 3.5% Ag and 3.0% Bi.

6. A sixth lead-free solder: 95.5% Sn; 4.0% Ag and 0.5% Cu.

7. A seventh lead-free solder: 96% Sn; 2.5% Ag; 0.5% Cu and 1.0% Bi.

The first aspect of the second test involves determining the melting temperature, Coefficient of Thermal Expansion (CTE) and specific gravity of the solder under test. The results of the first aspect of the second experiment are tabulated in fig. 5 and represented in the form of a curve in fig. 6.

As can be seen from the tables and graphs, the tests demonstrated that the ALLOY349 solder of the present invention has a coefficient of thermal expansion very close to that of conventional leaded solders, so that the concern about incompatibility between the solder of the present invention and existing parts and boards is greatly reduced.

The second aspect of the second test involves measuring the tensile strength, load at maximum load, yield strength and Young's Modulus (Young's module) of each solder. The results of these tests are all shown in the table shown in fig. 7, while the curves of fig. 8 show the tensile strength and yield strength of each alloy.

As can be seen in FIGS. 7 and 8, the results of these tests indicate that the ALLOY349 solder of the present invention has superior strength and Young's Modulus (Young's Modulus) compared to conventional leaded solders, thereby indicating that the fillet joint strength formed from the ALLOY of the present invention can be significantly higher than the joint strength formed from conventional leaded solders.

Test 3: fillet weld bulge

The increasing use of lead-free solders in various industries has shown a tendency for fillet mounding when using lead-free solders in circuit board vias of printed circuit boards employing OSP and Ni/Au coatings.

In a third test, the selected lead-free solders, i.e., ALLOY349 solder of the present invention and the following six conventional lead-free solders, were subjected to such a test for the occurrence of fillet humping defects:

1. first lead-free solder: VIROMET 217.

2. Second lead-free solder: 92.3% Sn; 3.2% Ag; 0.5% Bi and 4.0% In.

3. Third lead-free solder: 89.8% Sn; 3.2% Ag; 1.0% Bi and 6.0% In.

4. Fourth lead-free solder: 88.8% Sn; 3.2% Ag; 2.0% Bi and 6.0% In.

5. Fifth lead-free solder: 94.5% Sn; 4.0% Ag; 0.5% Cu and 1.0% Bi.

6. A sixth lead-free solder: 96.5% Sn; 3.5% Ag.

The results of the third test are shown in fig. 9, 10A and 10B, and fig. 9 shows the results in a list. FIGS. 10A and 10B show micrographs of two different scales of fillet joints formed on Ni/Au and OSP coatings, respectively, using ALLOY349 solder of the present invention. These results clearly show that the solder of the present invention can eliminate the fillet ridge defect formed in the Ni/Au and OSP coated vias in printed circuit boards.

Test 4: copper dissolution rate

The fourth test was conducted to compare the dissolution rates of copper in the lead-free solder of the present invention, a conventional lead-containing solder (63% Sn/37% Pb), and three other prior lead-free solders:

1. first lead-free solder: VIROMET 217.

2. Second lead-free solder: 99.3% Sn; 0.7% Cu.

3. Third lead-free solder: 95.5% Sn; 4.0% Ag; 0.5% Cu.

The test was performed by dipping a copper plate with a known weight of flux into the molten solder, followed by measuring the copper concentration in the solder using an inductively coupled plasma. The copper dissolution rate was then calculated from the ratio of the measured copper concentration in the solder to the weight of copper immersed in the solder.

The results of the fourth test are shown in fig. 11 and 12, which show the test results in a tabular and a graphical manner, respectively. As can be seen in fig. 11 and 12, copper has a slightly higher dissolution rate in the solder of the present invention than in the conventional lead-containing solder, but the solder of the present invention was also found to be one of the lowest copper dissolution rates among them in the lead-free solder subjected to the test.

Test 5: skimming

The fifth test was a suitability test for use with the solder of the present invention in a wave soldering machine. In one embodiment of wave soldering, the circuit board is held just above a number of molten solder surfaces within the barrel. A solder wave having a sufficiently high amplitude is then caused to propagate through the surface of the molten solder, with the wave crest contacting the surface of the circuit board. The wave is as wide as the circuit board (or as wide as the portion of the circuit board to be soldered) and as the solder wave passes across the surface of the molten solder, all components on the lower surface of the circuit board come into contact with the molten solder.

When using existing lead-free solders, it has been found in some cases that the level of slag present in the barrel is unacceptably high after several uses.

A fifth test was conducted to determine the degree of dross when ALLOY349 of the invention was used, and compared to the dissolution rates in conventional leaded solder (63% Sn/37% Pb) and three other prior lead-free solders:

1. first lead-free solder: VIROMET 217.

2. Second lead-free solder: 99.3% Sn; 0.7% Cu.

3. Third lead-free solder: 95.5% Sn; 4.0% Ag; 0.5% Cu.

In this test, the solder being tested was used in a molten solder bucket in a simulated conventional wave soldering machine. The solder machine is not changed when different solders are used and the circuit board is soldered using a wave solder machine in the same way as a conventional tin/lead solder. The wave soldering machine was operated at a barrel temperature of 245 c and normal atmospheric conditions to transport the circuit boards over the solder barrel surface at a speed of 1.4 to 1.8 m/min. At the end of each four consecutive 15 minute operating periods, the slag in the barrel was removed and the weight of the slag was weighed to determine the amount of slag produced during the wave soldering process during each period. The slag weights are then summed to give a measure of slag productivity per hour. The results of the fifth test are tabulated in fig. 13, which clearly shows that the solder of the present invention produces less dross than all other lead-free solders except one, and less dross than found in conventional lead-containing solders.

It can be appreciated from the above results that the present invention provides a lead-free solder that is well suited for use as a direct replacement for conventional lead-containing solders due to the comparable coating wettability, flowability, and compatibility, fillet mounding and dross characteristics of existing components exhibited by the solders of the present invention.

Therefore, the need for manufacturers to replace existing equipment, processes, or component coatings to accommodate the use of lead-free solders can be eliminated or substantially reduced by using the solders of the present invention. As a result, the process of converting the manufacturer's equipment to equipment using lead-free solder is greatly simplified and more economically viable than has been attempted heretofore.

The word "comprising" in the present specification means "comprising or consisting of … …".

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the function of the invention, or a method or process for attaining the result of the invention, are merely illustrative of the invention disclosed and it will be appreciated by those skilled in the art that such features, taken alone or in combination, may be utilized in various ways to achieve the same.

Claims (7)

1. A lead-free solder comprising 91.3% tin, 4.2% silver, 4.0% indium and 0.5% copper.

2. A method of making a lead-free solder comprising the step of mixing tin, silver, indium and copper such that the solder comprises 91.3% tin, 4.2% silver, 4.0% indium and 0.5% copper.

3. A soldering method comprising using a lead-free solder comprising 91.3% tin, 4.2% silver, 4.0% indium and 0.5% copper.

4. A lead-free solder comprising 91.39% tin, 4.1% silver, 4.0% indium, 0.5% copper and 0.01% phosphorus.

5. A method of making a lead-free solder comprising the step of mixing tin, silver, indium and copper such that the solder comprises 91.39% tin, 4.1% silver, 4.0% indium, 0.5% copper and 0.01% phosphorus.

6. A soldering method comprising using a lead-free solder comprising 91.39% tin, 4.1% silver, 4.0% indium, 0.5% copper and 0.01% phosphorus.

7. A method as claimed in claim 3 or 6, including the step of soldering the corrugations using a lead-free solder.