WO2005099961A1

WO2005099961A1 - Lead-free, bismuth-free solder alloy powders and pastes and methods of production thereof

Info

Publication number: WO2005099961A1
Application number: PCT/GB2005/001460
Authority: WO
Inventors: Hector Andrew Hamilton Steen; Gavin John Jackson
Original assignee: Henkel Loctite Adhesives Limited
Priority date: 2004-04-15
Filing date: 2005-04-15
Publication date: 2005-10-27
Also published as: GB2413565A; GB0408461D0

Abstract

The invention relates to a Pb-free, Bi-free solder alloy powder mixture made of two powders: a first higher melting point Sn or Sn alloy powder or Sn powder; and a second lower melting point Sn alloy powder or Sn powder, which melt to form a final solder alloy containing 0.1-5 % Cu and/or 0.1-10 % Ag and/or 0.1-5 % Sb, the remainder being Sn. The power mixture gives reduced tombstoning tendency due to its melting range. The invention also relates to a method of solder alloy powder production giving reduced tendency to tombstoning in attachment of a chip component to a printed circuit board, which comprises mixing a first Pb-free, Bi-free solder alloy powder and a second Pb-free, Bi-free alloy powder, which has a lower melting point than the first solder alloy powder, the two being meltable on soldering to produce a final alloy containing 0.1-5 % Cu and/or 0.1-10 % Ag and/or 0.1-5 % Sb, the remainder being Sn.

Description

LEAD-FREE, BISMUTH-FREE SOLDER ALLOY POWDERS AND PASTES AND METHODS OF PRODUCTION THEREOF

This invention relates to lead-free solder alloy powders and pastes for soldering components, predominantly to printed circuit boards (PCB's), with the aim of minimising the occurrence of tombstoning defects.

Tombstoning is a defect that affects discrete chip components reflow soldered using solder paste. It is caused by a difference in when the solder paste at opposite ends of a component melt in a reflow furnace. It occurs when the paste at one end (termination) of the chip melts and wets that termination before the termination at other end of the chip. The surface tension forces that develop when solder melts and wets the termination act to pull the chip upright, if the other termination is not simultaneously wetted. The result is that the electrical circuit of which the chip is a part is broken. Sometimes partial lifting may occur at one end, or the chip may twist in the horizontal plane, so that electrical contact is not complete.

Tombstoning is driven by differences in speed of wetting between the two ends of the component, and speed of wetting is influenced by a number of factors, including - solderability of the component, flux activity, and temperature differential between the two ends of the component. The forces developed are affected by factors such as component and termination size, alloy surface tension, and pad size and location. For example, small S T components are particularly prone to tombstoning. One approach to reducing the incidence of tombstoning is to choose a solder alloy which melts over a temperature range, and this approach has been used for tin-lead solders, by addition of silver, bismuth and antimony, to increase the melting range. Melting over a temperature range means that the surface tension forces on the component terminations develop more slowly, and the force differential between the two ends of the component is reduced.

Rather than using a single alloy with a melting range, it is possible to use in the paste a mixture of two powders with differing melting points such as tin-lead and tin-bismuth. Combinations of eutectic alloy powders, Sn63Pb37 (melting point 183°C) and Sn62Pb36Ag2 (melting point 179°C) have been used, an advantage being that the resulting alloy after reflow (melting) retains a narrow melting range of 179-183°C. However, due to concern over lead toxicity, leading to legislation restricting the use of lead in solders, lead-free solders are increasingly used in the soldering of components to printed circuit boards. The principal lead-free solders are based on the tin-copper, tin-silver, and tin-silver-copper eutectics, melting at 227C, 221 C and 217C respectively.

Where lead-free solders are concerned, no fully satisfactory anti-tombstoning solution has been proposed.

Patent application EP 1 245 328 A1 represents a very different approach to tombstoning from the initial work using powder mixtures. It covers the use of a paste containing a single lead-free solder alloy powder with a 'twin peak ' melting range. It discloses solders based on tin with 0.2-1.0 % Ag, and further elements potentially chosen from 'strength improving' elements Ni, Co, Mo, Fe, Mn, Cr, 'melting point lowering' elements Bi, In, Zn, and 'oxidation preventing' elements P, Ga, Ge.

The "twin peak" melting point range seems to show some success in combating tombstoning. However, it is noted in the citation that the solidification range should be in the range of about 10-15°C, whereas the examples only show alloys with a smaller range of about 8°, which thus are potentially not well-suited to reliable anti- tombstoning.

Furthermore, the lower temperature peak of heat absorption in a differential scanning calorimeter (DSC) curve is not particularly pronounced and this might not be sufficient to give a different melting behaviour of the solder from that of a single-peak alloy especially when the small melting range is taken into account.

The paper "Conquer Tombstoning in Lead-Free Soldering" by Benith Huang and Ning- Cheng Lee of the Indium Corporation of America, Clinton, NY presented at IPC Printed Circuits Expo describes the use of SnAgCu alloys with various melting ranges to reduce tombstoning, again with a single alloy rather than a mixed powder to achieve the desired composition. One of the alloys tested give twin peaks which are not particularly pronounced, and will therefore be expected to be less effective in reducing tombstoning. The paper also requires alloys in which the composition (Ag content) is significantly different from current standard lead-free solders. Patent application GB 2352200 relates to lead-free solder alloys made from two powders with different melting points having bismuth containing solder alloy powder as the lower melting constituent. Two powders are used in order to maximise the effect of the bismuth in lowering the liquidus temperature. Additionally it has been found advantageous to have the lower melting alloy in the form of a finer powder than the higher melting alloy; this allows the fine lower melting particles to melt first and form a network of liquid around the larger particles. This liquid network enables wetting to take place more readily and initiate reflow, and maximises the effective melting range of minimise tombstoning.

However, lead-free solders containing bismuth have some disadvantages; they are very sensitive to lead contamination, as any lead that may be present due to contamination gives rise to serious reliability issues. Bismuth forms together with lead a lower melting point alloy that can severely impact reliability. The lead may arise from SnPb solderable coating on components even in an otherwise lead-free environment. This is a particular concern, because component manufacturers are often unable to supply a lead-free finish on a particular component type.

As can be seen from the above discussion, increasing the melting range of a solder can inhibit tombstoning. However, a wide freezing range of solidification is undesirable, due to the possibility of hot cracking caused by thermal or other stresses during cooling through the pasty ranges. There is also sometimes a need to keep soldering temperatures low, to protect the delicate electronic components.

Thus, it is desirable to design a solder paste which minimises tombstoning and contamination problems whilst balancing other soldering requirements.

The present invention is defined in the independent claims. Advantageous features are set out in the sub-claims.

According to a first aspect of the invention there is provided a (Pb-free, Bi-free) solder alloy powder made of two powders: a first higher melting point tin (Sn) alloy powder and a second lower melting point Sn alloy powder, which melt to form a final solder alloy containing Sn and at least one of Cu, Ag and Sb. Putting this last phrase in another way, at least one of (and preferably both of) the powders contains Cu and/or Ag and/or Sb with a remainder of tin. According to a further aspect of the invention there is provided a method of solder paste or alloy powder production giving reduced tendency to tombstoning in attachment of a chip component to a printed circuit board, which comprises mixing a first (Pb-free, Bi-free) solder powder and a second (Pb-free, Bi-free) alloy powder, which has a lower melting point than the first solder powder, the two melting on soldering to produce a final alloy containing Sn and at least one of Cu, Ag and Sb, for example 0.1-5% Cu and/or 0.1-10% Ag and/or 0.1-5% Sb, the remainder being Sn.

There is also provided use of a solder paste or powder mixture as described herein in attaching a chip component to a printed circuit board and thereby reducing the incidence of tombstoning.

Sn-Ag-Sb-Cu and Sn-Ag-Cu alloy compositions are well documented as lead-free solder alloys. The concept of combining higher and lower melting point alloy powders to form a mixture which melts over a large temperature range is also well documented. However, the use of a lead-free, bismuth-free alloy made from a powder mixture with a resultant smaller melting range that still inhibits tombstoning is not known, and has not been documented.

As described above we have previously found that solder pastes made with mixtures of Sn63Pb37 powder and Sn62Pb36Ag2 powder, melting at 183°C and 179°C respectively give a significantly reduced tendency to tombstoning as compared to pastes made up with prealloyed powders, when used to solder chip components on printed circuit boards. However, further work lead in the direction of a wider melting range requirement for lead-free solders, with Bismuth employed to widen the range and lower the soldering temperature. It was believed that tombstoning is a more severe phenomenon in the lead-free applications, possibly due to the higher melting temperatures and/or different surface tension characteristics of the tin-based composition, and that a larger melting range was a necessity.

Surprisingly, however we have discovered that the use of a Pb-free, Bi-free powder mixture also has reliable anti-tombstoning effects. Surprisingly, it seems that an inherent advantage of using two powders here is that initial melting is more localised, in channels between the higher melting point constituent particles, so that melting occurs later and over a longer time period, so that melting at opposite terminals overlaps, despite a narrow melting range.

One advantage of the powder mixture is that the melting characteristics of the powder mixture differ from the solidification characteristics once the powder has melted to become an alloy. Thus, surprisingly, the mixture can be anti-tombstoning, but also avoid problems with hot cracking.

Furthermore it is hypothesised that the powder mixture will function as an anti- tombstoning solder better than a single alloy with the twin melting peaks because the heat absorption peaks of the powder mixture have better separation than for the single alloy.

The use of these powder mixtures allows a final solder joint composition closer to current standard SnAgCu lead-free solders to be achieved while at the same time achieving a pronounced twin peak in the melting range.

The dependent claims relate to particularly advantageous alloy compositions and other parameters.

Thus use of a mixture such as a SnAg alloy powder, (e.g. a SnAg3,6 powder), and a

SnAgCu alloy powder, (e.g. as the standard Sn96.5Ag3.8Cu0.7 powder), such specific alloys melting at 221 °C and 217°C respectively, in a lead-free, Bi-free paste, will also have an anti-tombstoning effect as well as enable the paste to reflow at a lower temperature than if prealloyed powder were used. Thus addition of a different powder to the standard powder can be used to inhibit tombstoning. This has advantages in keeping the composition close to the standard, and in fabrication and consumer acceptance.

All percentages expressed herein are on a weight basis. It is noted that unavoidable impurities may be included in the powder mixture up to 0.05%. For example, Cu, Ag and Sn may be included. 0.1 % can be seen as a minimum level of addition for any measurable effect to be seen.

We have further found that the reduction in tombstoning is enhanced if the lower melting alloy is present as a finer size powder than the higher melting alloy which is present as a larger size powder. Preferably, the lower melting point alloy is employed in the form of a powder of which the particle size is predominantly less than 30μm diameter, while the first solder alloy powder particle size is predominantly greater than 30μm diameter. More preferably, the lower melting point alloy particle size is predominantly in the range 15-38μm and the first solder alloy particle size is predominantly 20-45μm. By the word predominately, it is meant that more than 50% by wt, preferably more than 75% and most preferably all of the alloy in question has the indicated particle size. It is hypothesised that the fine lower melting powder particles melt first and form a network of liquid around the larger powder particles. This liquid network enables wetting to take place more readily and initiate reflow, and maximises the effective melting range to minimise tombstoning.

In preferred practice, there is used a starting alloy containing up to 3% Cu, up to 5% Ag, and up to 5% Sb, the remainder being Sn and at least one of the elements Cu, Ag and Sb being present in an amount of at least 0.1%. The second alloy contains constituents chosen from Sn and at least one of the elements Cu, Ag and Sb in an amount at least 0,1%.

Preferably, care should be taken to have the final alloy show the following analysis: Ag up to 6% Cu up to 3% Sb up to 5% Sn rest

The invention will now be further explained with reference to table 1 of example compositions and the preferred embodiments shown in the figures, in which:

Fig. 1 is a differential scanning calorimeter (DSC) curve for a powder mixture of SnAg3.5 and SnAg3.8Cu0.7;

Fig 2 is a differential scanning calorimeter (DSC) curve for a powder mixture of Sn63Pb37 and Sn62Pb36Ag2;

Fig. 3 is a differential scanning calorimeter (DSC) curve for a powder mixture of SnAg3.5 and SnAg3.8Cu0.7 in a different proportion; and Fig. 4 is a differential scanning calorimeter (DSC) curve for a powder mixture of SnCuO.7 and SnAg3.5.

Example 1 In this example a solder paste was made up containing a blend of 80% Sn96.5Ag3.5 type 3 powder (approximately in the size range of 45-25 μm) + 20% Sn95.5Ag3.8Cu0.7 type 4 (approximately in the size range of 38-20 μm) powder. DSC analysis of this blend gave two peaks, at 217°C and 221 °C, i.e. with the same 4°C separation as for the established Sn63Pb37 and Sn62Pb36Ag2 powder blend anti-tombstoning paste.

Initial results in the laboratory on boards manipulated to maximise tombstoning indicated significantly fewer tombstones with the anti-tombstoning blend than with a paste containing the standard Sn95.5Ag3.8Cu0.7 alloy. Fig. 1 shows DSC data for the 80%/20% blend. The tin-lead-silver anti-tombstoning paste is shown in a separate graph for comparison as Figure 2.

Example 2

In this example a solder paste was made up containing a blend of 60% Sn96.5Ag3.5 type 3 powder + 40% Sn95.5Ag3.8Cu0.7 type 4 powder. DSC analysis of this blend gave two peaks, at 217°C and 221 °C, but with a larger peak at 217°C and a smaller peak at 221 °C. In this way the proportion of liquid and hence the effective surface tension at a lower temperature in the melting range is increased. See Fig. 3.

Example 3

In this example a solder paste was made up containing a blend of 60% Sn99.3Cu0.7 type 3 powder + 40% Sn96.5Ag3.5 type 4 powder. This gives a longer melting range of 10°C instead of 4°C, but without a pronounced peak at the high temperature end of the melting range. See Fig. 4. Table 1

Ten alloy lead-free combinations that will inhibit tombstoning are shown in the table below. All these are combinations of two eutectic alloys or alloys with very narrow melting ranges, separated by between 4°C and 19°C.

Claims

CLAIMS:

1. A Pb-free, Bi-free solder alloy powder mixture made of two powders: a first higher melting point Sn powder or Sn alloy powder; and a second lower melting point Sn powder or Sn alloy powder, which melt to form a final solder alloy containing 0.1- δ%Cu and/or 0.1-10% Ag and/or 0.1-6% Sb, the remainder being Sn .

2. A method of solder powder production giving reduced tendency to tombstoning in attachment of a chip component to a printed circuit board, which comprises mixing a first Pb-free, Bi-free solder powder and a second Pb-free, Bi-free powder, which has a lower melting point than the first solder powder, the two being meltable on soldering to produce a final alloy containing 0.1 -δ% Cu and/or 0.1-10% Ag and/or 0.1-5% Sb, the remainder being Sn.

3. A solder powder mixture or method according to claim 1 or 2 wherein the first powder comprises one only of Cu or Ag or Sb, and the remainder Sn.

4. A solder powder mixture or method according to any of the preceding claims wherein the second powder comprises at least two of Cu or Ag or Sb preferably Ag and Cu only, and the remainder Sn, wherein its composition is more preferably Sn95.5Ag3.8Cu0.7.

5. A solder powder mixture or method according to claim 1 or 2 wherein the first solder powder contains from 0.1-6% Sb, preferably 4-6% Sb, and the second solder powder contains from 0.1-3% copper, preferably 0.5-1% Cu, or from 1-5%Ag, preferably 3-4% Ag, or from 0.1-3% Cu and from 1-5% Ag, preferably 0.5-1% Cu and 3-4% Ag.

6. A solder powder mixture or method as claimed in Claim 5 where the second solder powder contains additionally 0.1-2% Sb.

7. A solder powder mixture or method as claimed in claim 1 or 2, wherein the first solder powder contains from 0.1-3% Cu, preferably 0.5-1% Cu, and the second solder powder contains from 1-5%Ag, preferably 3-4% Ag, or from 0.1-3% Cu and from 1 -5% Ag, preferably 0.5-1 % Cu and 3-4% Ag.

8. A solder powder mixture or method as claimed in any of Claims 1-4, wherein the first solder powder contains from 1-6%Ag, preferably 3-4% Ag, and the second solder alloy powder contains from 0.1-3% Cu and from 1-5% Ag, preferably 0.5-1 % Cu and ^'3-4% Ag.

9. A solder powder mixture or method as claimed in Claim 7 or 8, where the first and/or second solder powders contain additionally 0.1-2% Sb.

10. A solder powder mixture or method according to any of Claims 1 , 2, 3, 5 or 6 where the first solder alloy is SnθδSbδ, and the second solder alloy is Sn99.3Cu0.7, or Sn96.4Ag3.6, or Sn95.5Ag3.8Cu0.7.

11. A solder powder mixture or method as claimed in any of Claims 1-5 where the first solder alloy is Sn96.4Ag3.6, and the second solder alloy is Sn95.6Ag3.8Cu0.7.

12. A solder powder mixture or method as claimed in any of Claims 1-3, 5 or 6 where the first solder alloy is Sn99.3Cu0.7, and the second solder alloy is Sn96.4Ag3.6, or Sn95.5Ag3.8Cu0.7.

13. A solder powder mixture or method as claimed in any of claims 1 , 2, 3 or 9, wherein the second powder comprises Sn only.

14. A solder powder mixture or method as claimed in any of claims 1 , 2, 4 or 9 wherein the first powder comprises Sn only.

15. A solder powder mixture or method as claimed in claim 14 wherein the second powder comprises only one of Cu or Ag and the remainder Sn.

16. A solder powder mixture or method as claimed in any of the preceding claims wherein the final alloy has an analysis:- Ag up to 6% Cu up to 3% Sb up to 5% Sn rest

17. A method as claimed in any of the preceding claims that is applied to the production of a solder pad affixing a component to the surface of a conductor on a dielectric substrate.

18. A solder powder mixture or a method according to any of the preceding claims, the powder mixture having a melting point range of 4 to 19°C, preferably 4 to 9°C most preferably 4 to 8°C.

19. A solder powder mixture or a method according to any of the preceding claims wherein the lower melting point powder particles are finer than the higher melting point powder particles.

20. A solder powder mixture or a method as claimed in any of the preceding claims, wherein the second, lower melting point powder has a particle size of predominately less than 30μm diameter, while the first solder powder particle size is predominately greater than 30μm diameter.

21. A method according to any of the preceding method claims comprising simultaneous or sequential addition of the powders to a flux medium, and mixing.

22. A Pb-free, Bi-free solder alloy paste formed from the solder alloy powder mixture defined in any of the preceding claims with the addition of a flux medium.

23. A Pb-free, Bi-free solder paste obtainable from the method of solder paste production as defined in any of the preceding claims.

24. Use of a solder paste or powder mixture according to any of the preceding solder powder mixture or solder alloy paste claims in attaching a chip component to a printed circuit board and thereby reducing the incidence of tombstoning.