WO1993003884A1

WO1993003884A1 - Lead-based solder alloy and its use in soft soldering

Info

Publication number: WO1993003884A1
Application number: PCT/GB1992/001459
Authority: WO
Inventors: Gordon Arbib; Hector Andrew Hamilton Steen
Original assignee: Multicore Solders Limited
Priority date: 1991-08-22
Filing date: 1992-08-06
Publication date: 1993-03-04
Also published as: AU2401092A; GB9118115D0

Abstract

Wave soldering of electronic component in particular if carried out in a substantially oxygen-free inert atmosphere utilising a quaternary solder alloy consisting of 1 to 15 wt% tin, 1 to 13 wt% antimony, 1 to 15 wt% bismuth with the balance being lead.

Description

EAD-BASED SOLDER ALLOY AND ITS USE IN SOFT SOLDERING This invention relates to the soldering of electronic components and is principally, but not entirely, concerned with the soldering of such components, when mounted on a printed circuit board (PCB), by means of wave soldering.

One commercially popular method of mass producing PCBs involves the use of a wave soldering machine in which a molten solder wave is caused to come into contact with a PCB carrying electronic components as it passes through the machine so as to cause the components to become soldered to the PCB in the required locations after the PCB has emerged from the solder wave and the solder has solidified. Conventionally, wave soldering has been effected under an oxygen-containing atmosphere, i.e. air, using a solder alloy which is a tin-lead eutectic, i.e. a 63:37 Sn/Pb alloy, or a close alloy thereto, e.g. a 60:40 Sn/Pb alloy. To ensure adequate wetting and bonding of the solder on the PCB, it is normally necessary to apply a flux material to the PCB, which results in the formation of flux residues which may then need to be removed, generally by cleaning with an organic solvent such as a chlorofluorocarbon (CFC) which gives rise to environmental problems.

When wave soldering is conducted in air, oxide which is formed on the solder wave intermixes with solder to form dross which must be removed periodically from the solder bath. In order to avoid such dross formation and in order to reduce the need for employing a flux material in the wave soldering process and thereby avoid the need for subsequent removal of flux residues by environmentally harmful CFCs, wave soldering of PCBs has recently been conducted in a non-oxidising atmosphere (i.e. <10 ppm oxygen), such as an inert gas, e.g. nitrogen (see "Inert Atmosphere Wave soldering", Electronic Production, May 1991, pages 11-12). However, it has been found that in inert gas wave soldering with the conventional tin-lead solder alloys, there is a greater tendency to bridge formation between joints, a defect known as bridging, which can cause short circuiting on the PCB. This is thought to result from the higher surface tension of tin-lead solder under nitrogen compared to that under air.

Because of the high tin content of the tin/lead alloys necessarily used in the soft soldering of electronic equipment, several proposals have been made for partially substituting the relatively expensive tin content by one or more less expensive alloying metals. There are, however, several constraints on how much the tin content can be reduced, firstly since the tin is the constituent that ensures wettability of the solder alloy on the material to be soldered, and secondly, since the modified alloy will no longer be a eutectic or near eutectic, so that there will be a substantial difference between the liquidus (i.e the temperature above which the alloy is completely molten) and the solidus (i.e. the temperature below which the alloy is completely solid), the solidus to liquidus temperature range being designated ΔT.

Thus in "Welding Journal", October 1989, pages 50-51 (S.F.Dirnfeld and J.J.Ramon), various quaternary solder alloys, a few of which consist of tin, lead, bismuth and antimony, are mentioned as cheaper alternatives for a number of tin/lead alloys. However, the authors state that it is difficult to obtain very narrow melting intervals (i.e. small ΔTs), thereby ruling out replacement of the near-eutectic tin-lead solder. Thus the solder alloy containing the four elements referred to above and having the lowest tin content, which has the composition 20 wt% Sn, 8.5 wt% Bi, 0.5 wt% Sb and 71 wt% Pb, has a ΔT of 70°C (compared with 0° for 63:37 Sn/Pb and 5° for 60:40 Sn/Pb) and is therefore only suggested as being useful in applications in which a 30 wt% Sn/70 wt% Pb solder alloy is normally used, the quaternary alloy being stated to have a lower tendency to copper-tin intermetallic buildup (which should improve solder joint strength). A similar quaternary solder alloy containing 1.4 wt% antimony and 70.1 wt% lead is disclosed by the same authors in their U.S. Patent No.4,975,244, but this also has a ΔT of 70°C.

We have now unexpectedly found that by the appropriate choice of antimony and bismuth contents, a quaternary solder alloy containing the same four elements but with a relatively narrow ΔT can be produced even with a low tin and high lead content, thereby rendering the alloy useful as a replacement for the conventional 63:37 and 60:40 tin/lead solder alloys, particularly in wave soldering under an inert gas atmosphere. We have further unexpectedly found that, whereas in conventional wave soldering in air, the soldering process needs to be effected at a temperature which is at least 50°C higher than the liquidus of the solder alloy for satisfactory joints to be obtained, with a quaternary solder alloy in accordance with the invention, it is possible to wave solder under nitrogen at only about 25°C higher than the liquidus.

Thus in accordance with one aspect of the invention, there is provided a method of mass producing PCBs utilising a wave soldering process, wherein the wave soldering is conducted in a substantially oxygen-free inert atmosphere and wherein the solder wave is formed from a quaternary solder alloy consisting of 1-15 wt% tin, 1-13 wt% antimony, 1-15 wt% bismuth with the balance being lead (except for impurities and incidental elements).

It will be appreciated that use of a quaternary solder alloy in accordance with the invention in a wave soldering process under a substantially oxygen-free inert atmosphere is also included within the scope of the invention. Hence in accordance with a further aspect of the invention, there is provided the use of a quaternary solder alloy in a wave soldering process under a substantially oxygen-free inert atmosphere, said alloy consisting of 1-15 wt% tin, 1-13 wt% antimony, 1-15 wt% bismuth with the balance being lead (except for impurities and incidental elements).

It is believed that most of the quaternary solder alloys which can be employed in the wave soldering process of the invention are novel per se and that those which have been disclosed in the prior art have not been described as being used or useful in the soft soldering of PCBs by wave soldering.

Hence in accordance with a third aspect of the invention, there is provided a quaternary solder alloy suitable for use in soft soldering, the solder alloy consisting of 1-15 wt% tin, 7-13 wt% antimony, 1-15 wt% bismuth with the balance being lead (except for impurities and incidental elements). Preferably, the alloys of the invention to be used according to the first and third aspects of the invention consist of 3-6 wt% tin, 7-10 wt% antimony, 3-10 % bismuth with the balance being lead, (except for impurities and incidental elements) The term "a substantially oxygen-free inert atmosphere" is used in this specification as meaning an inert atmosphere, e.g. nitrogen, containing less than 500 ppm oxygen, preferably not more than 10 ppm oxygen.

Solder alloys in accordance with the invention which have been prepared are:

Alloy wt% wt% wt% wt% Solidus Liquidus ΔT Sn Bi Sb Pb

Of these alloys, A and B are preferred because of the low ΔT value.

It will be appreciated that the solder alloy of the invention may contain small amounts of incidental elements such as those commonly incorporated in solder alloys, e.g. silver, provided that they do not significantly degrade the wetting properties or increase the solidus to liquidus range (ΔT). Impurities which may be present in the solder alloy in very small amounts include arsenic, iron, copper, aluminium, cadmium, zinc, phosphorus, sulphur, gold, indium and nickel.

The quaternary solder alloy of the invention may conveniently take the form of a solder bar, a plurality of which can be used to form or replenish a wave solder bath.

In tests carried out using the solder alloy A referred to above, double sided PCBs having plated through holes (PTHs) bearing leaded components were satisfactorily wave soldered under nitrogen (<10 ppm oxygen) at 250°C with no apparent bridging defects. The PCBs were pre-fluxed with a low residue flux.

Strength tests carried out on alloy A show that in bulk form it has higher tensile strength than 60/40 Sn/Pb solder, while ring and plug solder joints made with alloy

A have only slightly lower shear strength than joints made with 60/40 Sn/Pb solder.

Alloy Tensile Strength* Solder Joint Shear Strength+ N/mm² N/mm²

A 60 22 60/40 38 25

* At ambient temperature at a 1 πun/min strain rate + At ambient temperature at a 0.2 mm/min strain rate

Fatigue testing carried out on solder joints made with alloy A and 60/40 Sn/Pb solder show that joints produced with alloy A of the invention exhibit improved thermal fatigue resistance as compared with similar joints made using the Sn/Pb alloy under temperature cycling at 0 to 100°C with observations being carried out every 500 cycles. Joints made with Sn/Pb alloy tended to last for up to 2500 cycles and joints made with alloy A tended to last for up to 3500 cycles.

Whilst the solder alloys of the invention have been described as being particularly useful in wave soldering under an inert gas atmosphere, it will be appreciated that the quaternary solder alloys may also be used, even without an inert gas atmosphere, in other methods of soft soldering where the tendency to form dross is much less than in wave soldering, such as for example, in hand soldering with a soldering iron or in a static solder bath. For such purposes the solder alloy may be made up in forms appropriate to such use, such as, for example, in the form of solder wire (solid or flux-cored), solder cream or solder preforms.

Claims

CLAIMS :

1. A method of mass producing PCBs utilizing a wave soldering process, wherein the wave soldering is conducted in a substantially oxygen-free inert atmosphere and wherein the solder wave is formed from a quaternary solder alloy consisting of 1 to 15 wt% tin, 1 to 13 wt% antimony, 1 to 15% bismuth with the balance being lead (except for impurities and incidental elements).

2. A method as claimed in Claim 1, wherein the alloy consists of 3 to 6 wt% tin, 7 to 10 wt% antimony, 3 to 8 wt% bismuth with the balance being lead (except for impurities and incidental elements).

3. A method as claimed in Claim 1 or 2, wherein the alloy consists of 4.6 wt% tin, 8.0 wt% bismuth, 9.0 wt% antimony and 78.4 wt% lead (except for impurities and incidental elements).

4. A method as claimed in any preceding claim, wherein the wave soldering is carried out in an atmosphere of nitrogen containing less than 500 ppm oxygen, preferably not more than 10 ppm oxygen.

5. A quaternary solder alloy suitable for use in soft soldering, the solder alloy consisting of 1 to 15 wt% tin, 7 to 13 wt% antimony, 1 to 15 wt% bismuth with the balance being lead (except for impurities and incidental elements).

6. An alloy as claimed in Claim 5 which consists of 3 to 6 wt% tin, 7 to 10 wt% antimony, 3 to 8 wt% bismuth with the balance being lead (except for impurities and incidental elements). 7. An alloy as claimed in Claim 5 and consisting of 4.8 wt% tin, 9.5 wt% antimony, 5.0 wt% bismuth and 80.

7 wt% lead (except for impurities and incidental elements).

8. An alloy as claimed in Claim 5 and consisting of 4.6 wt% tin, 8.0 wt% bismuth, 9.0 wt% antimony and 78.4 wt% lead (except for impurities and incidental elements).

9. The use of a quaternary solder alloy in a wave soldering process under a substantially oxygen-free inert atmosphere, said alloy consisting of 1 to 15 wt% tin, 1 to 13 wt% antimony, 1 to 15 wt% bismuth with the balance being lead (except for impurities and incidental elements).

10. The use of Claim 9, wherein the alloy consists of 3 to 6 wt% tin, 7 to 10 wt% antimony, 3 to 8 wt% bismuth with the balance being lead (except for impurities and incidental elements).

11. The use of Claim 9, wherein the alloy consists of 4.6 wt% tin, 8.0 wt% bismuth, 9.0 wt% antimony and 78.4 wt% lead (except for impurities and incidental elements).