Method for soldering to a copper comprising object using a lead-free solder alloy
FIELD OF THE INVENTION
The invention relates to a method for soldering, using a lead-free copper comprising solder alloy, to a copper comprising object.
DESCRIPTION OF THE PRIOR ART
In solder alloys lead has conventionally been used. However, the use of lead has become a source of concern, due to possible health and environmental hazards. The use of lead-free solder alloys has been promoted and regulations have been published to ban the use of lead containing solder alloys. "Lead-free" within the content of the invention is to be understood to allow solder alloys to comprise small, non-intentional amounts of lead, in the order of a tenth to a few tenths of a percent.
When soldering to copper comprising objects, such as copper wire or copper foil, is performed a problem arises in that the copper contained in the objects may dissolve from the copper comprising object, leading to a poor solder connection, or even, in the case of thin wires, to a break in the wire.
In JP 2001-121286 a 5.5%-8% copper comprising lead-free solder has been proposed to reduce copper dissolution.
The inventors have found that the known lead-free copper comprising solder alloy in circumstances performs less than satisfactorily and often breakage of wires and poor solder joints occur.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the soldering process to enable better solder joints to be obtained. To this end the method in accordance with the invention is characterized in that the temperature of the solder alloy during soldering lies between 450 °C and 535 °C and the lead-free solder alloy comprises between 8.5% and 14% (by weight) Copper (Cu).
Often copper wires are covered with protective layers, such as urethane layers, and, to save time in the soldering process, the wire is not stripped, but the soldering process is
performed directly to the covered wire at a high temperature. The high temperature is used to strip the wire of the coating as well as to solder to the copper wire.
Experiments using copper comprising lead-free solders having a copper content of 9 to 11 % Cu have been reported in JP 2001-121286. However, these experiments are presented as proof that soldered joints made with such high copper content have a poor mechanical strength.
Surprisingly, the inventors have found that in the indicated soldering temperature range (a range not mentioned in JP 2001-121286), a copper content of 8.5% to 14% offers the possibility of better and stronger soldering joints, in contradiction to the teaching of JP 2001-121286.
It has been found that when soldering at the indicated soldering temperature range, a high copper content (of 8.5% to 14%) leads to an substantial increase of maximum soldering time, wherein maximum soldering time is the time one has to solder before a wire breaks. This enables better and stronger soldering joints to be made. At a lower copper content the maximum soldering time before breakage of the wire is substantially less and thus leading to either breakage of the wire (if the soldering time is too large) or bad solder joints if the soldering time too small.
The invention is of particular use when Litz wires are soldered. Litz wires are composite wires, in which a bundle of thin wires is used. These wires are often separately covered with a coating. Litz wires are used in high frequency applications such as in deflection units for CRT's or in high- frequency electrical transformers. Litz wires offer a lower high-frequency resistance. They pose, however, a challenge in so far as it is often difficult to make a good soldering joint to the wires. The relatively fragile nature (a bundle of thin wires) of Litz wires, combined with the fact that often each wire is separately coated, makes such wires difficult to solder to. There is a relatively great risk that either the wires break (when the soldering time is too large or the solder temperature is too high) or that the coatings are not effectively removed (when the soldering time is too small or the soldering temperature is too low), in either case leading to bad solder joints. However, using the method in accordance with the invention it has been proven that good, mechanically stable and sound solder joints are made.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereafter.
SHORT DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 illustrates a deflection unit with wires ends
Fig. 2 illustrates a detail of a deflection unit Fig. 3 illustrates a method to solder the wire ends
Fig. 4 illustrates the Sn-Cu phase diagram
Fig. 5 illustrates the soldering temperature range and the Cu content range of the present invention.
Fig. 6 illustrates the cross section of a Litz wire Fig. 7 illustrates the copper dissolution at 460°C for several soldering alloys.
The figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows a deflection unit 1 for a CRT (Cathode Ray Tube). The deflection unit comprises a yoke ring 2, and deflection coils 3 (in this example wound around the yoke ring) and 4 (not shown, being located inside the yoke ring). The deflection unit comprises a holder 5, having pins 6, upon which the ends of the coils 3 and 4 are wound and soldered. Figure 2 shows more in detail the holder 5 with the pins 6. The wire ends are shown around the pins and the solder is also visible.
Figure 3 illustrates a method for soldering. The wire ends of the coils 3, 4 are wound (with the coating intact) around the pins 6. The pins are then dipped into a bath 7, comprising a solder alloy 8 at a solder temperature Tsoider. The temperature is so high that the protective coating layers are melted and/or burnt away and the wire ends of coils 3, 4 are soldered to the pins. After this has been accomplished the bath is removed. The soldering time is of importance.
The present invention relates to a combination of one the hand the temperature of the solder alloy and on the other hand the composition of the solder alloy, more in paiticular the Cu content. It has been found that when soldering at the indicated soldering temperature range, a high copper content (of 8.5% to 14%) leads to an substantial increase of maximum soldering time, wherein maximum soldering time is the time one has to solder before a wire breaks. The longer the maximum soldering time, the smaller the possibility that the wire break during soldering and the better (when wires are coated) the coating may be
removed. This enables better and stronger soldering joints to be made. At a lower copper content (below 8%) the maximum soldering time before breakage of the wire is substantially less and thus leading to either breakage of the wire (if the soldering time is too large) or bad solder joints if the soldering time too small.
The below given table illustrates this effect at a soldering temperature of 500 °C.
wire SnAg3.8Cu0.7 SnCu3 g2Cu6 SnCu8.5 SnPb35 solid wire 1 8 15.5 solid wire 2 3 4.5 5.5 7.5 14.5
Litz wire 1 2.5 3.5 4 5.5
Litz wire 2 1.5 1.5 1.5 2.5 6.5
When one compares the soldering composition having 8% Cu (SnCu8.5) to the soldering composition having 6% (SnAg2Cu6) by weight of Cu, the table shows that the maximum soldering time is increased by 40-60%. This is remarkable since the increase in maximum soldering time going from 3% Cu to 6% Cu content (comparing the SnCu3 column to the SnAg2Cu6 column) is much smaller (from 0 to 22%) although the difference in Cu content is larger (a difference of 3% compared to 2%). Using SnCu8.5 (i.e. an alloy comprising 8.5% by weight of Cu) thus allows for longer maximum soldering times. The resulting solder joints have excellent mechanical strengths. The latter is surprising, since it seemingly is in contradiction with the teaching of JP 2001-2001-121286 which warns against the use of solder alloys having more than 8% Cu, because of the reportedly poor mechanical properties of the solder joint. Without being bound the any explanation of the effects, the following possible explanation may explain this surprising behaviour.
Figure 4 shows the phase diagram of Cu-Sn alloys. Figure 5 shows a part of this phase diagram illustrating the Cu percentage range of interest to the present invention. The inventors have realized that it is not just the Cu percentage that is of importance for the Cu dissolution, but more importantly, the relation between the soldering temperature and the liquidus temperature (the line 51 (Tiiquidus) in figure 5). The Cu dissolution seem to be acceptably low if the soldering temperature is between 20 and 45 °C above the liquidus temperature. Using a higher temperature increased the Cu dissolution, while using a lower temperature requires long soldering times, thus also increasing the
problems. Preferably the soldering temperature is between 20 and 40, most preferably between 25 and 35 C above the liquidus temperature. For soldering temperatures between 450 and 535 °C the Cu percentage ranges between 8.5% and 14%. The preferred range is given by the arced area in figure 5, i.e. between 20 and 45 °C higher than the liquidus temperature Tιiquidus- More preferred ranges are, for simplicity not indicated in figure 5.
The method is of particular importance (without such to be understood to be an inherent restriction to the method) for soldering Litz wires. Figure 6 illustrates a Litz wire 60. A Litz wire 60 comprises an outer protective coating 61, within which a bundle of smaller wires 62 is comprises, each having an outer protective coating 63 and an inner core (Cu or Cu alloy) 64. Typically the diameter of wires 64 is 100-120 micrometers. The invention is in particular useful for soldering Litz wires having wires 64 with a diameter of less than 125 micrometer.
Such wires are used for high frequency applications (such as line coils for deflection units or high frequency transformer or coils). This types of wires has relatively much coating and relatively small wires. The increase in soldering times and the excellent mechanical properties the method of the invention provides are of particular importance for these types of wires.
It is remarked that, within the concept of the invention the solder alloy may comprise small amounts of other elements such as Ag or Ni. Figure 7 illustrates for a copper wire with a diameter of 0.180 mm the difference in total dissolution time (i.e. the time it takes for a wire to break) at the vertical axis for different soldering alloys as a function of temperature (horizontal axis). At a soldering temperature of 460°C, using a soldering alloy SnCu8.5 (i.e. having 8.5% of Cu) it takes twice or more times as long before the wire breaks than when a soldering alloy having slightly less copper (SnC7 and SnAg2Cu6) is used, namely 22 seconds vs 11 or 10 seconds. Thus at 460°C and 8.5% Cu (approximately at the center of the range indicated in figure 5) the invention offers a great advantage over points (460°C, 7% Cu, and 6%Cu) outside the range.
In embodiments in which thick layers of insulation cover the wires to be soldered, high temperatures (above 480°C) and high percentages of Cu (above 10%) are preferred.
It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of
characteristic features, even if not explicitly recited in the claims. Reference numerals in the claims do not limit their protective scope. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.