Low temperature solder

The present invention includes a low temperature solder composition having a mixture of elements including indium, tin and silver. Less than about 75% of the composition by weight is tin.

BACKGROUND
 Windshields and rear windows of vehicles such as automobiles often include
 electrical devices located within or on the glass. Typically, the
 electrical devices are antennas or defrosters. In order to provide an
 electrical connection to such an electrical device, a small area of
 metallic coating is applied to the glass which is electrically connected
 to the electrical device. An electrical connector for connecting to a lead
 or the lead itself is then soldered to the metallic coating on the glass.
 The solder typically used has a melting point of about 193.degree. C.
 (380.degree. F.) and requires about 750-800 watt seconds of energy to melt
 the solder. Heat of this magnitude flows to the glass and often damages
 the glass in regions near the solder joint, for example, causing cracking.
 SUMMARY OF THE INVENTION
 The present invention is directed to a solder composition that has a
 considerably lower melting temperature than solder compositions currently
 available for soldering to glass. As a result, when the present invention
 solder composition is employed for soldering components onto a windshield
 or rear window, the amount of heat that flows to the glass is low enough
 so that the glass experiences little or no damage. The solder composition
 of the present invention has a mixture of elements including indium, tin
 and silver. Less than about 75% of the solder composition by weight is
 tin.
 In preferred embodiments, more than about 25% of the solder composition by
 weight is indium. More typically, less then about 50% of the composition
 by weight is tin and more than about 50% of the composition by weight is
 indium. The solder composition includes the element copper and preferably,
 the elements within the solder composition have a weight percentage of
 about 65% indium, about 30% tin, about 4.5% silver and about 0.5% copper.
 A preferred range of the weight percentage of the solder composition is
 64.35%-65.65% indium, 29.7%-30.3% tin, 4.05%-4.95% silver and 0.25%-0.75%
 copper. The solder composition preferably contains no more than about
 0.75% antimony, about 0.08% gold, about 0.2% lead, about 0.08% aluminum,
 about 0.03% arsenic, about 0.005% cadmium, about 0.005% zinc, about 0.25%
 bismuth, about 0.02% iron and about 0.005% nickel. The solder composition
 preferably has a liquidus of about 121.degree. C. and a solidus of about
 118.degree. C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention provides a solder composition that is suitable for
 soldering electrical components to glass for electrically connecting to
 electrical devices within or on the glass. Referring to FIG. 1, the rear
 window 10 of an automobile is employed as an illustrative example. Window
 10 includes a window defroster 12 consisting of electrically resistive
 defrosting lines 14 embedded within or deposited on the inner surface of
 window 10. The defrosting lines 14 are electrically connected to a pair of
 electrical contacts 16 located on the inner surface of the window 10. The
 electrical contacts 16 consist of a conductive coating deposited on the
 inner surface of window 10. Preferably, electrical contacts 16 are formed
 from silver.
 Referring to FIG. 2, the solder composition 20 of the present invention is
 employed to solder an electrical connector 18 to each electrical contact
 16 on window 10. Power lines 22 can then be electrically connected to
 electrical connectors 18 to provide power to window defroster 12 (FIG. 1).
 The present invention solder composition 20 has a solidus temperature of
 about 118.5.degree. C. (245.3.degree. F.) and a liquidus temperature of
 about 121.5.degree. C. (250.7.degree. F.) as depicted in FIG. 3. This
 allows solder composition 20 to be melted during soldering with a low
 energy input level so that the solder composition 20 and electrical
 connector 18 are at temperatures which are low enough not to damage window
 10. While existing solder compositions typically require at least 750-800
 watt seconds of energy input in order to melt, the present invention
 solder composition 20 can be melted with energy input levels as low as 250
 watt seconds. However, energy input levels of between about 300-650 watt
 seconds provide the greatest strength. Although solder composition 20 has
 a melting temperature of only about 250.degree. F., the melting
 temperature is still high enough not to melt during the normal use of a
 car, for example, when the car is in the sun with the windows closed.
 The low melting temperature of solder composition 20 is achieved by the
 particular elements present and the relative amounts of those elements
 within solder composition 20. In the preferred embodiment, solder
 composition 20 contains about 65% indium (In), 30% tin (Sn), 4.5% silver
 (Ag) and 0.5% copper (Cu) by weight as depicted in FIG. 4. Solder
 composition 20 also can contain trace amounts of antimony (Sb), gold (Au),
 lead (Pb), aluminum (Al), arsenic (As), cadmium (Cd), zinc (Zn), bismuth
 (Bi), iron (Fe) or nickel (Ni). Solder composition 20 preferably has the
 following composition ranges:

Indium 64.35-65.65%
 Tin 29.7-30.3%
 Silver 4.05-4.95%
 Copper 0.25-0.75%
 Antimony 0.750% max.
 Gold 0.080% max.
 Lead 0.200% max.
 Aluminum 0.080% max.
 Arsenic 0.030% max.
 Cadmium 0.005% max.
 Zinc 0.005% max.
 Bismuth 0.250% max.
 Iron 0.020% max.
 Nickel 0.005% max.
 Levels of antimony, gold, lead, aluminum, arsenic, cadmium, zinc, bismuth,
 iron or nickel greater than the above specifications can effect the
 material properties of solder composition 20.
 Cracking of automotive window 10 during soldering is limited by limiting
 the amount of tin in solder composition 20. At tin levels of about 30%,
 cracking of window 10 is substantially limited or prevented. High
 percentages of tin tends to crack automotive glass due to the dissimilar
 coefficients of expansion of automotive glass and tin. The high percentage
 of indium causes solder composition 20 to have a low melting point or
 temperature. The presence of silver in solder composition 20 prevents
 solder composition 20 from scavenging silver from the electrical contact
 16 into the solder composition 20. Solder composition 20 preferably should
 have at least about 3% silver to prevent silver scavenging from electrical
 contact 16. Finally, the addition of copper to solder composition 20
 raises the melting temperature of solder 20 to the desired level.
 As can be seen, no lead is added to solder composition 20. As a result,
 solder composition 20 is lead-free except for the small level of
 acceptable contamination allowed for as indicated above. Under normal
 conditions of use in an automobile, solder 20 does not corrode, pit or
 melt. The low melting point of the present invention solder is at least
 about 50.degree. C. lower than currently available solders for automotive
 glass but is still high enough to be above temperatures experienced by
 automotive glass during the normal use of an automobile as previously
 mentioned.
 To make solder composition 20, ingots of indium, tin, silver and copper are
 melted and mixed together in the amounts of about 65% indium, 30% tin,
 4.5% silver and 0.5% copper. The mixed solder composition 20 is then
 extruded into a ribbon suitable for soldering. Alternatively, the elements
 can be melted from powder form.
 Although particular percentage figures have been given for the present
 invention solder composition 20, the percentages of the various elements
 as well as the presence of particular elements can vary, For example, in
 another preferred embodiment, solder composition 20 is about 64.95%
 indium, 30% tin, 4.31% silver, 0.71% copper with the remaining 0.03% of
 the solder composition having traces of bismuth, cadmium, iron, magnesium,
 nickel, lead and thallium. Further, depending upon the application at
 hand, the amount of indium within solder composition 20 can be as low as
 about 25%. In addition, the amount of tin can be as high as about 75%.
 However, it is more likely for the amount of tin to be less than 50% and
 the amount of indium to be greater than about 50%.
 In use, soldering of the electrical connectors 18 to the electrical
 contacts 16 on window 10 with solder composition 20 is preferably
 conducted by resistance soldering. Alternatively, any conventional
 soldering apparatus can be employed, for example, a soldering iron. The
 energy input level for melting solder composition 20 is preferably between
 250-650 watt seconds with 300-650 watt seconds being more preferable. This
 is evidenced by the test results described below from a series of tests
 where electrical connectors 18 were soldered to a piece of glass and then
 the force required to pull the electrical connectors from the glass was
 measured.
 EXEMPLIFICATION
 Example 1
 In the first test, nine groups of four samples were tested. In each group,
 the electrical connectors 18 were soldered with solder composition 20 to
 an electrical contact on a glass plate at energy input levels ranging from
 250 watt seconds to 650 watt seconds. The electrical connectors 18 were
 then pulled from the glass and the required force was measured in pounds.
 The pull strength test was performed at room temperature. The results are
 listed below as follows:

1) 58.8 lbs
 2) 67.2 lbs
 3) 80.4 lbs
 4) 54.2 lbs
 5) 63.3 lbs
 6) 57.7 lbs
 7) 61.8 lbs
 8) 48.8 lbs
 9) 53.9 lbs
 10) 55.4 lbs
 AVG. 60.1 lbs
 Example 4
 In the fourth test, six groups, each containing five samples were tested.
 In each group, the electrical connectors 18 were soldered with solder
 composition 20 to an electrical contact on a glass plate at energy input
 levels ranging from 250-500 watt seconds and employing type R
 electro-rosin flux. The pull strength test results are as follows: