A method is described for forming sharp tips on thin wires, in particular phosphor bronze wires of diameters such as one-thousandth inch used to contact micron size Schottky barrier diodes, which enables close control of tip shape and which avoids the use of highly toxic solutions. The method includes dipping an end of a phosphor bronze wire into a dilute solution of sulfamic acid and applying a current through the wire to electrochemically etch it. The humidity in the room is controlled to a level of less than 50%, and the voltage applied between the wire and another electrode in the solutions is a half wave rectified voltage. The current through the wire is monitored, and the process is stopped when the current falls to a predetermined low level

Green, G.

Mattauch, R. J.

English

NASA Technical Reports Server

United States Patent 4,375,396 
Green et al. [45] ' Mar. 1, 1983 
[54] THIN WIRE POINTING METHOD 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
James M. Beggs, Administrator of 
National Aeronautics and Space 
Administration, with respect to an 
invention of Gordon Green; Robert J. 
Mattauch, both of Charlottesville, 
Va. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
322,316 
Nov. 17, 1981 
[51] Int. C l . 3  ................................................ C25F 3/02 
[52] U.S. Cl. ........................... 204/129.55; 204/129.75 
[58] Field of Search ....................... 204/129.55, 129.75 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,628,936 2/1953 Albano ........................... 204A29.55 
3,006,827 10/1961 Capuano .................... 2W129.75 X 
40 - 
Primary Examiner-G. L. Kaplan 
Assistant Examiner-Nam X. Nguyen 
Attorney, Agent, or Firm-Paul F. McCaul; John R. 
Manning; Thomas H. Jones 
P71 ABSTRACT 
A method is described for forming sharp tips on thin 
wires, in particular phosphor bronze wires of diameters 
such as one-thousandth inch used to contact micron size 
Schottky barrier diodes, which enables close control of 
tip shape and which avoids the use of highly toxic solu- 
tions. The method includes dipping and end (12) of a 
phosphor bronze wire (14) into a dilute solution (16) of 
sulfamic acid and applying a current through the wire 
to electrochemically etch it. The humidity in the room 
is controlled to a level of less than 50%, and the voltage 
applied between the wire (14) and another electrode 
(18) in the solution is a half wave rectified voltage. The 
current through the wire is monitored, and the process 
is stopped when the current falls to a predetermined 
low level. 
7 Claims, 4 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19830011676 2020-03-22T00:40:13+00:00Z
U.S. Patent 
40 
12 
Mar. 1, 1983 
38 
FIG. I 
IO 
36 f ,30 
4,375,396 
A 
4,375,396 
1 
THIN WIRE POINTING METHOD 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 42 USC 2457). 
BACKGROUND OF THE INVENTION 
A pointed metal wire of a diameter in a range from 12 
to 50 micrometer (0.5 to two thousandths inch) is often 
used to contact a micron size Schottky barrier metal 
anode. The extreme tip of the wire must have a small 
radius of curvature, and certain high frequency (above 
100 GHz) applications also require a given cone angle at 
the tip for electrical and mechanical reasons. Materials 
commonly used in the wires or whiskers for high fre- 
quency applications, are gold and its alloys, phosphor 
bronze, and tungsten. Stress relieved grade C phos- 
phor bronze wire is especially useful as a contact 
whisker material for high frequency applications, be- 
2 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
FIG. 1 illustrates a system 10 which is used to form a 
5 point at the end portion 12 of a wire 14. The wire end 
portion 12 is dipped into a bath 16 of an electrolyte, and 
a current is passed through the wire and the electrolyte 
to electromechanically etch the wire end portion 12. 
This is done by applying a voltage between the wire 14 
10 and a platinum or other electrode 18 which is immersed 
in the solution. The process can be conducted to closely 
control the shape of the final wire end, to enable its use 
as an element for contacting the anode of a micron size 
Schottky diode or making other contacts in micron size 
FIG. 4 shows the shape of the tip 20 required at the 
end portion 12 of the wire, to enable its use as a contact. 
The curvature, or radius of curvature of the extreme tip 
22 and its width W along which it is curved should be 
20 small such as I/lOth the diameter D of the wire and 
should be relatively uniform all about the wire, to make 
reliable contact with the anode of the Schottky diode 
over a small area. In addition, certain high frequency 
applications where the Schottky diode is operated 
15 areas. 
40 
45 
50 
55 
cause of its high mechanical strength and springiness. *25 above 100 GHz, require close control of the cone angle 
One technique for forming a pointed phosphor A of the tip, to an angle such as about 60". In most 
bronze wire or whisker of the desired cone angle and applications the cone angle must not be too small or the 
extreme tip diameter or radius involves electrochemical tip end may collapse. Since the wire diameter D is 
etching in chromium trioxide. Since chromium and its small, between about 12 to 50 micrometer (about 0.5 to 
compounds are extremely toxic, there is considerable 30 2 thousandths inch) it can be difficult to control the 
need for a less objectionable pointing solution. A point- shape ofthe tip. 
ing technique which avoided the use of highly toxic FIG. 2 shows a first step in the Process conducted 
mateials and which enabled the pointing of wires of with the equipment of FIG. 1, wherein the end portion 
controlled cone angle and extreme tip diameter, would 12 of the wire has been dipped into the electrolyte 16- 
35 The wire is initially immersed to a length of about five be of considerable value. 
times its diameter, or about 125 micrometer for a wire of 
25 micrometer diameter. The electrolyte 16 is hydro- 
philic to the wire, that is, the electrolyte forms a menis- 
cus 26 about the wire. It is believed that the meniscus 26 
has an important effect on the formation of the tip, by 
allowing progressively greater etching rates at progres- 
moderate shows the wire 
wherein the tip 2o has been formed to the shape shown 
stopped and the wire is rinsed, dried, and observed 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present 
invention, a method is provided for the pointing of fine 
wires. The end of the wire is dipped into an electrolytic 
solution to electrochemically etch the wire. In the etch- 
solution is utilized as the electrolyte. The etching can be 
across the wire-electrode system monitoring the cur- 
and current is through the wire and sively lower levels along the meniscus, to produce the 
angle of the tip. FIG. ing Of phosphor bronze wire, an aqueous sulfamic acid end portion 12 at the end of electrochemical etching, 
by a predetermined in FIG. 4. At this time, electrochemical etching is 
rent and stopping the etching OFra t ion  when the 
rent drops to a predetermined level. The humidity in under a microscope to check the form of the tip. Previous pointing of phosphor bronze wires has been the environment can be controlled at a level of less 
than 50% to increase the reliability of pointing. The 
wave 
rectified sinusoidal current, which is found to provide 
more reliable control of the point. 
The novel features Of the invention are Set forth with 
particularity in the appended claims. The invention Will 
be best understood from the following description when 
read in conjunction with the accompanying drawings. 
accomplished by electrochemical etching of the wire in 
a solution containing chromium trioxide, which is ex- 
tremely toxic. It has been found that well formed wire 
tips can be consistently produced using an electrolyte 
16 which is an aqueous sulfamic acid solution. A solu- 
tion which has been found to produce good results 
includes 4% sulfamic acid crystals (H2NS03H) by 
weight dissolved in deionized water. Sulfamic acid is 
used chiefly as a weed killer, in cleaning metals, and in 
the form of salts as a flame proofing or softening agent 
for papers and textiles. Its relatively low toxicity allows 
the pointing method to be carried out without extreme 
precautions against poisoning. A variety of other elec- 
trolytes have been tried, but they resulted in a wire 
point that is too blunt, or which is concavely tapered, or 
which has a rough surface. Also, a variety of sulfamic 
acid concentrations were tried, ranging from under 1% 
of sulfamic acid crystals by weight in water, to a satu- 
rated solution of about 50% sulfamic acid crystals, but a 
current can be a substantiah' 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a simplified side elevation and schematic 
view of a pointing method conducted in accordance 
with the present invention. 
FIG. 2 is an enlarged view of the area 2-2 of FIG. 1. 
FIG. 3 is a view similar to FIG. 2, but shown at the 65 
FIG. 4 is an enlarged view of the final wire or whis- 
6o 
end of the electrochemical etching step. 
ker tip produced by the method of the invention. 
3 
4,3153 96 
4 
solution of about 4% was found to produce the best can be accomplished by electrochemical etching, using 
results. an aqueous sulfamic acid solution as the electrolyte. 
The system 10 used for pointing the wire 14, includes Control of the length of the etching process can be 
a current source 30 which produces a half wave recti- determined by monitoring the current flowing through 
fied output of constant peak voltage. The particular 5 the wire as a voltage is applied, between an electrode in 
source 30 includes a pair of transformers, including a the solution and the wire, that is constant (the peak in 
first one 32 connected to a 110 volt 60 Hz power outlet each voltage cycle is the same, and the cycle time is 
to provide an adjustable voltage, and another one 34 orders of magnitude smaller than the process time). The 
which provides a voltage stepdown. The output of the electrochemical etching is stopped when the current 
second transformer 34 is connected through a switch 36 10 falls to a predetermined level. The current is preferably 
and a diode 38 to the immersed electrode 18 and to the a half wave rectified current. Control of the humidity in 
wire. The output of the voltage source 30 is a halfwave the environment to a low level that is preferably below 
rectified sinusoidal voltage. Various voltage devices 50%, further enhances the reproducibility of the pro- 
have been tried, including those that produce substan- cess. 
tially ripple-free DC, sinusoidal AC, and full wave rec- 15 Although particular embodiments of the invention 
tified AC, as well as the half wave rectified AC. Of all have been described and illustrated herein, it is recog- 
of these, the half wave rectified AC output has been nized that modifications and variations may readily 
found to provide more consistent wire tip shapes. occur to those skilled in the art and consequently, it is 
It is important to determine precisely when the elec- intended that the claims be interpreted to cover such 
trochemical etching process should be halted. This is 20 modifications and equivalents. 
accomplished by monitoring the current flowing What is claimed is: 
through the wire, which is found to rapidly decrease as 
electrochemical etching progresses, and by ceasing the 
process when the voltage reaches a predetermined 
level. In the system of FIG. 1, the output of the voltage 25 
source 30 had a peak voltage of 17 volts, which resulted 
in a peak current through the wire 14 of about 5 milli- 
amperes at the beginning of the process. After about 5 
seconds, the current had dropped to about 1 milliam- 
pere and the process was then stopped. A switch 36 was 30 
initially closed to begin current flow and the switch was 
opened as soon as the predetermined current level was 
reached. An oscilloscope 40 was used to monitor the 
pointing cell voltage and current, with the horizontal 
input to the oscilloscope connected to the high side of 35 
the voltage source 30 and with the vertical input con- 
nected across a current sensing resistor 42 on the low 
side. In FIG. 1, the screen of the oscilloscope 40 shows 
in solid lines the display at the beginning of the process, 
and in phantom lines the display at the end of the pro- 40 
cess. 
It has been found that the moisture in the environ- 
ment around the electrolyte bath 16 affects the wire 
pointing process. When the humidity is relatively high, 
the point shape is not as consistent as when the humidity 45 
is low. A relative humidity of less than about 50% is 
desirable. In tests that were conducted, the humidity in 
a room environment was reduced by using a dehumidi- 
fier and an air conditioner to lower the relative humid- 
ity, although the minimum obtained was about 40% 50 
when the outdoor humidity was high. It is believed that 
the low humidity is useful in controlling the meniscus 26 
about the end portion of the wire, as by permitting 
evaporation of the upper end of the meniscus to pro- 
duce a wide cone angle in the tip. Of course, most of the 55 
weight and volume of the electrolyte is water, so that 
moisture in the air affects the vaporization rate of the 
water. While acceptable results have been obtained 
even at high humidity levels considerably above 50%, it 
is found that lower humidity levels significantly im- 60 
prove the reliability of the process. 
Thus, the invention provides a method for forming 
points or tips of controlled shape at the ends of thin 
wires, and especially of phosphor bronze wires. This 
1. A method for pointing phosphor bronze wire com- 
dipping an end portion of a wire to be pointed in an 
electrolyte, and passing current between said wire 
end portion and said electrolyte; 
said wire formed of phosphor bronze, and said elec- 
trolyte consisting substantially of sulfamic acid 
diluted in water. 
prising: 
2. The method described in claim 1 including: 
maintaining the humidity in the region above said 
3. The method described in claim 1 wherein: 
said step of passing current includes applying a half- 
wave rectified substantially sinusoidal current to 
said wire. 
electrolyte below 50%. 
4. The method described in claim 1 including: 
said step of applying current includes applying a 
controlled voltage between said wire and an elec- 
trode immersed in said electrolyte, monitoring the 
current passing through said wire, and ceasing the 
passage of current through said wire when the 
current falls to a predetermined level. 
5. The method described in claim 1 wherein: 
said electrolyte consists of qbout 4% sulfamic acid by 
weight and about 96% water by weight. 
6. A method for pointing a wire, comprising: 
immersing an end portion of said wire in a bath of a 
liquid solution which wets to said wire so that a 
liquid meniscus forms around the wire and etch- 
ing away wire material at said meniscus; 
said wire is formed of phosphor bronze; and 
said solution consists of about 4% by weight of sul- 
7. A method for pointing a wire comprising: 
immersing a clean end portiop of a phosphor bronze 
wire in a bath of sulfamic acid; 
applying a controlled voltage between said wire and 
a location of said bath, to electrochemically etch 
the wire end portion; and 
monitoring the current passing through said wire, and 
ceasing the electromechanical etching when the 
current falls to a predetermined level. 
famic acid crystals dissolved in water. 
* * * * *  
65 


Thin wire pointing method

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server