Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For properties of solid material
Reexamination Certificate
1994-11-08
2001-04-10
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For properties of solid material
C204S414000, C204S434000, C205S775000
Reexamination Certificate
active
06214210
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods of electrochemical analysis and, in particular, to the use of gel electrolytes for electrochemical analysis of metal or semiconductor surface characteristics.
BACKGROUND OF THE INVENTION
A major cost problem experienced by the electronics industry is the loss of solderability of electronic components, particularly during storage. Poor solderability of electronic component leads and printed wiring boards accounts for a large percentage of solder joint failures. Previous studies have determined that oxidation of the surface and underlying substrate and/or intermetallic layers of solderable components is a cause of this degradation. Solderability and the basic methods of sequential electrochemical reduction analysis and restoration of solderability are described in U.S. Pat. Nos. 5,262,022 and 5,104,494, the teachings of which are hereby incorporated by reference.
In the method of sequential electrochemical reduction analysis, surface oxides that interfere with the solderability of metals are detected by electrochemical reduction. The resulting data yields both the types and amounts of oxides present. For analysis of printed wiring board (PWB) through-holes and surface pads, an electrolytic solution is brought into contact with the area to be tested and electrical contact is made through another PWB feature that is electrically interconnected with the test area. Large component leads, such as resistor wires, can be evaluated conveniently by immersing the portion to be tested in the electrolyte solution and making electrical contact to a part of the lead above the level of the solution. However, testing of fine-pitched component leads is difficult because electrolyte solution tends to extend up the lead to the component body by capillary attraction. Because little or no unwetted area remains on the component lead, the cathode connecting lead generally comes into contact with the electrolyte solution. Thus, the cathode lead must have a high hydrogen overvoltage (i.e., the same as or higher than that of the tested component) and must be pre-reduced to avoid measurement errors from its reaction with the electrolyte. Also, the upper part of the component lead, where capillary attraction or “wicking” occurs, is usually not typical of the area to be soldered and can give misleading results when included as part of the analyzed area. Furthermore, penetration of electrolyte solution into non-hermetic seals between leads and the component body can result in damage to the device and grossly erroneous data from electrochemical analysis.
Another problem with many electrochemical analytical methods, including sequential electrochemical reduction analysis, is interference from oxygen that is present initially or is introduced through leaks in the electrolyte solution containment system. Electric current associated with electrochemical reduction of oxygen tends to mask the processes of interest and introduce errors into the analysis data. Thus, there is a need for an improved, quantitative, nondestructive method of analysis that is easily adapted for electrochemical testing of various electronic and corrosion resistant components.
SUMMARY OF INVENTION
The present invention comprises a method and apparatus for electrochemical analysis of metal or semiconductor surfaces using a deoxygenated gel electrolyte. Use of a gel electrolyte is applicable to known processes, such as sequential electrochemical reduction analysis for determining solderability of electronic components which is described in U.S. Pat. No. 5,262,022, and electrochemical surface analysis of metallic corrosion resistant materials, for example. In sequential electrochemical reduction analysis, performed according to the method of the present invention, a solderable portion of an electronic component or circuit board to be tested is placed in contact with a gel electrolyte, such as a deoxygenated borate buffer solution having a gelling agent. The solderable portion of the component is connected to the cathode of a direct current power source. A second, counter electrode, typically comprising an inert material such as platinum or stainless steel, for example, and a third, reference electrode, such as a saturated calomel electrode (SCE), are also placed in contact with the gel electrolyte. A small current is passed between the counter electrode and the solderable portion of the component in contact with the gel electrolyte while the potential between the cathode and the reference electrode is recorded as a function of time to analyze the solderable portion of the component for surface oxides. In systems where the counter electrode has a stable voltage at the low currents used, it can also function as the reference electrode, thereby eliminating the need for a separate reference electrode.
Use of a deoxygenated gel electrolyte provides several advantages over a conventional liquid electrolyte in processes such as sequential electrochemical reduction analysis. Compared to a liquid electrolyte solution, a gel electrolyte prevents capillary attraction or “wicking” up and/or along an electrical component lead. Prevention of wicking allows electrochemical analysis to be localized to the area of interest, such as the portion of the lead to be soldered, and keeps the electrolyte from coming into contact with the cathode lead from the power source. Also, the interfering effects of atmospheric oxygen are minimized for deoxygenated (or deaerated) gel electrolytes because convection mixing of atmospheric oxygen is practically absent and diffusion is generally a very slow process in a gel. For a gel electrolyte saturated with inert gas, for example, brief exposure to oxygen as test specimens are changed is not sufficient to affect the electrochemical analysis. In some cases, it is possible to perform the electrochemical analysis in ambient atmosphere rather than in an inert gas atmosphere as is generally required in the prior art.
A principal object of the invention is improved electrochemical analysis of metal or semiconductor surfaces. A feature of the invention is the use of a deoxygenated gel electrolyte in electroanalytical methods such as sequential electrochemical reduction analysis to determine solderability of electronic components. An advantage of the invention is electrochemical analysis of metal or semiconductor surfaces where the presence of oxygen introduces errors into the analysis and where capillary attraction of liquid electrolytes is undesirable.
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Tench D. Morgan
White John T.
Deinken John J.
O'Shaughnessey James P.
Rockwell Technologies LLC
Tung T.
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