June 2010 Archives

Chemical and electrolytic etching have been in use as the fundamental technology in all areas of applications in electronics, precision machinery, and printing industries. In some application areas etching has been found obsolete by the emergence of more advanced technologies, but etching still remains to be an important processing technology. Some examples are introduced below.

(1)Thin film etching for circuit forming

Electronic circuits used for computers and communications equipment are comprised of printed circuit boards with integrated circuits, resistors, and capacitors of microscopic sizes. Demands for increased density for the integrated circuits require these resistor and capacitor components to become smaller year after year. For case of resistors as an example, conventional wire wound construction where a fine resistance wire wrapped around a insulator has limitations for miniaturization. To overcome the limitations, the resistors are now produced by etching a thin resistance film material formed on a insulator surface, where the etching produces a resistance circuit pattern with a desired resistance value on the insulator substrate.

For the resistance material, nickel/chrome alloys and tantalum film are typically used. The etchants used are mixtures of nitric acid, hydrochloric acid, and water.

Conventionally, the thin films are formed by a vapor deposition method such as sputtering, but electroless nickel plating and other methods are becoming popular in order to reduce the production costs.

For the electroless nickel plating, there are Ni-B alloy and Ni-P alloy types. A plating solution that uses sodium borohydride as a nickel redox agent produces a nickel alloy containing boron B, and a solution using sodium hypophosphite produces a nickel alloy containing phosphorus P. These nickel alloy films Ni2P, Ni3P, Ni2B, and Ni3B require considerably strong etchants for the etching processes since there would be some remaining phosphorus and boron smuts that are hard to etch. Typically, a solution of sulfuric acid, nitric acid, and water (1:1:1) heated to 40~50 deg. C is used. Therefore, the resist material used for protection of non-etched areas must also withstand this etching solution.

(2)Photo-etching use in printing industry.

Printing is a technology to transfer ink images such as characters, graphics, and photographs on paper, metal, or plastic surfaces, and requires printing plates.

Historically, printing technology has advanced through woodblocks, lead movable type, lithography, copper intaglio, and has now evolved into the photomechanical processes in use today. There are some type setting techniques in use for character printing, but graphics and photograph printing all rely on photo-etching technology. This is a technique of producing printing plates by creating concave or convex images of photos and graphics on metal plates. By photo-printing the acid resistant images on metal plates, then an etching process used to produce these printing images. For relief printing, zinc, magnesium, and copper are used for the metal substrates. These metal contain special metal additives to promote printing precision enhancements by reducing the sizes of metal crystals. For the etchants, nitric acid, water solution of sulfuric acid + hydrochloric acid, and in case of copper plates ferric chloride are used.

A type of lapping operation using electrolytes in place of the usual lapping fluids is called Electrolytic Lapping. With this method, the lapping action progresses as the surface layer is removed by electrolytic etching. Instead of the fixed abrasives used in electrolytic grinding and honing, loose abrasives mixed in the electrolytes are used with this method. There are three types of electrolytic lapping, as shown in [Fig.1], depending on the applied method.

In the method (a), the work piece (anode) and the lap (cathode) is spaced at a certain distance (0.05~0.2mm), and a solution mixture of electrolyte and abrasives (called Electrolytic Slurry) flows in the gap. The slurry flow is created by the lap's rotation and removes the electrolytic byproducts from the surface by abrasion.

With the method (b), the work piece and the lap are pressed against each other with the abrasives in between. To avoid the work piece and the lap from electrically shorting, pressing pressure, abrasive size, and the mixture ratio are carefully adjusted. The lap's shape and slurry injecting method is carefully engineered also.

In the method (c), the lap is made of electrically insulating material (i.e. hard urethane), and the cathode surface is made a slightly concave shape in relation to the lap surface. In this case, there will be no concerns for electrical shorts even when using small abrasive sizes, so fine finish surfaces can be obtained by using fine abrasives in 2,000~3,000 mesh sizes. However, the gap distance will be larger than (a) or (c), and the metal area is relatively small, resulting in slower processing speeds. This type of electrolytic lapping is called Compound Lapping.

The electrolytic lapping can be used for a variety of machining applications depending on the lap shapes and motion patterns. From the industrial application stand point, the electrolytic lapping can be classified into the following categories. One is called "Electrolytic mirror finish lapping" where the work piece may be either too hard and brittle or too soft, but needs to be finished into mirror smooth surface. The other is for grinding hard materials at speeds similar to electrolytic grinding, and is typically called "Electrolytic lapping".

There is a process very similar to grinding called honing. This process uses oil grinding stones mounted to a retaining device called "hone". Applying pressure and rotary/reciprocating movements between the grinding stones and the work piece while injecting large amounts of kerosene or diesel, the work is processed. As compared to grinding, this process removes less material but the finished surfaces are extremely fine. This is applicable for final finishing of surfaces after grinding for precision parts.

Combining the mechanical honing outlined above with electrolytic etching is the electrolytic honing. The mechanism is shown in [Fig.1] below.

As can be seen in the figure, the work piece is the cathode, and the tubular metal tube tool is the anode. This tube tool is the equivalent of the hone in the mechanical honing, and is made of stainless steel for corrosion resistance nature. The gap between the two is maintained to 0.07mm~0.12mm and the metal tube is rotated and reciprocated. The tube has several slots that hold the bar/stick shaped grinding stones. The mechanism is configured similar to that of mechanical honing where the grinding stones are pressed against the work piece at a constant pressure.

The tubular cathode tool has a length enough to cover the entire process area during the reciprocating motion. There are many small holes provided laterally.

The electrolyte (normally a solution of NaCL) is supplied through the interior of the cathode tool, and flows out of laterally placed holes. When the rotary/reciprocating motion is applied to this arrangement with direct current applied, the grinding stones would remove small peaks on the metal surface while the etching action would keep fresh metal exposed, thus honing and etching actions are optimally combined.

Electrolytic honing, in comparison to mechanical honing, causes less wear on grinding media, causes no burrs, and leaves no residual stress and heat induced damages on fished surfaces. The electrolytic honing process uses relatively low current density of 20~45A/cm^2, while the pressure required to supply the electrolyte into the working gap is high at 10kg/cm^2. In some cases, better finish can be obtained by keep honing with grinding stones for several seconds after the electrical current is turned off.

Electrolytic grinding is most suitable for parts that are difficult to remove burrs from. Even if mechanical grinding may be more efficient in material removal, for parts that are troublesome time consuming with burrs and deformations, electrolytic grinding is more advantageous.

[Fig.1] is an automotive valve body made of cast aluminum alloy, as an example.

The material is not particularly difficult to grind, but the part has a complex pattern on its surface and is very time consuming for burr and deformation removal if ground with conventional mechanical methods. By electrolytically grinding this part, grinding efficiency will be lower but the total merit would be high since there is no issue with burrs and deformations.

The above is true for grinding honeycomb shaped ends of parts also. Mechanical grinding will cause burrs on the honeycomb structure, but electrolytic grinding will avoid this.

For thin parts that require a relatively large amount of material removal, conventional mechanical means would have to grind small amount at a time from both sides, many times back and forth to avoid the material from deforming. Electrolytic grinding is not limited by such shortcomings.

For example, gauge plates (made of special tool steel) with 0.5mm thickness have 0.3mm material removal required. With a conventional machine grinding, only 50 parts per day was the production limit, but electrolytic grinding enabled this limit to be raised to 300 per day.
The example shown in [Fig.2] are knitting machine needles. The needles are made of SK5 alloy, hardness HRC40, the work table chuck fixtures 200 needles, cutting depth is 0.7mm, applied current is 75A, process time per part is 0.8 seconds, and the accuracy of machining is ±0.01mm.