December 2009 Archives

Chemical milling is also called "Chemical contour machining", and is a metal milling technique by utilizing chemical etching process. With this technique the etching depth is limited to the thickness of the material to create three dimensional features, unlike through etching techniques.
Chemical milling typically follows a process shown in [Fig.1]. There are four major process blocks: Pre-process; Masking; Etching; and Post process.

When milling the entire surface by etching, proceeds directly to the etching process since the masking process is not needed. When creating steps on the surface, multiple "etch and mask removal" processes are repeated.

(1) Masking

Masking techniques for chemical milling are: (1) with photo resist; (2) screen or offset printing; (3) by scribe and peel; (4) mechanical masking.
The photo resist method, as described previously, creates very fine and accurate but thin resist patterns that provide small chemical resistances to strong alkali and acidic etchants. It is mostly used only for small depth applications.

The screen printing method uses screens (mesh) made of synthetic fiber such as nylon and polyester, or fine filament of stainless steel. Negative images of the needed patterns are created on the screen, then resistive ink is press-roller applied onto the substrate through the screen to create the final patterns.

With the scribe and peel method, masking material is first applied on the entire surface of the substrate, then the masking is cut along a template with a sharp cutter and peeled off to expose the area to be milled.

Mechanical masking method uses a flexible and corrosion resistant rubber template mechanically affixed to the substrate to cover the non-milled area of the substrate. Due to its nature, the mechanical masking methods can only be used for simple patterns.

An appropriate method is chosen from these methods based on required accuracy, repeatability, process speed, and economy.

Challenges regarding photo forming are the process for the plate separation and plating layer thickness management. Oxide, chromate, sulfide film are used for the separation layer. [Table.1] shows some examples of the process configurations.

[Table.1] Separation film forming processes

Electroplating is usually used for its speed of deposition. A variety of plating solutions can be used, but copper and nickel are mostly used. In rare instances gold and silver are used.
Electroplating is typically performed with a system such as the one shown in [Fig.1]. Continuous filtering of the solution is a special point to note in order not to allow precipitation of foreign objects within the solution and from anode slime. A fine meshed bag is also used on the anode to prevent slime from intruding. Organic impurities that degrade the plating glossiness and physical properties are removed by activated charcoal treatment.

In order to ensure unified plating layer thickness, the plating bath solution is stirred, as well as the substrate is rotated and rocked. This is to adjust the distance from the anode and to account for the effects of agitating the solution. [Fig.2] shows how the plating layer grows and reaches the desired thickness by continued plating.

Good products are obtained when the plating layer thickness is within the resist's thickness. But when the plating becomes thicker than the resist layer, the plating spills over the pattern and the accuracy is degraded. The excess plating spilled over is called Side Spreading.

Photo Forming is a technique that combines the photo resist pattern forming (previous tutorial) and Electro-forming and/or Chemical Plating. This forming technique does not utilize any chemical etching, but uses an identical pattern forming methods as photo-resist patterning and often used in the same field where photo etching is used.

The photo-forming is a process of forming metal parts by building up electro-plating or chemical plating layers on a substrate. A photo resist is applied on a substrate to create a photo etching pattern, and by growing a thick plating layer where photo resist is void (on bear metal portion) a metal part can be harvested.

Photo etching and photo forming are compared in [Fig.1] below.

As can be seen in [Fig.1], the pattern forming techniques are the same with both processes but the substrate materials used are different. With photo etching, the resist material is applied on a metal work piece, where conductive film is applied on a substrate with photo forming. The substrate does not necessarily need to be metal. It can be insulating material such as plastic, glass, and wax where conductive film can be applied and plated. The final product is peeled off of the substrate after desired amount of plating is deposited.

Since the final product is to be peeled off of the substrate after the plating is done, the substrate surface must be pre-processed to promote ease of peeling. Commonly used substrates are made of steel, nickel, stainless steel, aluminum, copper, and brass for workability and economy reasons. These metal substrates are de-greased, pickled, and activated using the standard methods, then photo resist is applied, exposed, and developed. After the desired patterns are formed, thin film of oxide, sulfide, and chromate is deposited, then plated.

Etching methods that use chemical resistant photosensitive film are called Photo Etching, or Photo Engraving.

The photo etching is a process of obtaining intended patterns by: depositing a film of photo-resistive material on a metal work piece > light expose a pattern on the film using a separately created photo-mask template, causing the exposed portion developer solution insoluble > dissolve the unexposed portion by immersing in developer solution, leaving the final pattern intact. (There are types of photo-resist material that exposed nature is opposite.)

This method is also used for chemical blanking as well as chemical milling. In either case, pre/post processes are the same. Typical photo etching process is shown in [Fig.1].

To perform a micro-fabrication process with etching requires a creation of a Photo-Mask. This is an important process step since the final product quality is largely affected by the accuracy and quality of the photo-mask. Typically, the photo-masks are produced by photographic processes where enlarged original design drawings are reduced into film or dry-plates of the actual work sizes. The process of making the enlarged original drawings is called "Artwork", and making of the working size photo masks is called "Camera work".

For economical and handling purposes, multiples of small parts are made on the same photo mask original. By doing so, many small parts can be produced from a single metal piece at once.

There are two types of photo-resists, Negative resist and Positive resist. The negative resist is a type where exposed portion becomes insoluble to developer solution, and the positive type becomes soluble where exposed. To crate photo-resist patterns on work pieces, a negative image photo mask for the negative resist, positive image photo mask for the positive resist are used. Typically, the negative resist is used. After the exposure and develop processes are completed, the work can be processed for chemical blanking.