November 2009 Archives

#021 Chemical Etching - Post Processes

Chemical blanking post processes include: removal of masking material; rust inhibitor treatment; finishing and inspection.

Parts that are created by chemical blanking are seldom used with the masking material still remaining on the surfaces. The remaining mask material must therefore be removed by some means.

There are various masking materials used and there are dedicated removers specified for them. In other cases, the mask materials are removed by immersing the parts in trichloroethylene, xylene, or benzene.

Typically, the masking materials are more easily dissolved in alkali. Photo-resist material (material used for creating patterns by light exposure) are especially so and usually removed by immersion in heated caustic soda 10~30% solution.

If the masking material is not easily removed by simple immersion alone, scrubbing with gauze and/or brush is applied. For mass production schemes, machines that remove the masking material by spraying are used.

After removing the masking material, water wash and drying are performed. If necessary, rust inhibitor application process is to follow.

A point of caution for etching process is Side Etch. The corrosion of etching process advances vertically as well as horizontally. The horizontal direction advancement is called Side Etch (s), or Side Cut. The figure [Fig.1] below is a cross sectional view of etched surface. The area of side etch (s=(w2-w1)/2) increases as the etching depth (d) increases.

Nature of the side etch phenomena vary with the metal property, composition of etchants, and etching conditions. Little issues exist when the metal thickness to be etched is small (i.e. several micrometers), but thicker metal will be troublesome.

To minimize the side etch effects, selecting proper etchant becomes important. Etchants with corrosion inhibitors are often used. Etching methods also affect this. The spray type and paddle type etching systems where the etchant is applied perpendicular to the targets will result in smaller side etch as compared to immersion etching method.

[Fig.1] Cross section view of etched surface

#020 Chemical Etching - Etchants

 Many etchant (corrosive liquid) compositions and temperature settings are used according to the metal material and process speed requirements. (Table 1) shows the etchant compositions for representative metals and process conditions.

(Table 1) Etchants used for chemical etching
Metal materialEtchant compositionLiquid temperature (℃)Process speed (mm/min.)
Aluminum (AL) and alloysNaOH(150-180g/L)+AL(6-20g/L)70-800.018-0.025
FeCL3(120-180g/L)500.025
HCL(50vol%)+HNO3(5vol%)500.025-0.05
FeCL3(30Be,3.8L)+HCL(38%,34ml)45-
Copper (Cu) and alloysFeCL3(30-40g/L)500.025-0.05
(NH42S2O8(8-16g/L)32-500.013-0.025
CuCL2(30-35g/L)550.013-0.015
CrO3(22-30g/L)500.025-0.035
Chrome (Cr)FeCL3(42Be,2vol)+HCL(38%,1vol)80-
HCL(38%,3vol)+H2O(7vol)--
Gold (Au)HCL(75vol%)+HNO3(25vol%)32-380.025-0.05
NaCN+H2O2--
MgHNO3(12-15vol%)32-500.025-0.05
MoH2SO4(25vol%)+HNO3(25vol%)550.025
HNO3(30vol%)+HCL(30vol%)--
Nickel (Ni) and alloysHNO3(48vol%)+H2SO4(5.5vol%)+H3PO4(11vol%)+HC2H3O2(5.5vol%)43-500.025
FeCL3(34-38g/L)500.013-0.025
HNO3(1vol)+HCL(1vol)+H2O(3vol)--
Silver (Ag)HNO3(50-90vol%)38-500.013-0.025
Fe(NO33(30g/L)550.020
CrO3(40g/L)+H2SO4(20mL)+H2O(2L)--
Steel alloysHNO3(20vol%)+H2SO4(5vol%)+H3PO4(5vol%)57-700.018-0.025
FeCL3(34-38g/L)500.025
HNO3(10-15vol%)500.025
Stainless steelHCL(50vol%)+HNO3(5vol%)+H3PO4(2.5vol%)650.01
FeCL3(34-38g/L)650.02
Tin (Sn)FeCL3(34-38g/L)55-
TiHF(10-50vol%)30-500.013-0.025
Metal materialEtchant compositionLiquid temperature (℃)Process speed(mm/min.)

#019 Chemical Etching - Chemical Blanking

Chemical blanking is used for parts that are otherwise typically produced by mechanical blanking presses from thin plates and foil material. With mechanical presses, vibrations, backlash, and part distortion will make smaller parts difficult to produce. Chemical blanking becomes a favorable solution in such a case.

(1) Process steps

Chemical blanking employs the following four steps.

a) Pre-process

Material cleaning - Metal surface is cleaned by degreasing, pickling and grinding.

b) Masking

Metal portion not to be removed by etching is covered with chemical resistant coating. For extra precise applications, photo-resist material is used.

c) Through-material etching

Since the material is to be penetrated through, etching is applied from both sides simultaneously.

d) Post-process

Removal of the resist material, and washing off etchant. If the resist material is non water soluble, organic solvents are used.

(2) Through-material etching

There are two application methods: Immersion type and Spray type. With the immersion type, the part is immersed in the corrosive liquid, and the liquid is constantly stirred. Air injection is the widely used stirring method.

Paddle type etching machine

The spray method of etching employs specially designed etching machines. Spray type machine has nozzle(s) that spray the etchant, and Paddle type has a rotating wheel with paddles that spray the etchant. Both types are made in vertical and horizontal configurations, as well as single sided etching and dual side etching types.

Spray type etching machine

#018 Chemical Etching - Chemical Blanking

Processing techniques utilizing the chemical etching are collectively called "Chemical machining". This is a process of obtaining desired shapes and dimensions by restricting and focusing the metal dissolving chemical on substrates.

Chemical etching can be divided into the following categories.

(1)Chemical blanking
By etching through the material, parts of various shapes can be cut out (blanked) from thin plates and foil material.
(2)Chemical machining
By limiting the etching depth to less than the thickness and only partially removing the relatively thick material, three dimensional features are created.

These two etching methods are different only in terms of whether going through the material or not, and the methods of protective masking the areas no to be removed with resist coatings as well as the etching methods are the same. By this way of categorizing, the etching process to create precision parts and electronic components from thin plates correspond to Chemical Blanking, and the etching process used for printing plates and name plates correspond to Chemical Machining.
The etching processes used for printed circuit boards and integrated circuits can be considered as chemical blanking since the subject copper material is etched through, but since the plastic portion of the substrate remains completely unaffected, it is considered somewhat different from the conventional chemical blanking of all copper substrate.
These processes employ some means of applying protective resist material to manage and concentrate the effects of the etching, and to protect non-etched area of the material. As opposed to many other means of processing/machining, the etching processes do not use the similar tools or tool equivalents. Various machining, electro-discharge machining, electrolytic machining, and ultrasonic machining processes use tools or equivalent electrodes, and their relative movements against the materials to be processed result in the final products.
As explained above, chemical blanking and chemical machining differ only as to whether the material is bored through or not, but the process characteristics will vary according to this difference and the application areas may also widely differ.
An example of a printed circuit board.

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