Page 42 - PCMI Journal July 2018
P. 42
David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining of Aerospace Mater5i/a3ls1/18
Issue 131 July 2018 PCMI Journal 41
Sodium hydroxide etchants for aluminium (3)
Problem
Solution
Evolution of H2 – clings to resists’ edges and impairs etching
Add potassium ferricyanide:
General Motors US Patent 3,532,569 of
1967
Highly exothermic – destroys resists
Reformulate etchant
GM Patent etchant too slow
(0.33μm/min at RT)
Optimised formulation of Gerlagh and Baeyens (1975) gave etch rate of 9 μm/ min at 55°C
Poor etch factor (~1) and slow etch rate of Gerlagh and Baeyens formulation
Raise temperature of etchant to 90°C?
Alkaline etchants can strip aqueous photoresists
Use acidic etchants
The effect of potassium ferricyanide addition to sodium hydroxide etchant
Without potassium ferricyanide Al + -OH + H2O → AlO2- + 11⁄2H2
With potassium ferricyanide
Al + 4-OH + 3Fe(CN)63- → AlO2- + 2H2O + 3Fe(CN)64- ferricyanide ferrocyanide
i.e. hydrogen gas evolution has been prevented
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