For Printed Circuit Board Developing Lines Using Alkaline Etchants, Can Commercially Pure Titanium Replace Tantalum as an Immersion Heater Sheath Material?

This is a particular material challenge in the development lines for printed circuit boards using alkaline etchants, usually sodium or potassium hydroxide solutions at 50-60oC. Tantalum has been the material of choice for immersion heaters in this service because of its excellent resistance to caustic attack. However, the high cost of tantalum (around 15-20 times that of titanium) maintains the interest in economically pure titanium as a viable substitute. The question is not if titanium survives in alkaline etchants - it does, under some conditions - but if it can match the reliability and service life of tantalum in the specific environment of PCB developing lines where etchant chemistry contains not only hydroxide but also complexing agents, surfactants and trace metal ions from dissolved copper. The corrosion resistance of Grade 2 titanium and tantalum in alkaline PCB etchants is compared and the conditions for the successful substitution of titanium for tantalum are determined.

Corrosion Behavior of Titanium in Alkaline Media
Titanium possesses an amphoteric oxide coating that is stable in acidic and alkaline environments, although its stability range is limited. In sodium hydroxide solutions at concentrations below 20% and temperatures below 80°C, Grade 2 titanium will retain a passive layer with corrosion rates below 0.02 mm per year. With rise in concentration or temperature, the passive film starts to dissolve and titanium produces soluble titanate species (Na 2 TiO 3 or Na 4 TiO 4 ). Grade 2 titanium will corrode at ~0.03-0.05 mm/yr at 60C in 15% NaOH, typical of PCB alkaline developing lines, well within acceptable limits for a 1.65 mm wall tube expected to last 5-7 years.

The more serious problem is caustic stress corrosion cracking (SCC). Titanium can be vulnerable to SCC in concentrated hydroxide solutions at elevated temperatures (over 70°C) when residual tensile stresses are present from tube bending or welding. The danger of SCC is minimal but not negligible at the average PCB development temperature of 55 to 60 C. Welded tube ends and mechanically expanded tube sheets have residual strains that might induce cracks if etchant concentration drifts upward or temperature excursions occur. However, tantalum is virtually impervious to caustic SCC up to 150°C.

Effect of Etchant Additives and Copper Contamination
Commercial alkaline PCB etchant additives have dramatic effect for titanium performance. Organic chelating chemicals like EDTA or ammonia-based compounds may chelate titanium ions and speed up the disintegration of the passive layer. Surfactants can adsorb at the surface and protect it or diffuse into flaws of the film and assault the surface. Most importantly, the dissolved copper from the etched circuit boards collects in the etchant bath. By use of displacement reactions (Ti + Cu2+ → Ti2+ + Cu0) it is possible to deposit metallic copper on titanium surfaces as copper ions (Cu2+). This copper deposit forms a galvanic couple: copper is cathodic to titanium in alkaline solutions, causing anodic dissolution of the titanium at the borders of deposits. Localized attack under copper deposits has perforated 1.65 mm titanium tubes in PCB duty in 12-18 months, whereas tantalum is noble enough that copper does not deposit or produce galvanic assault.

Application Matrix: Titanium vs. Tantalum for Alkaline PCB Heaters Parameter Typical Bath Conditions for EtchingTitanium Rather Than Tantalum?Expected Titanium Life (1.65 mm) Suggested Material
fresh developer no copper clean system 10-15% NaOH or KOH 55°C no additionsYes, complete 5-8 years Grade 2 titanium.
Production developer, low copper (<5 g/L) Same plus 2-5 g/L CuYes, with monitoring 3-5 years Grade 2 titanium
Production developer, high copper (5-15 g/L) Same plus Cu 5-15 g/LNo, rapid attack 12-24 months Tantalum or Grade 7 titanium Etchant with EDTA or complexing compounds NaOH 10-15% + chelatorsNo, fast film attack 6-12 months Tantalum exclusively
High temperature line (65-70°C) 15-20% NaOH, thermal stressSCC risk No <12 months Tantalum
Stripper line (base + organic solvent)Organic amines (variable)* Titanium attacked No. * <6 months Tantalum or steel lined
Conclusion for PCB Heater Spec.
Commercially pure titanium may be considered as a substitute for tantalum as an immersion heater sheath material in alkaline PCB developing lines, but only under certain, highly regulated conditions: hydroxide concentration less than 15%, temperature less than 60 °C, dissolved copper less than 5 g/L and no aggressive complexing agents. Under such conditions, Grade 2 titanium, with 1.65 mm wall thickness, offers 3-5 years of service at about 5-10% of the cost of tantalum. If copper build-up on the production line is greater than 5 g/L, or if the line employs chelated etchant or any service beyond 60°C, then titanium will fail prematurely from galvanic attack or caustic SCC. In these circumstances tantalum continues to be the proven standard and attempts to substitute titanium result in unanticipated downtime and bath contamination that far outweigh any initial material savings.