Specialty Test
   

After the base chemistry stress profile has been established, it is important to define effects process variables have on it. Solution temperature increase, for instance, in a sulfamate nickel bath, will shift the entire stress profile curve lower, towards the more compressive values. Once such understanding has been gained for all or at least a few critical variables, it is relatively easy to optimize the process for the most desirable stress profile, the highest possible plating rate or any other objective. A useful outcome of this part of the study will also be the distinction between critical and trivial process variables in terms of stress control.

Naturally, for this effort to be worthwhile all work needs to be done in a system that is known to be free from impurities and otherwise representative of the process.

The second important characteristic an electroformer needs to be aware of is the process window, or the range of internal stress levels within which parts can be successfully electroformed. This should be established empirically, but is well worth the effort and expense of doing. Comparing the process window to the bath stress profile will help define the appropriate current density range for successful electroforming and give an overall process estimate in terms of its suitability for a given task. Processes with steep stress profiles are usually less suitable for electroforming, while a relatively flat stress-current density curve makes for an easy and versatile operation.

Armed with the knowledge of the optimized process stress profile and the process window, one can now determine how s/he wants to control stress in the system. Decisions need to be made as to what variable(s) to use for stress control, what level of stress to maintain at what current density, and how frequently to measure and adjust stress in the bath. It is easy to see that the process window defines the desirable internal stress range.

The number of ways people control stress during electroforming is as great and diverse as the number of variables that affect it. Among the most notable methods are: average current density adjustment during deposition6 , temperature adjustments, addition of organic stress-reducing agents8 such as saccharin and naphthalene-sulfonic acid derivatives, varying solution agitation rates, etc. It is imperative in this approach that all process variables except for the control ones should be kept constant, while the control variables are changed only in response to observed stress changes in the system. A sophisticated computerized stress control system6 based on these principles has been successfully implemented in an industrial application.

Regardless of the chosen method of stress control, it is important that stress readings in the process are taken and adjustments to control variables are made as needed to maintain the desired stress level at regular intervals. These intervals can be established once an evaluation of the rate of stress variations in the system has been made. Regular stress measurements will also help detect increased levels of contaminants in the bath or other process deviations that usually result in abrupt unexplained stress changes.

The outlined approach to stress control during electroforming does not exhaust all the possibilities at our disposal. Even processes with relatively high as-plated internal stress levels can sometimes be successfully used for forming objects with tight dimensional tolerances. This can often be achieved, as long as the electroform does not loose its integrity in the process, by appropriately heat treating the finished form prior to removing the mandrel. In many instances, stressed electrodeposits will respond to heat treating in a fashion similar to cold worked metals. A complete progression from normalization through stress relief and recrystallization to full annealing can be observed in sufficiently pure electrodeposits.

Another sometimes practiced procedure for removing stressed forms from reusable mandrels includes heating or cooling the electroform/mandrel assembly so as to facilitate their separation due to unequal coefficients of thermal expansion. Obviously, this method will not assure dimensional stability of the form after separation.

Conclusions
Despite the multitude of factors affecting stress in an electrodeposit, stress control during electroforming can be successfully implemented once a clear understanding of the bath stress profile and the process window have been developed. Regular stress measurements, good housekeeping to prevent bath contamination and deliberate selection of control variables for stress maintenance are critical to the success of this approach. Post-plating heat treatments can be useful in reducing stresses in electroforms with high internal stresses.

Stress Control Techniques

The numerous published theories of stress in electrodeposits seem to agree on one point: it is hard to find a process variable that does not influence deposit internal stress. Indeed, internal stress is perhaps the most integral characteristic of an electrodeposition system. A typical list of variables affecting internal stress in a deposition process usually looks like this:

Acknowledgments

Thanks are due to my friends and coworkers at Reflexite PTC for the stimulating atmosphere of creativity and constructive criticism and to Reflexite Corporation for the permission to publish this paper.
References
1. Stoney, G.G., Proceedings of the Royal Society, A82:172; 1909
2. Armyanov, S. and Sotirova-Chakarova, G., Metal Finishing, 90:61, November 1992
3. Brenner, A. and Senderoff, S., J. Res. Natl. Bur. Std, 42:89, 1949
4. Kushner, J., Plating, 41:10, 1954
5. Popereka, M. Ya., Vnutrennie Napryazheniya Elektroliticheski Osazhdayemych Metallov. Novosibirsk, 1966
6. U.S. Pat. No 4648944
7. Dini, J.W., Electrodeposition, Noyes Publications, 1993
8. Watson, S. A., Additions to sulphamate nickel solutions, Nickel Development Institute, 1989
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