Since real deposits always form in two-dimensional layers, the unidimensional model of stress is but a convenient scientific abstraction. Yet, it can help illustrate that, just like a spring, a stressed deposit displays internal elastic forces causing it to expand or contract if the bond between the substrate and the plated layer can be broken. Whenever the adhesion to the substrate is strong, the latter, depending on its thickness and elastic properties, will be distorted to a convex or concave shape and partially relieve internal stresses in the deposit. This phenomenon has been successfully utilized by Stoney(1) who developed on its basis an extremely sensitive method for measuring internal stress in plated deposits known as the bent strip (cantilever beam) technique.

Given the tendency of stressed electrodeposits to expand or contract once the bond between the deposit and substrate is broken, difficulties facing practicing electroformers are quite formidable. Even moderate (7,000 -10,000 psi) tensile or compressive stresses in electroforms may lead to geometric distortions and loss of reproduction fidelity. Stresses in electroforms may also result in form "shrinkage" or warping causing difficulties in mandrel separation after forming, often referred to as "locking". In more severe cases, higher stress levels will result in spontaneous early form-from mandrel separation, leading to loss of form and/or mandrel. Extreme stress levels in electrodeposits are known to cause metal cracking. Processes with such high deposit stresses are, therefore, not suitable for electroforming. In cases where post-machining of electroforms is required, additional difficulties may arise due to machining heat effects. Those may cause further electroform distortion or damage if substantial residual internal stresses are present.

The foregoing, then, leads to a logical conclusion: successful electroforming depends on our ability to measure and control stress during electrodeposition.

A number of modifications to the original bent strip test method, mentioned earlier, found their way into the research laboratory over the years(2). Other methods were also developed. The spiral contractometer was invented by Brenner and Senderoff(3) in 1949, Kushner's stresometer(4) - in 1954. In 1958 the length-change (dilatometric) stress determination method was proposed by Popereka(5.) An electronic strain gauge apparatus6 constituting a modification of the stresometer concept was patented in 1985. All of these as well as the more recent optical (laser and interferometric) techniques have been used for laboratory studies of stress during electro- and electroless deposition.

For a test method to be accepted and routinely used in an industrial environment, as opposed to a research laboratory, a number of fairly stringent additional requirements need to be met. First and foremost, the test has to be fast, simple to use and interpret, the equipment - reliable and robust, the results - accurate, repeatable and meaningful. In the case of electroforming it also means, for reasons to be discussed later, that stress tests must be performed directly in plating tanks in a non- or minimally invasive fashion. This last requirement simply means that carrying out a stress test should not significantly alter the established electrochemical or hydrodynamic patterns in the process tank. The ability to take measurements continuously and ease of automation are also desirable features for an industrial stress measurement method.

Most of the known today stress testing methods fail to meet at least one of these requirements and, therefore, are suited more for laboratory than for industrial use. So, the spiral contractometer is bulky, requires calibration prior to each test and deposit stripping afterwards. The two disk membrane devices - the stresometer and its electronic modification are fairly complicated yet not sensitive enough in the low stress level region (1,000 psi) and require periodic deposit stripping, too. The same is true about the dilatometric stress measurement method, whose accuracy is additionally affected by temperature variations.
The nature of electroforming itself dictates that of most practical interest to the process engineer should be low stress levels which, as noted earlier, cause the least distortion in the electroform. With this in mind, the simplest yet sensitive enough measurement technique for industrial applications is still the bent strip method. One of it's present commercial versions(7) utilizes disposable brass two-legged strips whose opposite sides are plated and the resulting leg deflection caused by deposit stress is measured on a simple scale (Fig. 2).