More equations for this method are presented in the General Motors Engineering Standards.1 General Motors used this method for stress tests for many years. However, the measurement of the degree of deflection is difficult to determine and accuracy remains questionable.

Several other methods for deposit stress measurement include the spiral contractometer2, and the two disk membrane devices – the stress-meter3 and its electronic modification. The spiral contractometer (Fig. 2), measures deposit stress levels up to 140 MPa (or 20,000 lb/in2) tensile or compressive, but is not applicable in the low stress value range. The spiral basis material is 0.033 cm (0.013 in.) thick. It is made from stainless steel and has an electroplating surface area of about 77 cm2 (12 in2). The manufacturer recommends a deposit thickness of 16 µm (approximately 0.6 mil) for deposit evaluations. A given spiral will contract or expand with use depending upon whether the deposit stress is tensile or compressive in nature. One end of the spiral being utilized is held in a fixed position while the other end is linked to a measuring dial that measures the degrees of spiral movement. The spirals must be calibrated prior to use. This test method is somewhat bulky to use and calculation of the results is time consuming. But the spirals are strippable and can be used for many test determinations as long as the deposit can be removed without destruction of the spiral material.

The stress-meter and its electronic modification (Fig. 3) are rather complicated to use, but this method also permits periodic stripping of the membrane disk. These methods of deposit stress measurement are not applicable for low stress values approaching zero.4

A more recent method of determining internal deposit stress values is the Deposit Stress Analyzer. This method utilizes discardable, economic test strips that offer super sensitivity with a capability of measuring a wide range of values from almost zero to 276 MPa (40,000 lb/in2). A small surface area of 7.1cm2 (1.1 in2) allows for minimal use of plated deposits, making it a desirable choice for precious metal evaluations. The test strips are calibrated during manufacture, so calibration is not required at the point of use. They offer an easy read of the deflection caused by an applied coating.

The test strips are similar to a tuning fork configuration, having two legs that are placed in a working bath or in a small laboratory set up for use. A solution volume as small as 600 mL is sufficient for test purposes. Since one leg is resist coated on one side and the other leg is coated with resist on the opposite side, deflection of these legs during the deposition of a given coating occurs in opposite directions. If the stress is tensile in nature, the legs spread outward on the coated side of the test strip legs. For compressive stress, the legs spread outward on the resist coated side of the test strip legs (Fig. 4). A measuring stand supports coated test pieces over a scale from which the total increments spread can be determined (Fig. 5).