Specialty Test
   
Fig. 2
A formula is used to convert units of spread into the deposit stress level. Tests can be performed directly in the plating tank by using a slotted tubular cell (Fig. 3) which has the added advantage of precluding any possible errors during electrolyte transfer to a laboratory cell through dilution, contamination, temperature variations, etc.
Fig. 3

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:
1. Current density;
2. Concentration of every major component of the plating bath (metal salts, conductive salts, buffering agents, wetters, etc.);
3. Concentration of additives (organic or inorganic);
4. Concentration of impurities (chemical or particulate), including trace amounts;
5. Bath temperature;
6. Agitation rate;
7. Solution pH;
8. Plating cell geometry;
9. Composition and condition of anodes;
10. Anode/cathode surface area ratio;
11. Quality of DC power (ripple);
12. Nature and condition of the substrate;
13. Numerous others.

This seemingly endless list of factors may make any attempt at stress control look futile at best. And yet, a methodical approach coupled with tight process control and good housekeeping makes stress control during electroforming perfectly possible. An important note to make here is that a particular plating tank represents such a unique combination of variables listed above, that it is preferable to measure stress directly in the tank rather than hope to recreate the same conditions in a laboratory cell.

A somewhat closer look at the list of variables above will reveal that at least some of them are either constant for a particular plating process/tank combination, or can be kept constant with relative ease (concentrations of main components, temperature, pH, cell geometry, anode/cathode ratio, agitation). Once a particular plating chemistry has been selected and initial process variables fixed, one needs to map an important process baseline - the bath stress profile. Stress profile is the relationship between deposition current density and the corresponding deposit internal stress. This can be easily established by repeating a stress test several times at varied current densities keeping the rest of the variables constant. Sample stress profiles for several sulfamate nickel electrolytes are shown in Fig. 4.
Fig. 4
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