63J Chen and PD Knudsen US patent 5076841 1991 US patent 5120578 1992 Controls

Electroless Copper Solution. The electroless copper solution requires a variety of controls, all aimed toward maintaining steady-state performance in terms of plating rate, deposit physical properties, and bath stability (both during operation and on storage). First and foremost, the principal chemical parameters (copper, formaldehyde, and caustic content) must be kept within specification. Normally, by following replenishment and bailout instructions with respect to these primary components, all other bath chemical components will remain within the recommended ranges. However, occasionally chelate and stabilizer levels will also need independent control. Other than the chemistry, operating temperature is the most critical operating parameter. Too low a temperature will shut the plating off; too high may cause excessive plating rate and instability. Other key operating parameters include specific gravity (a measure of byproduct buildup (see the section "Bath Chemistry" in this article), air agitation, filtration, circulation, panel loading, and others. Specific methods for maintaining bath stability during periods of shutdown are also specified: plating down to 70 to 80% copper content in the case of room-temperature operating baths, or simply reducing the temperature for storage in the case of heated baths. Air agitation is usually employed to improve storage stability.

In general, baths providing higher film thicknesses have tighter requirements in terms of equipment parameters such as heaters, air agitation, circulation, filtration, replenishment mode, and so on. Full-build baths in particular have stringent requirements for agitation, circulation, and filtration, and they require close control over plating rate consistency. The needed controls are well characterized for all current electroless formulations, and most plating facilities can readily maintain the required controls on a regular basis. For a discussion of automatic bath controllers, see the section "Equipment—Control Equipment" in this article.

Other Process Solutions. General controls for other process solutions include the temperature and concentration of critical components, as specified by the supplier. Immersion time may also be critical in certain cases (e.g., PTH microetch), but not so long as a specified minimum immersion is attained (e.g., catalyst promoter). Supplier recommendations should be followed. Due to interaction between various pretreatment chemistries, as well as with the electroless chemistry, it is strongly recommended that a single supplier be used for the entire process. This practice also facilitates prompt resolution in troubleshooting situations.

In the gross pretreatment sequence, certain general controls must be considered. For PWB desmear, the permanganate (Mn7+) to manganate (Mn6+) ratio must be maintained at a specified level. Similarly, for POP adhesion promotion, the (Cr3+) level must be controlled as specified. In POP it is strongly recommended that the temperature of the neutralizing solution be higher than that of the etch, to ensure that all hexavalent chromium residues are leached from any porous surfaces.

Adherence to specified chemistry controls is also necessary in the direct pretreatment sequence. In the PTH microetch, deviation from the prescribed parameters can lead to over- or underetching. In the catalyst solution, important parameters include specific gravity (to control salt content, which helps maintain colloid stability), Sn(II) level (which protects the colloid from oxidation and sedimentation), and colloid content (to control catalytic activity). In addition, certain metallic contaminants that can poison the catalytic activity (such as iron and chromium) must be monitored and controlled. Attention must be paid to the temperature and immersion time of the accelerator solution, to avoid under- or overacceleration. In addition, especially in chloride-containing accelerators, the dissolved copper content must be controlled to avoid formation of a rough deposit (believed to be CuCl) on the panel surface.

Miscellaneous. The importance of adequate rinsing, using good-quality water, can hardly be overemphasized. Heated rinses may be recommended after the catalyst promotion step to aid in removal of excess surfactant film. Rinsing after catalyzation requires maintenance of slightly acidic water containing no multiply charged cations, in order to prevent agglomeration of the negative colloid and surface roughness on the panel. Thorough rinsing in clean water is a general requirement in electroless copper processes, to prevent both panel surface defects and shortened bath life due to contaminated solutions.

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