...inroads the 1980s but disappeared later almost completely for a number of reasons we shall take a look at. It is conceivable that full-build electroless copper may stage a comeback due to the fact that novel patterning processes can eliminate some of the drawbacks of the original full-build electroless process.

[ILLUSTRATION OMITTED]

The electroless copper process requires a source of copper ions, a reducing agent, typically formaldehyde, and a catalyst that initiates the reaction. Formaldehyde is oxidized to formic acid in the presence of strong alkali, generating two electrons that reduce cupric ions to metallic copper (see Equations A and B).

(A)2HCHO+4[OH.sup.-][right arrow]2[HCOO.sup.-]+[H.sub.2]+2[H.sub.2]O+2e

(B)[Cu.sup.2]+2e[right arrow][Cu.sup.0]

To prevent the formation of copper hydroxide precipitate, copper ions are complexed with a complexing agent such as ethylenediamine tetraacetic acid, tetrakis-(2-hydroxypropyl)-ethylenediamine, or a variety of alpha-hydroxy-carboxylic acids. The bath is stabilized by cyanide, substituted quinoline sulfonic acids, (1) 2-mercaptobenzothiazole, or a variety of other stabilizers. The bath composition can be automatically controlled. Formaldehyde is titrated with bisulfate, the copper concentration is measured colorimetrically, the pH is measured with a pH probe, and the cyanide concentration is checked with an ion-selective electrode. (2)

The classic full-build electroless copper process, pioneered by the Kollmorgen Corporation and licensed to a number of companies, including Hitachi and IBM, provides the palladium catalyst embedded in the dielectric base material. After drilling the through-holes, a photoresist is laminated to the surface, is exposed, and developed. Standard aqueous processable dry-film photoresists cannot be used in this process because they don't survive the long exposure to the strong alkali of...

NOTE: All illustrations and photos have been removed from this article.



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