One of the most challenging tasks for electronic materials suppliers might well be bonding chemically dissimilar materials in packages to assure mechanical integrity of the package through thermal cycling, thermal shock, or other environmental stresses or chemical attacks. Conductors and insulating materials tend to have different coefficients of thermal expansion (CTEs) and are chemically sufficiently different (e.g., hydrophilic/hydrophobic, polar/non-polar) so that they don’t like to stick to each other. The solution to improved chemical adhesion comes often in the form of a chemical modification of the copper surface (e.g., the formation of a copper oxide layer), other chemistries that are alternatives to oxide formation, conversion coatings, or plasma or corona treatments to the dielectric. These surface treatments often go hand in hand with the micro-roughening of the copper surface to boost the chemical adhesion with mechanical adhesion, which is a bit of a misnomer because it really amounts to an increase in surface area that offers a larger anchoring surface for the same chemical forces; true mechanical interlocking (tongue and groove) hardly ever occurs. Micro-roughening may also be applied to the dielectric surface (e.g., in the form of desmear [swell and etch]) to further boost the adhesion.

It is interesting to think of how many different metal/dielectric interfaces there are in a typical electronic package where the metal is typically surface treated. The classic copper surface treatment (single-sided or double-treat) is done by the laminate supplier to enhance the copper foil adhesion to the FR-4 resin. At the PWB fabricator, a multilayer bonder treatment is applied to the circuitized surfaces of innerlayers before they are interleafed with prepreg for multilayer pressing. An adhesion promoting coating may be applied to the copper surfaces to enhance photoresist adhesion to avoid under-etching or under-plating. After the metallization of through-holes, the plated copper surface in the through-hole may be modified to improve via-fill resin adhesion to the copper wall. Then there is solder mask over bare copper, but the copper may not be all that bare. An adhesion promoting layer may have been applied before the solder mask is coated. If the package has build-up layers, adhesion between the copper circuit traces and the build-up resin needs to be improved by copper surface treatment. And the solder mask of a BGA flip chip may also need a little help to adhere to the top metal layer of the motherboard. So, there are many opportunities for interfaces to become unglued, and good opportunities to sell adhesion promoters for many processing steps along the way.