Preventing hole-wall pullaway: the first steps to take to ensure good electroless copper adhesion.

ONE OF THE most aggravating problems (besides voids) associated with the metalization process is the issue of hole-wall pullaway (HWPA). Simply stated, HWPA is an area or areas within the PTH exhibiting a lack of adhesion of the electroless copper deposit. This defect can be as subtle as minor blisters or appear as larger areas exhibiting what is known as lack of bonding (FIGURES 1 and 2). It has several causes:

1. Overeactive electroless copper process. Essentially, the deposit is being laid down much too fast, leading to a stressed condition. The inherent stress in the deposit causes the copper to pull away from the substrate. Check the deposition rate on the electroless copper rate panels: was an increase noted, possibly due to an over-temperature condition in the electroless copper solution? Is the chemistry out of balance (check the formaldehyde, caustic and copper content)? Is the process being controlled within the operating window of these additives? A higher-than-normal caustic concentration will lead to a higher deposition rate. Examine the grain structure of the electroless copper deposit. Compare to results when HWPA did not exist.

2. Desmear operation leaves a less-than-desirable surface topography on the resin. This is typically an issue related to the permanganate process. Higher Tg resins are more resistant to desmear chemistry and thus the surface topography and extent of resin removal are less than optimal. Weight loss measurements utilizing a small coupon made of the same resin system and processed through the desmear process under the current conditions will yield some evidence. A SEM taken of the PTH will give a good picture of the topography on the resin after desmearing.

[FIGURES 1-2 OMITTED]

FIGURE 3 depicts an excellent example of optimum resin topography.

[FIGURE 3 OMITTED]

It is conceivable that the solvent swell (the first process step in the alkaline permanganate process) is not sufficiently weakening the polymer-polymer bonds in the resin. This will result in reduced resin removal and insufficient topography. However, overpenetration of the solvent into the resin matrix can also be detrimental. One relies on the permanganate solution to not only oxidize the weakened polymer bonds of the resin matrix, but to oxidize the excess solvent as well. Excess solvent will lead to HWPA and potentially other problems.

Avoid the situation shown in FIGURE 4. The topography is very smooth and will minimize catalyst adsorption and subsequent uniform metalization. The smooth topography will reduce copper-to-resin adhesion as well. Typical causes of the smooth topography are:

* Inadequate contact time with solvent step.

* Solvent not compatible with resin system.

* Time and temperature in alkaline permanganate solution inadequate.

* Manganate build-up in permanganate solution--keep manganate below 20 grams per liter.

* Caustic content of permanganate solution too low.

[FIGURE 4 OMITTED]

Next time, we will investigate additional causes of HWPA and provide tips on desmearing various resin systems with alkaline permanganate.

MICHAEL CARANO is vice president of sales and marketing at Electrochemicals (Minneapolis, MN). He can be reached at [e-mail].