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Trouble in Your Tank: Copper-to-Copper Peeling

Introduction

Yes, it is annoying, even critical. Your customer returns product to you complaining about poor adhesion of plated copper to the surface. So what is the genesis of this defect? Is it electroless copper peeling from base laminate copper? Or is it electrolytic copper peeling from the electroless copper? One can experience copper peelers in pattern plating, panel plating or in electroless copper. Peeling can be seen on the conductors, within the PTH or in the microvias.

Thermal excursions will exacerbate the problem as thermal stresses will increase the opportunity for one layer of copper to separate from another. As is often the case, separation of the copper from the post interconnect (aka interconnect defect) is considered peeling. However, I have presented ICD issues in previous columns, so this month the focus is on copper-to-copper peelers.

Electroless Copper Peeling

First, one must consider achieving good electroless copper-to-copper foil adhesion. One must start with a copper foil surface that is free from oxides, soil greases and other organic materials. In addition, a micro-etching solution, whether it be persulfate or peroxide-sulfuric acid-based, should be used to ensure a clean and active copper surface that will provide sufficient surface roughness to promote and enhance adhesion of the copper to the surface. Not to be forgotten is the need to get solid bond of the electroless copper to the innerlayer copper (post-innerconnect) and to the capture pad of a blind via.

In previous columns, I described several reasons that electroless copper could separate from the foil copper, copper interconnect and/ or copper capture pad. One such cause is the use of a particular type of cleaner-conditioner prior to the electroless copper deposition process. Some of these chemicals, while helping to promote strong palladium catalyst adsorption to the surface, may also leave a surfactant-like film on the copper. This film in turn will act as an adhesion barrier, causing the copper plated deposit to peel from the foil copper. One suggestion is to carefully evaluate any cleaner-conditioner to ensure compatibility with the electroless copper.

There are other possibilities that would lead to this type of adhesion failure. One such failure mechanism is related to resin smear remaining on the capture pad of a blind via. Figure 1 shows significant blistering of the copper from the blind via capture pad most likely due to resin. One way to fix this issue is to design the laser process to ablate the resin material down to the capture pad. Even with this scenario, the concern with UV YAG is that once the beam reaches the copper pad, the energy is “stepped down” to minimize further attack on the copper pad. This can lead to resin and ablated byproducts to remain on parts of the pad. In turn this provides a mechanism for adhesion failure. Any remaining resin material can act as a fulcrum, forcing the plated copper to peel.

To read the full version of this column which originally appeared in the April 2017 issue of The PCB Magazine, click here.