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70 PCB007 MAGAZINE I JULY 2018 Flexible Metalization, Part II Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY In previous columns and elsewhere in liter- ature, concerns about electroless copper peel- ing from polyimide material have been report- ed. The main concern with is the creation of a void caused by peeling or blistering (Figure 1). It is hypothesized that the blisters originate from one of, or a combination of, the potential causes listed below: • Excessive catalyst adsorption causing a hyperdeposition of electroless copper • Highly stressed electroless copper deposit • Plasma residues causing poor bondability of electroless copper • Overactive resin surface due to plasma • Lack of surface profilometry due to plasma Often, fabricators resort to double passing the circuits through the electroless copper pro- cess with the hope that the second pass will cover the void and prevent further blistering. Just remember, "Hope is not a strategy." It is recognized that plasma desmear leaves the surface in somewhat of an inert state. It is important that the fabricator work very close- ly with the plasma supplier to ensure an opti- mized process. It is also critical that residues from the plasma desmear process be removed prior to electroless copper plating. As is often the case, adhesive-based flexible circuit constructions present another challenge related to achieving void-free metalization. Due to the difficulty in plating flexible mate- rials (particularly when plasma-treated only), one would think that alkaline permanganate treatment could be used to enhance the plat- ing of a void-free deposit. Unfortunately, using a full permanganate cycle can have disastrous consequences (Figure 2). The suggestion is if one has voiding issues with certain flexible or rigid-flex builds, use a modified permanganate cycle after plasma. Figure 1: Flexible circuit. Plasma desmeared prior to plating. Figure 2: Excessive attack on the adhesive due to a full permanganate cycle.

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