Oxide Alternative Processes

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Introduction

It is all about optimizing the performance of the oxide alternative chemistry. This includes close monitoring of the main reactive ingredients of the process chemistry. And one of the first issues that the industry had to address, whether one is using reduced oxide chemistry or oxide alternatives, is pink ring. While the industry has enjoyed moderate success at minimizing the dreaded pink ring defect with reduced oxides, the desire has long been to simplify the bonding treatment process. Secondly, improving bond strength on high-performance and advanced resin materials is critical to the successful implementation of any interlayer treatment process. While the latter attribute is well documented, the ability of the oxide alternative bonding system to resist pink ring must be established. In all instances, where multilayer PWB test coupons (some fabricated with reduced oxide, others with alternative bonding treatment) were subjected a severe acid test, no pink ring was detected. The coupons were immersed in an aqueous solution of 17% hydrochloric acid for 15 minutes. After removal, the coupons were horizontally ground down to remove the outerlayer pad. Coupons were then inspected under 30x microscope. No pink ring was detected on the coupons fabricated with the organo-metallic (oxide alternative) bonding process. However, the reduced oxide bonding process showed pink ring.

It is important to recognize that the existence of pink ring does not suggest that the PCBs are rejectable. In fact, pink ring is defined in the IPC-600H (acceptability of printed circuit boards) as a process indicator. No evidence exists that pink ring affects functionality. In fact, the presence of pink ring concerns a number of manufacturing processes. The focus of concern should be the quality of the lamination bond, the laminate materials, drilling, desmear metallization processes. As an example, etchback prior to metallization can open up a wedge between the B-stage material and the oxide (or oxide alternative). The wedge defect thus can act as a via allowing acid chemicals to seep in, removing some of the treated copper foil coating leading to the appearance of pink ring.

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Editor's Note: This column originally appeared in the July 2014 issue of The PCB Magazine.

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