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28 PCB007 MAGAZINE I JULY 2018 Feature by Rick Nichols, Sandra Heinemann, Gustavo Ramos, Dr. Lars Nothdurft, Dr. Britta Schafsteller and Hubertus Mertens ATOTECH DEUTSCHLAND GMBH, BERLIN, GERMANY Abstract The symbiotic relationship between solder masks and selective finishes is not new. The solder mask application is one of the key considerations to ensure a successful ap- plication of a selective finish. The selective fin- ish is the final chemical step of the PCB man- ufacturing process; this is when the panels are at their most valuable and are unfortunately not re-workable. Imperfections are not tolerat- ed, even if they are wholly cosmetic. Quality issues often manifest themselves in the form of a 'ping pong' conversation between the fab- ricators, the solder mask suppliers and the se- lective finish suppliers. Without tangible evi- dence these discussions are difficult to resolve, and the selective finish process is usually re- garded as responsible. Solder masks identified as 'critical' in the field have been tested using state-of-the-art technology to assess whether performance markers could be found. This article will focus on the chemical characteristics and use them to predict or identify potential issues before they occur rather than specifically name 'criti- cal' solder masks. It is also the intention of this article to address the potential of a solder mask to react to common yield improvement practic- es such as UV bumping. It is hoped that this awareness will help fabricators to ensure maxi- mum yields by asking the right questions. Critical solder masks impact all selective finishes. In this article, immersion tin will be used to highlight the relationship between crit- ical solder masks and some of the issues seen in the field. The article will conclude with a novel approach to identify redeposited vola- tiles post reflow. Background The function of a solder mask is to pro- vide protection to the active circuit and def- inition for the selective finish. These funda- mental attributes include photosensitivity for lithographic imaging and sufficient production Countering Solder Mask Residue Production Concerns

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