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30 PCB007 MAGAZINE I JULY 2018 environment resistance to protect the active circuit. These functions are simple in princi- ple, but the resultant implications can be far reaching. The impacts that will be focused on in this article will be surface imperfections and where relevant, immersion tin will be used as an example. The hazards of a critical solder mask are pri- marily two-fold (excluding developing resi- dues): Leach-out into process chemistries and solderability issues during multiple assembly steps. Solder masks were originally designed to facilitate hot air solder leveling (HASL). The need for co-planarity has seen the introduction of chemical selective finishes such as ENIG, ENEPIG and immersion tin to name a few. This change in application has obvious implications attached. Whereas HASL is a hot solder shock for 10 seconds (approximately 260°C for eu- tectic solders and 280°C for lead-free solders), the wet chemical selective finishes introduce longer dwell times in harsh chemical environ- ments, often at high temperatures. In an ideal world, the selection of the solder mask would be optimally coupled with the selective finish employed. In the real world, however, the cost factor is prevalent as a selection tool. To coun- ter this notion, an attempt will be made in this article to raise awareness of the components within a solder mask that render it potentially 'critical' to production. It is equally perplexing that in this day of high-tech six sigma process control, the qual- ity of the solder mask process is still judged by weight gain, Stouffer wedges, breakpoint tests and the temperature of the baking ovens. Whilst all these controls ensure a good coat- ing, they provide no information on the status of curing or the potential for volatile release during selective finish processing or assembly. It must be stressed at this stage that solder masks fulfill a fundamental part of circuit man- ufacture and that, in the main, this is achieved very successfully. The virtual eradication of solder mask residues after developing is testa- ment to the advancement in the imaging arena. For this article we will assume that resist resi- dues have been totally eradicated and focus on the impact of leaching. Leaching of the solder mask into the chemistry has been shown to have lifetime and quality impacts. This is true of all the mainstream selective finishes. This article will identify solder mask constit- uents that can be critical to production using state-of-the-art analysis equipment and dem- onstrate that traditional industry fixes such as UV bumping, if adopted, have little impact on eradicating the potential risks to yield. The sol- der masks selected as 'critical' are based on field experience. The Identification of Potentially Process— Critical Compounds in Solder Masks Methodology For this investigation eight samples were se- lected based on field experience and four sam- ples are taken from 'critical' customer panels (identified as A, B, C and D). These are de- scribed in Table 1. The analysis methods employed, as listed in Figure 1 are: • Elemental analysis (CHNS): Quantification method for the chemical composition of carbon, hydrogen, nitrogen and sulphur • Fourier Transform infrared spectroscopy (FTIR): A fingerprint/identification of functional groups Table 1: A description of the test population.

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