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JULY 2018 I PCB007 MAGAZINE 37 Pyrolysis and GC-MS A sample of less than 200 µg is scraped from the material to be examined. This is then ex- posed to temperatures of 550°C. The impact of this is to fragment the molecules. The frag- mented molecules are then separated by gas chromatography and subsequently identified by mass spectrometry. This method can be used to identify the polymeric compounds of solder mask. Due to the sensitive nature of the industry, response charts will not be included in this article as they may infringe upon con - fidential know-how. The takeaway message, however, is that, whilst the signal responses for critical solder masks are similar, they dif- fer from those of non-critical solder masks. The Impact of Traditional Yield-Hiking Tricks on Critical Solder Masks A typical trick within the industry to increase yields is to use ultraviolet (UV) bumping. This is a pre-selective finish treatment aimed to completely harden or polymerize the solder mask and prevent release of volatiles during the selective finish process or reflow process. Figure 8 demonstrates that the signal is the same for the critical solder mask before and af- ter UV bumping. This is an indication that the solder mask is not polymerized or that there is too much photoinitiator in the solder mask matrix. Figure 9 demonstrates that percent conver- sion (polymerization) is related to irradiation time. The labels on the curves refer to three different monomer systems. The best situa- tion is full polymerization in a short irradia- tion time(s). The figure also demonstrates that Figure 8: GC-MS results for critical and non-critical solder masks before and after UV bumping. Figure 9: A representation of polymerization [1] .