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SEPTEMBER 2020 I SMT007 MAGAZINE 87 From these results, it can be summarized that when OSP-coated PCBs undergo ther- mal excursions, two main changes take place: (1) oxide layer at the organic metal interface thickens and (2) active OSP molecules get rear- ranged to form supramolecular fiber-like struc- tures. Oxide thickening requires both heat and oxygen, while fiber formation is affected by the elevated temperature alone. OSP Flux Ingredient Interaction Now, with our understanding of the changes in OSP after thermal excursion, we wanted to find out which chemicals, often contained in liquid fluxes, could overcome the effects of these changes. In the soldering flux industry, few organic solvents are loosely termed as OSP cutters because of their ability to dissolve OSP coatings. We studied the relative OSP cutting efficiency of a few such solvents. As-coated and 2x reflowed laminates of fixed dimensions with ENTEK Plus HT finish were dipped in a fixed volume of each of these sol- vents (at 70–80°C) and 5% HCl. OSP coating dissolved in solvents was quantified by mea- suring the UV absorption of the resultant solu- tions. 5% HCl was known to dissolve as-coated ENTEK Plus HT completely, and the absorp- tion value of this solution was taken as 100% OSP concentration. The absorption value of other solutions was converted into a relative percentage of OSP dissolved (Figure 5). From the absorption plot, it is easily under- stood that most of the solvents taken can easily dissolve as-coated OSP, but solubility decreases drastically once the coating under- goes two reflows. Only solvent SOL1 and 5% HCl can dissolve 40–50% of the pre-reflowed coating under the experimental conditions. Solvent SOL1 is one of the strongest organic solvents and can break down the supramolec- ular arrangement in pre-reflowed OSP. On the other hand, HCl can react with benzimidazole (which is a base by nature) and remove it into the solution. If inorganic acids can dissolve thermally treated OSP, can organic acids do the job? To confirm this, a 1% solution of two organic acids was prepared in SOL3 (having poor per- formance on reflowed OSP), and a dissolu- tion study was carried out as earlier. Results showed that both of the acid solutions could dissolve ~80% of the as-coated OSP. But for the pre-reflowed OSP—while solvent can dis- solve only 5–10% of the OSP layer—activator solutions showed better efficiency, dissolving 20–30% (Figure 6). However, dissolving the organic layer is only half of the job. For solder to wet and form a joint, the oxide film beneath the organic layer needs to be removed. Historically, organic or inorganic acids, halides, halogens, etc.— collectively known as activators—are used for this purpose. To understand how they react with oxides, differential scanning calorimetry (DSC) was used. Commercially available, analytical-grade cuprous, or cupric oxide, was mixed with an activator (90:10 ratio) by dry mixing and was heated in a DSC pan at 10°C/min ramp Figure 5: Relative percentage of as-coated and reflowed OSP dissolved in different solvents. Figure 6: Relative percentage of as-coated and reflowed OSP dissolved in solvent and activator solutions.