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86 SMT007 MAGAZINE I SEPTEMBER 2020 OSP molecules attach themselves to the cop- per surface by forming coordination bonds with surface metal ions, as well as by elec- trostatic attraction. When OSP-coated copper pads undergoe thermal exposures (baking or reflow), the oxide layer at the metal-organic interface gets thickened depending on the nature of the exposure (Figure 1). The oxide layer formed under different ther- mal conditions was quantified using sequen- tial electrochemical reduction analysis (SERA). The results showed that the oxide layer consists of both Cu 2 O and CuO, and it increases with the increase in exposure temperature or time (Figure 2). The study also indicated that there might be a temperature range above which OSP coating softens and changes its morphol- ogy. During this transi- tion, the coating becomes less protective and allows ex t e r n a l g a s ( ox y g e n ) to penetrate through it, which increases the oxide layer thickness. We have studied this morphology change of OSP coating by atomic force microscopy (AFM). Three-dimensional AFM images (Figure 3) showed that the as-coated OSP surface consists of an extensively large number of fine-grained micro-peaks, which largely disappeared after Pb-free reflow with the appearance of larger grain-like features. This morphology change is not noticed when coupons are exposed to 165°C or less, independent of exposure dura- tion. These observations reinforce the hypoth- esis of coating softening above 165°C as sug- gested in SERA study. The obvious question here is, "What types of changes in the organic layer result in larger grain morphology after OSP is exposed to higher temperatures?" Images obtained from field emission scanning electron microscopy (FESEM) showed the presence of small-size random grains present on the surface of the as-coated OSP, which was converted into elon- gated fiber-like structures upon standard Pb- free reflow (in air or nitrogen) (Figure 4). The formation of such fiber may be attributed to the supramolecular structure formation of sub- stituted benzimidazole molecules. Figure 1: Schematic diagram of oxide thickening at the Cu-OSP interface. Figure 2: Oxide thickness in OSP coating after different thermal excursions. Figure 3: Three dimensional AFM images of as-coated and thermally treated OSP coating. Figure 4: FESEM images of as-coated and a thermally treated OSP coating.

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