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JULY 2019 I SMT007 MAGAZINE 35 are used for dispensing applications, such as jet printing. Type 6 solder paste is also used for other ultra-fine-feature applications [5 & 6] . Aside from the printing capabilities given by smaller solder powders [7] , other performance changes occur when smaller solder powders are used. Stencil and shelf life of the solder pastes may be shortened when using smaller solder powders. Smaller solder powders have a higher potential for random solder balling and graping. Voiding behavior can also be affected by a change in solder powder size. The goal of this study is to quantify the performance for IPC Type 3, 4, 5, and 6 SAC305 (Sn / Ag 3.0% / Cu 0.5%) solder powders in both water-sol- uble and no-clean solder pastes. Experimen- tal data for each solder paste is compared and contrasted, and recommendations for the opti- mal use of each solder paste are given. Methodology Surface Area of Solder Powder and Reactivity As the solder powder size decreases, the surface area of solder powder increases for a given mass (Table 3) [8] . These surface areas were calculated using the middle value in the main particle size range. The surface area of the solder powder is important because it plays a role in the reac- tivity of the solder powder. As the surface area increases, the rate of reaction increases. Imag- ine trying to dissolve a cube of sugar in a cup of water. It takes time and a lot of stirring for the sugar to dissolve completely. If the same mass of granular sugar is mixed into a cup of water, it dissolves much more quickly (Figure 3). After one minute of mixing, the granulated sugar dissolved completely while the cube of sugar was partially dissolved. The granulated sugar has a higher surface area than the cube of sugar, which enables the granulated sugar to dissolve more quickly. The same principle is true with solder pow- der. The higher surface area of smaller solder powder types causes the rate of reaction to be higher than the larger solder powder types. Therefore, smaller solder powder types are more susceptible to oxidation when exposed to air [9] . The chemical reactions for oxidation of tin (Sn) are as follows: Sn (s) + ½ O 2 (g) = SnO (s) Sn (s) + O 2 (g) = SnO 2 (s) As oxygen reacts with the solder powder, metal oxides are created. The primary oxide that forms on SAC305 alloy is SnO [10] . The solder paste flux removes the metal oxides and helps to slow further oxidation [8] . Oxida- tion of the solder powder can continue, albeit slowly, as long as the solder paste is exposed to air. Mixing and increased temperature accelerate this process. This reaction process Figure 3: Granular sugar (L) versus cubed sugar (R) dissolved in water with a 1-minute mix time. Table 3: Solder powder size and surface area for a 1 Kg mass.

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