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32 SMT007 MAGAZINE I JULY 2019 Feature by Tony Lentz FCT ASSEMBLY Abstract Solder powder size is a popular topic in the electronics industry due to the continuing trend of miniaturization of electronics. The question commonly asked is, "When should we switch from Type 3 to a smaller solder powder?" Sol- der powder size is usually chosen based on the printing requirements for the solder paste. It is common practice to use IPC Type 4 or 5 sol- der powders for stencil designs that include area ratios below the recommended IPC limit of 0.66. The effects of solder powder size on the printability of solder paste have been well documented. The size of the solder powder affects the performance of the solder paste in other ways. Shelf life, stencil life, reflow performance, voiding behavior, and reactivity/stability are all affected by solder powder size. Testing was conducted to measure each of these solder paste performance attributes for IPC Type 3, 4, 5, and 6 SAC305 solder powders in both water- soluble and no-clean solder pastes. The per- formance data for each size of solder powder in each solder paste flux was quantified and summarized. Guidance for choosing the opti- mal size of solder powder is given based on the results of this study. Keywords: Solder powder size, solder paste performance, solder paste printing, reflow, voiding, solder paste stability Introduction Size matters. That simple statement is true for many things in life. Small hands are bet- ter for speed texting. Large people are better at playing offensive or defensive line in the national football league. Antibodies are micro- scopically small but they play a key role in our health and well- being. Who doesn't want a large cup of coffee (Figure 1)? Size also matters in the world of solder paste. The size of the solder powder used in a solder paste has Figure 1: Size matters.