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November 2017 • SMT Magazine 43 In IPC 610 section 1.5.2, the following defi- nition can be found: "Cold Solder Connection: A solder connection that exhibits poor wetting and that is characterized by a grayish porous appearance. (This is due to ex- cessive impurities in the solder, inadequate cleaning prior to soldering, and/or the insufficient applica- tion of heat during the soldering process.)" In section 5.2.5, this is further explained by including rosin connection. Cold and rosin connections are both classified as a defect for IPC Class 1-3. In section 5.2.3 of IPC 610 Soldering Anom- alies – Reflow of Solder Paste, incomplete wet- ting is classified as a defect for IPC Class 1-3. Below is an example using a 0.4 mm pitch CSP with 0.25 mm round apertures and a 0.1 mm thick laser-cut stencil. This shows that, by increasing the ramp rate above 180°C from 0.7°C/s to 1.2°C/s, the HIP defect detected in functional testing is reduced from 1.5% to 0% (Table 3). In this case, nitrogen (<1,000ppm of O 2 ) re- flow (Table 3) has the same positive effect on the functional yields. One thing to keep in mind is that this is functional test yield loss, and many HIP defects might not be captured because there could be marginal mechanical and electri- cal connections. Figure 11: Graping illustration. Figure 13: Impact of aperture size on amount of graping. Table 3: Yield loss on an actual product at different ramp rates and in air vs. nitrogen reflow for a 0.4 mm pitch CSP. Figure 12: Actual images of graping of 01005 components (top and lower left), and a good solder joint on 01005 lower right). DFX ON HIGH-DENSITY ASSEMBLIES