Issue link: https://iconnect007.uberflip.com/i/895789
40 SMT Magazine • November 2017 Copper markings should be used in the out- er layers if the design can accommodate this. This applies to CSPs and other parts that have a critical placement tolerance. The effect of silk- screen on SPI performance can clearly be seen in Figure 8 and Figure 9. Solder Paste and Alloy Selection Today, the most common solder paste alloy is SAC 305 (Sn96.5/Ag3/Cu0.5) for high-densi- ty assembly technologies. When increased ther- mal cycling performance is required, a higher silver-content alloy such as SAC 387 (Sn95.5/ Ag3.8/Cu0.7) is typically used. However, using a higher silver (Ag) content alloy will increase price and could have a negative impact on me- chanical reliability. Before making the solder paste selection it is important to evaluate key material properties such as: • Printability • Voiding • Cold and hot slump • Solder balling and solder beading • Wetting • HIP resistance/oxidization barrier • SIR (surface insulation resistance) The selection of powder size varies between type 3, type 4, type 4.5, and type 5 (Table 1). With the correct flux formulation, a more ex- pensive type 5 powder can in many cases be avoided but still meet an acceptable Cpk of 1.67 (Table 2). Besides a cost increase, type 5 powder also increases the risk of HIP and graping due to an increased surface area and oxide content. DFX ON HIGH-DENSITY ASSEMBLIES Figure 7: PCB silkscreen stack-up with stencil. Figure 8: Process capability graph on a 0.4 mm pitch CSP without silkscreen on the PCB. Volume (mil³) specification limit is 50-150%. The mean value is very well-aligned to the target volume. Figure 9: Process capability graph on a 0.4 mm pitch CSP with silkscreen on the PCB. Volume (mil³) specification limit of 50-150%. The mean value shifted to the upper specification limit due to the impact of silkscreen.