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42 SMT Magazine • November 2017 Even though reflow in air is possible, nitrogen reflow is most common in the market for type 5 powders. Process From a process point of view, screen print- ing has always been the major contributor of yield loss. Many investigations over the last few years have shown that printing can contrib- ute to 60-70% of the overall process yield loss, followed by reflow soldering ranging between 10-20%. Voiding, Graping, and HIP Recently, QFN voiding has become a very hot topic. In some cases, reducing QFN void- ing can be done with the help of a hot and long soak, burning off more flux residues that could lead to excessive voiding. The drive for reduced QFN voiding can lead to an increase of graping on 0201 and 01005 passive parts and an increase of head-in-pillow (HIP) on fine-pitch CSPs and PoPs. Many stud- ies show that graping does not have a negative impact on shear strength, but it is a clear pro- cess indicator for HIP. In both cases, flux ex- haustion is caused by excessive heat and time before the solder joints are formed. With regards to HIP, there are also other fac- tors such as warpage and ball/bump contami- nation that impact the total amount of failures. The head-in-pillow defect (Figure 10) is an open solder joint in a BGA or CSP where the solder paste deposit does not coalesce to the ball on the component. The result is an appar- ent solder joint with a gap between the solder reflowed to the PCB paste and the solder ball itself. This defect is particularly troublesome because it is very difficult to detect, even with X-ray inspection. In addition, sometimes there is mechani- cal contact between the two solder deposits al- lows it to pass functional testing but it fails lat- er in the assembly process or in worst case on the field. Graping (Figures 11-12) is also in many cas- es referred to as cold solder. Not understanding the true root cause of the issue drives a different set of potential process solutions that in many cases magnifies the issue. Graping typically happens on smaller solder paste deposits (Figure 13) due to fact that the smaller amount of solder pastes results in small- er amounts of flux to remove oxides. Table 1: Powder types according to J-STD-005. Table 2: Cp and Cpk comparing type 4.5 and type 5 powder on a 0.2 x 0.2 mm aperture and 0.08 mm thick stencil (AR=0.625). Figure 10: Typical HIP issue when using too long of a soak time and too high of a soak tempera- ture. This issue could pass a functional test and end up as a field return. DFX ON HIGH-DENSITY ASSEMBLIES