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40 SMT Magazine • August 2015 several industry consortia efforts are currently in progress to help characterize factors influencing package warpage behavior in an attempt to de- fine mitigation measures. These include the iN- EMI Package Warpage Qualification Criteria and the HDPUG FCBGA Package Warpage projects. As discussed in Part 1 of this paper, the au- thors worked on a number of mitigation prac- tices to minimize HoP defects on product. How- ever, it also sparked interest from the different participating OEM and EMS companies to dis- cuss the need to address the root cause of HoP in the industry. It is the belief of the authors that there are a number of factors that lead to the formation of the HoP defect, but package warpage during reflow is viewed as the most sig- nificant contributor. In this paper, a number of real product examples collected by different participating members will be presented. Current industry specifications governing package warpage will be reviewed and recommendations for revised process warpage criteria for BGA packages with a pitch of 0.8 mm and above will be made. Head-on-Pillow Description HoP is "characterized by complete melting of both the solder paste and the BGA solder ball but with insufficient coalescence to produce well-formed solder joint" [2] . known HoP Mitigation Solutions It is not the intent of this paper to focus on process mitigation strategies as these have been adequately described in previous papers. How- ever it is worth listing here the four most com- mon process modifications employed for HoP mitigation. 1. Stencil aperture modification is by far the most common method applied by the EMS to mitigate the risk of HoP. A stencil aperture with bigger openings [2] will deliver more paste to targeted areas to help increase the chance of proper soldering in the areas most affected by component warpage. The image shown in Fig- ure 1 is just one of many stencil designs in use in the industry which illustrates progressively larger stencil openings in the 4 corners of the component land. 2. Reflow profile adjustment is another so- lution used by the EMS. By varying the profile type (soak or ramp), dwell time; atmosphere (air or nitrogen); different assembly houses have found varying degrees of success in decreasing the prevalence of HoP on the assembly. 3. Solder paste type has also been tested in order to understand which solder paste prop- erties can be more effective in mitigating the HoP defect. As shown in Figure 3, multiple types of solder paste can be considered for evaluation. 4. As discussed in Part 1 of this paper, with proper engineering effort, AXI was successful at detecting more than 90% of the HoP defects on the cards used for that study. Further valida- tion tests using products with confirmed HoP defects will lead to greater refinement of such tools thus decreasing the possibility of defects escaping to the field. HoP Defect Hypothesis in the Process Flow Mitigation practices will help to minimize the issue but cannot be expected to completely eliminate it. In this paper a hypothesis is put forward by the authors in order to explain the importance o f addressing the package warpage as a means to eliminating the HoP defect rather than minimize it. A number of examples col- lected from OEMs & EMS firms are then used to support revised warpage acceptance criteria. The hypothesis recognizes that process figure 1: Mitigation through stencil aperture modification. WARPAGE ACCEPTANCE PROPOSAL continues FeAture

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