SMT Magazine

SMT-Nov2017

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38 SMT Magazine • November 2017 influence the quality of the assembly process. With the reduced pitch and component spac- ing, the capabilities for solder paste, assembly, and PCB fabrication will be tested to its limits and beyond, with the need for using statistical tools is becoming a requirement to develop and maintain good yields. Even with the right solder paste selection, good assembly process, and high-quality PCB and components, failures still will occur, lead- ing to yield losses and extra cost. Once these failures occur, it is very easy to jump to conclu- sions and start making changes without know- ing the root cause of the failure. As engineers, we quite often tend look for the most difficult solution, but in many cases, the simple solution is the right solution to solve the issue. PCB and PCBA Design When moving to high-density assembly, it is very important to understand PCB design and its limitations. With increasing density, the PCB fabrication becomes much more challeng- ing. Some of the key PCB design requirements include: • 50 µm copper/copper spacing (inner and outer layers) • 50 µm solder mask slivers • 25-40 µm solder mask registration tolerance • 60 µm microvias • 200 µm microvia capture pads in outer and inner layers Understanding DFx and design is in many cases critical to be able to identify the root cause of the issues that occur on the production line. When it comes to PCB and PCBA design, there are many important aspects. In this paper, we have selected a few that have a critical impact on SPI and overall yields. • PCB stretch and shrink • Filled vs. unfilled microvias • Silkscreen PCB Stretch and Shrink The maximum PCB stretch is 0.05 mm over the entire panel, and this must be clearly speci- fied on the PCB fabrication drawing. This is very important for fine-pitch parts and high-densi- ty spacing products. The pads and apertures for 01005 passives and 0.3 mm pitch CSPs is 0.2 mm; a stretch or shrink of more than 0.05 mm leads to solder paste that will be 25% of the pad. Microvia Issue Excessive voids in some cases lead to solder bridging. This could be caused by oversized or "leaking" microvias in pad in combination with normal process variations. An issue on a smart- phone can be seen below; failure rate for bridg- ing was 6% in this case (Figure 2). One primary issue today is that there are no clear specifications as to how big the microvias are allowed to be or where to measure the actual size of the microvia (Figure 4). Since there is no DFX ON HIGH-DENSITY ASSEMBLIES Figure 1: Typical PCBA panel layout for consumer products. It is important to ensure that the overall stretch/shrink tolerance is no more than 0.05 mm. Figure 2: Solder bridge and excessive voids on a 0.4 mm pitch CSP due to microvia size issues.

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