Taking on the 0.3 mm ultra-fine pitch device challenge in PCB design
Answering the 0.3 mm Ultra-Fine Pitch Challenge
However, not all is doom and gloom when it comes to resolving the 0.3 mm pitch challenge the industry faces. Some leading edge CMs and EMS providers are successfully creating innovative new PCB design layout rules, which meet both DFM requirements and 0.3 mm pitch CSP component requirements.
Rule one is not to follow current design guidelines – calculating CSP ball-joint pad from the 20 percent reduction formula. However, certain numerical experiments and yield improvement process developments are at the forefront to achieve the optimized pad dimension, which can meet both fabrication requirements and CSP SMT assembly requirements.
At our company, we’ve been involved in more 40 PCB projects using these proprietary 0.3 mm pitch device guides have resulted in 100 percent yield, thus proving they are successful.
A major consideration for these new developments is the board fabrication approach, itself, so that a PCB to be populated with 0.3 mm pitch micro CSPs and micro BGAs is properly fabricated with two objectives in mind. One is fabricating it within reasonable cost; two is meeting final solder-joint quality and reliability demands.
The route to get to these objectives involves a combination of methodologies to solve both small pad and mask-layer quality issues. The idea here is that the pad should be large enough and a solder-mask layer must exist between two pads.
Pad dimension is an especially crucial element for controlling the overall 0.3 mm project success and helps to ensure a high success rate for the rest of the steps. If pad dimension is less than the required critical dimension, the remaining processes won’t be successful, no matter the adjustments made. In short, a fixed-diameter pad dimension is the linchpin for both PCB design layout engineers and OEM customers to focus on, thus eliminating ambiguous pad dimension ranges.
One solution to the problem
At Nexlogic, we’ve implemented a new way to achieve +/-1 mask-aperture accuracy that we think effectively resolves 0.3 mm pitch fabrication issues. A number of factors are involved in attaining this goal.
They include reducing issues associated with fiducial mark tolerance, CSP pad position tolerance, mask-aperture accuracy, using laser direct imaging (LDI), small mask gap quality and others within a high-end fab house’s capability range.
A major part of this workable formula is a unique stencil approach for applying solder paste to 0.3 mm pitch micro CSPs. Traditionally, most assembly houses rely on a gel-flux only recipe for ultra-fine pitch CSPs. However, they fall short on acceptable yields and reliability. However, the right solution ensures there is sufficient solder paste for all CSP joints within the current area ratio (below 0.70.)
This specially developed stencil produces a highly efficient paste release. A key benefit is it prevents solder ball/solder splash normally existing around ultra-fine joints during the paste assembly process. Moreover, paste registration and volume control are also excellent, thus precluding solder bridging and insufficient solder problems.
Another part of this 0.3 mm pitch formula is adjusting the pick and place and reflow processes with proprietary processes in order to effectively handle 0.3 mm pitch devices. This normally comes as a result of working on a number of PCB projects involving 0.3 mm pitch devices.
Michael Yu is senior manufacturing engineer at NexLogic Technologies, Inc., San Jose, CA with extensive experience in process control, SMT process and manufacturing resources management. He has been in the EMS industry for over 16 years with work experience at Bema Electronics and Pactron Electronics. He has a BS in Materials Engineering from Shanghai Jiao Tong University and an MS in Mechanical Engineering from South Dakota School of Mines & Technology.
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