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smt fall-out -- acceptable rates

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smt fall-out -- acceptable rates | 14 July, 2010

We are running SMT products and have been for some time now. First, I wanted to check with you folks to see what an acceptable FPY rate, or what you would consider and acceptable rate, is. Secondly, we are having issues primarily with product that use QFN packages with 20 and 24 pins. We are experiencing solder shorts and deficiency of solder. Some are 30 mil arrays and seem to have more issues than the 60 mil. Thise are also small pcbs, 20/24-up panelizations.

I want to know what your experience is with such issues, possible fixes, and what you would consider an acceptable FPY rate. We know what ours is but don't have a benchmark to compare it to. On the products in question, we are running about 95% on functional and 80% on ICT. This points to a definate process problem (which is why I ask about the soldering issues we are having).

I feel an FPY rate of 97% on the ICT and 99% on FT is acceptable and achievable, but we have to work out these issues to get there. Thanks in advance for any suggestions you may have.

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smt fall-out -- acceptable rates | 14 July, 2010

I can't offer much advice on acceptible rates or #'s. From a process standpoint, most of your bridging or insufficient originates in your printer. Look into buying dedicated tooling specifically designed for the build. We use "h-towers" on all high volume builds which are designed and made by "Rapid Tooling" in Texas. We have seen a huge reduction in solder defects. It's one of those things you need to try to believe; then you won't want to run without.

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smt fall-out -- acceptable rates | 14 July, 2010

98% to 99% pass yield should be your target. The previous company I worked for, we were able to meet these goals with most all of our products. It took us some years to achieve this level of pass rate. Of course analyzing and good reporting from some type of QIS system is very important.

I recently signed on with a company that is new to making boards (OEM), and basically starting from scratch. I am achieving about 94% on the high mix with fine pitch 20mil parts. My problem is in the printing process and I have worked extensively with different types of pad finishes, as well as aperture sizes to get better results. My current problem is achieving optimum board support and I agree with the person who recommended dedicated fixture support. 3 months ago I was around 70% on pass rate so improvements can be made on your 80%.

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smt fall-out -- acceptable rates | 14 July, 2010

Thank you. I spoke with our SMT guy, and he stated that there is plenty of board support on the pcb. So it's not a matter of flexing. There just seems to be inconsistancy in the solder application. He did mention that part of the problem is a lack of, in some cases a total absense of solder mask in between the leads, which would explain our solder bridging. Based on your response, my expectations for FPY are pretty close on the mark. You are correct, getting the FPY rate up starts with good data analysis. The reporting mechanism on the ICT is pretty generic and limited. However, I plan on creating a LabView application to interface with the ICT for real-time reporting and data analysis. I understand the tools required to get me there, but pin-pointing specific known problems is a good start. Thanks again for your input.

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