The SnCuNi alloy (SCN) has been used in wave soldering applications because of its applicability in achieving acceptable soldering results and its lower rates of reaction with copper (used in PCBs and components) and iron (used as a based material in wave soldering equipment). The lack of precious metals like silver makes it less expensive, and the cosmetic appearance of the final joints is similar to that of SnPb joints; but these benefits are offset by its higher melting point temperature of 227[degrees]C in reflow applications. The higher melting point may necessitate reflow profiles with higher peak temperatures and/or longer time above liquidus (TAL) than that of SAC 305 to obtain complete and homogenous mixing of the paste deposits with the component lead/bump. Concerns such as possible damage of heat-sensitive components and joint reliability arise.
The addition of the Q2 ION expands Bruker Elemental’s range of metal analysis systems even further. Mr. Andreas Kunz, Sales Director for Bruker Elemental, commented: “We can now offer an even wider selection of metal analysis solutions to our customers. We believe the Q2 ION represents a breakthrough in best-in-class performance and analytical capabilities for an ultra-compact system with a very attractive price/performance ratio.”The test vehicle was a 0.062" thick, four-layer FR-4 PWB with Cu-OSP surface finish and non-solder mask-defined pads. Each board was populated with 16 256 I/O BGA-CSP components. The design of experiments included different peak temperatures (238[degrees]C and 248[degrees]C) and TAL (50 and 75 sec.). The corresponded TAL above 227[degrees]C was 30 and 50 sec., respectively. Levels for each factor were based on the current SAC 305 process window and recommendations from the SCN solder paste supplier. All boards were reflowed in air, and a small batch of pure SCN joints were reflowed in nitrogen (<100 ppm).High quality ultraviolet (UV) OES analysis in a compact system is an innovative feature that represents important progress for ultra-compact spark-OES systems. UV-OES enables Q2 ION customers to analyze metals for important additional elements, such as carbon, phosphorus, sulfur and even nitrogen for duplex steels. Other key features like high analytical stability through patented active ambient compensation, standard-less calibration, proprietary ClearSpectrum™ algorithms, low argon consumption and minimal maintenance give Q2 ION users additional confidence in its reliable analytical results at very low operating costs.(1.) F. Song, J. Lo, J. Lam, T. Jiang and S.W.R. Lee, "A Comprehensive Parallel Study on the Board Level Reliability of SAC, SACX, and SCN Solders," Electronic Components and Technology Conference, May 2008.An x-ray automatic program was used to inspect voiding. All solder joints from 12 boards with 16 components each were tested. The percentage of the single largest and overall voiding per solder ball was recorded. Results showed pure SCN shows fewer and smaller voids than the other two metal systems. However, the overall sizes of voids were insignificant. The overall sizes were on average less than 3.5% for the SAC and mixed systems and less than 1.2% for SCN systems. The size of these voids passed IPC-A-610D, which sets acceptance criteria for Class 1, 2 and 3 at a maximum 25% of the ball x-ray image area.The company you are buying your DIY solar kit from is a primary concern also. If you are buying your kit from a store, are the people friendly there and willing to offer advice? Do they seem knowledgeable about the solar system you are considering constructing? If you are searching through online companies: Do they have a well laid out website with frequently asked questions? Are the instructions on their website clear when you are searching and purchasing (unclear instructions here might indicate additional difficulty you will have with instructions in your kit)? Do they offer resources for you if you have questions on the product, or might have questions when putting the product together? Putting a bit of research into the company you are purchasing this item from might save you a lot of grief later.For More Information:Early failures, which correspond to 1% of the failure data, showed a different trend. In this case, SAC systems had higher numbers of cycles-to-failed, followed by mixed (90 fewer cycles) and SCN (204 fewer cycles) systems. An improvement in early failures was observed when nitrogen was used in SCN joints, resulting in similar behavior to SAC systems.A study was carried out to develop reflow processes for SCN solder paste using SAC 305 and SCN-bumped BGA-CSP components. Assembly characterization was performed using cross-sectional analysis, vibration testing and thermal cycling. The objective was to characterize the performance of pure SCN joints and compare them with pure SAC 305 solder joints and mixed SCN paste/SAC 305 sphere solder joints. This was accomplished by designing reflow soldering profiles that reached the same peak temperatures and TAL (above 217[degrees]C) optimized for typical SAC 305 assemblies.For more information on Q2 ION, please visit www.bruker-elemental.comFor information about Bruker AXS - Elemental and Bruker Corporation (NASDAQ: BRKR), please visit www.bruker.comGood solder joint formation and collapse was observed on SCN and mixed assemblies when reflowed at 238[degrees]C and TAL of 75 sec. Microstructure analysis showed the main difference between these two systems was the presence of [Ag.sub.3]Sn intermetallic in the mixed joints. Another difference was the thickness of the intermetallic between the PCB and joint. A 60% thicker intermetallic was observed in the mixed system. This might be an indication that the nickel content inhibits the growth of CuSn intermetallic. (1)
Ursula Marquez de Tion is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com.
Author: Ursula Marquez de Tion
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