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Reflow Profiling: KIC's Brian O'Leary Shows New Methods, New Understanding
March 2, 2010 |Estimated reading time: 3 minutes
BOSTON — Reflow profiling has gained importance as SMT assemblers struggle to meet the lower-temperature reflow needs of components, and the higher-temperature needs of lead-free solder alloys. Profiling can involve a lot of guess work, wasted boards, and lost time. KIC, along with the Rochester Institute of Technology (RIT), studied the most accurate way to profile a BGA, and achieved nonlinear results. The methodology and design of experiment described here, from Brian O’Leary’s presentation to the SMTA Boston Chapter, can be applied to other profiling situations.
With high-mass BGA components occupying PCB real estate next to small passives, profiling is about achieving a balance, reflowing all of the solder joints without damage to components and as few defects as possible. KIC’s first step in the profiling experiment was to test the main methods of thermocouple (TC) attach. Between various epoxies, high-temperature solder, aluminum tape, and Kapton tape, aluminum tape interfered the least amount with true temperature readings, offered more reliable attach than Kapton, and is nondestructive. “Aluminum tape proved the closest thing possible to a naked TC,” summarized O’Leary, adding about 1°–2°C ΔT difference from actual temperature. The study also used a flattened thermocouple design.
KIC and RIT performed the profiling study on BGAs because these components tend to be the most expensive on the circuit board, and thus supply managers and customers are reticent to purchase extras to try out different profiles. The researchers discovered that PCB thickness and BGA size differences don’t always produce linear changes in reflow profiles. The smaller BGA (15 × 15 mm, 1-mm pitch, weight: 0.4 grams) produced a smaller offset in temperature to reflow; however, a midsize BGA (23 × 23 mm, 1.27-mm pitch, 1.5 grams) can have a larger ΔT in reflow than a more massive BGA (35 × 35 mm, 1-mm pitch, 5.7 grams). Two PCB assemblies per combination were tested, and the researchers performed two runs through reflow for each assembly.
Once the BGA’s offset is understood, reflowing can proceed without destroying BGAs. Now, the profile must be developed with the passives in mind. Reflowing for the offset of the BGA, while knowing that you’ll fry small components around it, isn’t time- or cost-effective either, O’Leary points out, but it does happen.
O’Leary gave a practical example of this balancing act, with a BGA 160 (the small size mentioned above) on 62-mil FR-4 and an offset of 3°C. The BGA component body cannot handle reflow temperatures above 250°C. The BGA joints require at least 238°C (235°C + 3°C offset) to reflow properly. 248°C is the ideal temperature to ensure reflow and avoid component body damage. However, electrolytic capacitors located near the BGA have a component max temperature of 245°C. If the BGA is reflowed to 248°C, the ELCAPs will need rework post-assembly. The reflow profile must be tweaked to hit between 239° and 243°C, reflowing the BGA balls while protecting the capacitors.
This is a simplified example, and KIC’s business is reflow profiling software products that consider these variables along with the hundreds of other components, oven variations, etc. encountered in a real profiling situation. With this RIT study, KIC was able to more accurately understand the reflow nature of BGAs, and thermocouples, to produce more accurate nondestructive reflow profiling methods.
Reflow profiling software can calculate the ideal in-spec profile for a complicated assembly, as our simplified example shows. It also can tailor profiles to meet production objectives, like minimizing power usage or increasing line throughput. Reflow profiling products have the dual purpose of reducing time/waste at profiling, and preventing reflow from bottlenecking the line or sending assemblies to the rework station.
Brian O'Leary is the national sales manager for KIC and has presented at various SMTA chapters showing innovative concepts for improving processes and yields. He also developed an independent book titled Profiling Guide for Profitability. His Website, www.profilingguru.com, hosts discussions on profiling tips, energy conservation, thermocouple (TC) attachment as well as more advanced concepts. The web site features articles, videos and podcasts.
Written by Meredith Courtemanche, executive editor Also Read: A Shift in the Perception of Reflow Profiling by Paul Austen, ECD and Thermal Profiling: A Practical Approach to Reflow Profiling by Liyakathali Koorithodi, Indium Corporation.
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