Circuits Assembly - March 2008 - (Page 60)

Tech Tips Diagnosing Intermittent BGA Connections Mixed leaded and Pb-free PCBs may require nonstandard profiles. I The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly. ntermittent BGA connections are a commonly noted The board design may also interfere with solder ball problem. When pressure is applied to the center of reflow. A large ground plane near the BGA may conduct the BGA, performance improves. Several issues proenough heat from the BGA to prevent proper reflow. duce this symptom: solder paste reflow profile, printed Boards designed for conductive cooling may have a simicircuit board design, solderability, microcracks, black lar issue. Preheating the board and adjusting the reflow pad, or poor adhesion of pads to substrates. profile will reduce this problem, but a board designed To identify the cause of the failure, for extreme conductive cooling may review the board finish and ball alloy. If become a BGA soldering challenge. the assembly is a mix of SnPb and PbConfirming incomplete solder ball free components, it is possible that an collapse, large voids, shorts and poor incorrect solder paste reflow profile was solder intermetallics requires destructive used. While SnPb solder pastes can be analysis, such as cross-sectional analysis used with SAC BGAs, the standard SnPb and dye and pry test. Destructive failreflow profile should not be used. SAC ure analysis is required to investigate BGAs need a hotter profile in order to less obvious failure mechanisms such as Figure 1. This solder ball crosscollapse the SAC solder balls. If the problack pad, micro-cracks (Figure 1) and section shows a micro-crack file is too cool, the lead in SnPb pastes poor pad-to-substrate adhesion. between the solder ball and pad. will not fully mix with the SAC balls and Dye and pry test. The board and BGA good intermetallics will not form. This are immersed in red dye and a vacuum would cause inferior solder joints to is applied. The vacuum permits the form between the balls and lands. dye to penetrate micro-cracks. After In a completely Pb-free assembly the dye dries, the BGA is pried from (such as this case, where the Pb-free the board and is examined optically BGAs were soldered with SAC paste), (Figure 2). The presence of red dye on it is important to verify the solder and surfaces indicates cracks were present reflow profile. If the EMS firm assembles Figure 2. Typical example of a prior to removing the BGA (Figure Pb-free and SnPb assemblies at the same BGA package pried off a board 3). The way the solder joint fractures facility, it is possible that an incorrect after dye penetration. also indicates whether there was strong solder or profile was used. Nondestrucwetting to the pads and good solder tive testing such as x-ray fluorescence reflow. A clean failure between the (XRF) can confirm use of Pb-free solder BGA land and substrate could indicate while x-ray inspection can be used to thermal shock from reflow, wave sollook for shorts and proper reflow of soldering or rework. der balls. Void calculations of the solder Cross-sectional analysis. A solder balls can confirm the void percentages ball row is cut and polished and the meet IPC standards (25%). cross-section examined with a metalCertain types of BGAs may require Figure 3. Close-up of a BGA side lographic microscope and SEM. Solder nonstandard Pb-free reflow profiles. after dye and pry testing. The and land composition are investigated Super BGAs have metal backs that can solder ball covered in red dye with electron dispersive spectroscopy impede interior balls from reaching (circled) indicates a micro-crack (EDS). The presence of expected interreflow temperature and forming good existed prior to BGA removal. metallic compounds confirms good solder joints to bond pads. Even large wetting of the solder to pads. Crossplastic BGAs may not experience enough heat for the sections can also confirm minimal voiding in the solinterior solder balls to reflow properly. Poor interior der balls. Cracks (Figure 1), and the presence of large solder joints may produce symptoms of an intermittent amounts of phosphorous in the intermetallics between open that improve under finger pressure to the BGA centhe land and ball, may indicate black pad failure ter. Tweaking the reflow profile or preheating the board mechanisms. Finally, the pads are examined for good may help the interior solder balls to reflow. adhesion to the substrate. ■ Circuits Assembly MARCH 2008 circuitsassembly.com 60 http://www.aciusa.org http://www.aciusa.org http://circuitsassembly.com

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Circuits Assembly - March 2008 Caveat Lector Letters Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing What Drives the Crowd? Mastering ESD Control in Automated Handling Systems Beating the RoHS Heat Trade Secrets True to Its Roots The Road Abroad - Strategic Alliance or Greenfield Facility? Tech Tips Wave Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Materials World Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - March 2008

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