Circuits Assembly - April 2008 - (Page 34)

Cover Story terminal strips with gold finish and a pin diameter of 0.025". The three Experiment Materials. A 16-layer, FR-4 board with two thicknesses (0.093" and plastic dual inline package (PDIP-16) had 16 pins and 100% tin matte 0.125") was used in the experiment (Figure 1). The board material was finish. They had a pin pitch of 0.100" and pin diameter of 0.015". The 25 TU 752 with a Tg of 170°C and Td 350°C. The solder mask was Probi- axial resistors had two leads with 100% tin finish with a pin diameter of . mer 65. The board dimensions were 5.5" by 7". The finish was Entek Plus 0.022” For all PTH components except DIPs, leads were mechanically trimmed to achieve lead protrusions of more HT copper OSP with a specified thickness of than 0.140". DIPs had a visible lead protru20 µm. The surface finish was selected for its sion on the assembled 0.093"-thick boards popularity and sensitivity to temperature and only. IPC-A-610D defines a minimum lead air environment. protrusion where the end is discernible in OSP provides the copper surface a temsolder for Classes 1, 2 and 3, with exception porary layer of protection from oxidation. of pre-established lead length components During soldering the organic coating is pen(DIPs) where the lead protrusion may not be etrated and dissolved by flux and temperadiscernible.12 ture. Multiple exposure to thermal cycles results in coating cross-linking, making it Figure 2. Oxygen levels (ppm) for the full tunnel Flux. Flux 1 was a VOC-free no-clean with less penetrable by weak organic acids used in configurations. 4.5% solids content. Flux 2 was a resin alcohol actual flux formulations. Based on the OSP base no-clean with 7% solids content. supplier’s specifications, this surface finish has Alloy. The alloy was SnAg3Cu0.5 (SAC the ability to withstand more than three Pb305). free thermal cycles without significant wetting Design of experiments. A full factorial experdegradation.10 iment was designed (Table 2). A total of two repetitions were performed for each condiMost metals show a strong tendency to tion. For reflow, five oxygen atmospheres and form compounds or oxides with O2. For two methods of supply nitrogen were used. Pb-free systems, which use alloys high in tin For wave, two fluxes and two wave ambient and copper surfaces, SnO (mainly), SnO2 and Figure 3. Oxygen levels (ppm) for the reflow-only were used on two board thicknesses. Cu2O oxides are formed. They form imme- configurations. Assembly process and equipment. A total of diately under room temperature and grow 144 boards were assembled under various thicker under reflow temperatures above atmospheric conditions, using two fluxes and 200°C. The surface tension of oxides is much two board thicknesses. lower than the values of their corresponding Printing process. A DEK Galaxy printer metals. This issue affects alloy and surface finwas used to screen-print the adhesive. The ish wetting behavior.11 printing parameters were: Flextronics performed the board design. • Print speed: 20 mm/s. It contains footprints for surface-mount and • Print deposits: 2. PTH components. Table 1 shows the design • Squeege: metal with 60° angle. dimensions for the PTH components. • Stencil thickness: 0.006". Components. Selected surface-mount and • Stencil type: laser stencil and electropolPTH components were used in this experi- Figure 4. Main effects plot for bridging. ished. ment. Because these components were intendProper volumes of adhesive were printed for all ed for wave soldering, a glue dispensing process folcomponents except the SO-16, which had a higher lowed by curing was developed for the surface-mount standoff than the actual deposit. A thicker stencil components. The adhesive material was Loctite 3627. (more than 0.010") should be used for this application. PTH components were manually placed immediately Additional adhesive material was dispensed using a prior to wave soldering. semiautomatic dispenser to the SO-16 pads. Resistors. Forty 0805 resistors were placed perpenPick-and-place. A Universal Instruments GSM was dicular to the board and 32 1206 resistors were placed used to place surface mount components on the boton both orientations. Both sets of resistors had 100% tom side of the board. tin finish over nickel. Reflow. A 13-zone (nine heating, four cooling) SOCs. Twenty SOT-23s were placed parallel and Figure 5. Solder bridging on forced convection oven (model XPM3 940) was used perpendicular to the direction of the board. They had SO-16 – counts as three pins. to reflow the assemblies. Five atmospheres were evaluthree pins and 100% tin finish. Thirteen SO-16s were also used (both orientations). They had 16 pins with gullwing leads. ated. The atmospheres are denoted by the oxygen content in parts per million (ppm): 25, 500, 1200, 2500, and 210,000. The four lowest O2 levThey were Pb-free with 100% matte tin finish. PTH. All PTH components were rated for Pb-free processing and were els were used for both nitrogen supply methods. These methods will be Pb-free finished. The three 64-pin connectors were high-temperature referred to as “full tunnel” and “reflow only.” Note: The air atmosphere 34 Circuits Assembly APRIL 2008 circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - April 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Design and Modeling of High-Speed, High-Density 3-D CSPs and Memory Modules The ‘Big Brush Off’ Revisited Impact of Soldering Atmosphere on Solder Joint Formation Beyond Moore’s Law ESD Control For Class 0 ESDS Devices Growing Your Brand This Year’s Model Tech Tips Reflow Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - April 2008

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