Circuits Assembly - November 2007 - (Page 40)

Reflow Soldering PCBs populated with BGA and CSPs require extraorFirst article inspection will minimize or eliminate dinary care during reflow. The mistake designers make stencil thickness issues. Pre- and post-reflow quality is to place vias next to BGA pads or around BGA periphassurance (QA) inspections on the first article are vital eries. This hinders perfect soldering because solder for catching and correcting these and other discrepis sucked into vias during ancies. Those can include, reflow, rather than being for example, component consumed by the BGA balls. damage or tilted or twisted That occurs when vias are components as a result of not properly masked or higher-than-typical oven tented. temperature or airflow. The better approach is to Wrong aperture sizes are mask or tent BGA vias duranother root cause of opens ing PCB design. Even when and shorts. Therefore, it this step is omitted, a DfM is important to perform a check mapped at the plandesig n-for-manufacture ning stages or CAM level (DfM) check after a postcan catch this. Otherwise, layout database is converted reflow becomes a major to Gerber files, which crechallenge and undetected ates the stencil apertures vias with solder paste traces decodes. Gerber files are a can cause highly evasive, non-intelligent combina- Figure 1. Framed (left) and non-framed stencils. Framed stencils typiintermittent BGA connection of lines through X-Y cally provide superior paste uniformity. tions. coordinates. Consequently, Thermal profile. A 100% sometimes there’s a chance accurate thermal profile of a software glitch that goes hand-in-hand with wrongly defines X-Y coordireflow soldering. The pernates or aperture sizes. This fect thermal profile precisecauses opens and shorts at ly follows the curves and Gerber levels. A DfM check specifications set forth by is of utmost importance at the solder paste manufacthe CAM level, then, before turer (Figure 2). A typiPCB files are released to cal reflow oven has seven fabrication. to eight thermal zones, Jeopardizing the image although a larger oven can is an aperture-related issue have more. A thermal proto consider. This occurs file is composed of three when careful attention isn’t specific cycles before it paid to apertures. Creatcools. One is preheat, in ing a wrong aperture openwhich a board is processed ing can define too large an in two initial zones to heat image, creating a short or it to a specific level. Second too thin an image, creatis soak time, in which two ing an open. Also, an aperFigure 2. A perfect thermal profile precisely follows the curve and to three zones soak the solture defines trace thickness, specifications set by the paste supplier. der paste and activate the which affects board impedflux. Last is ramp for the ance. If trace thickness is reflow at peak temperature. The board is kept under properly defined, perfect impedance is achievable. specific temperature cycles for these time zones before it Non-framed stencils can pose even more problems. is cooled. Figure 3 shows an example of careful thermal Without rigid framing, a stencil is not uniform. Nonprofile monitoring; the computer screenshot shows 170°C uniformity can prevent a stencil from dispensing solder in zones 3 and 4 and 195°C in zone 5. paste evenly on all surface mount pads. Again, during As mentioned, a common practice is to deploy one of reflow, too little paste creates voids or cold solders; too three types of thermal profiles, depending on PCB size much causes shorts or opens. A framed stencil typically and largely based on guesstimates. Consequences include provides the crucial uniformity to prevent these problems a high probability of cold solder joints or damaged com(Figure 1). 40 Circuits Assembly NOVEMBER 2007 circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - November 2007

Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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