Circuits Assembly - September 2008 - (Page 51)

Common Wave and Selective Soldering Defects and Solutions From bridging to shrinkage, where there’s a cause, there’s a cure. may be seen with some solder masks due to the increased o reduce defects, materials selection is critical, and soldering temperatures associated with Pb-free soldering. process optimization is paramount. Following is a Solder balls in between the pins may be caused by poor summary of the most common soldering defects flux activity. Preventive actions: Change solder mask; optiin the wave and selective soldering processes, and some mize solder temperature, or correct flux. of the preventive steps that alleviate them. Solder shrinkage (Figure 3): This pheSolder bridging: This is a buildup of nomenon is the result of solder reducsolder between leads or pads, causing tion during solidification of the joint. a short. Solder bridges occur when the The surface experiences significant solder does not separate from two or stress and relieves it by forcing the surmore leads before it solidifies. Prevenface to contract quickly. This phenomtive actions: Use a correct design: short enon is influenced by cooling rate, flux, component lead length and small pad alloy, surface finish, component type and pitch between the pins. Use a strong and component finish. Preventive action: flux and optimize the amount. Use a Figure 1. Solder lifting. Solder shrinkage cannot be avoided, but debridging tool if available. the addition of nickel in the alloy might Excess solder: Occurs when too much minimize micro cracks. solder is used to form the joint. PosVoids: These are holes in a solder sible causes include poor solder sepajoint. Voids can decrease electrical and ration conditions or barrel cracking. thermal conductivity of the interconRework is not recommended, as it will nection path and cause thermal failure. not improve joint reliability. Preventive Outgassing in the plated through-holes action: Optimize drainage conditions by Figure 2. Solder spikes. during soldering may produce holes using a different wave former. in the solder; another cause can be Fillet lifting (Figure 1): This is a condicontamination of the surfaces. Prevention in which the solder joint lifts off the tive actions: Improve board quality and PCB as the board cools and contracts clean surfaces of components. Pre-bake after soldering. The primary reason for boards; increase soak time, or use nitrofillet lifting is a coefficient of thermal gen. expansion mismatch of the materials. Skip joints: This occurs when solder Preventive actions: Keep solder temperapads and components are not wetted ture low; avoid low melting alloy comwith solder. Reasons include insufficient binations, and do not use SnPb-finished Figure 3. Solder shrinkage. flux on the pad during soldering, oxicomponents. dized copper pads, solder wave shadowPad lifting: A condition in which the ing, and blocking of the chip wave. Presolder pad lifts from the PCB as the ventive actions: Improve layout design, board cools and contracts after the solcheck fluxer, clean chip wave, check dering process. The primary reason for for pad wettability. If the solder does fillet lifting is CTE mismatch of the not cover the pad on the topside of the materials. Preventive actions: Keep solder board and has not traveled to the top of temperature low; avoid low melting solthe plated through-hole, then the joint is der contaminations, and optimize mateFigure 4. Insufficient hole penincompletely soldered. rial selection. Insufficient hole penetration (Figure Solder spikes (Figure 2): These occur etration. 4): This occurs when the solder has not when excess solder solidifies in a longtraveled to the top of the plated through-hole and does drawn pointed form on pads or terminations. Preventive not cover the pad on the board topside. Preventive actions: actions: Use a stronger flux or use nitrogen. Increase solder temperature, stronger flux activity, or use Solder balling: Occurs when tiny balls form around a “smart” wave. components. An increased incidence of solder balling n circuitsassembly.com Circuits Assembly SEPTEMBER 2008 Wave Soldering T Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com. 51 http://vitronics-soltec.com http://circuitsassembly.com

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Circuits Assembly - September 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Reflow Soldering with a SnCu Eutectic Pb-Free Alloy Improving OEE in High Mix Facilities Effectively Managing RF Design in Utility Metering Applications Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys Tech Tips Wave Soldering Pb-Free Lessons Learned Materials World Process Doctor Equipment Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - September 2008

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