| Common Wave and Selective Soldering Defects and Solutions |
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| Written by Ursula Marquez de Tino | |||
| Sunday, 31 August 2008 19:00 | |||
From bridging to shrinkage, where there’s a cause, there’s a cure.
To reduce defects, materials selection is critical, and process optimization is paramount. Following is a summary of the most common soldering defects in the wave and selective soldering processes, and some of the preventive steps that alleviate them.Solder bridging: This is a buildup of solder between leads or pads, causing a short. Solder bridges occur when the solder does not separate from two or more leads before it solidifies. Preventive actions: Use a correct design: short component lead length and small pad and pitch between the pins. Use a strong flux and optimize the amount. Use a debridging tool if available. Excess solder: Occurs when too much solder is used to form the joint. Possible causes include poor solder separation conditions or barrel cracking. Rework is not recommended, as it will not improve joint reliability. Preventive action: Optimize drainage conditions by using a different wave former. Fillet lifting (Figure 1): This is a condition in which the solder joint lifts off the PCB as the board cools and contracts after soldering. The primary reason for fillet lifting is a coefficient of thermal expansion mismatch of the materials. Preventive actions: Keep solder temperature low; avoid low melting alloy combinations, and do not use SnPb-finished components.  Pad lifting: A condition in which the solder pad lifts from the PCB as the board cools and contracts after the soldering process. The primary reason for fillet lifting is CTE mismatch of the materials. Preventive actions: Keep solder temperature low; avoid low melting solder contaminations, and optimize material selection. Solder spikes (Figure 2): These occur when excess solder solidifies in a long-drawn pointed form on pads or terminations. Preventive actions: Use a stronger flux or use nitrogen.  Solder balling: Occurs when tiny balls form around components. An increased incidence of solder balling may be seen with some solder masks due to the increased soldering temperatures associated with Pb-free soldering. Solder balls in between the pins may be caused by poor flux activity. Preventive actions: Change solder mask; optimize solder temperature, or correct flux. Solder shrinkage (Figure 3): This phenomenon is the result of solder reduction during solidification of the joint. The surface experiences significant stress and relieves it by forcing the surface to contract quickly. This phenomenon is influenced by cooling rate, flux, alloy, surface finish, component type and component finish. Preventive action: Solder shrinkage cannot be avoided, but the addition of nickel in the alloy might minimize micro cracks.  Voids: These are holes in a solder joint. Voids can decrease electrical and thermal conductivity of the interconnection path and cause thermal failure. Outgassing in the plated through-holes during soldering may produce holes in the solder; another cause can be contamination of the surfaces. Preventive actions: Improve board quality and clean surfaces of components. Pre-bake boards; increase soak time, or use nitrogen. Skip joints: This occurs when solder pads and components are not wetted with solder. Reasons include insufficient flux on the pad during soldering, oxidized copper pads, solder wave shadowing, and blocking of the chip wave. Preventive actions: Improve layout design, check fluxer, clean chip wave, check for pad wettability. If the solder does not cover the pad on the topside of the board and has not traveled to the top of the plated through-hole, then the joint is incompletely soldered. Insufficient hole penetration (Figure 4): This occurs when the solder has not traveled to the top of the plated through-hole and does not cover the pad on the board topside. Preventive actions: Increase solder temperature, stronger flux activity, or use a “smart” wave.  Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
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