The devil's in the details: tombstoning revisited: you've checked the oven, the oven profiles, the solder paste and the nitrogen levels--why are you still encountering tombstoning?
A: Although using a nitrogen-enhanced soldering atmosphere benefits wetting characteristics during soldering, tombstoning of small devices is often associated with low oxygen levels. However, other attributes of the thermal profile, oven setup, conveyor operation or assembly characteristics may be contributing to, or causing, the tombstoning.
Check the Oven, Nitrogen and Oxygen
The control of the gas flow in a forced convection oven is very important. Inadvertent introduction of undesirable turbulence, from shop conditions and/or oven curtain damage, could add to the possibility of tombstoning concerning small components. Depending on the oven design and gas flow control, the total flow rate may be too high. But, more likely, a maintenance or oven setup issue ks causing an undesirable directional impingement of the oven atmosphere on the assemblies.
To get the full benefit of nitrogen enhancement wetting forces, a maximum of 2,000 ppm or 500 ppm is often quoted as a desirable upper level for oxygen. However, some facilities use nitrogen to drive the oxygen level down to 100 ppm--a process that is not always necessary. The question relates that tombstoning only occurs when nitrogen is in use and only on specific boards. This fact suggests that the wetting speed may be fast enough, but inadequate on specific pads--leading to the conclusion that the profile might be in need of attention. Raising the level of oxygen could, perhaps, prevent the problem.
Profiling the paneled assemblies, with and without nitrogen flowing and with thermocouples placed at locations where the tombstones occur, may also help. Unequal heating detected at specific pads may be the result of the attachment of one of the component pads to a groundplane. In such a case, the addition of a thermal relief to the groundplane is a good solution for a problem that cannot be solved with a profile adjustment.
The Solderability Issue
Remember, tombstoning is related to variations in wetting forces and wetting speeds--all components and boards should exhibit adequate and consistent solderability. The solderability issue is often overlooked and "dial twiddling" machine settings are substituted as a solution to solder joint defect issues. Solderability is particularly important with 0201 and 0402 components, as small differences in wetting characteristics can make big differences with such components. Any significant temperature or solderability differential between the terminations, at opposite ends of any susceptible chip component, can result in tombstoning.
Variation in solder mask thickness is also an important, though often overlooked, issue that can contribute to tombstoning--particularly with small, lightweight components. Thickness variations are sometimes encountered with a hot air leveled solder mask and can sometimes cause a "teeter-totter" condition between pads--lifting one component termination off the pad. Tombstoning has also been associated with large, wide pads that may allow the component to slide during wetting, moving one termination off the pad.
Personal communication with Andy Mackie, Praxair, Inc.
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DISCLAIMER: We believe the information provided in this work is relieve and useful, but it is furnished without warranty of any kind from the author. Readers should make their own determination of the suitability or completeness of any of the material included for specific purposes and adopt any safety, health and other precautions as may be deemed necessary by the user.
Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; (623) 544-4646; e-mail: firstname.lastname@example.org.
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|Title Annotation:||Ask Les|
|Date:||Oct 1, 2002|
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