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February 4, 2008
Lead Float
We are placing and soldering TSOP-56pin I.C 's. We have done some investigation, checking the component before loading into the SMT machine, as well as checking the solder applied on all pins.
We continue to have the problem.
Why and how does lead float occur?
G. H.
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Ask the Experts Comments
February 4, 2008
How big is the die in the TSOP? A number of companies are reporting issues with this kind of behavior with Alloy 42 TSOPs with large die, especially on bigger, thicker boards with more copper.
Dr. Craig D. Hillman, CEO & Managing Partner
DfR Solutions
chillman@dfrsolutions.com
 Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification (commodity and engineered products), passive component technology and printed board failure mechanisms. Dr. Hillman has over 30 publications and has presented on a wide variety of reliability issues to over 150 companies and organizations.
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February 4, 2008
There could be a variety of reasons for this particular condition to occur with this particular type of component. There is no mention made of the lead plating material, the type of solder paste used and the loading of the solder paste flux in the question and all of these parameters must be taken into consideration when trying to figure out this problem.
Basically the component floats because the surface area of the leads overcomes the weight of the component, the same as a ship floats on the water, it displaces its own weight. The temperature of the profile, the z axis placement force should all work together to heat the leads, allowing the flux from the paste to reduce the surface tension on the leads thereby allowing the solder to wet the leads and pull the component down into the molten solder.
There are many other issues to consider and I would be willing to discuss this if you get in touch with me.
Leo Lambert, Vice President, Technical Director
EPTAC Corporation
leo@eptac.com
 At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee, covering areas of mounting, soldering, solderability and assembly.
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February 4, 2008
Due to the light weight of these components and the surface tension of the solder this may be causing the components to float out of alignment during reflow. I would also look for some type of vibration occurring as the PCB is moved thru the reflow oven OR a cooling fan blowing the part out of alignment.
Try the following:
Rotate the PCB 90 or 180 degrees on the oven belt or conveyor to see if it is caused by a cooling fan.
Check the components leads for oxidation that is preventing the solder from wetting to the leads.
If this doesn’t help try a gluing operation to secure the TSOP in place or reduce the solder volume in the TSOP locations.
Edward Zamborsky, Regional Sales Manager
OK International Inc.
 Mr. Zamborsky serves as one of OK's technology advisors to the Product Development group. Ed has authored many articles, and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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