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March 21, 2018

Toe Fillet Requirements on Gull Wing Components

Is a toe fillet required on gull wing components? If it is not a requirement why is that so? We have some gull wing components that have no toe fillet, but all the other requirements for the solder connection are met.


Expert Panel Responses

Per IPC-610, section 8.3.5 Flat Gull Wing Leads and section8.3.6 Round or Flattened (coined) Gull Wing Leads, there is no requirement fortoe fillets. The only requirement is that if the lead overhangs the pad or landarea it does not violate the minimum electrical spacing requirements of theproduct. The reasoning for not having a requirement for toe solderfillets is that the method of manufacturing for the gull wing lead framescreates a condition on the end of the lead which may or may not be solderable.Therefore we did not create a criteria for a solder fillet at the end of theleads, if it happens it is ok and if it does not happen that is stillacceptable. The strength of the solder joint for gull wing components is in theheel fillet of the component and the length of the component lead, so thesolder joint at the toe of the component does not add anything to the strengthand reliability of the solder joint. If all the other requirements are met except the toe fillet ismissing as mentioned the condition is acceptable. I hope this answers your question.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
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.

Per J-STD-001, there is no requirement:
  • There is no reference to a minimum toe fillet height (Class 1, 2, and 3)
  • Toe overhang is permitted where it does not violate electrical clearances (Class 1, 2 and 3), in which case by definition there is no toe fillet.
While this may seem odd, we need to realize that the toe of a gull-wing lead is under little or no mechanical stress, because the component is attached at the heel end. In practice, nearly all failures of gull-wing joints begin at the heel and propagate toward the toe. In addition, the fact that most gull-wing leads are formed and cut after plating leaves the toe relatively unprotected from oxidation, and so getting a toe fillet may be difficult.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

The NASA requirement is the width of the endsolder joint (toe) must be greater than 75% of the lead width.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and 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.

There is arequirement to have the pad extend past the toe of the lead... however, thesolder presence cannot be left to a toe fillet only. The design of thepads, coupled with the dynamics of the reflow will pull the solder from heel totoe and vice/versa. For this reason, thetoe fill maximum requirement is often noted, but the minimum, due to control isnot defined as a defect.

Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Structurallythe most significant elements of a gull-wing solder joint are the solderbetween the pad and the bottom of the gull wing lead and the solder at the heelof the lead (heel fillet). Toe fillet can't be counted on nor does it addany appreciable strength. Many gull-wing devices are sheared from theirlead-frames after lead-frame plating. This results in exposedbasis material at the point of plating buss shearing. So if the basis isunsolderable or tough to solder, as most are, it may not wet during thesoldering process. A good example of this would be a copper alloy lead-frame. Copper (sans OSP-coating) is notably difficult to flux with typical No-Cleanflux formulations. Plating is added to such lead-frames to allowreasonable solderability during SMT processing. Alloy 42 (Kovar) isanother lead-frame alloy this is not solderable without the addition of aplating. Therefore, there can be no expectation for toe fillet formationduring soldering. Bottom Termination components (BTCs) are another class ofcomponent with sheared lead and no reasonable prospect for toe fillet formation. Lastly, recall that IPC 610 allows for some gull wing toe overhang beyond thePWB's solder land. Certainly on an overhanging lead one could not expectto have a toe fillet. Concentrate on getting the rest of the solder jointright, especially an in-spec (per -610) heel fillet.

Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

Per IPC-610, there is no requirement for toefillets. If all the other requirements are met except the toefillet is missing as mentioned the condition is acceptable. My recommendation is to try to stay away from a commonbad practice: finding an issue where there is none.A solder joint is conforming or non-conforming there isnothing in between.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment
The reasons given for lack of a fillet and its contribution to solder joint strength make good sense. How do you reconcile the statements about no requirement for a toe fillet with Figure 8-89 in IPC-A-610F and the definitions for a defect being "minimum end joint width (C) is less than 50% lead width (W)"? If my customer points to this figure for a condition where there is no toe fillet, I don't see how I can defend this position.
Jeff McCoy, ARQ Electronics

Reader Comment
The answer to Reader Comment from Jeff McCoy is, the height of the end joint is Dimension G--not any part of the toe height. If wetting is evident for minimum 50% of the lead width, the requirement is met.
John De Leeuw, TE Subcom